Membranes in the Dairy Industry - ACS Publications - American

Chapter 11

Membranes in the Dairy Industry

Downloaded by UNIV OF GUELPH LIBRARY on June 26, 2012 | http://pubs.acs.org Publication Date (Web): October 31, 2011 | doi: 10.1021/bk-2011-1078.ch011

Heath H. Himstedt and Jamie A. Hestekin* Ralph E. Martin Department of Chemical Engineering, University of Arkansas, Fayetteville, AR 72701 *E-mail: [email protected]. Phone: 479-575-3492.

Membranes have significant uses in the dairy industry. Many different types are used including reverse osmosis, nanofiltration, ultrafiltration, microfiltration, and electrodialysis. New applications are being developed although much of the classic technology continues to be used. This chapter explores the different uses of membranes in the dairy industry, discusses the challenges, and gives recommendations for technical solutions in the future.

1.1. Introduction Membranes in the dairy industry are widely used and yet many significant challenges and opportunities remain to their even further use. As of 2001 it was estimated that in terms of total square footage the dairy industry used 400,000 for ultrafiltration, 300,000 for nanofiltration, 100,000 for reverse osmosis, and 50,000 for microfiltration with as much as 20-30% of the total membranes used in the manufacturing industry coming from dairy (1) . The sheer number of separations is based on the nature of the feed stream and the myriad of products that can be produced from milk. Milk is a very complex mixture ranging from fat, often several microns in size when in native globule form, to salts, which are only a few daltons. On average, milk will be approximately 3%+ protein, 4%+ lactose, 3.5%+ fat and 1% ash. Several other components exist in more dilute concentrations such as amino acids, vitamins, and flavors. Because of this highly diverse composition, membrane processes can produce economical separations. Figure 1 shows many separations using different pressure driven processes. In this figure the membranes get tighter as you go from left to right and thus more and more components are rejected. That is, reverse osmosis rejects almost everything while microfiltration rejects only particulates. Membranes, however, © 2011 American Chemical Society In Modern Applications in Membrane Science and Technology; Escobar, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


also separate on the basis of electrical driving force (in which charged salts can be separated) and volatility (in which water and flavors can be separated). All of those processes will be described in the section below except for volatility driven separations, which have found little use in the dairy industry because of the low concentration of flavors in milk. In all cases, a historical and a current overview will be presented and suggestions for future processing will be made. Hopefully by describing the characteristics of the streams as well as giving membrane flux information, this chapter will lead to the development of new ideas and help facilitate new applications in the dairy industry.

1.2. Microfiltration Microfiltration (MF) separates a given liquid feed into two product streams according to mean particle diameter or size (2). The mean pore diameter, or pore size, of MF membranes is not definite and ranges approximately between 0.1 μm and 20 μm (2, 3). This range is particularly beneficial to the dairy industry because it allows for the selective separation of bacteria and microorganisms, whey proteins, casein micelles and milk fat globules (3). Many of the different techniques used in these separations will be described below.

Figure 1. Separation of Milk with Pressure Driven Flow.

172 In Modern Applications in Membrane Science and Technology; Escobar, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


1.2.1. Bacteria Removal and Increased Shelf Life One major application for MF in the dairy industry is the removal of harmful microorganisms from raw milk before the milk undergoes additional processing into numerous dairy products (2, 4). Milk with a lower bacteria count forms more hygienic dairy products (5), leads to longer shelf life (1, 6–8), can prevent negative side effects associated with bacteria and their enzymes (3, 9), and can improve taste (1, 6, 10, 11). Almost all the microorganisms present in milk can be retained using a 1.4 μm pore diameter membrane (1, 2, 6, 9, 10, 12). MF was first applied to bacteria reduction and shelflife improvement by Alfa/Tetra Laval, who named the process Bactocatch® (7). When defatted skim milk is fed into the Bactocatch®, it permeates essentially all of the milk components through the membrane while most microorganisms are retained. Skim milk is used since milk fats can be easily removed by centrifugation prior to the membrane process, and fats rapidly clog the membranes if left in the MF feed (3, 13). The bacteria-laden retentate is sterilized and recombined with the low-bacteria permeate, or the retentate is concentrated and then discarded (7, 11). By maintaining a constant trans-membrane pressure, fluxes of 500-700 L/m2*hr m2 or greater have been obtained for pressures around 0.5 bar (1, 2, 6, 7). This is a high flux for this type of separation. Today, MF plants are capable of removing over 99.5% of the bacteria found in milk and are used commercially by Cravendale in the UK, Natrel in Canada, and TetraPak in Switzerland among others (10, 14–17). MF offers many advantages over pasteurization, the traditional bacteria reduction technique (2, 9, 18, 19). When compared to pasteurization, MF is more energy efficient and results in a significantly lower, and more uniform, bacteria count (3, 15). All somatic cells present in milk are totally retained by MF membranes of an appropriate diameter; consequently, the resulting milk will not be degraded by the thermoduric enzymes the bacteria produce, which traditionally would require high temperature pasteurization to destroy (4, 20). Pasteurization on the other hand results in areas of varying degrees of sterilization due to the uneven application of heat and/or conglomerates of organisms requiring more heat to sterilize than if the organisms were not conglomerated (3). It has also been shown that the nutritional and bioactive properties of milk and its components will remain intact or be damaged as little as possible in products derived from microfiltered milk compared to the rather harsh process of pasteurization (2). In addition, MF is able to remove heat-resistant organisms that pasteurization cannot eliminate or destroy without using extremely high temperatures. Perhaps the greatest advantage of MF over pasteurization is that MF removes harmful microorganisms and spores while pasteurization merely kills, but does not remove the dead cells. Since the spores are still present in pasteurized milk and are extremely difficult to destroy, they can often lead to spoilage or more harmful degradation products (2). Spores are of special concern since they exist in strains of the Lactobacillus or Clostridium bacteria, which are common in milk and are not destroyed by traditional heat treatment (21). These spores, under certain conditions, can be fatal.



Figure 2. Example Process of Spore Removal by Membrane Microfiltration. An example of a MF process for bacteria and spore removal, based on a review of the literature, is given in Figure 2. Before being fed to the membrane bank cream, a fatty stream separated from the raw milk by centrifugation to form skim MF (around 1.4 µm) milk feed because the milk fat will clog the membranes. The bacteria and spores are successively removed from each membrane stage while low-bacteria skim milk passes through the membranes. The bacteria-rich retentate from each stage is combined along with the cream from the centrifugation to be pasteurized at ultra-high temperatures. This stream is then combined with the low-bacteria MF milk permeate to be homogenized and cold-pasteurized to obtain a longer-lasting, better-tasting, reduced bacteria product such as described by (3). The recombination of the two reduced-bacteria streams (cream and raw milk) produces a “non-pasteurized”, very near-sterilized milk whose nutritional, hygienic and organoleptic (taste and texture) qualities have been shown to be superior to traditionally pasteurized milk for use in various dairy products (1, 6, 11). Therefore the second pasteurization can be performed at a very low temperature, cutting costs and equipment load (10). This process achieves a more uniform sterilization than pasteurization alone and retains all of the fat present in the cream, an advantage in certain applications compared to microfiltering the entire feedstock, which removes bacteria as well as fat. For this example process, it is assumed that a MF membrane rejects 99% of a common spore. In this case, if the feed contains ~10 (6) spores, 86% of the flow can be treated by MF in a three-stage process; however, spores are still present in the range of ~100. Or simply stated, MF cannot be used for complete spore removal (sterilization) unless very high rejections and several stages are used. Industrial efforts have produced fluxes around 500 L/m2*hr at pressures around 0.5 bar using membranes such as Sterilox and Membralox with pore sizes of 1.4 µm (2, 3). If 99.9% rejection is assumed, more of the feed can be treated, but in any 174 In Modern Applications in Membrane Science and Technology; Escobar, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


case it is difficult to certify that no spores or bacteria pass through the membrane. Therefore, at this time, the US FDA will not allow membrane processes to replace heat treatment for legal pasteurization and therefore is not practical in the US at this time other than product improvement. Microfiltered milk is not only safer to consume, but also lasts significantly longer (17) since the bacteria count—units of cfu or colony forming units—is lower in microfiltered milk than in pasteurized milk (18). Processes for the production of microfiltered milk containing less than one bacterium per ml have been reported; a vast improvement compared to several hundred per ml in pasteurized milk (10). This lead to a doubling of milk shelflife for MF milks compared to pasteurized milks (10, 17, 18). Recently used 1.4 µm Membralox membranes at fluxes of approximately 200 L/m2*hr to achieve bacteria counts as low as 0.24 cfu/mL from raw milk containing 2,400 and 1,475 cfu/mL (15); a 3.79 log reduction in count. Pasteurization of the MF permeate milk achieved an additional 1.84 log reduction, slightly higher than the typical 1.67 log reduction of raw milk pasteurization. The combination of processes yielded a 5.6 log reduction from 2,400 or 1,475 to 0.005 cfu/mL. Additionally, coliform and spore counts were below detectable levels (15). The authors defined shelflife as the time when milk contained less than 20,000 cfu/mL as per the Pasteurized Milk Ordinance; greater than 20,000 was termed spoilage. The authors stored raw and pasteurized MF milk for 92 days at 6.1, 4.2, 2.0 and 0.1°C. All of the control samples reached spoilage by 22, 29, 57 and 92 days for the four temperatures. In contrast, only 50% and 12% of the 6.1°C and 4.2°C pasteurized MF milk was spoiled at the end of testing period. Moreover, no samples stored at 2.0°C or 0.1°C even reached detectable bacteria counts, i.e. greater than 1,000. The authors note that no milk is considered to have an indefinite shelflife because of proteolysis within the milk leading to an off-flavor (22). However, even taking this into account, the pasteurized MF milks still attained much higher shelf lives showing that MF milk not only contains a greatly reduced number of bacteria, but also that this count increases with time at a slower rate than that of pasteurized milk (15). 1.2.2. Recombined Milk and Cheeses As mentioned previously, the high-bacteria, high-fat MF retentate is often treated separately and then recombined with the whey protein-containing permeate. The resulting mixture is then sent for further use or dried to form a recombined milk powder, which can be used as feedstock to produce dairy products such as yogurts, drinks, butters and cheeses (23–25). Normally, recombined milk requires modifications to the product procedure; however, this is not the case with recombined milk that has first been microfiltered (23, 26, 27). Product yield, taste and structural properties have been shown to benefit from the use of recombined milk (26, 28–31). The most prevalent use of recombined milk is in the production of cheeses. They are made using milk powders or liquid feeds specifically enriched in casein and depleted in proteins (26, 28–32). With MF it is possible to design a process using a casein-enriched retentate as cheese feedstock which does not require any modifications to existing cheese manufacturing techniques (33). These cheeses are 175 In Modern Applications in Membrane Science and Technology; Escobar, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


depleted in fat and salt and free of rennet and starter cultures. This reduces costs and increases yields since the need for downstream whey purification operations is decreased or eliminated (33). Various cheeses have been produced through the use of MF including cheddar (31, 34–36) and mozzarella (12, 23, 37) using both high and low concentration factor MF. One example is the skim milk powder depleted in whey protein for use in countries with shorter milk supplies by removing roughly 67% of the whey proteins (23). They report cheese making yields were 7.3 ±1.8% higher for cheeses produced with the powder rather than traditionally made cheese. This lead to a notable reduction in the volume of whey being discharged as well as a more economical process. Chemical additions were also reduced, leading to a more environmentally friendly, better tasting and healthier product (23). Nelson and Barbano describe a process which removes serum proteins from raw milk for use in cheese manufacture (38). The process consists of a three-stage MF system coupled with two three-stage Ultrafiltration (UF) system, which function as a diafiltratin step. Diafiltration, discussed at length later, is a technology which allows for increased concentrations to be obtained in membrane retentates through the use of successive diluting and concentrating. The overall process demonstrated by Nelson and Barbano is shown in Figure 3. Raw bovine milk was centrifuged to produce a skim milk feed. Membralox 0.1 µm membranes ran at about 0.4 MPa were used to remove the serum proteins from the milk. The MF bank was operated such that the retentate weighed one third of the skim milk feed. The retentates were diluted with two parts permeate from the UF systems. This mixture was recirculated through the MF membrane to obtain a retentate with a 3x protein concentration increase. This was repeated for each of the three MF stages. The MF permeates from each stage were collected and passed through the UF banks (flat sheet plate and frame units using 10,000 MW polysulfone membranes) at fluxes of approximately 80 L/m2*hr to form serum protein concentrates by concentrating the MF permeate feed to 20x. The UF permeate served as the diluting agent for the diafiltration of the MF retentate as described previously. Additionally, the balance of the UF permeate not used for diafiltering serves as an excellent feedstock for producing high purity lactose. The retentate of the third MF stage, after being diluted to a concentration useful for cheese production, had roughly 95 ± 1.1% of the serum proteins present in the feed skim milk removed. This final retentate has 3.1 times the concentration of casein present in the feed. The combination of high casein and low serum proteins made this very attractive for cheddar and mozzarella cheese manufacture (38). The allure of this process is that no acid whey is produced, which is a low-value byproduct of cottage cheese manufacture. The serum protein depleted MF retentate described above is diluted with water and then ultrafiltered to reduce the lactose and mineral content to the composition typical of commercial cottage cheese. This UF retentate is then ultrafiltered twice more to obtain a cottage cheese feedstock with the necessary physical properties. Coagulant and flavorings are then added to yield cottage cheese on a continuous basis with no acid whey production.



Figure 3. Process for Removal of Serum Proteins from Raw Milk for use in Cheese Manufacture described by Nelson & Barbano (38). MF retentates were also used by to produce a low-moisture, part skim mozzarella cheese (39). Pasteurized skim milk was microfiltered using a Megaloop continuous MF system equipped with a 0.1 µm ceramic membrane. The skim milk was concentrated to a retentate concentration factor of 6, 7, 8 or 9, which removed approximately 66 to 71% of the proteins. The MF permeate was very similar to cheese whey but did not contain the additives used in cheese production. This could lead to better functional properties of whey protein concentrates (WPC) and isolates (WPI) compared to those produced from cheese whey. The MF retentate was mixed with pasteurized cream to standardize the casein to fat ratio at unity. A stirred curd, no brine cheese method as described by Barbano et al. (40) was then used to make low-moisture, part skim mozzarella. The cream and MF mixture was allowed to set before additives were introduced. This passed through a vatless coagulator before proceeding to cutting. A minimal amount of whey was then drained. The mixture was then cooked at 38°C continuously, salted, and then stretched and cooked in a salt solution at 57°C. Finally the product, a low-moisture part-skim mozzarella with a similar composition as commercial cheeses, was cooled and packaged Production of cheese through the use of MF leads to a reduction in industrial footprint since large vats can be replaced with smaller continuous coagulators. Additionally, rennet use was cut by 93% and whey loses were reduced (39). 1.2.3. Fat and Casein Removal Another use for MF in the dairy industry is the separation and fractionation of milk fat; that is, separating the solid curds from the liquid whey. Removal of milk fat is achieved by a process analogous to bacteria removal; the only major difference being that the separated fat can be processed to form consumer products rather than being discarded or recombined as with bacteria (1, 6). The difference between the process used here as compared to the one used for bacteria removal is the pores of the membrane are smaller and thus are not clogged by milk fat. Once the fat is removed, the permeate can be used to produce fat-free 177 In Modern Applications in Membrane Science and Technology; Escobar, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


dairy products, or the fats can be isolated and can serve as feedstocks. The fat is present predominantly in globules varying in diameter from 0.1 to 15 μm (Walstra et al, 1999). Small globules—diameters less than 2 μm—are separated from the large globules (11). Professional tasting panels have attested that small fat globules lead to more texturally pleasing products, with the exception of butter for which large globules are preferred (1, 2, 6, 11). A similar method is used to remove residual lipids from whey to increase the purity and value of whey protein concentrates (WPC) and isolates (WPI) produced by subsequent UF (3)(7) MF increases the opportunities for fractionation of milk as the micellar phase can be either retained or passed into the filtrate dependent upon MF pore size (1, 6). It is highly desirable to retain the casein micelle phase while permitting the soluble components of milk, namely whey proteins, to pass through. This typically requires pore sizes below 0.2 microns (3). When milk is microfiltered, a fat-free serum permeate containing the whey proteins—discussed in detail in the UF section—is obtained (41). The retentate, which contains casein as well as the fats, is processed further into many different milk products ranging from powders to cheese. Dried milk powders are sought as a texture and curding enhancer, whitening agent and nutraceutical components (3, 42). The production of isolated casein on an industrial scale would usher in a completely new platform for exploiting casein’s functional properties, which have previously only been available with major modifications to the structure—rennet casein—or in a completely disintegrated form—acid caseinate (43, 44). Powdered micellar casein, if obtainable industrially, holds great potential for use in fortifying cheese milk, replacing cheese curd in process cheese methods and isolating individual proteins to produce purified nutraceutical derivates from milk (45, 46). Additionally, partial removal of whey proteins by MF increases yield by significantly reducing the detrimental effects of heating on rennet coagulability and the cheesemaking abilities of milk, since there are fewer proteins which can become denatured by heating processes (23). To the best knowledge of the authors at the date of publication, no industrial scale process has been implemented, but processes have been proposed. It is believed that relatively pure casein can be obtained by beginning with skim or fatfree milk. One such procedure was performed by Gautier et al. on the laboratory scale (47). Milk was circulated along a 0.1-0.2 μm MF membrane to obtain a permeate with a composition close to that of sweet whey, known as “ideal whey” because of its great potential in WPC and WPI manufacture (48). The permeate is clear and will remain sterile and viral free so long as recontamination is prevented by appropriate measures on all downstream equipment. The retentate obtained from this process is an enriched milk solution of native micellar casein which can be dried to form a casein powder (47). Saboya and Maubois proposed a three-stage MF system to concentrate the casein (2). The highly concentrated retentate is spray-dried to obtain purified micellar casein. Many variations of the process that show its versatility and potential as an industrial-scale solution for the production of micellar casein have been described (49, 50). As the worldwide demand on the dairy industry continues to grow, an increasing supply of natural casein is needed; a need which MF has the potential to fill (1, 6, 11). The eventual production of micellar casein on an industrial scale 178 In Modern Applications in Membrane Science and Technology; Escobar, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.

will depend heavily on the profitable use of the lipid-free serum (MF permeate). It can be further processed and concentrated to obtain WPCs and WPIs with high concentrations of total proteins and certain target proteins, respectively. For example, the permeate from a MF casein removal process could be ultrafiltered to separate α-lactalbumin for use in human infant formulas, or for recovery of minor whey proteins showing promise in pharmaceutical applications (3).


1.2.4. Microfiltration Conclusions It is apparent that MF has the potential to serve as a vital technology in meeting the increasing demand on the worldwide dairy industry by providing healthier, better tasting and more nutritional products in a more efficient, cheaper and purer manner (1, 6). As mentioned previously, MF eliminates the organoleptic problems induced by the high temperatures of pasteurization and provides a gentler environment for processing (1, 6). The advantage of MF is that it can perform multiple tasks previously dedicated to independent unit operations while avoiding the irreversible physical changes often associated with dairy processing—such as protein denaturation or flavor degradation. These changes very often lead to loss of the ability to further modify the functional properties of certain milk components. MF performs the necessary operations while preserving the components of milk in their naturally-occurring forms, thus retaining the ability to exploit and specifically adjust the functional properties to produce healthier and more nutritional products (43, 44). Three identified opportunities include: 1) guarantee 100% bacteria and spore removal when using microfiltration membranes for ultimate FDA approval of spore removal as pasteurization, 2) design membranes easier to clean so they can be ran for longer periods of time, and 3) developing a process where fats can be incorporated in microfiltration separation for bacteria removal.

1.3. Ultrafiltration Following MF in terms of application in the dairy industry is ultrafiltration (UF). UF is generally characterized by molecular weight rather than diameter as with MF. Although there is no definitive molecular weight range for UF, molecular weight cutoffs (MWCO) between 1,000 and 1,000,000 are widely accepted as UF (9, 51, 52). In the membrane industry molecular weight is often reported in units of Daltons (Da) or molecular weight units. Although not strictly equal, Daltons are, for engineering purposes, interchangeable with MWCO. In dairy, the most common UF module is a 10,000 MWCO because it rejects nearly all of the casein and why proteins while letting components like lactose and ash permeate through the membrane (9, 51, 53, 54). UF is exceptionally versatile when applied to the dairy industry since membrane pressure, flow rate, concentration and temperature (except in the case of calcium compounds) have shown little to no effect on UF retention of milk and whey compounds. UF is unique in that it can reject 100% of fat, nearly 100% of proteins, but permits lactose, salts, ash and water-soluble minerals to pass through (9, 51, 55). Thus, UF can potentially 179 In Modern Applications in Membrane Science and Technology; Escobar, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.

increase product yield and reduce waste by selectively concentrating milk and the full range of milk proteins so that as little as possible is discharged. Compared to traditional methods such as vacuum evaporation, UF consumes less energy. UF also requires a smaller industrial footprint since it can simultaneously perform tasks which previously required multiple unit operations. Currently, UF serves many interests in the dairy industry ranging from novel cheese techniques to protein fractionation, as discussed below. Additionally, the definition and benefits of diafiltration are also presented.


1.3.1. Cheese Production There have been many efforts to develop simpler and quicker processes for making cheese and cheese-type products through the use of UF. For instance, UF can be used to pretreat milk and modify the concentrations of the proteins before proceeding to traditional cheese-making techniques such as inoculation starters, acidification and heat treatment (56–65). It is estimated that the yield increase on cheeses, specifically soft cheeses, manufactured from UF retentate can be as high as 10-12% resulting in increased profits of 3-5% (51). The permeate can be used as feed in whey processing with advantageous properties compared to traditional whey. This is because the UF permeate whey is free of by-products and additives of traditional cheese manufacture (48, 66–69). UF precheeses are advantageous because they lead to more uniform product consistency, texture and weight (53, 70, 71). Many cheeses including feta (51, 72, 73), camembert (53, 74), mozzarella (72, 75, 76) and cheddar (48, 51, 77–79) have been produced using UF technology. Ferreira et al. made reduced fat mozzarella using UF concentrate with their process consisting of using low fat milk (1.6%) and acidifying to pH 5.65 and heating to 50°C (76). This temperature increases the membrane flux and allows longer runs. They used Koch Hollow Fiber Polysulphone Membranes of 10,000 MW cut-off with an inlet pressure of 0.16 MPa. They were able to easily concentrate to 2.5 concentration factor but found that the cheese was inferior in terms of elasticity and firmness (76). Soft and fresh cheeses have proven more successful when made with UF than hard cheeses, such as cheddar and mozzarella (38, 39). By producing cheese in such a manner, it is possible to produce cheeses with higher calcium content, less fat, better body and favorable taste compared to traditional techniques (48, 58–60, 75). Additionally, these processes can be performed in a continuous manner; a feat impossible for traditional cheese production. Previous attempts to commercialize UF cheddar and mozzarella have failed since the processed cheeses were not fully comparable to traditional cheeses (72, 79, 80). Processed cheeses with improved qualities have now been produced, which will hopefully lead to successful commercialization (28–30, 38, 48, 51, 70, 75). Trecker et al. developed one such process whereby lactic acid is used to acidify pasteurized milk to a pH of 6.0 to 6.3 (81). The resulting milk is subjected to UF and optionally diafiltration as seen in Figure 4a. A UF retentate with a volume concentration ratio between 4 and 7 is produced. This retentate contains 15-50% total solids and 2.5-4% lactose. Salt, to prevent coagulation, and additional lactic acid are then added. A precheese with a total solid content of 30-70 % is formed through the evaporation of this mixture. Emulsifiers, flavorants 180 In Modern Applications in Membrane Science and Technology; Escobar, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


and other additives, such as preservatives, are combined with the precheese at a temperature between 150 and 240°F before being subjected to conventional cheese production techniques. The resulting homogenous process cheese-type product is ready for immediate packaging and requires no dedicated curing step since all necessary curing can take place under refrigerated conditions during transport and storage time between cheese production and consumption, normally one to two weeks. There is no formation or separation of curds and whey as well as no added enzymes, cultures or microorganisms. Cheeses ranging from full-fat to non-fat are produced having similar organoleptic properties to traditional techniques. In addition, the processed cheese is more nutritious compared to traditional process cheeses because the whey proteins normally discarded with the separated whey are retained in this process. This process can be employed as batch, semi-continuous or continuous. Processing time is only about five hours for a batch process and about two hours for a continuous process. Figure 4b presents a process, described by Trägardh, for continuously producing cheddar cheese via UF (7). The retentate from ultrafiltered milk is divided with 10% being pasteurized and 90% reserved for downstream processing. Starter culture is added to the pasteurized retentate. The mixture is then fermented before combining with rennet and the 90% unmodified UF retentate. This new mixture is coagulated continuously and cut. Continuous syneresis produces ready to consume cheddar cheese and whey ready for fractionation via NF, RO, or additional UF. Agrawal and Hassan describe a process to produce reduced-fat cheddar cheese (70). Raw milk was separated into skim milk and cream. The skim milk was then ultrafiltered using a spiral wound 10,000 MW Koch membrane. The retentate was then blended with cream to standardize the casein to fat ratio at 1.35. Cheddar was manufactured from this cheesemilk without modifications to standard techniques. The resulting cheeses contained higher moisture than reduced-fat cheddars from regular milk, which verified previous reported results (82). The study showed that bitterness required in sharp cheddar cheese containing high moisture levels could be produced by UF with a 1.2x concentration factor. Another process describes the production of stirred-curd pizza cheese from ultrafiltering sweet cream buttermilk (28–30, 83). Studies have shown that UF buttermilk improves the texture and moisture content of low-fat cheeses (75, 84–87). In one embodiment, buttermilk was ultrafiltered at 28 bar using 10,000 Da polyethersulfone membranes to achieve a retentate concentration of 18 to 20% solids (83). The resulting cheesemilk could then be standardized with cream and processed using standard cheese making techniques (28–30, 83). The fat and casein contents of UF cheese were slightly higher than the control, but overall the cheeses had similar compositions. The increased fat and casein lead to higher yields in the UF cheese with no differences detected in flavor attributes and lower free oil (28–30). It was determined that UF of sweet cream buttermilk was attractive since it produced a product with improved qualities while still retaining the functional properties necessary for quality pizza cheese.



Figure 4. Cheese Manufacture from UF Milk via two different methods described by a) Trecker et al. (81) and b) Trägardh (7). Processed cheese and cheese products produced from UF-concentrated feedstock are beneficial to the consumer as well as the manufacturer. These products can be tailored to the health needs of specific groups of consumers including reduced sugar products for diabetics and lactose-free products for intolerants (88). The manufacturer is able to simplify the production process because UF can simultaneously perform multiple tasks, which previously required multiple unit operations. Additionally, UF allows plants to process more product with a smaller capacity by concentrating feedstocks to higher solids content, i.e. lower water content. This reduces the steam and energy requirements for drying operations, the volume of waste discharge, and overall process costs (7, 89, 90). 1.3.2. Diafiltration Diafiltration (DF) is a process that is coupled with membrane processes, particularly UF, to increase the purity of the concentrate beyond what is practical, or sometimes possible, with standard membrane filtration. DF uses UF membranes to first concentrate the feedstock until the practical protein purity limit has been reached, remembering that flux decreases and power requirements increase as the concentration of the retentate increases (52, 88). After the first concentration, the retentate is diluted (either continuously or with equal parts 182 In Modern Applications in Membrane Science and Technology; Escobar, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


deionized water and retentate). This diluted retentate is then filtered again in a batch diafiltration mode. This process is repeated—each time more protein is retained—until the desired protein purity in the retentate has been obtained. There are three methods of diafiltration: continuous, discontinuous by sequential dilution and discontinuous by volume reduction. With continuous diafiltration, the low molecular weight species in the retentate is diluted by adding water at the same rate as filtrate is generated. As a result, the retentate volume and product concentration do not change during the diafiltration process. Sequential dilution involves diluting the sample with water to a predetermined volume and then concentrating the sample back to its original volume. Volume reduction diafiltration works in the opposite manner. The sample is concentrated to a predetermined volume and then diluted back to its original volume with water. Both discontinuous methods are repeated until the desired purity is reached. Continuous diafiltration is regarded as a gentler process, especially when dealing with biological molecules, since the retentate concentration gradually changes, rather than the sharp peaks of the discontinuous methods (88). Continuous diafiltration is typically easier to perform in an industrial setting and therefore the most prevalent; however, it does require more volume of diluting agent, water, than the volume reduction method. DF can also be used to concentrate one component in the presence of another as in Table 1a and Table 1b. A theoretical example membrane process is given in Table 1a which separates 3% protein (100% rejection) from 5% lactose (0% rejection). Although this represents an ideal separation, a 10,000 MWCO membrane would yield a separation very similar to ideal for these components. If one concentrates to five times (5x) protein concentration and then adds equal an equal volume of water before concentrating again—this repetitious process is an example of batch wise diafiltration—it is observed after four such cycles that nearly 100% of the fed protein is now in the retentate while 98.8% of the incoming lactose passes through into the permeate which contains no proteins. This results in a retentate protein to lactose ratio of 75:1 compared to the original 3:1. The 100% rejection of protein results in a 100% yield. This type of procedure is a common way to produce various protein concentrates and isolates from both milk and whey. Using membranes with roughly 100% protein rejection, 50% ash rejection, and 0% lactose rejection, MPC 37, MPC 53, MPC 80, as well as MPI 90 can be produced. WPCs and WPIs can be produced by this process as well. Table 1b presents an ideal case of a 30,000 MWCO membrane with 10% α-lactalbumin rejection and 90% β-lactoglobulin rejection. As shown in the table, a β-lactoglobulin to α-lactalbumin ratio of 35:1 is obtained in the retentate after 3 diafiltration cycles as described above. However, only 69.6% of the β-lactoglobulin fed is retained; that is, 30.4% is lost in the permeate. The α-lactalbumin concentration in the retentate lowers with each cycle before reaching 4% in the final retentate. Obviously this is a large percentage and represents a tremendous opportunity for innovation to increase the yield. To fully separate proteins by ultafiltration, many researchers have fully utilized diafiltration for the separation of whey proteins (91–93) preparation of lactose and sodium reduced skim milk and milk protein concentrates (51, 94, 95) and various unique casein fractions from milk (96–99). 183 In Modern Applications in Membrane Science and Technology; Escobar, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.

Table 1. Equal Volume Batch Diafiltration of Whey using a) Ideal Membrane and b) Real Membrane a) Ideal DF: 0% Lactose rejection, 100% Protein rejection


Starting concentrations: 3% Protein, 5% Lactose Relative Permeate Volume (Permeate/ Feed)

Protein (Lactose)

Retentate Concentration

Permeate Concentration

% in Retentate

% in Permeate

Relative Membrane Area

5x

15 (5)

0 (5)

100 (20)

0 (80)

1

0.8

1-DF

15 (5)

0 (2.5)

100 (10)

0 (90)

1.3

1

2-DF

15 (5)

0 (1.25)

100 (5)

0 (95)

1.6

1.2

3--DF

15 (5)

0 (0.63)

100 (2.5)

0 (97.5)

1.9

1.4

4-DF

15 (5)

0 (0.31)

100 (1.25)

0 (98.8)

2.2

1.6

b) Hypothetical DF: 10% α-Lac rejection, 90% β-Lac rejection Starting concentrations: 2.5 g/L α-Lac, 5 g/L β-Lac Relative Permeate Volume (Permeate/ Feed)

β-Lac (α-Lac)

Retentate Concentration

Permeate Concentration

% in Retentate

% in Permeate

Relative Membrane Area

5x

21.3 (2.9)

0.9 (2.4)

85.2 (23.2)

14.8 (76.9)

1

0.8

1-DF

19.9 (1.6)

0.9 (0.8)

79.6 (12.8)

20.4 (87.2)

1.3

1

2-DF

18.6 (0.9)

0.8 (0.4)

74.4 (7.4)

25.6 (92.8)

1.6

1.2

3--DF

17.4 (0.5)

0.8 (0.3)

69.6 (4.0)

30.4 (96.0)

1.9

1.4

1.3.3. Protein Fractionation UF is also used to produce protein concentrates or isolates from milk and whey, abbreviated MPC/MPI and WPC/WPI respectively. These can be processed further and utilized for their high nutritional value as food additives or prestock for dairy product manufacture to produce a healthier and more economical product (7, 100–104). Whey, the liquid component of milk which has been separated from the solid curds, contains lactose, proteins, salts and residual fats as well as roughly 20% of the milk proteins (101, 102, 105, 106). Milk proteins have been used in the production of dairy and therapeutic products, among others (7, 51, 88, 107). These proteins can be recovered and concentrated using UF while simultaneously removing the lactose, ash and salts present in the whey (7, 51, 108–110). This reduces the volume of waste discharge, alleviating the major 184 In Modern Applications in Membrane Science and Technology; Escobar, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


environmental problems caused by the discharge of most of the industrial whey produced in the past. Powdered WPCs containing all the whey proteins are currently produced by UF followed by spray drying. The concentrates generally have a protein content ranging from 35% to 85%—isolates contain greater than 90% protein—and have become increasingly vital in satisfying the needs of the food, dietetic and pharmaceutical industries (101, 102, 105, 111, 112). Table 2 summarizes properties of the four major whey proteins. A major use for UF is the separation of α-lactalbumin and β-lactoglobulin, often to obtain a high concentration of α-lactalbumin for use in formulas for infants (113). β-lactoglobulin is not present in human milk and often occurs as a dimmer having a effective separation molecular weight of roughly 35,000. Immunoglubulins and Bovine Serum Albumin (BSA) are other proteins that can be separated from whey. Many research efforts have sought to economically and effectively separate the various proteins in whey, including ion exchange (114), chromatography (115, 116), selective precipitation using salts (115, 117–120), pH exploitation (115, 121, 122), and heat treatment (123–125). Many studies have focused on the preparation of enriched α-lactalbumin and β-lactoglobulin fractions since these two proteins account for more than two-thirds of the total whey proteins. Simple UF cannot sufficiently separate these because their molecular weights are very similar as shown in Table 2. Often the required processes required to isolate these proteins incur large expense and are difficult to scale up (95, 111, 115). The protein α-lactalbumin research is prevalent, due to its appeal as an additive in infant formulas and link to early childhood brain development (101, 102, 107, 126). Human milk contains much more α-lactalbumin, about 40% by weight of the total proteins, compared to bovine milk, about 5% of total proteins. In addition, human milk contains no β-lactoglobulin, an allergen to human infants present in bovine milk (101, 102, 107, 126). It is therefore desirable for whey proteins in breast-milk substitutes to have a low concentration of β-lactoglobulin to reduce the concentration of allergen and a relatively high concentration of α-lactalbumin to make it more similar to human milk (101, 102, 107, 126). Most attempts to concentrate α-lactalbumin were not capable of scaling up to industrially viable processes because of complexity, poor economics, low yield (111), degradation of the products due to intensive heat treatments (124), very alkaline pH (122), or high salt content (118). However, UF is an attractive alternative to produce designer protein concentrates (88, 115, 127), such as the process described by Wu below (126). In this process whey is first ultrafiltered through a 10-100 thousand MWCO membrane. The retentate can be dried to form a WPC while the permeate is then acidified to a pH of 3.5 to exploit a characteristic of α-lactalbumin. When present in acidic conditions below pH 4, α-lactalbumin undergoes a physical confirmation and forms a dimer or tetramer (126). In addition, UF under acidic conditions avoids the off-flavors and unsatisfactory rheological properties resulting from too high calcium content resulting from milk ultrafiltered at its natural pH (7). The proteins in the acidified whey protein feed are concentrated using another 10-100 thousand MWCO membrane UF/diafiltration process until the calcium to protein ratio in the retentate is less than 0.001. Removal of calcium allows the 185 In Modern Applications in Membrane Science and Technology; Escobar, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


α-lactalbumin molecules to become more concentrated in the precipitation step later in the process. The α-lactalbumin is precipitated from the low-calcium whey protein retentate by first diluting with deionized water to facilitate precipitate formation and promote subsequent separation. Then, the pH is adjusted to a range of 4-5 to precipitate a portion of the proteins. The precipitated proteins (α-lactalbumin, BSA and Immunoglobulin) can be separated from the soluble proteins (β-lactoglobulin and casein peptides) by centrifugation or MF. The precipitate stream is enriched in α-lactalbumin, and can be used in the manufacture of various products such as infant formulas (101, 102). The stream containing the soluble proteins is low in fat and can be made into whey protein isolate after further concentration (126). The author describes a particular interpretation of the invention, see Figure 5a, which has already been implemented. Cheddar cheese why was concentrated with a 10,000 MWCO Amicon S10Y30 membrane. A volume of 18 L of UF retentate was acidified and passed through the system. The final 3.5 L retentate contained 18% total solids. This retentate was then diafiltered, pH neutralized, centrifuged twice, and dried to form a WPC powder containing 76% protein and 9.7% fat by weight. The α-lactalbumin level was 28.4% of the weight of the protein. Another method, shown in Figure 5b, was proposed by Konrad and Kleinschmidt (128) utilizing an enzymatic method (129) to isolate pure β-lactoglobulin. The key is that bovine β-lactoglobulin is readily degraded by tryptic digestion whereas α-lactalbumin is highly resistive (130). Sweet whey which had been skimmed was fed into a pilot-scale UF system equipped with 150 kDa Koch polysulfonic membranes operated at 2 bar and 5-20x concentration. The permeate was of interest for this process since not all of the α-lactalbumin was retained by the UF. In fact, higher concentration ratios resulted in increased α-lactalbumin transmission with a maximum permeate yield of 25% α-lactalbumin. The retentate was a desirable whey protein concentrate because of the enriched β-lactoglobulin and immunoglobulin concentrations (66, 128). The pH of the permeate was adjusted to 7.7 and appropriate aliquouts of trypsin were added. This trypsin hydrolysis digested the β-lactoglobulin present in the UF permeate. The β-lactoglobulin deficient mixture was ultrafiltered, heat treated and then spray dried to obtain a concentrate which contained roughly 15% of the α-lactalbumin present in the whey feed. The overall process produced nearly no waste and yielded an almost pure α-lactalbumin concentrate 93 ± 2% comparable with literature (108–110, 131).

Table 2. Properties of Whey Proteins



Figure 5. Protein Fractationation Processes via Ultrafiltration via two different methods described by a) Wu (126) and b) Konrad & Kleinschmidt (128). Whey concentrated through UF can be used independently or in the production of consumer goods. For example, a WPC can be used as a supplement for liquid or powder milk to fortify yogurts with proteins and calcium while contributing no lactose. There is agreement in literature that replacement levels up to 25-30% milk solids non-fat are possible (48). The use of WPC in this manner has been studied, and no influence on the course of yogurt fermentation was found (132). It was also shown that the addition of WPC to yogurt formulation could produce an enhanced yogurt (132, 133) which may increase the survival of live culture bacteria (134). Furthermore, it is possible for this yogurt to be utilized as an additive in cheese manufacture analogous to the ultrafiltered precheeses reviewed above. Curd, the solid portion of milk, presents unique opportunities for utilization of membrane filtration as well, namely the isolation of casein, a protein widely used in the food industry. It is typically produced by lowering the pH of milk to approximately 4.6 through the addition of acid; however, this results in products that are water insoluble. In order for these casein concentrated products to be best utilized, they must be made water soluble (135, 136). In the past, this 187 In Modern Applications in Membrane Science and Technology; Escobar, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.

was accomplished through treatment with heat and alkalis: typically calcium, potassium or sodium hydroxide (117, 120, 137). Once concentrated, casein can be utilized as a food additive. It is successfully used to improve the consistency and texture, especially when melted, of cheese substitutes, even those containing no other dairy compounds (51, 138). Much work is being done to develop an industrial scale system capable of producing purified casein, which would allow manufacturers to produce foods with increased protein concentration without resorting to traditional harsher methods.


1.3.4. UF on the Farm Another use of UF is the concentration of milk on the farm. One group heat treated milk directly from the milking line before feeding into spiral wound membranes capable of 1000 pound per hour operation (139, 140). The fresh, whole milk was fractionated and then either used as a starter for cottage cheese production, or shipped to a factory at reduced cost. They noted no difficulties in cheese production from the UF concentrate and foresaw no modifications necessary to classical procedures when using the concentrate. The results indicated increased yields of up to 5% for the cheese, and therefore this group endorsed the use of UF on the farm, not merely in the factories. Slack et al. published two in-depth papers on both the technical feasibility (141) and economic value (142) of UF on the farm. Raw whole milk was concentrated to 3x at 340 kPa and 150 L/min in spiral-wound modules with rejections of 100% for fat and 99% for proteins. The 137 liter batches took approximately 45 minutes to reach the desired concentration. No difficulties were noticed in the field compared to laboratory operation, and the machinery did not disturb the cattle or normal routine. The resulting permeates could be stored for up to four days without pasteurization without becoming rancid. The authors concluded that incorporating UF into the milking lines on the dairies is a viable alternative to UF at a dairy factory. This would lead to reduced costs of refrigeration and transportation, as well as increasing storage capacity at the farm. The factories would benefit from reduced storage costs, increased production rates, and decreased whey production (141, 142). The authors noted that some modifications, changes in cooking times for example, to processing may be necessary in order to use this permeate as is. This process was deemed economically viable for herds greater than 100 cattle (142). This is because the capitol and operating costs of the UF system are two large for the small herd. A detailed description of the various assumptions is given in the literature (142). Economic calculations were performed for herds of 50, 100, 500, and 1000 cattle; all but the 50 head herd saw economic benefits from on-farm UF. It was noted that the savings increases with the herd size, such that the case of milk from 1000 cattle concentrated to 3x results in a savings of approximately 50%. An excellent review was written by Howie, which described various technical and economic factors affecting UF on farms (143). The author notes that the “rate of savings” decreases with increasing concentration; that is, the farmer would save less and less compared to the additional costs of concentrating the milk. Additionally, the author presents some economic terms and regulations which must 188 In Modern Applications in Membrane Science and Technology; Escobar, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.

be well understood before introducing milk which has been processed through membranes into the consumer sector.


1.3.5. Ultrafiltration Conclusions UF is an incredibly powerful and versatile tool in the dairy industry. It is able to increase product yield, reduce waste, and increase profitability by concentrating products to a high purity and decreasing operational footprint by performing the duties of multiple traditional operations. That is not to say; however, that UF is a dormant technology in the dairy industry. The role of UF in dairy operations will only grow as manufacturers investigate new methods for producing products with added proteins. New developments guarantee that the most value and nutrition will be gathered from every component of milk in more beneficial ways than traditional methods. Three major opportunities that have been identified are: 1) tighter pore membranes with controlled separation characteristics for protein fractionation, 2) reduced productivity loss with diafiltraion due to better system design, and 3) improved cheese properties of UF cheese through a combination of membrane design and system design.

1.4. Reverse Osmosis Reverse Osmosis (RO), formerly known as hyperfiltration, is a separation and concentration technique that relies upon the diffusion of water through a porous membrane (144). RO membranes are characterized by high selectivity of almost all components from water (145, 146). RO offers several advantages over thermal evaporation, the traditional technology of choice for comparable separations. These include lower energy consumption and a smaller footprint for RO (145, 147–150). RO is used extensively throughout the world to obtain usable water from a variety of sources (151–153). In the dairy industry, RO is used primarily to concentrate whey (1, 6, 154), but it is also widely used to concentrate waste effluents and milk (155–160), as well as to reduce transport costs of dairy products through pretreatment (146). The clean water permeate can be recycled within the process to reduce fresh water intake. The concentrates contain nutrients which are advantageous when applied to the manufacture of liquid milks, yogurts, butters, ice creams, milk powders and other dairy-based consumables (161–166). Current applications for RO in the dairy industry are reviewed below, but novel applications are constantly being explored making the growth potential of RO for dairy processes promising. 1.4.1. Waste Reduction One of the main applications of RO in dairy is the production of purified water (151, 153, 159, 160, 167). This ranges from seawater desalination to process water recycling. Dairy effluents (process water, UF permeates, cleaning solutions, etc) contain high levels of dissolved organics, lactose, ash, urea, proteins and other compounds (146). These are extremely expensive to sewer and 189 In Modern Applications in Membrane Science and Technology; Escobar, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


treat due to their high BOD and COD levels. In fact, there are times where the stream cannot even be sewered due to its composition (51). It is apparent that a means for purifying the effluents while recovering the valuable constituents would be of tremendous benefit. RO has been shown to be a convenient way to achieve this by producing purified water while simultaneously concentrating the dairy constituents in the feed (151, 161–166). This achieves two valuable goals: 1) the dairy compounds can be used to create value-added products for consumption as well as non-food applications (161–166, 168); 2) the purified water can be reused in the plant for process water makeup, cleaning, diafiltration washing, or even for irrigation (159, 160, 169). Vourch et al. performed a study to characterize wastewater from different dairies and develop standard operating conditions for treatment of this wastewater (160). They contacted 11 French dairies and found that wastewater ranged from 0.4 g/L to 71 g/L dry matter and 0-22% fat and were the results of rinses, cleaning in place, etc. To process this wastewater they used a commercially available TFCHRSW 2540 RO spiral-wound membrane obtained from Koch, which has the characteristics of 99.5% NaCl rejection. Using these membranes and various feeds obtained from dairy processing, they found a flux of 20-30 L/m2*hr (at 2 MPa and 25°C) at 0% water recovery and around 10 L/m2*hr at 90% water recovery. Using these experiments they found that a 540 m2 RO unit would be required for a 100 m3/d dairy wastewater treatment facility with 95% water recovery. A stream of special interest for dairies is evaporator condensate, which represents a large water load which cannot be reused directly due to its organic contents. RO is often used to concentrate the organics to produce clean water for reuse to reduce fresh water intake. One large cheese plant has reported reducing discharge loads by 150,000 gallons per day by treating evaporator condensate to produce high-quality water permeate (170). Similarly, the Michigan Milk Producers Association uses RO in a large milk processing plant to process condensed milk condensate. The RO system treats 80,000 pounds per hour of evaporator condensate containing 30 ppm organic material with 72,500 pounds per hour—145 gallons per minute—filtered water with 3-5 ppm organics produced. The reclaimed water is used in the plant’s cleaning systems and as boiler feed makeup. Most often the retentate is put in the plants wastewater discharge. It is important to note that effluent compositions are extremely variable and no one process system can serve as a solution for every plant. Additionally, it is often not possible to obtain purified water of suitable quality, depending on the target use, with only a single RO step (151). However, RO is quite versatile and can be used in series with additional RO units or in conjunction with other techniques (MF, UF, NF, evaporation, etc) to achieve any desired composition from any variety of feed (164, 165). 1.4.2. Whey and Milk Concentration Another high volume use for RO specific to the dairy industry is concentrating whey and lactose (1, 6, 154, 171–174). UF is first used to fractionate and concentrate proteins in whey as discussed previously. The resulting permeate 190 In Modern Applications in Membrane Science and Technology; Escobar, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


contains most of the lactose along with other valuable nutrients originally present in the feed. Using RO, it is possible to concentrate these for use in products by removing the water present in whey to achieve roughly 20-25% dry matter (145, 161–166). Beyond this, high osmotic pressure, fouling and high viscosity diminish performance (1, 6). This increases evaporator capacity and yield, and saves energy by reducing evaporator load (149, 175). The concentrated feed is already at roughly 25% solids; therefore, the evaporators do not need to remove as much water compared to raw feed (149, 175–177). This allows extremely pure compounds to be produced. Concentrating whey and lactose have already been shown to be successful in industry. Kosikowski reports that preconcentration of whey to roughly 12% solids before proceeding to evaporation has become the industrial standard in the United States; the standard for Europe is upwards of 24% (178). As an example, one large cheese plant concentrates UF permeate from WPC production to obtain a concentrated lactose product (179). The UF permeate feed passes through two parallel RO banks, each with three RO operations in series. A lactose-rich concentrate stream is produced and sent for final processing while the permeate stream is sent to a second RO (or NF) system to be demineralized. A third RO system is used to recover clean water to be recycled as diafiltration water for the UF system. Milk also presents unique challenges that can be handled using RO. Raw milk can be concentrated directly to improve dairy product taste, increase yield and reduce processing costs (156, 158, 180). Most dairy products have been satisfactorily made using milk RO concentrates (1, 51, 53, 178, 181). Concentrates can be stored at temperatures above refrigeration for extended periods of time with no adverse effects, providing manufacturers much flexibility for feed usage and more uniform feed composition (145, 154, 155, 161). For years, satisfactory liquid milks have been produced by reconstituting dried concentrates with water (158, 162, 178). According to Glover et al., RO can also be used to produce a consistent milk composition by concentrating raw milk to 30% solids (53). This yields a product with minimum level of undesirable “cooked” flavor, a problem sometimes encountered through membrane filtration of milk. RO concentrates are used in the production of various dairy consumables such as milk, yogurt, ice cream, cheese, and butter. When RO concentrates are used in the manufacture of ice cream and yogurt in lieu of powdered milk, benefits such as a less grainy texture and more appealing taste are obtained (53, 162, 163, 166). One commercial yogurt in France is produced using milk concentrated to 5% solids, and has been preferred by consumers over yogurt produced using raw milk fortified with milk powder (182). Recombined or reconstituted milk made with RO concentrates can lead to a more nutritional beverage (158). The increased retention of whey solids leads to quality cheddar and cottage cheeses with yield increases of 3% and 5%, respectively, when milk volume is reduced by 20% (161, 183). Butters manufactured using RO concentrated creams enjoyed increased curd contents, improved churning losses and increased yields (162, 165). Ur-Rehman et al. obtained a patent which describes a variety of dairy products which can be produced through a combination of membrane technologies (184). These products are tailored to contain certain levels of proteins, lactose, calcium, 191 In Modern Applications in Membrane Science and Technology; Escobar, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


fat, and lactose for use in the manufacture of a number of dairy products for consumption. Specific compositions are presented, but the procedure is nearly identical for the various products. As shown in Figure 6, the milk fats are removed from whole milk for possible reuse while the skim milk is ultrafiltered (184). The UF permeate is then nanofiltered resulting in a retentate ready for use and a permeate which is then stored for future use or passed through a reverse osmosis unit, which concentrates the solids within the milk. The retentates of the RO, NF, and UF are then mixed. Alternatively, some of the NF permeate can be mixed in with the RO, NF, and UF retentates without passing through the RO first. Another claim of the invention includes a DF unit for processing the UF retentate. The DF unit is run with a combination of RO permeate and fresh water. The DF retentate is mixed with the other retentates just like the UF retentate was without the DF unit. Yet another claim mixes a fraction of the originally separated cream with the membrane retentates. Depending upon the desired product composition, the retentate mixture undergoes a series of heat and enzymatic treatments before being stored. The authors describes compositions suitable for yogurt, ice cream, butter, cheese, and a variety of other products. 1.4.3. RO versus Evaporation and RO on Site Despite the benefits of RO, it would not enjoy industrial success without clear advantages over the competing technologies. For RO the competing technology is thermal evaporation (176, 177). The advantages for RO over evaporation are many, but the primary advantage is the lower energy consumption (185). RO, which requires a smaller footprint than evaporation, does not need steam and thereby costs less and consumes approximately 60% less energy (149, 150, 175). Since RO can be performed at low temperatures (4°C-15°C), there is no microbial growth within the system or protein degradation (145). RO consumes less energy than even the most advanced and efficient evaporation techniques: mechanical vapor recompression (MVR) and vacuum evaporation (146, 149, 175). However, since RO cannot achieve concentrations of more than 25-30% solids, it must always be paired with evaporation in order to obtain high concentration products (146). Typically the most economical solution is to concentrate by RO to roughly 25% solids before proceeding to evaporation (146, 176, 177). An extension of this is using RO to concentrate dairy effluents--such as whey for protein concentrates and milk for milk powders--at the dairy site before being transported to central plants for further processing. By concentrating, i.e. removing water, before transporting, dairies can effect large savings. Since there is often little advantage in scale for large units compared to smaller units (176, 177), outlying dairies with no treatment facilities could install small operations and not be forced to transport the large amounts of water contained in raw whey. Various combinations of concentration and transport costs for whey have been studied. de Boer and Hiddink found that it was most economical to use small RO units on remote dairies to concentrate whey to roughly 22% solids before transporting the concentrate to a central facility for processing into high-purity consumer products (146). In some cases this reduced transport loads by two-thirds. Additionally, the discharge volume of detrimental effluents from 192 In Modern Applications in Membrane Science and Technology; Escobar, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


smaller dairies would be reduced, simultaneously saving money through avoided sewer costs, and reducing environmental impact (176, 177).

Figure 6. Milk Concentration via Reverse Osmosis and Nanofiltration described by Ur-Rehman et al. (184). Numerous studies have attested to the energy saving benefits of RO. When the US Department of Energy evaluated the future energy use of the country, it found a need to reduce energy usage in the food industry. The department estimates that preconcentrating whey by RO as described above would save 3, 3.6, and 4 trillion BTU per year if whey would be concentrated to 12, 15, and 20% solids, respectively, before evaporation (170). Preconcentration of whey and milk before shipping would save approximately 2.4 trillion BTU per year, assuming roughly 25% of all whey processed whey in the US originates from smaller, outlying dairies (165). The predicted energy crisis is forcing the dairy industry to decrease energy consumption in the near future, and these statistics show that RO is a ready and viable solution. 1.4.4. Reverse Osmosis Conclusions RO is internationally recognized for its ability to produce purified water in an advantageous manner. However, the dairy industry has developed novel ways to use RO to enhance existing processes or create entirely new avenues. Dairy waste and filtration permeates can be concentrated to reduce the discharge of streams with high BOD, COD and organic content. Whey and milk (or any filtration permeates originating from these feeds) can be filtered to obtain compounds that provide advantageous nutritional and organoleptic properties when used instead of traditional feed materials. These concentrates also lead to greater production 193 In Modern Applications in Membrane Science and Technology; Escobar, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


efficiency and lower costs as described above. An appealing extension of this is installing small RO units in outlying dairies to concentrate whey and milk before shipment to centralized processing plants, thus reducing transportation costs. The clean water produced from the various RO processes described can be used for process water makeup, cleaning solution, irrigation, diafiltration washing, or other uses. This greatly reduces sewer costs, fresh water intake and discharge of harmful effluents. Compared to evaporation RO consumes less energy, costs less, is easier to install, and contributes a smaller footprint. A principle need that continues for RO in the dairy is cleaning since chlorine cannot be used on RO membranes and it is difficult to clean without chlorine (52).

1.5. Nanofiltration Nanofiltration (NF) is a pressure-driven membrane process used to selectively separate components having a molecular weight lower than 1,000 Da, typically 200-1,000 Da (1, 6, 52). This cutoff corresponds to pore sizes roughly one nanometer (nm) in diameter, hence the name NF (52, 186). NF is a very useful process because of the intermediate selectivity between the MWCO exclusion of UF and the diffusion filtration of RO. It possesses great versatility due to a broad range of operating conditions: pressures between 250 and 400 psi, temperatures up to 150°F, and pH values between 2 and 10 (187). It is unique among the major membrane technologies (RO, NF, UF and MF) in that monovalent ions such as sodium and chlorides pass through more easily while divalent ions such as calcium and sulfates are highly rejected (154, 187–190). Additionally, chlorides pass through at a much higher rate than monovalent cations (188, 191, 192). In fact, NF is alternatively defined as “the pressure-driven membrane separation of electrolytes” for its ability to separate mono- and divalent ions (193). Sugars, small organics and other higher molecular weight compounds such as urea and proteins are also rejected (150, 165, 194). Since the rejected species are simultaneously concentrated, i.e. water is removed, while being separated, NF presents an attractive means of modifying dairy compositions to produce more valuable and nutritional products (186). Suarez et al. performed an interesting study comparing the membrane performance of whey and milk ultrafiltration permeate (192). The major difference between these two feeds is that whey has a significant amount of protein while ultrafiltration protein does not. They performed their experiments with an NF pilot plant using a DK22540C membrane supplied by Osmonics. When comparing the whey and the milk ultrafiltration permeate, an interesting result was seen. Until about 2 MPa, both performed quite similarly with fluxes at 2.0 MPa of around 40 L/m2*hr at a temperature of 16 C. However, at this point the whey flux flattened out while the milk ultrafiltration permeate increased nearly linearly to pressures of at least 4.0 MPa (80 L/m2*hr flux). The reason for the difference is that whey contains over 0.5% protein while milk ultrafiltration permeate has negligible protein and the protein causes a gel permeation layer which leads to no further flux increase at increased pressures. So although both feeds had high rejections of calcium and magnesium (>90% at all pressures) and lower rejections of sodium and potassium (about 60% at 2.0 194 In Modern Applications in Membrane Science and Technology; Escobar, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


MPa), the milk ultrafiltration permeate is a much better target because it can be concentrated further, up to a volume recover factor of 5, with little loss in flux or membrane performance. This is an example of how the other components in milk can affect the performance of the membrane system (192). NF was first introduced into the dairy industry in 1984 to demineralize salt whey drippings to improve sweet whey yield (193, 195). This was achieved by combining the demineralized concentrate with the sweet whey stream. The nutritionally unfavorable NaCl and KCl were removed while the nutritionally beneficial calcium compounds were recovered. This allowed dairy processors to reclaim valuable, nutritional whey solids and greatly reduce waste (187, 193, 195). Since then many uses of NF have been developed including 1) demineralization and deionization of UF permeate and whey (1, 6, 191, 192, 194, 196, 197); 2) separation and concentration of small, high-value milk components (190, 198–201); and 3) waste reduction or treatment (159, 175, 187, 202–206). NF processes increase the value of previously wasted and undervalued dairy constituents, especially whey proteins (88, 200, 201, 205, 207). 1.5.1. Demineralization A major function of NF in the dairy industry is the demineralization of sweet whey, or simply whey; however, NF is also used to process acid whey and the permeate resulting from the ultrafiltration of milk or whey (191, 192). These so-called “wastewaters” obtained during dairy processing contain valuable, beneficial components such as proteins (8.4 g/L in whey) and lactose (79 g/L in whey). Whey has been viewed as a nuisance and is often used as a component in animal feeds or treated as bulk waste (208–211). These components show great promise as additives and flavorants in various dairy products; however, the very high mineral salt content must be reduced before they can be consumed (150, 212). Discharge without treatment is not an option either due to the high biological and chemical oxygen demands (BOD and COD respectively) (191, 192). NF for demineralization presents a means of isolating and concentrating the valuable components while simultaneously increasing efficiency and reducing disposal loads and costs (51, 209, 213, 214). Traditionally these feeds are concentrated by evaporation before demineralization is carried out through ion exchange (IE) or electrodialysis (ED) (150, 215). Such processes are plagued by high capital and operating costs, large volumes of waste, and regeneration complications (187, 216, 217). In addition, IE and ED unit operations must be large in order to concentrate the high water volume of process whey (169). NF is therefore advantageous since it demineralizes and concentrates concurrently (191, 192, 196) and is less expensive (149, 150, 213). NF is, however, limited in the extent to which it can demineralize feeds: up to 50% alone (1, 6, 187, 193, 213) or up to 90% when coupled with DF (1, 6). NF is competitive with ED and IE, and is often more selective than ED (1, 6). Monovalent ion reduction of up to 90% with DF and divalent ion reduction as low as 3% with DF has been achieved compared to only 62% and 43% respectively for demineralization with ED. Note, since NF selectively fractionates mono- and divalent ions, it is more selective than ED since it reduces 195 In Modern Applications in Membrane Science and Technology; Escobar, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


more monovalent but fewer divalent ions (1, 213, 218). In addition, lactose and non-protein nitrogen (NPN) losses are lower by 6% and 25% respectively than in ED (212, 217), and protein losses are less for NF as well (1, 213). For products requiring very high demineralization—90% or more—NF can be used to preconcentrate the feed for ED or IE. NF competes favorably against evaporation and even RO in this duty since it requires less capital and energy (7, 51, 214). Preconcentration by NF allows smaller ED and IE operations to be installed and leads to increased yields, efficiencies, and savings (187, 193, 209, 213, 214). When NF is used in conjunction with ED or IE, products of 90-95% demineralization or greater can be achieved, separations impossible for NF alone (187). The economics of choice between NF, ED, or IE—or any combinations thereof—depend on the feed, demineralization achieved, and plant capacity; however, NF is generally more economical the lower the desired demineralization. Indeed, NF alone is the best choice for products requiring roughly 30-35% or less demineralization (193). It should be noted that NF does not perform independent of the feed. For example, higher salt retention have been observed for wheys, but higher fluxes are observed for the UF permeates of milk because of the lower protein concentration (191, 192). Protein and lactose retentions of 99+% and 98+%, respectively, are routinely obtained for both wheys and UF permeates (191, 192).

1.5.1.1. Demineralization of Sweet Whey Whey is produced at a rate of 8-10 pounds for every pound of cheese. It contains all the water soluble constituents and 12-20% of the proteins present in milk. This large nutritional source—70% of the nutritive value of milk (169)—has traditionally been underutilized or simply discarded (176, 177, 209). Due to the advent of advanced membrane technologies, an increasing amount of whey is processed into whey powders such as WPCs, WPIs and other high quality protein rich nutritional products (208). Because high mineral content is detrimental for human consumption, an interest in desalted whey products is growing rapidly since products with lower mineral content are more desirable and profitable (150). The general process of demineralizing whey begins with the separation of fines and fat from whey. The whey is then pasteurized, cooled, and sent to NF for concentration and demineralization (187, 209). The resulting products can be used as more economical replacements for milk powder in products such as infant formulas, ice creams, frozen desserts and snack foods (53, 126, 154, 196, 200, 208, 209, 219–222). Whey powders offer additional benefits in that they offer complete and bio-available amino acids, and offer food producers functional benefits such as improved sensory characteristics, emulsification and moisture retention (196). Phillips patented a process where NF instead of IE can be used to produce a mineral-rich stream from whey which can be processed into a mineral whey powder suitable as a food additive, particularly for low-sodium foods (200). The author acknowledges that increased use of membrane filtration in downstream processing would result in the same product, and is therefore possible for future modifications. Specifically, membranes could replace the current methods for 196 In Modern Applications in Membrane Science and Technology; Escobar, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


removing calcium phosphate deposits. Fouling was eliminated by removing calcium phosphate before final processing which subsequently increased the efficiency of the process and will result in increased useful life of the equipment. NF was used by Kelly et al. to demineralize sweet whey to a 4x concentrated product containing 22% total solids by achieving the reductions shown in Table 3 (193). Another process is described by Minhalma et al. where NF is used to produce a product concentrated in lactose and free amino acids for use in food manufacture (209). The study focuses on second cheese whey (SCW), a by-product of curd cheese manufacture which is highly concentrated in organic matter, lactose, mineral salts, vitamins, and free amino acids. It is common practice for the SCW to be treated as common waste and mixed with domestic sewage for disposal. The NF process shown in Figure 7 produces a nutritional product while simultaneously reducing waste (209). First, reduced fat “serpa” cheese whey is passed through an UF system to produce a protein concentrate. The UF permeate is combined with reduced fat SCW and fed to a NF system operating between 1.5 to 3 MPa at 9.21 L/min. The NF permeate is highly concentrated in salts, specifically NaCl, and can be used elsewhere within the plant. The retentate is readily prepared as a product roughly 5x more concentrated in nutrients than the feed. The SCW can now be successively processed into two beneficial streams instead of simply being discarded.

Table 3. Mineral Reductions in a Nanofiltration Process described by Kelly et al. (193)



Figure 7. Nanofiltration Product High in Nutritional Benefits Made via Demineralization described by Minhalma et al. (209).

1.5.1.2. Demineralization of Acid and Salt Whey NF demineralization can be extended to acid and salt whey, both historically difficult and inconvenient to process, to produce nutritional products. Acid whey, or sour whey, is obtained during the production of “acid-type” cheeses such as cottage cheese. The properties of acid whey, namely the high ash and lactic acid contest as well as the acidity, have led to many technical difficulties and limited its industrial processing (196). Attempts to produce powders from acid whey have been unfruitful because the powders are so insoluble due to the ash and acidity 198 In Modern Applications in Membrane Science and Technology; Escobar, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


(196); insoluble powders are of little to no benefit to the food industry. Also, lactic acid is responsible for difficulties during spray drying—the process previously investigated for acid whey processing—leading to lumping and caking of the whey powder particles (207, 223). Lactic acid also increases whey viscosity and lowers the quality and value of whey products. This obstacle has been overcome previously by neutralizing the feed or adding skim milk (224). However, the neutralization process causes an increase in ash content of the product which contributes to an unacceptable salty taste and poor product quality. NF has been used to minimize these issues by decreasing the acidity, mineral content and ash content of the whey in one operation (16, 191–193, 196, 197). This produces powders with qualities comparable or better than sweet whey powders in a more efficient process which requires less power (213). If the NF retentate is then neutralized, an even higher quality product is achieved which contains less acid, especially lactic acid (196). After spray drying, the resulting whey powders can be added to dairy food products such as ice cream and yogurt to improve physical properties, organoleptic qualities and nutritional value (201, 206). Barrantes and Morr obtained 85% whey solid recovery using a NF/DF process with a VCR of 3 (194). Monovalent ion concentration was reduced by up to 99.6% while polyvalent ions were reduced by up to 80% (194). Dec and Chojnowski produced demineralized acid whey powders with an average moisture content of 2.66% containing an average of 71.06% lactose, with a max of 75.1% (196). This compared favorably to traditional sweet powders which contained 76.3% lactose. NF systems are now producing quality nutritional food components from a source previously thought of as waste (7, 201, 213). Salt whey, obtained through the processing of hard-pressed cheeses such as Cheddar, has been as difficult to handle as acid whey. The high sodium chloride content of salt whey complicates the sweet whey process since salt whey must be handled separately to prevent a salty taste in the final product (51). Although only 4% of the original milk volume becomes salt whey during processing, the annual volume is rather significant and presents an opportunity for obtaining valuable nutritional food components. NF can be used to lower the sodium chloride level while concentrating the desired milk constituents. Spray drying produces a valuable and nutritional whey powder for consumption thereby reducing waste volume (51). Gregory describes a process that removes approximately 90% of the sodium chloride while recovering 80% of the nutritional whey solids (195). The process consists of 1) concentrating the initial salt whey volume to 60% using a NF/DF combination where DF water is added at one-half of the permeate flux rate and 2) NF/DF at constant volume until the end-point is reached (90% NaCl removal). 1.5.2. Milk UF Permeate Milk UF permeate, like whey, has been traditionally viewed as an inconvenient byproduct with disposal being the best or only option. The composition of the permeate is similar to that of skim milk except for a negligible amount of proteins. It mainly contains minerals, lactose and riboflavin, but no enzymes and microorganisms due to the UF (190–192). The high concentration of 199 In Modern Applications in Membrane Science and Technology; Escobar, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


lactose and divalent salts makes recovery of these components both an economical and environmentally beneficial option. NF processes could be used to separate unwanted monovalent salts from lactose and desired multivalent ions, particularly calcium (197). Suarez et al. reported monovalent cation demineralization of 40-50%, 36% ash removal and 77% chloride removal when UF permeate was concentrated 4.7 times (191). The techniques used are analogous to those used in whey demineralization. Leruyet et al. patented a process, consisting only of membrane processes, to create a concentrated dairy ingredient which retains the proteins, lactose, calcium, and phosphorous, but is low in chlorides, sodium, and potassium (190). A schematic of the process is shown as Figure 8. The invention covers a range of membrane sizes and fluxes resulting in various product compositions; however, the process order is identical. First, skim milk is microfiltered to remove any microorganisms or remaining large fat globules, and reduce the casein concentration. The MF permeate containing the water soluble components is then ultrafiltered to separate the proteins from the NPN, lactose, and mineral salts. The UF permeate is then passed through a NF system to remove sodium, potassium, and water. The NF and UF retentates are then combined to produce the final product. According to the authors, the procedure could also be run without the UF bank, which would result in a slightly different product.

1.5.3. Recovery of Lactose As UF becomes more prevalent in the dairy industry, an increasing volume of UF permeate presents new processing opportunities. Whey UF permeate contains large amounts of lactose and few proteins. These qualities make it the perfect feed for purified, dried lactose production (169, 225). Currently, concentration by evaporation or NF followed by spray drying can be used to obtain lactose powder from the whey UF permeate (154, 207). NF has been shown to increase lactose crystallization yields. However, the resulting mother liquor has a very high salt content, cannot be utilized, and must be discarded. NF can be used to demineralize and preconcentrate the feed selectively between lactose and ionic species, thereby obtaining a lactose-rich retentate and a mother liquor with a significantly reduced salt and mineral content (193). This reduced volume mother liquor can then be processed into commercial dairy products (207). By processing the whey UF permeate through NF before drying, lactose crystallization yield is improved up to 60% or more with DF with little effect on lactose purity (1, 207, 213). This generates a commercial avenue for the lactose mother liquor thereby reducing disposal volume. Additionally, NF leads to less strenuous operating conditions during the evaporation and crystallization steps because the modified feed leads to less deposit formation and lactose crystal washing respectively (193, 207).



Figure 8. Process for Demineralized Milk UF Permeate described by Leruyet & Mauga (190).

1.5.4. Other Uses of Nanofiltration As is discussed in the RO and UF sections, concentrating dairy effluents on the farm or dairy site to reduce travel expenses is a growing use for membrane technologies. Small NF systems on outlying dairies are used to concentrate and demineralize whey and UF permeates before being shipped to a processing plant or waste disposal facility (187, 226). While NF cannot achieve the purity capable with RO, it is more economical for achieving moderate purity levels since RO operating pressures are higher than those of NF. This corresponds to an energy savings of 20-45% for NF depending upon feed conditions and the desired product (187). If the effluent must also be demineralized, the economics become even more favorable for NF since the single demineralization function removes both the minerals and water from the milk or UF permeate feed (187). A prime example of this use of NF is in Australia, where the Murray Goulburn Co-op has installed a 10,000L/m2*hr NF plant that concentrates Cheddar cheese whey to 21-22% TS with a consequential 35% demineralization. This concentrate is shipped to a central processing plant to be demineralized further by IE for use in milk-added products. The savings in shipping costs can be passed on to the consumer and/or used to subsidize the dairy (193).



Another emerging use for NF is the fractionation of milk to produce higher protein contents, up to three times that of raw milk. Using such a preconcentrated feed, cheese facilities could increase output since less of the feed become whey compared to raw milk. On the other hand, if this concentrated product were dried, it could be stored as a feedstock to produce valuable specialty cheeses during the slack season because the protein-rich product would have a longer shelflife than milk. This would establish a more consistent pattern of annual cheese production and would help to level prices since the supply would not be as dependent upon season (176, 177). 1.5.5. Nanofiltration Conclusions As mentioned throughout this discussion, NF compares favorably with the competing technologies: evaporation, ion exchange and spray drying. The main advantage for NF is that it can demineralize and concentrate simultaneously; feats impossible for the other technologies (186). It also costs less to install and operate (149). When used in conjunction with other technologies to produce highly purified products, NF increases yield and allows for smaller unit operations (187). It also allows dairy producers to manufacture more nutritional or more specialized products. Finally, NF can be used to standardize dairy feeds so that product properties are more uniform and product supply is not as dependent upon season. NF is a young technology which has proven highly beneficial to the dairy industry. It has increased the capabilities of the current processes and ushered in entirely new products which are more nutritional and less expensive. NF is replacing electrodialysis in many applications and will continue to find new applications as fractionation ability increases.

1.6. Electrodialysis As compared to the other technologies that have been discussed, electrodialysis (ED) does not function by using a pressure driving force. Instead, it uses an electrical driving force to move ions from one stream to another. Using a current, anions move towards the anode and cations move towards the cathode. Charged membranes determine whether ions will permeate the membrane or be rejected. Cation exchange membranes (CEM) are negatively charged so that only cations can pass and anion exchange membranes (AEM) are positively charged so that only anions can pass Neutral species, such as sugars, are uncharged so they will not move through the membrane. Because the membranes are dense, typically the limit in separation is charged species of 300 or less although recent work has shown the separation of even larger charged molecules using charged ultrafiltration membranes instead of traditional CEM and AEM membranes (227). Thus, differing from pressure driven processes where the major component, water, is what also is passing through the membrane, in the case of ED the minor component, the charged species, actually transports through the membrane. Thus, the more dilute the species to be removed the more economically viable ED is compared to pressure driven processes such as RO or NF. However, this ion 202 In Modern Applications in Membrane Science and Technology; Escobar, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


movement causes ED to have one significant drawback called the limiting current density. The limiting current density is when the applied current is so great that ions cannot be removed fast enough by diffusion-electrical attraction and thus excess current is used to split water. In this case, although power is drawn no further ions of interest are separated. Thus, there is a minimum concentration of separated ion that ED can achieve and most processes are operated well below this limiting current density. In the following discussion, we will show many different applications of ED in the dairy industry as well as talking about new areas of research that are being explored. For an even more complete look of electrodialysis in the dairy industry, Bazinet has published two excellent reviews (228, 229). 1.6.1. Whey and Milk Demineralization ED for whey demineralization has been around since the late 70’s (230). Demineralized whey powder has been proposed useful for alcohol production (208), baby food (231, 232), yoghurt (232, 233), and biologically active peptides (232) among others. Demineralization has been investigated in both a continuous fashion and in a batch basis. In the classic work by Hiraoka et al. cheese whey and skim milk were demineralized by ED (230). The membrane unit used was by Tokyama and the Neosepta membranes used were also distributed by Tokyama. Powdered cheese whey was reconstituted to 20-30% total solids and skim milk was reconsistuted to 8-30% total solids to simulate an actual commercial whey and\or milk process. The authors found that in this case that they could reach 90% demineralization at 98% current efficiency. They also found that this separation happened well below the limiting current density so that indeed 90% demineralization could be achieved on a consistent basis. Several other studies have been proposed for the demineralization of whey by electrodialysis (150, 215, 234). Longergan et al. used ED to remove calcium from frozen milk and skim milk concentrates (235). They found it necessary to remove the calcium to avoid destabilization of the casein micelle. An Ionics electrodialysis membrane stack was used with up to 30 cell pairs, each cell pair containing 220 cm2 active membrane area. The concentrate, diluate, and rinse streams were all circulated at rates of 3.5-4.4 L/min at pressures of 190-205 kPa. The voltage across the stack was varied from 0-100 volts direct current. In this case, the limiting current density of the membrane was found to be approximately 12 milliamps/cm2 when used in conjunction with skim milk. After 130 minutes processing time, 72% of the ash, 70% of the calcium, and 55% of the phosphorous were removed. The removal of this amount of ions increased the stability of frozen skim milk from 1 week to >53 weeks (235). However, as described above, many of the currently installed ED units for whey and milk demineralization are currently being combined with nanofiltration (1, 6). This is because NF membranes can achieve high rates of demineralization at low power consumption with limited fouling. Further, NF selectivity can often be easier manipulated than ED. An exception to this trend is the use of bipolar membranes. Bipolar membranes first appeared in industry in the late 1980s (236). Bipolar membranes are membranes which contain both an AEM and a CEM with 203 In Modern Applications in Membrane Science and Technology; Escobar, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


a thin layer of water in between. Because of the two different membrane types, no single ion will diffuse all of the way through the membrane. For instance, a positively charged cation would be able to travel through the negatively charged cation exchange membrane but would not be able to pass through the positively charged anion exchange membrane. Thus, no ions from one solution are able to make it all the way through to the other solution. Where the membrane has high value is that at the water interface in between the membranes, water splitting can occur. Thus, the membrane can support current by sending an H+ one direction and an OH- the other direction. Or, in the case when bipolar membranes are alternating with traditional AEM and CEM membranes, one ion can be removed from a solution at a time and replaced with either an H+ or OH-. Thus, the pH of a food can be lowered while removing undesirable ions such as sodium. One example where bipolar membranes have been applied is the electrochemical acidification of milk by whey desalination (237). This hybrid process accomplishes two goals simultaneously since, as described above; whey must be demineralized for taste purposes and property purposes. Second, it is desirable in certain cases to acidify milk in order to shift it past the iso-electric point and recover casein. However, this process normally produces large amounts of salts which are undesirable. This process does not produce any undesired salt because it comes from the whey which is already a component of milk. In this study the authors used Neosepta ion exchange membranes and BP-1 bipolar membranes—both from Astom Corporation, Japan. They then used various configurations to produce both an acid stream and a base stream. Their worked showed that they could achieve the desired product at very high current efficiencies (86% and 87% for the production of acid and base, respectively) when part of the casein whey is mixed with the acid. However since the casein whey had a tendency to precipitate in the membrane stack, this configuration was the only desirable one. 1.6.2. Production of Protein Fractions Protein fractions are extremely valuable because they can produce products with differing properties at potential lower overall concentration of protein. As partially described above, one way to remove proteins from solution is by dropping them to their isoelectric point. One of the first demonstrations of this process was by Amundson et al. in which UF was used for water removal, the pH was adjusted for partial precipitation, and ED was used to demineralize ions (125). In this application, β-lactoglobulin in a precipitate is separated from α- lactalbumin solution by centrifugation. The ED is important because it allows the fractions to be demineralized with minimum loss of solute. Much of the work on electroacidification today is taking place with bipolar membranes. Bazinet et al. conducted an interesting study to look at different casein isolates produced by bipolar electroacidification and their functional properties (238). There studies showed that bipolar membrane acidification produced isolates that were similar to those made by normal chemical additions. Thus, it was concluded, that the environmentally friendly membrane way of producing isolates could be a widely applied technology. Another study by 204 In Modern Applications in Membrane Science and Technology; Escobar, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


Bazinet et al. found that pH and temperature are important properties during electroacidification and these properties can be varied to modify the structural properties of whey proteins (239). Specifically, electroacidification was found to be responsible for most of the structural changes (67%) while temperature and pH also had an effect (33%). Finally, an interesting recent study has been presented by Mier et al. (240) In their study, they attempted to investigate variables such as milk concentration and current density on the effects of the electroacidification with bipolar membranes. Milk concentration was varied between 2.5 and 10 wt % and the current density was varied between 100-400 A/m2. The authors found at the lowest energy consumption (0.38 kWh/kg product), a casein purity of 86% was obtained while at the highest energy consumption (0.6 kWh/kg product) a product purity of 95% was obtained. This compared favorably to traditional chemical acidification which obtained a product purity of 85%. It was also found that this casein purity could be obtained at different milk concentrations making the electroacidification an attractive technology. Further, it shows that varying power consumption can deliver different quality products. 1.6.3. Removal of Lactic Acid during Lactose Fermentation Another application where ED is finding a niche in the dairy industry is through the removal of organic acids. One such study was conducted by Boyaval et al. (241) In this study the authors attempted to make lactose (from sweet whey) more valuable by fermenting it to lactic acid with the bacteria Lactobacillus helveticus. When the lactic acid processed without a dialyzer in total cell recycle, it produced a concentration of 64 g/L while, when using the electrodialysis system, 85 ± 5 g/L was obtained. Another more recent study was conducted by Boonmee et al. in which they attempted to use electrodialysis to remove lactic acid from dairy starter culture (242). Dairy starter culture is a way of quick starting the cheese process. In this case, Lactococcus lactis NZ133 was used in batch fermentation with 80 g/L lactose as the starting substrate. The ED unit used was from BDH Chemicals in Australia with a total membrane area of 0.04 m2 cation exchange membrane and 0.03 m2 anion exchange membrane. The diluate and the concentrate were operated at a flowrate of 60 ml/min while the electrode rinse was operated at 47 ml/min. The unit removed lactate at a flux of 502 g/m2*hr at 35 volts. However, the ED performance was definitely affected by the medium components in the starter culture and for this reason the ED was not that effective in increasing starter culture growth. In this system, ED processes also seemed to struggle with cell adhesion and the uncontrolled pH at the end of each batch cycle seemed to affect the cell growth rate. It was also possible that nutrient efficiency was occurring across the ED membrane (243). Thus, ways of increasing the selectivity of ED must be explored for this technology to be fully utilized. 1.6.4. Electrodialysis Conclusions The emergence of bipolar membranes onto the scene allowed for ED to receive a revival in popularity. These applications have allowed for acidification with simultaneous demineralization and has opened many new exciting possibilities 205 In Modern Applications in Membrane Science and Technology; Escobar, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.

in whey demineralization, electroacidification, etc. It has also been reported that the flavors produced with bipolar ED membranes are much more palatable than those produced using normal acification (244). Further, other applications have been shown to have electrodialysis could be operated in an allergen free type environment which could open the possibilities for applications even more (245).


1.7. Comprehensive Example Process Although each of the major membrane technologies is powerful and beneficial in its own right, when they are used to complement each other, raw milk can be fully utilized. Most often a plant will proceed in a “stair-stepping” manner in which a product stream from the previous, less selective process will serve as the feed for the subsequent stage. Dunker et al. obtained a patent which typifies the scope of membrane filtration in a dairy plant (78). The process begins by coarse filtering raw milk to remove any macro impurities; note, this step is not always necessary. The milk is then microfiltered to remove the fat globules and any microorganisms present. With further processing, the fat can be used as an additive in food products. The MF permeate, equivalent to skim milk, is then ultrafiltered to remove the proteins for protein concentrate and isolate manufacture. The lactose present in the UF permeate is removed by the subsequent NF stage and processed to obtain a dried powder which can be used as a food additive. RO then removes the minerals and salts present in the NF permeate; allowing only clean water to pass through the entire process. The minerals can be used to make value-added products, and the clean water can be recycled within the facility to reduce fresh water intake, or used for a number of other purposes. In this way, no beneficial component of milk is unused. When used in conjunction, the major membrane technologies open up new avenues for dairy manufacturers and allow more nutritional or specialized products to enter the market. Membrane technologies have undoubtedly changed the dairy industry, but what is even more exciting is that this innovation is not stagnant. Procedures utilizing multiple membrane technologies are prevalent throughout the dairy industry and becoming more so with each passing year (28–30, 77, 78, 100, 133, 166, 184, 209). Membrane technologies have improved and revolutionized the potential of the dairy industry, and with new advancements yearly, the future is truly wide open.

1.8. Membrane Maintenance and Cleaning One critical design aspect of any membrane separation system, which to this point has not been addressed here, is proper maintenance and cleaning of the membranes to remove deposits of proteins, minerals, and other foulants. Proper cleaning is extremely important not only for adhering to all applicable regulations, but also retaining maximum membrane performance (246). The latter is extremely important since membrane lifetime in the dairy industry is mostly dependent upon the frequency and nature of the cleaning cycles, not the time spent in service for dairy production. Regulations vary by location, but the US FDA requires complete 206 In Modern Applications in Membrane Science and Technology; Escobar, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


membrane cleaning a minimum of once per 24 hours in order to be certified as Grade A dairy. Local jurisdictions can set more stringent requirements if deemed necessary based upon the characteristics of the local milk supply or the products being made. Regulations in other countries can vary drastically from those in the US, including using membrane operations where US dairy manufacturers cannot, as outlined in the appropriate sections above. Since the products being made are for consumption, the necessary procedures to keep equipment sanitary and product specifications accurate can often dictate the overall economics of the process (246). Although protein deposition is the most prevalent dairy foulant (247), membrane performance can also be hindered by deposits of minerals, fats, carbohydrates, and bacteria (186, 197, 202, 246, 248–250). Each of these requires a different method of cleaning and/or prevention, and no cleaning method is without disadvantages. Alkaline cleaning solutions can be used to dissolve proteins which have deposited onto the surface of the membrane; however, they tend to precipitate out metals from solution, which can also be problematic. Oxidizing agents can be used to hydrolyze precipitated proteins—often increasing their solubility—but these can only be used with MF, NF, or UF because they are damaging to RO membranes. Enzymes, specialized catalysts which hydrolyze a particular foulant, can often be used to clean membranes with no detrimental side effects; however, the conditions in which they are used must be carefully controlled (251–253). The two most important aspects of enzymatic activity are time and pH. The duration of enzyme application must be within a certain range. If applied for too long the enzyme can attack the polymer structure of the membrane; if too short the enzyme will have no noticeable effect on the foulant (251–253). The pH of the solution must be carefully controlled in order for proper enzymatic activity to occur. The effective pH range for each enzyme must be obtained from the manufacturer. Certain foulants can be more easily removed at increased temperatures; however, all membranes have a maximum temperature designated by the manufacturer, less than 125°F for most polymeric membranes, which one must be certain to operate below. This can be problematic since some foodstuffs, gelatin for example, are only soluble above a given temperature. It must be noted that heightened temperatures do not always make cleaning easier. Denaturation of proteins at elevated temperatures can result in structural changes which make the protein insoluble and difficult to clean. Vibration (254) or ultrasonic agitation (255) can also be used to reduce precipitations onto the membrane surface; however, care must be taken to prevent damage to the membrane. Practicality and efficacy of these additions must be evaluated on a process and composition specific basis. For more detailed information, please see the excellent review by D’Souza and Mawson (246). The best form of cleaning is process optimization to minimize and/or prevent deposition of dairy components. For instance, if fat is a major foulant in a given process, but not a key component of the product, perhaps a skim milk feed should be used instead. This will greatly reduce membrane fouling due to fat while still obtaining the same product. Feed composition, operating temperature and pH can be varied such that the solubility of foulants is at a maximum leading to reduced precipitation. Fouling cannot be completely avoided, and membranes 207 In Modern Applications in Membrane Science and Technology; Escobar, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.

must be properly maintained and cleaned regularly. The details of the cleaning regiment must be determined for each individual process, bearing in mind what is the optimum for each unit operation. With proper design and optimization fouling can be minimized, and the membranes can remain healthy and effective longer.


1.9. Conclusions Many different membrane technologies have been discussed in this review, some that have been conducted for more than 30 years, and others which are more recent. An example of how all these processes could fit in a dairy is given in Figure 9. As shown, many different membrane processes, some used multiple times at different points, can be/are used in the dairy. This figure contains the processes and where they are commonly found as well as reference numbers of where this process has been studied. Finally, Table 4 is a reference list of current membrane and membrane product manufacturers as well as points of contact. If you are considering membranes for use in a dairy, these companies and individuals will serve as a good starting point. Membranes provide many current and future opportunities in the diary industry and their continued implementation will reduce costs, allow new products, and produce more environmentally friendly solutions.

Figure 9. Uses of Membranes in the Dairy Industry.



Table 4. Important Membrane Contacts in the Food Industry

Acknowledgments The authors would like to acknowledge the Ralph E. Martin Department of Chemical Engineering at the University of Arkansas for its support during the authoring of this chapter. Further, we would like to thank the Honors College at the University of Arkansas and a state SURF grant for its support of Heath Himstedt during the authoring of this book chapter. Finally, we would like to thank Dr. Roy Penney for his careful and thoughtful review of this chapter.

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