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Chapter 5
Starch Extrudates as Sustainable Ingredients in Food and Non-Food Applications Romain Milotskyi,1 Christophe Bliard,*,1 Richard Venditti,2 and Ali Ayoub2 1Institut
de Chimie Moléculaire de Reims, ICMR, CNRS UMR 7312, URCA, B18, UFR SEN, Moulin de la Housse, Chemin de Roulliers, BP 1039, F 51 687, Reims, Cedex 2, France 2College of Natural Resources, Department of Forest Biomaterials, North Carolina State University, 2820 Faucette Dr., Campus Box 8001, Raleigh, North Carolina 27695, United States *E-mail:
[email protected].
The technique of extrusion has been applied to plasticized starch and starch products to induce radical physicochemical changes to the semi- crystalline polymer complex in the native granules, such as destructurization, amorphization and homogenization. The addition of small molecular components such as plasticizer, water or supercritical CO2 in the pressurized melt creates the condition for the formation of widely expanded foam with a unique structure, by rapid decompression at the nozzle. The melted polymer compound has also been used as a reaction medium to perform chemical modifications.
© 2018 American Chemical Society Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Introduction Natural and synthetic polymers have been physically and chemically modified at various stages of their fabrication processes to adapt properties or reach specific functionalities. Starch is the main ubiquitous storage polysaccharide found in plants. In its native state, starch is found in the form of highly organized semi-crystalline micrometric granules (1). Beside its general nutritional value, starch is already used in all areas of industry due to its wide panel of physicochemical properties and widespread availability. Starch is used as a thickening agent and stabilizer; in pharmaceuticals as a disintegrant; as binder in paper and cardboard and surface sizing agent in coated paper as well as many other domains of applications (2). This large range of uses has been largely extended by tuning the desired properties through specific chemical modifications. Starch can be chemically modified by reacting the anhydro-glucose HO groups to achieve the desired properties. One of the problems encountered while trying to chemically modify a polymer molecule is the accessibility to the site to be modified by a reagent. The use of solvents can be beneficial to homogenize the reactive medium and ease reagents penetration towards the reacting sites, but polymers can sometime be difficult to solubilize and solvent removal will add to both process and environmental costs. Removing the solvent molecules can accelerate the reactions but the larger the molecular weight the higher the viscosity. The obtained concentrated polymer solutions can sometimes be difficult to manage. On large scales, reaction medium can be particularly difficult to homogenize by using mechanical stirring. Furthermore low molecular mobility is associated with poor mixing states and challenging temperature control, lowering yields and adding safety concerns to exothermic reactions. Carrying reaction on fused polymers can in rare cases be done with particularly stable low molecular weight molecules, but in natural polymers such as polysaccharides the degradation temperature is reached long before the melting can occur. An early developed solution to address this problem was to take advantage of the property of plasticized polymers to directly carry out chemical reactions at the melt stage within the extruder. One of the properties of amorphous polymers is the glass transition temperature (Tg) at which segments of the macromolecules acquire enough mobility to transfer local deformation through cooperative movements within an amorphous phase. This phenomenon can be strongly enhanced by increasing interactions with small plasticizer molecules. Thus, the addition of small portions of plasticizer drastically lowers the glass transition temperature. The glass transition temperature variation follows the phenomenological model of Couchman and Karaz (3) (eq 1), depending on the molar fraction and calorific capacities differences of the molecular components:
wI, weight fraction of components n et n+1 (starch and plasticizer) ΔCpi, change in heat capacity at the glass transition temperature Tgi Tgi, glass transition temperature of components n et n+1 90 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Plasticized polymer compounds can then be processed by extrusion at temperatures much lower than both the fusion and decomposition temperatures. The extrusion technique has long been used in the food transformation industry to prepare cereal extrudates such as pasta (4). The use of chemical transformation during the extrusion process was a direct development of the technique, allowing the mixed chemicals to react onto the polymer melt. Thus chemical reactions can be advantageously carried out in the melt state, providing that care is taken to prevent undesirable interferences between the plasticizer and the reagent. In the plasticized stage the obtained compound can then be easily melted, extruded in single or twin-screw extruders and molded under pressure at low temperatures by the action of thermo-mechanical energy. This process called reactive extrusion (REX) gathered the advantages of efficient mixing technology in a small thermo-controlled volume, accelerating the reaction by concentrating the reactive medium. During extrusion, starch undergoes drastic physicochemical transformations. The granular structure is destroyed, the semi crystalline order disappears and some hydrolysis of the polysaccharidic chains takes place due to the high shear imposed upon the very large macromolecules. The obtained products are more homogenous and often display reduced intrinsic viscosity. The plasticizing and melting process destructurizes the granular organization and liberates the access to the hydroxyl groups to react upon. Starch can be plasticized by small molecular polyol such as glycol, glycerol and even larger polyols such as mannitol or sorbitol. A recent paper (5) describes how low DP dextrins can also be used as plasticizer. It was also noticed that a partial chemical modification induced a noticeable decrease in the Tg. As a matter of fact, when carrying out several consecutive chemical modifications on starch, including a purification stage where the plasticizer was washed off, it was noted that the presence of plasticizer was not necessary from the second modification stage and on (6). After grafting pending residues at a degree of substitution (DS, or average number of reacted OH groups per glucose monomer) level of 0.5 onto the polysaccharide, further extrusions could be performed directly without the addition of plasticizer. This phenomenon could only be partially explained by a macromolecular size reduction, consecutive to the strong thermomechanical input, but also by the fact that the presence of pending functionalities on the polymer eases the energy transfer along the polymer chain. Most classical cationic, anionic or neutral chemical modifications can be easily applied to starch in order to impart an ionic –cationic or anionic- or hydrophobic character to the resulting polymers. These modifications can be done by attaching the modifying residue onto -OH groups of the polysaccharide backbone through a common ether, ester or acetal bond. High pressure supercritical CO2 has been added within the pressurized mix to modify the physicochemical properties of polymer compounds inside the extruder (7). It has also been largely used to enhance expansion of starch melts outside of the dye in order to create foams (8). Post-extrusion UV or β irradiation of starch has been used in combination to modify the final properties of the starch extrudates by creating cross-linking points or graft hydrophobic residues (9, 10).
91 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Starch Foam The world-wide production and consumption of plastics made from petroleum sources has increased enormously in the past 20 years. Disposal of used plastic products has become a public concern because of their non-biodegradability (11). Much effort has been put into producing environmentally friendly alternatives to plastic products to alleviate widespread concerns about their long-term resilience in landfills and toxic by-products from their incineration. Another challenge is that since 2010, chlorofluorocarbons (CFCs) have been banned globally because of their adverse impact on the planet’s protective ozone layer. One industrial activity that has been significantly impacted by this ban is the manufacture of plastic foams—lightweight alternatives to solid plastic that are valued for their flexibility and ability to insulate, as well as their cushioning ability and enhanced flotation in marine applications. Plastic foams are created by combining two chemicals that would otherwise form a solid plastic or by melting an existing solid (12). A third substance, often a CFC, is then added as a “blowing agent.” This agent vaporizes at the reaction temperature, releasing gas bubbles into the molten plastic. Today, the goal of the plastic foam industry is to make a new material that remains lighter than solid plastic but has many of the same qualities of durability and rigidity as the solid version, and to do so without having to rely on ozone-depleting gases. There is more carbohydrate on earth than all other organic material combined. Polysaccharides are the most abundant type of carbohydrate and make up approximately 75 percent of all organic matter (13). The most plentiful polysaccharide is cellulose, found in plant cell walls. It is the most abundant organic compound on earth and alone accounts for 40 percent of all organic matter. It forms the structural fiber of plants, keeping the cell wall intact and giving it strength. Well over 150 billion pounds of cellulose are produced commercially each year (14). Starch is also a very abundant biopolymer, used by plants as the major storage material for carbohydrates (13). Starch is a high molecular weight mixture of two glucose-based polymers, amylose (linear) and amylopectin (branched). Starch-based foams have been prepared for many decades. Through a combination of heat, water and shear, the starch-water mixture behave thermoplastically and can be extruded into shapes (15). This method has been used to produce cereals and more recently starch-based packing material. These types of foams have pore diameters in the order of 1 mm and are considered to be macro-porous foam.
Starch Microcellular Foams (SMCF) When water and heat are applied to starch granules the granules swell. The amylose material in particular extends from the starch granules and forms a hydrogel in the water phase. If the water is removed in a normal drying process, large capillary forces act on the starch and collapse the starch structure. In order to preserve this structure, two methods have mainly been used: freeze drying and solvent exchange (16–18). Solvent exchange involves the replacement of the water with another solvent (or sometimes a succession of solvents) of lower 92 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
surface tension. Final drying from the solvent with low surface tension, e.g., liquid or critical CO2 or ethanol, avoids the large compressive forces caused by the drying of high surface tension water and better preserves the SMCF pore structure. The micro-structure of the bulk region of SMCF sheets formed from wheat starch dried from ethanol, consists of a network of strands of amylose forming irregular pores generally less than 2 micrometers in diameter (18). The specific surface area of SMCF sheets dried from critical point CO2 were found to be very high (18), ranging from 50 to 145 m2/gram, which compares favorably with the specific surface areas of common inorganic fillers and pigments used in paper, coatings and paints: kaolin clay (5-8 m2/g) , calcinated clay (16-17), calcium carbonate (2-6), silicates (40-130), and titanium dioxide (8-25) (19). Ayoub et al. have explored the use of foamed starch as a paper filler. Very high brightness pigments have been reported (see progress report). The use of SMCF in the field of drug delivery and as hosting molecules for different organic materials has been explored (20, 21). Porous microspheres have also been prepared which simulate pollen and carry active agents for controlling the parasites that infect honeybees (21). SMCF also has been used as precursors for carbonaceous materials with tunable parameters (22).
Starch Microcellular Foams via Supercritical Fluid Extrusion A new, low-temperature, and low-shear extrusion technology, called supercritical fluid extrusion (SCFX), has been invented at Cornell University by Professor Syed Rizvi at the Department of Food Science (23). The technology involves injection of SC-CO2 during an extrusion process to produce microcellular extrudates. SC-CO2, formed by putting CO2 gas under increasing temperature and pressure, has been used as ‘an environmentally sound replacement for other toxic chemicals, including the solvents used in the manufacture of plastics’. The SCFX process has been successfully applied to various formulations of starches and proteins for continuous generation of microcellular foam (24, 25). The effects of process variables and formulation on foam expansion, cell size, cell density and mechanical properties have been studied and reported. The process involves introduction of SC-CO2 into a gas-holding matrix within an extruder especially modified and configured for this purpose. The use of SC-CO2 allows for simultaneous bubble nucleation, expansion and reduction of matrix viscosity (due to large solubility effects). It is well-known that changing the extruder nozzle diameter and length can control the pressure drop rate across the nozzle. The nucleation of cells and their subsequent growth is a function of both the solubility of CO2 in the polymer/gas matrix (and thus the operating pressure) and the rate of pressure drop across the nozzle (24–27). These authors found that the microstructure of extruded foams can be precisely regulated by varying operating parameters. A similar degree of control of the microstructure can be achieved by varying the operating pressure and nozzle pressure drop rate in the case of starch-based SCFX extrudates. Investigations over the last few years by Rizvi and co-workers have shown that a large variety of macro- and microcellular starch-based 93 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
extrudates can be prepared using the SCFX process. The morphology of the extrudates can be varied from closed-cells to open cells. Analysis by SEM of the cellular structure showed that the average cell size ranged from 65 to 244 microns, and cell density varied between 2.3 x 106 and 6.8 x 106 cells/cm3. Expansion ratios of the extrudates varied from 2.3 to 15.8, and bulk density from 0.09 g/cm3 to 0.58 g/cm3, depending on the formulation and the drying temperature used (24, 25, 28, 29). However, the major drawback of using thermoplastic foam starch-based polymers to produce biodegradable plastics is their hydrophilic characteristic and their poor mechanical properties. Thermoplastic starch will, if immersed in water, rapidly absorb moisture and lose most of its functional properties. One approach toward solving this problem is the chemical modification of the starch backbone by converting the hydrophilic hydroxyl groups to esters via reaction with carboxylic acids (30). Extruded starch acetate foam typically has low mechanical properties. To address this shortcoming, biodegradable polymers (polylactic acid, Mater-Bi and Eastman Copolyester) have been blended with starch acetate. The greatest obstacle to the development of starch acetate thermoplastics is cost. Compared to native thermoplastic starch, the cost of manufacturing starch acetate is 10 times higher. Recently, Ayoub et al. (31) developed the SMCF which was generated using native corn and pre-gel starches provided by Cargill Industrial Starch. Native and pre-gel corn starches are mainly considered since they are the least expensive and are widely available. Sodium hydroxide (1%) was used. Cross-linking [XL] and acetylation [Ac] are widely used methods to prepare modified starches. The benefits from this dual modification are that cross-linking makes the starch more resistant towards acidic medium, heat and shearing while acetylation increases hydrophobicity and is thus useful in increasing the water resistance of starch. The main objective of this part of our work was to reduce barriers that prevent the usage of starch-based foams by understanding the effect of dual-modification sequence of crosslinked (XL) and acetylated (Ac) starch in one continuous supercritical fluid extrusion (SCFX) process on wetting properties, physicochemical properties and cellular structure of solid foam. The starch (native and pregelatinized) was reacted with epichlorohydrin (EPI) and acetic anhydride (alkaline conditions) in a twin-screw extruder in the presence of supercritical carbon dioxide. An increase in epichlorohydrin EPI concentration from 0.00 to 3.00 % increased the degree of cross linking as measured by DSC and confirmed by the quantification of the glucose units in the solution after acid hydrolysis. We observed a reduction of the glucose units from 93.07% for 0.00% EPI) to 6.85% when 3.00% EPI was added. In fact, EPI was able to react with the glucose molecules (crosslinking step) and/or itself (polymerization step), and this leads to a decrease in glucose unit in the starch. The most prominent feature of the esterified starches was their increased hydrophobicity as determined by contact angle measurement. The increased hydrophilicity of starches is attributed to the replacement of hydrophilic hydroxyls by the relatively hydrophobic ester groups. With crosslinking/acetylation processing, contact angle was higher for modified starches, indicating that chemical treatments induced dramatic changes in their surface polarity. Compared with native starch, the contact angle for 3%XL-15%Ac starch increased from 43.1° to 91.7° which indicated its lower 94 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
wettability. In the FTIR spectra, we detected the carbonyl group’s peak at 1,740 cm-1. The intensity of this peak increased with a decrease in the peak intensity of the hydroxyl groups at 3,000-3,600 cm-1, indicating that the hydroxyl groups on starch were replaced by the acetyl groups. The average density (dav) of the SCFX extrudates shows that the values depend on the concentration of SC-CO2 used, the degree of chemical modification achieved by the addition of (EPI) and (Ac), and the nature of the starch (native or pregelatinized). The addition of small concentrations of (EPI) and (Ac) to the formulation reduced the density of the extrudates and increased the expansion ratio. The average cell size in the extrudate, determined by scanning electron microscopy, was also found to decrease from 150 μm to 34 μm (65% of the pore size was less than 25 μm and 10% less than 800 nm) by the addition of the two reagents (EPI and Ac) (Figure 1). The unmodified extrudate had a larger spread than extrudates modified with 3% (EPI) and 15% (Ac). This indicated that dual modification also increased the uniformity of cellular structure. The more uniform cellular structure of extrudates with EPI can be explained on the basis of the rate of cell nucleation. Nucleation of cells usually takes place over a period of time, and competes with the diffusion of gas into the cells, which leads to cell growth. A uniform cellular structure is important for developing a product with isotropic mechanical properties and provides greater control over its texture. SCFX extrudates also exhibited a non-porous skin surrounding the internal cellular morphology. This skin comprised of unexpanded starch, and very small cells. Rapid diffusion of CO2 out of the sample creates a depletion layer near the edges in which the gas concentration is too low to contribute significantly to cell growth. A combination of these factors caused the formation of a non-porous skin. The skin reduces water penetration and delays onset of water-related changes, which may be a desirable characteristic. Under the texture profile analysis (TPA) test mode, we performed the measurements of hardness and adhesiveness on each SCFX extrudates. The peak force of the first penetration was termed as hardness and the negative force after the first penetration was reported as adhesiveness. Moreover, the dual-modification (XL-Ac) of starches provided more hardness and adhesiveness to the extrudates than was observed for the unmodified starches. The results indicate that the addition of acetic anhydride can impact the water resistance of starch foams significantly and suggest that the combination of acetylation and crosslinking may be a powerful tool to produce solid foam reactively in one step when combined with supercritical fluid extrusion. This finding clearly demonstrates the positive effects of dual modification on the structure and the pore size distribution of solid foams.
95 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Figure 1. SEM micrographs of cross-linked starch extrudates at different concentrations EPI crosslinker.
Rationale and Significance of This Study It has been shown through the first section of this chapter that SMCF structures have great potential in industrial application. This study is a significant example of how SMCF could be an advantageous replacement of polystyrene foams for packing products. The intellectual interests of this activity were multi-fold. First, a better understanding of the use of SC-CO2 as a blowing agent and the simultaneous crosslinking of natural polymers to produce expanded foam-like micro-porous extrudates was developed. Second, relationships between chemical composition of natural polymers and the mechanical properties of these polymers was determined. Finally, a better understanding between cellular solid structures and their industrial application was addressed. This knowledge, currently lacking, will assist other chemist, materials chemists, and engineers to better utilize natural polymers for modern product applications. The US dependence on petroleum based plastics is not sustainable, based on the extremely volatile oil and energy situation, coupled with major changes in supply and demand patterns. Recently, the price of naphtha, the oil derivative used heavily in plastic production, has risen dramatically, climbing close to $600 per ton, significantly up from its price in the $400 range. The oil price also spiked to above $100 a barrel in January 2008. The severe cost increases are a challenge but in addition, the limited feedstock availability is tightening and impacting supply and demand worldwide, and putting the industry under tremendous pressure. In the United States, plastics now make up a significant part of a typical municipal solid-waste stream, and represent the fastest growing component. Forty-four billion pounds of plastics enter United State’s municipal solid-waste stream each year, equivalent to half a pound per day per person. In the United States, plastics on average account for 10% of municipal solid-waste weight, more than metals (8%) and glass (6%). The cost of waste management is now a matter of great public concern. Waste management has become a large and expensive affair. The public is extremely sensitive to the proximity of landfills to residential areas. Public concern has periodically erupted in the form of local restrictive legislation. The use of biodegradable, starch and wood fiber based 96 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
foams as proposed here would be a significant improvement to the municipal solid waste management issues facing the US. Furthermore, starch-cellulosic matrix do not have to be put in landfills after use. They can find an ultimate application as viable feedstocks for enzymatic conversion to ethanol or as high heat value material suitable for combustion.
Extrusion: A Process To Manufacture Nutritionally Superior Carbohydrate-Based Food Products Rice is one of the most popular carbohydrate staple foods in the world. Literally hundreds of varieties of rice are grown including long, medium, and short grain varieties with different colors, tastes and textures. Such mass scale consumption makes it an excellent choice as a nutrient or medication delivery system to target populations facing malnutrition or diet-induced chronic illnesses. In America today, the rice consumption per capita has increased rapidly to an all-time high of 27 pounds annually, mainly due to the growth of Asian and Hispanic dining influences, yet far from the potential of 100+ pounds per year found in many developed and developing countries. By the milling process, the rice grain is converted to the polished white rice, with nutrients such as vitamins, minerals, bran and fiber stripped away. Made by relatively less extensive milling, brown rice retains the bran, fiber, and germ components of the rice grain, making it a healthier choice. However, it has a characteristic rubbery texture and nutty flavor profile deviating from the mainstream preference for white rice. Moreover, brown rice requires at least twice the amount of preparation time and has a shorter shelf life due to potential rancidity caused by enzymes found in bran. It appears that there has yet to be found or developed an all-round tasty and convenient wholesome rice. Rice fortification is an existing practice and has been readily practiced by surface coating and transgenic techniques. Coating by fortified solution and drying is effective for water-soluble micronutrients such as vitamins and minerals; however, it prevents the consumers from their habitual rinsing of the rice prior to the cooking. Golden rice, the well-known genetic modified rice variety, has been shown in recent research efforts to produce increased levels of target beta carotene; however, the enhanced composition is limited to the investigated micronutrient and the uncertainty of public acceptance and preference for genetically modified foods. Most recently, simulated rice grain-like premix pellets by extrusion are making progress with highly concentrated doses of micronutrients (32). The practice of blending these colored pellets with regular rice kernels in the 100 to 1 ratio however does not have a favorable visual appeal. Moreover, upon cooking these extruded rice grains develop an unfamiliar firmer and stickier product texture, likely resulting from the severe extrusion-induced heat and shear treatment. Ayoub et al. (33) proved that there exists a sufficient gelatinization level of rice starch in the extruder to produce good quality and shaped rice grains. This result was obtained through a combination of more thermal and less shear input in order to minimize rice starch degradation and hydrolysis, providing softer and 97 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
less sticky rice products. Thereafter, the research team at Cornell University in the Department of Food Science, developed an effective and economically attractive continuous low shear extrusion of nutritionally superior rice kernel products. This team evaluated and quantified the critical thermal, sensory, nutrient retention, and textural properties (Figure 2). Comparisons of the fortified extruded rice with commercial white and brown rice was also conducted to validate product acceptability and superiority.
Figure 2. Schematic illustration of the manufacture of Nutritionally Superior Rice.
Extruded rice samples were produced using a pilot-scale TX-57 Magnum corotating twin-screw extruder system (Wenger Manufacturing, Sabetha, KS). The extruder was configured into two barrel lengths, a short barrel (SB) configuration with a L/D ratio of 15 and a long barrel (LB) configuration with a L/D ratio of 28.5 respectively (Figure 3 and 4). Such extruder setup with varying barrel length allows for distinct differences in residence time and shear-mixing treatments. The pressure profile across the extruder barrel will be established and built up to a peak pressure of about 500 psi using a customized set of screw configurations and a restrictor valve just upstream from a specially configured die support having multiple rice-shaped die inserts. The temperature of the extruder zone was set respectively at 30, 60, 60, 30 and 30°C from zone 1 to 5 for the LB configuration and at 30, 60 and 30°C from zone 1 to 3 for the SB configuration (Figure 3). The extruder was set at a screw speed of 180 rpm. Dry feed rate was set at 70 kg/hr. The rice dough moisture content in the barrel is targeted at 33% by simultaneously adding steam injections at the preconditioner (5-10% dry basis) and water at the extruder barrel (28-33% dry basis). The rice dough exiting the kernel-shaped dies was heated to about 85-90°C and then immediately cut by a rotating blade on the die surface at 1000 to 1500 rpm into individual kernels. Extruded kernels were collected on perforated aluminum trays and placed into a closed convection dryer at room temperature to 60°C to achieve shelf stable product moisture of 8-10%. In this final stage, the extruded prototypes were stored in sealed dark plastic containers for further product evaluations. 98 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Figure 3. Long and short barrel screw configurations for extruded rice production.
Figure 4. Pilot-scale TX-57 Magnum co-rotating twin-screw extruder system configured with long barrel of L/D ratio of 28.5:1 and short barrel of L/D ratio of 15:1 at Cornell University, Department of Food Science, Ithaca, New York. (photo courtesy of A. Ayoub). The fortified extruded rice hereby produced is unique and distinct from other extruded rice grains studies (32), which are substantially fortified to be strictly used as a 100:1 premix blend with natural rice grains. This product aims to be served independently in the place of natural rice grain and every kernel contains a homogeneous mixture with all ingredients evenly distributed when served. Use of rice flour derived from lower market value broken rice is economically attractive and may promote further cost-saving and enable the delivery of a more affordable and feasible program to alleviate worldwide malnutrition. Moreover, 99 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
fortification of functional ingredients including soluble fiber and bran can readily exceed the levels found in natural rice varieties to achieve potential health benefits such as better control of blood sugar, reduced fat absorption, and lowering of blood cholesterol. These are benefits not commonly associated with the nutrient composition of existing natural rice varieties. The commercialization potential of this project research is very high for consumer markets worldwide. In addition, the U.S. Federal Government could potentially use the manufacturing technology of nutritionally superior extruded rice developed from this proposed research to help the malnourished population in the States and worldwide. There are no major competitive products in this field since the nutritional composition of superior rice concept far exceeds that of conventional rice variety. Moreover, given the growing public anxiety over genetically modified food products, nutritionally superior extruded rice products offer distinct advantages over existing rice fortification technology, including surface coating since coated fortificants are easily washed away during rinsing.
Cationic Starch by REX A previous study (34) demonstrated that starch cationisation could be performed by using REX in very efficient conditions. A wide range of DS could be obtained in the melted state within minutes in reproducible conditions. Moreover both the viscosity and solubility of the products could be tuned on demand by changing the process parameters as opposed to the batch process where the cationic group distribution along the polymer chain and the average molecular size were fixed. Cationic starches are an important group of chemically modified starches widely used in papermaking and textile industries. They are used as additives during the paper formation to improve the retention of short fibers and drainage rate of the pulp and to increase the strength of the final sheets. In batch processes the reaction is carried out for several hours at high temperature in very large mixed slurry batches. The reaction is followed by a maturation period where the reaction mix is stored for a long time under controlled temperature, the overall process taking several days. The DS and viscosity range are limited by the starch source and reaction conditions. The degrees of substitution (DS) commonly used are in the range of 0.01 to 0.1. Most cationic starches used in industry are produced by grafting hydroxypropyltrimethyl ammonium (HPTMA, Figure 5), by reaction of glycidyl propyl trimethyl ammonium (2-epoxypropyltrimethylammonium chloride (QEp), or its hydrochloride precursor (3-chloro-2-hydroxypropyltrimethyl-ammonium chloride (QCl) onto native granular or hydrothermally destructurized starch. Several teams have described the preparation of cationic starches by using the REX technique. Della Valle et al. (35) studied the cationization of wheat starch with 3-chloro-2-hydroxypropyl-trimethyl ammonium chloride in a twin-screw extruder (Clextral BC 45). They found that the reaction efficiency depends mainly on the temperature of the reaction and the order in which the reagents and the catalysts were injected. According to their work, the SME had no 100 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
significant influence on the DS. The highest DS (0.045) was obtained with a reaction efficiency (RE) of 82% after a 48 hour post-extrusion maturation period. These authors claimed that the low viscosity of the obtained cationic starches was an advantage for papermaking due to the degradation of starch during extrusion. Carr et al. (36) investigated the cationization of corn starch with 3-chloro-2-hydroxypropyl-trimethylammonium chloride using sodium hydroxide as a catalyst in a Werner ZSK 30 extruder. The studied parameters influencing the reaction were temperature, reagent / starch ratio and screw speed. The authors showed that the reaction efficiency increased significantly with the reagent / starch ratio and the temperature and much less strongly with the screw speed. In our study (37) cationic starch was synthesized in a mini twin-screw extruder (Minilab Rheomex CTW5) using both reagents: QCl and QEp with glycerol as plasticizer. The influence of various parameters such as plasticizer amount, reagents concentrations, addition order, temperature and starch granule destructurization were studied. The highest reaction efficiencies (RE) 40% and 94% for a reagent/anhydro glucose molar ratio of 0.05 with (QCl) and (QEp), respectively, were obtained in one step after 5 minutes of reaction (Figure 6). The cationisation of starch strongly increased the water sorption properties. A theoretical model was applied to study reaction kinetics at different temperatures between 100 and 140 °C (38). It was shown that starch cationisation in the extruder appeared to follow a second order reaction. The reaction was complete in a few minutes. A decrease of molecular weight was observed on the REX modified product as in other extruded products. On the other hand, the cationization reaction conditions did not influence the MW distribution as the recorded size exclusion chromatograms of two samples DS 0.02 and DS 0.13 were identical. Tara el al. (39) also reported starch cationisation using a laboratory scale co-rotating twin-screw extruder (Clextral BC 21). They found that several important parameters such as screw speed, feed rate, and barrel temperature affect the DS and RE of the reaction.
Figure 5. Hydroxypropyltrimethylammonium residue and reagents used in starch cationization.
101 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Figure 6. Influence of the reaction time on the DS and RE. Adapted with permission from ref. (37). Copyright 2003 John Wiley and Sons.
Anionic Starches by REX Anionic starches are another important group of compounds found in several fields of application. They are used as thickeners in the food industry, excipients in the pharmaceutical industry and as additives in the process of textile printing (40). In addition, starch-based anionic flocculants are very useful for the sedimentation of fine clay particles, in porcelain wastewater, and phosphatic clay waste (41). One of the best known anionic starches is carboxymethyl starch (CMS). In 1924 Chowhury (42) performed the first synthesis of CMS, by reacting starch in an alkaline solution (40% NaOH) with sodium monochloroacetate. Classically, CMS is prepared by a slurry batch reaction. However, long reaction times and large amounts of solvent are required for this method (43). Reactive extrusion offers a simple and eco-friendly process with low amounts of plasticizers and shorter reaction time (44). Gimmler et al. (45) described the synthesis of low DS (0.05 to 0.3) carboxymethyl starch using reactive extrusion. These authors proposed an analytical model to find optimal extrusion conditions of potato starch carboxymethylation. They also determined the effect of extrusion conditions on rheological properties, RE and DS. Bhandari et al. (46) synthesized CMS with higher DS in a twin-screw extruder. In this study aqueous ethanol (50% by weight) was used as a plasticizer/solvent. A DSmax of 1.54 with a RE of 42% was obtained by increasing the number of kneading element (0-2). In our study (5) a mini twin-screw extruder (Minilab Rheomex CTW5) with two different screw geometries (co- and counter- rotating) was used to synthesize carboxymethyl potato starch. The influence of several process parameters (screw rotation speed, screw geometry, SME and reaction time) on RE as well as structural changes in starch molecules was investigated. It was found that screw geometry plays an important role in starch carboxymethylation via REX. The RE with different reagent sodium monochloroacetate (SMCA)/anhydroglucose unit (AGU) molar 102 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
ratios was systematically higher for co-rotating than contra-rotating geometry (Table 1).
Table 1. Influence of co- and counter-rotating screw geometry on the DS for different reagent molar ratio SMCA/AGU
DS (co-rotating)
DS (counter-rotating)
0.5
0.38
0.34
1
0.57
0.52
2
0.8
0.72
3
0.91
0.83
The mixing capacity is higher in the co-rotating mode. In this mode the specific screw geometry allows the material to follow an eight-shaped helical (∞) trajectory and pass very easily from one screw to the other in a ‘continuous channel’ type flow. However, in the case of counter-rotating screw geometry the material stays in a C-shaped volume around each screw and mainly recirculates around the same screw (Figure 7). The quality of the mixture is therefore limited as already observed (47).
Figure 7. Different flow types: a) C-chamber, b) continuous flow. Specific mechanical energy (SME) is a key parameter in starch chemical modification via REX. SME is the work input from the drive motor into the material being extruded, expressed per unit mass of the material. This parameter provides a good characterization of the extrusion process (48). SME (eq 2) depends on screw rotation speed, torque (mechanical work) and feed rate of the process (49) :
ω, screew speed (rpm) τ, torque (N m) φ, feed rate (kg h-1) 103 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
In process extrusion machines, the number of kneading blocks and counter pitch screw elements can be changed in order to modify SME. The screw geometry being fixed, the screw speed was increased in order to investigate the influence of SME on the RE and structure of the product. The obtained results were somewhat surprising: as the screws rotation speed increases, RE increased, despite the shorter residence time. The RE increased from 56 % to 64 % (Figure 8). As screw speed increases, SME in the extruder machine increases, thus allowing the reagent to better diffuse into the melted macromolecular mass and to react more easily, resulting in a more efficient mixing capacity and higher RE values. The hypothesis of a reaction boost due to a local increase in the temperature of the melt at higher screw speed was invalidated by running the reaction at different temperatures, keeping all other parameters constant. No noticeable improvement in RE was observed between 90°C and 120°C. Above 120°C the RE started to decrease due to product degradation.
Figure 8. Effect of screw rotation speed on the residence time (●) and reaction efficiency (■). Reproduced with permission from ref. (5). Copyright 2009 John Wiley and Sons.
Chemical modification of starch by reactive extrusion is a complex phenomenon because it includes both the physical modification of this polymer and the changes in the chemical structure. Starch fragmentation not only leads to changes in the molecular weight, but also structural changes of the molecules, which is not surprising given the nature of the starch which is a mixture of amylose (linear or branched long-chain molecules) and amylopectin (highly branched molecules). To understand this phenomenon we mesuared both intrinsic viscosity and molecular distribution by size exclusion chromatography. 104 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
It was shown (Figure 9) that significant exponential decrease in intrinsic viscosity with SME was observed as compared to native starch (η = 280 ml/g). This experiment confirms that chain degradation takes places during REX process.
Figure 9. Variation of intrinsic viscosity with SME for CMS samples extruded with different screw speed. Reproduced with permission from ref. (5). Copyright 2009 John Wiley and Sons.
It was shown that the intrinsic viscosity of amylopectin is more sensitive to shear intensity than that of amylose. The high sensitivity of amylopectin, compared to amylose, is due to its higher molar mass rather than its branched structure (50). To confirm the results of intrinsic viscosity, we applied SEC chromatography which gives important information about molecular mass distribution. As can be seen from Figure 10, starch molecular chain degradation occurred during the extrusion process. The elution profile of native starch on Sepharose CL-2B shows an amylopectin peak located at the exclusion volume (V = 110 ml), representing 69% of the total material, followed by the smaller fraction of amylose included in the eluted fractions later. In the case of the extruded CMS sample with a low SME action (30 kJ/kg), the peak of the high molecular weight component is predominant (Vel = 125 - 165 ml). However, for higher values of SME, lower molecular weight fractions (Vel = 220-320 ml) are mainly observed. These results are in good agreement with those obtained by intrinsic viscosity. 105 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Figure 10. SEC Chromatograms of native and carboxymethylated (CMS) starches. Reproduced with permission from ref. (5). Copyright 2009 John Wiley and Sons.
Starch Plasticizers During REX Carboxymethylation of Starch The main plasticizer used in the thermoplastic starches (TPS) compositions is glycerol (51). The non-toxicity and overall safety of glycerol is a significant advantage for its use as a plasticizer. At the same time, the price of glycerol remains highly variable and is related to its degree of refinement, ranging from US $ 167 / ton for crude glycerol to US $ 267 / ton for technical glycerol and US $ 1,000 for bi-distilled glycerol (52). We tested three new natural eco-friendly starch plasticizers as well as glycerol. The two studied dextrins are starch hydrolysis products, H-Maltor® 70-80 is rich in maltose 70% maltose, 18% maltotriose with the remaining in glucose and oligomers; and Glucor® 30-72 is rich in glucose (Dextrose Equivalent of DE = 24). EP2 is the crystallization liquor from sugar beet refinery. In Table 2, the RE obtained with different plasticizers are compared. Starch plasticized with Glucor® provided a higher DS compared to starches plasticized with H-Maltor®. Considering the difference in SME between these two dextrins, we can assume that starch plasticized with H-Maltor® shows a poorer melting behavior than when it is plasticized with Glucor®, which can influence the reaction efficiency. The plasticization with EP2 has a lower DS than in the case of Glucor® with close melting behavior. The advantage of these new plasticizer systems is their low cost and the fact that they share chemical similarities with starch. 106 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Table 2. Influence of the type of plasticizer on the DS, RE and SME for a DSt = 1 Sample
RE %
SME (kJ/kg)
CMS (glycerol 25%)
57
75
CMS (Glucor® 25%)
41
140
CMS (H-Maltor® 25%)
15
225
CMS (EP2 25%)
22
120
Reaction Kinetics in REX Conditions The kinetics of the reaction via REX is radically different from the that of batch processes. This result can be explained by the fact that in the case of REX no solvent is used; as a result the reaction temperature and the reagent concentrations are often much higher in the melt state than in solution. It is interesting to observe the resulting kinetics of starch carboxymethylation in different reaction processes. The team of Tijsen et al. (2001) described the carboxymethylation of potato starch in batch using different solvent systems. RE of up to 70% was obtained using a mixture of isopropanol – water with total reaction around 26 hours (43). In REX conditions, we found that the reaction displayed 2 types of kinetics: very rush start, resulting in 56% reaction efficiency in less than sixty seconds, then the reaction levels out after 5 minutes and plateaus at a RE of 64% (Figure 11) (6). It is important to mention that the OH groups of the glycerol plasticizer can react with a reagent as well. This could be a cause for the limitation of RE in REX obtained CMS .
Distribution Patterns of CMS Samples Starch possess three hydroxyl groups which can be substituted with carboxymethyl groups. All the three OH groups display different reactivity (53). The reactivity of hydroxyl groups depends on several factors including the state of activation, reaction medium and degree of crystallinity. It was interesting to study how REX conditions can affect the substituent distribution in anhydroglucose unit (AGU). Samples with different DSs were synthesized using single or multistep carboxymethylation via REX (6). Proton NMR spectroscopy on the hydrolyzed CMS samples was used to calculate the DS and analyze the substituent distribution in positions 2, 3 and 6 of the AGU. For all samples, the substitution preferably takes place in position 2 then 6 and 3 (Figure 12). This phenomena was observed for carboxymethylation reaction in solution (54). The high regioselectivity of etherification in position 2 could be explained by the preferred deprotonation of the most acidic hydroxyl group at position 2 of AGU (55). Amylose from potato starch was modified using the same conditions. In amylose the substitution in position 2 and 6 were preferred, however, the relative reactivity of hydroxyl remains unchanged: 2>6>3. 107 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Figure 11. RE of carboxymethylation as a function of the reaction time for DSth = 1 at 90 °C with a reagent monomer molar ratio of 1 in a co-rotating screw geometry, an equimolar NaOH / reagent ratio. Screw rotation speed was 60 min-1, the reaction time was varied between 55 seconds and 10 minutes by using an internal recirculation mode. The samples were plasticized with 25% glycerol.
Figure 12. Total and partial DS in positions 2, 3, 6 of CMS samples. Created using data from ref. (6). 108 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Another important parameter in CMS synthesis is the distribution of carboxymethyl functions along the polymer chains. It’s clear, that this distribution can have an effect on the properties of starch derivatives. HPLC chromatography of hydrolyzed CMS was used in order to quantify different repeating units (unmodified glucose (G0), mono-(G1), di-(G2) and tri-(G3) substituted glucose) in REX conditions. The first theoretical model of multiple carboxymethyl substituents distribution was described by Spurlin (56). According to Spurlin (Figure 13), the statistical model assumes that during the reaction of carboxymethylation, the reactivities of the three OH groups in the AGU are constant throughout the course of the reaction and are independent of the DS of the already modified polyglucosidic chain. The state of functionalization at the positions 2, 3 or 6 within the same AGU is considered as having no influence on the further substitution of these positions. The tetra-substituted carboxymethyl derivatives are not included in this model because of their low potential amounts. This model was used by several teams in order to compare the experimental data of carboxymethylation in solution with Spurlin statistics. Lazik et al. (57) studied the carboxymethylation of potato starch in methanol. They found that the reaction follows the theoretical Spurlin statistics model.
Figure 13. Proportions of unsubstituted (G0), mono-(G1), di-(G2) and tri-(G3) carboxymethyl glucose units according to Spurlin model. It was interesting to compare these results to the ones from reaction in REX conditions. As we can see from Figure 14, from the beginning the un-modified glucose (G0) and mono-substituted glucose (G1) do not follow the statistics. Di-substituted (G2) fractions seem to follow the statistics until the DS 1 is reached (Figure 15). Surprisingly, the level of tri-substituted glucose (G3) do not follow the statistics but display a linear increases from DS 0.8 and on (Figure 109 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
16). This completely unusual distribution seen in reactive extrusion conditions tends to indicate a block type of substitution of the polyglucosidic chains during the reaction as opposed to a statistical modification type. This heterogeneous substitution can have an influence on the CMS properties.
Figure 14. Values of the glucose G0 (▲), mono-O-carboxymethyl glucose G1(♦) mole fractions compared with Spurlin statistical model (lines).
Figure 15. Values of the mono-O-carboxymethyl glucose G1 (♦) and di-O-carboxymethyl glucose G2 (●) mole fractions compared with Spurlin statistical model (lines). 110 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Figure 16. Values of the glucose the di-O-carboxymethyl glucose G2 (●) and 2,3,6-tri-O-carboxymethyl glucose G3 (■) mole fractions compared with Spurlin statistical model (lines).
Conclusions Reactive extrusion is a powerful technique particularly well adapted to natural polymers such as polysaccharides. It can bring radical physicochemical changes to the polymers. It allows the production of expanded products with unique properties and can lead to fully destructurized product. Chemical modifications can be performed with a much higher efficiency and kinetics. The range of properties and chemical modification levels can be extended. The structural nature and properties of the products are different from similar products obtained from batch reactions.
Acknowledgments We would like to thank the Champagne-Ardenne regional council for the financial support of the cationic and anionic starch modification research projects “AMIVAL” and “POLIMER” and G. Stockton for proofreading this manuscript.
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