Quality Factors of Fruits and Vegetables - ACS Publications

Chapter 27

Ultrafiltration of Fruit Juices with Metallic Membranes

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R. L. Thomas and S. F. Barefoot Department of Food Science, Clemson University, Clemson, SC 29634-0371

Fruit juice production has been greatly simplified by the use of the Ultrapress process. This process utilizes formed-in-place metallic membranes on sintered stainless steel tubes for simultaneous pressing and ultrafiltration of fruit purees. The fruit purees are enzymatically depectinized and pumped directly through the metallic membrane system in a single pass to obtain highly clarified juice which can be aseptically packaged. Insignificant changes in the volatile composition of apples were noted using the Ultrapress process. The fruit juice industry is constantly seeking improved methods of obtaining highly clarified juices. Most of the research effort has been directed to the clarification of pressed juice by ultrafiltration (UF) membranes, although some effort has been made to obtain clarified juices by centrifugation and/or filtration of enzymatically liquified fruit pulps. Hollow fiber or thin-channel polymeric membrane systems were the first to be used in the ultrafiltration of fruit juices (1), but membrane fouling with increased feed concentration or as a result of compression effects as transmembrane pressure increased resulted in very low flux rates (2, 3). Large pressure drops across hollow fiber or thin-channel UF membranes cannot be achieved. Furthermore, virtually no suspended solids can be tolerated, since blockage of the flow channels would readily occur. Open tubular UF designs have been more successfully applied to juice clarification, since solutions high in suspended solids and viscosity can be accomodated due to large, circular flow channels. The disadvantage to open tubular designs is the low membrane packing density and increased tube support needed for higher 0097-6156/89/0405-0346$06.00/0 ο 1989 American Chemical Society

Jen; Quality Factors of Fruits and Vegetables ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

27. THOMAS & BAREFOOT

Ultrafiltration of Fruit Juices

347

pressure operation (4). However, i f open tubular systems can be operated in a single pass the best possible average flux can be obtained, since a l l increments of the membrane area are exposed to the minimum possible concentration. Also, i f very high solids could be accomodated by an open tubular UF system operated in a single pass, the need for conventional f r u i t pressing would be eliminated. Instead, a process for clarifying liquified f r u i t pulp directly would be possible. The concept of producing clarified f r u i t juice by a single pass of enzyme-treated fruit puree through a tubular, metallic ultrafiltration membrane system was recently described by Thomas et a l . (5, 6). The process is known commercially as the Ultrapress process, since pressing and u l t r a f i l t r a t i o n are accomplished simultaneously (7). The metallic membrane u l t r a f i l t r a t i o n system is composed of sintered stainless steel tubes of varying diameters with membranes formed-in-place within the porous matrix of the tubes by deposition of various metallic oxides. Metallic oxides in combination with polymers are also possible. Earlier studies (5) u t i l i z i n g 5/8-inch diameter tubes showed that high juice recoveries were possible, but only with extremely large pressure drops. With 5/8-inch diameter tubes pressure drops increased dramatically as juice recovery increased and were in the range of 700-800 psi at 85% juice recovery. In the single pass design viscosity effects are greatest near the end of the tube, thus most of the pressure drop will occur in this region. Large, dramatic pressure drops near the end of the system created i n s t a b i l i t i e s which made operational control d i f f i c u l t . Subsequent studies (6) produced a mathematical model of the process which defined the importance of tube diameter on pressure drop. The design flow in a channel was found to be proportional to the 2.0 to 2.5 power of the tube diameter. Thus, a prototype u t i l i z i n g 1 l/4-inch diameter tubes was constructed and operated stably at 85% juice yields with much lower pressure drops (250-350 psi). This same study (6) also addressed optimization of viscosity reduction of the puree and membrane flux u t i l i z i n g commercial liquefaction enzymes. Viscosity reduction was readily obtained with even small amounts of liquefaction enzymes, and further increases in enzyme concentration did not appreciably affect viscosity reduction. However, steady state flux was proportional to the level of enzyme used up to 0.044%. Membrane flux correlated very well, as expected (3), with reduction of total pectin. It was evident that enzyme pretreatment should be further developed with the goal of enhancing flux rather than reducing viscosity, especially since increased tube diameter could be used to overcome pressure drops imparted by the viscosity of the retentate at high juice recoveries. The previous studies also suggested that the Ultrapress process offered the potential for higher quality juice by reducing process time and also the potential for microbiologically stable juice, which could also provide an improved product by eliminating

American Chemical Society Library

St.,and N.W. Jen; Quality1155 Factors16th of Fruits Vegetables D.C.Society: 20036 ACS Symposium Series; Washington, American Chemical Washington, DC, 1989.

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heat pasteurization. Further optimization of enzyme pretreatment, determination of microbial rejections by the membrane, evaluation of the overall quality of the product, and potential new applications of the Ultrapress system are current objectives in the further development of this process. Materials and Methods Enzyme Treatment. Apple puree was produced as described in previous studies (5, 6). Two commercially available enzymes were added at varying concentrations to the purees, which were stirred constantly at 35°C for 1 hr for a l l enzyme treatments. Rohapect 7016, a liquefaction enzyme preparation (Rohm Tech, Inc., Maiden, MA), was added at a concentration of 0.044% (w/w) to serve as the basis for comparison to increasing amounts of a pectinase preparation, Rohapect DIL (Rohm Tech, Inc., Maiden, MA). The liquefaction enzyme was successively reduced by one-half in other treatments as the amount of pectinase was increased by approximately 50%, starting with an i n i t i a l pectinase concentration of 0.049% (w/w). After the 1 hr enzyme treatments, the viscosity of the purees were measured and they were ultrafiltered with metallic membranes as described by Thomas et a l . (6). Steady state fluxes were measured after 30 min of operation and reported as gallons/ft /day (GFD). 2

Microbial Challenges. Metallic UF membranes were as described by Thomas et a l . (6) and were challenged with microbial cultures prepared as indicated by Barefoot et a l . (8). Challenge cultures were Pseudomonas diminuta ATCC 19146 (American Type Culture Collection, Rockville, MD), Bacillus coagulans 43P, Saccharomyces cerevisiae NRRL Y-2034 (9), baker's yeast kindly provided by Dixie Yeast Corp., Gastonia, NC, or Pénicillium roqueforti blue mold powder (Dairyland Food Laboratories, Waukesha, WI). Cultures were added to either apple puree or peptone (0.1% aqueous). For microbial challenges and juice production, the system was operated in a 50 L recirculating mode as previously described (8). In summary, feeds consisting of apple puree or aqueous peptone were mixed with microbial suspensions in a temperature-controlled steamjacketed kettle. Feeds were pumped from the kettle by a centrifugal suction booster pump feeding a diaphragm pressurizing pump. Flow was regulated by a manual pressure control valve and monitored by a pressure gauge at the inlet. Concentrated feeds were recirculated to the kettle. Permeates passed from the shell through a steam or permeate outlet to two-way valves that diverted flow to a controlled atmosphere glove box for aseptic collection or back to the feed kettle. Metallic UF membranes were sterilized as described by Barefoot et a l . (8) and connected via sterile permeate lines to the glove box. Permeate (usually 150 mL) was collected in sterile containers and transported to the laboratory for microbiological analysis. Populations of challenge microorganisms in feed and permeate samples were determined by viable counts according to


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Ultrafiltration of Fruit Juices

standard methods (10). Apple juice was stored at 20-25°C and observed for turbidity indicative of microbial growth. Volatile Analysis. Volatile profiles of enzyme treated apple puree and ultrafiltered juice were obtained by gas liquid chromatography. A Hewlett Packard 5880-A gas chromatograph equipped with a flame ionization detector, a 5880-A series level four computerized data system and a DB-5 (30 m χ 0.25 mm I.D.) fused s i l i c a capillary column (J&W Scientific, Folsom, CA) were used. Puree was centrifuged at 10,000 χ g and the supernatant was injected onto the column (direct aqueous injection). Ultrafiltered juice was injected directly with no sample preparation. Sample size in a l l cases was 5mL split 15:1 with helium as the carrier gas at a flow rate of 1 ml/min. The oven temperature was held at 60°C for the i n i t i a l seven minutes of the injection time. It was then increased to 200°C at a rate of 5°C/min and was held there for twenty five minutes. Total volatiles were obtained by summation of integrated peak areas on the chromatograms. Percent areas of each peak were also calculated. Results and Discussion With the objective of enhancing membrane flux by enzymatic degrada tion of pectin in the puree, the liquefaction enzymes normally used in the process were reduced while a commercial pectinase preparation was used in increasing amounts as a replacement (Table I). Table I.

Cost Comparison of Commercial Liquefaction Enzyme Versus Replacement with Commercial Pectinase

Enzyme Treatment

Cone. (%)

Cost ($)/gall

0.044

-

0.054

(Rohapect 7016) (Rohapect DIL )

0.022 0.049

0.027 0.014

0.041

(Rohapect 7016) (Rohapect DIL)

0.011 0.074

0.014 0.020

0.034

(Rohapect 7016)

0.006

0.007

(Rohapect EIL)

0.098

0.024

A (Rohapect 7016 ) 2

3

leased on 200 gal/ton on 85% juice yield 2

A commercial liquefaction enzyme

3

A commercial pectinase


0.031

350


The commercial liquefaction enzyme typically used (Rhoapect 7016) contained predominantly cellulase and hemicellulase activity but with some pectinase activity. The use of more pectinase and less liquefaction enzyme represented a more favorable treatment economically and was actually more effective in reducing viscosity (Table II). Also, as predicted, the steady state flux of the membranes was higher when pectinase predominated. To assess microbial rejections, sterile membranes were challenged with J\ diminuta, j$. coagulans, S. cerevisiae, and P_. roqueforti spores suspended in apple puree or 0.1% peptone. Reductions achieved by ultrafiltration of cells in peptone at normal operating pressure (300 psi) were 5.1, 6.0, 7.6, and >9.0 logs for smallest to largest organisms (Table III). Membranes typically retained more cells from apple puree than from peptone at normal operating pressures. Retention of cells from apple puree was more than 1.5 logs greater than retention of cells from peptone by the same membrane (data not shown). The acid tolerant microflora in apple pulp averages approximately 10^ colony forming units per gram and consists primarily of fungi; 30 to 40% of the microbial population is mold and 60 to 70% is yeast (11). Thus, rejection of yeast and mold spores as shown here demonstrates that organisms most commonly found in apple puree that proliferate in and spoil juice would be retained completely by the metallic membranes. These results were verified in a straightforward manner by aseptically bottling apple juice from uninoculated apple puree in the glove box. The juice was shelf stable for over a year. These microbial rejections and the fact that the metallic membrane and their stainless steel supports are inherently thermostable demonstrate that a commercially sterile product can be produced by the Ultrapress process. Fruit juice quality was assessed by a direct comparison of the volatile composition of the c l a r i f i e d , Ultrapress juice with that of juice obtained from apple puree by centrifugation. A direct aqueous injection technique was employed for this comparison. The volatile profile for clarified juice from Mcintosh apples was nearly identical to that of the centrifuged juice obtained from the purees of the same apples with the exception of volatiles with retention times of 2 to 3 minutes (Figures 1 and 2). A reduction of 50 to 80% was observed for these volatiles from the Ultrapress juice, which included four peaks representing approximately 1.5% of the total volatiles. However, notable differences in peak areas were not observed for peaks with retention times of 6 min or greater. Saper et a l . (12) identified the volatile components of Mcintosh apples that were responsible for the characteristic aroma of the ripe f r u i t . These included ethyl acetate, ethanol, ethyl proprionate, ethyl butyrate, ethyl 2-methylbutyrate, hexanal, 2-methylbutanol, trans-2-hexenal, and hexenol. These compounds had retention times of 8 min or greater on a GC column with operating conditions similar to that used in this study. Poll (13) showed the rapid accumulation of these same volatiles in Mcintosh apples at the stage considered "ripe for eating," which had the highest f r u i t aroma score.


27.

THOMAS & BAREFOOT

Table II.

Ultrafiltration ofFruit Juices

351

Percent Viscosity Reduction and Steady State Flux with Various Enzyme Treatments Viscosity Reduction (%)

Enzyme Treatment*

Steady State Flux (GFD)

2

A Β C

40 37 53

30.6 24.7 34.0

D

74

37.0

^Enzyme treatment as described in Table I 2

After 60 minutes of exposure

Table III. Operating pressure (psig)

3

325-400 575-650

Logarithmic Reduction Values for Microorganisms in Peptone Achieved by Metallic Ultrafiltration Membranes LRV for P. diminuta P. roqueforti B. coagulans D

5.1 3.6

6.0 NA

7.6 6.1

^ S. cerevisfâë >9.0 >9.0

Samples were taken at 5 min intervals after 10 min operation at each pressure. The system was operated for 100 min at 300-400 psig and 30 min at 575 to 675 psig.

a

^Logarithmic reduction values (LRV) were calculated by the formula: LRV = Log io CFU/mL Feed - Log IQ CFU/mL permeate.

Although the peaks in Figures 1 and 2 were not identified, i t was apparent that the Ultrapress process caused insignificant changes in the volatile composition of the apple, indicating that fruit juices with aroma profiles nearly identical to that of the fresh f r u i t can be obtained.


352


»0

Retention Time (min)

Figure 1.

Volatile profile of Mcintosh apple puree.


27. THOMAS & BAREFOOT

Ultrafiltration ofFruit Juices

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Retention Time (min)

Figure 2. Volatile profile of apple juice manufactured by the Ultrapress process using Mcintosh apples.


354


Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

Heatherbell, D. Α.; Short, J. L.; Stauebi, P. Confructa 1977, 22, 157. Breslau, B. R.; Killcullen, Β. M. J. Dairy Sci. 1977, 60, 1371. Kirk, E. D.; Montgomery, M. W.; Kostekass, M. G. J. Food Sci. 1983, 48, 1663. Paulson, D. J.; Wilson, R. L.; Spatz, D. P. Food Technol. 1984, 38(12), 77. Thomas, R. L.; Westfall, P. H.; Louvieri, Ζ. Α.; Ellis, N. D. J. Food Sci. 1986, 51, 559. Thomas, R. L.; Gaddis, J. L.; Westfall, P. H.; Titus, T. C.; Ellis, N. D. J. Food Sci. 1987, 52, 1263. Thomas, R. L.; Titus, T. C.; Brandon, C. A. U.S. Patent 4 716 044, 1987. Barefoot, S. F.; Tai, H. Y.; Brandon, S. C.; Thomas, R. L. J. Food Sci., 1988, In Press. Martini, Α. V.; Kurtzman, W. W. Int. J. Syst. Bacteriol. 1985, 35, 508. Speck, M. L. Compendium of Methods for the Microbiological Examination of Foods; American Public Health Association: Washington, DC, 1984. Swanson, K. M. J.; Leason, S. B.; Downing, D. L. J. Food Sci. 1985, 50, 336. Saper, G. M. J. Food Sci. 1977, 42, 44. Poll, L. Lebensm.-Wiss. U.-Technol. 1985, 18, 205.

RECEIVED May 31, 1989


Quality Factors of Fruits and Vegetables - ACS Publications

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