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Characterization of Proteinaceous Membrane Foulants and Flux Decline During the Early Stages of Whole Milk Ultrafiltration
Characterization of Proteinaceous Membrane Foulants and Flux Decline During the Early Stages of Whole Milk Ultrafiltration P. S. TONG, 1 D. M. BARBANO, and M. A. R U D A N Department of Food Science Cornell University Ithaca, N Y 14853 ABSTRACT
facture of frozen desserts, cheese, yogurt, and other dairy foods (5, 6, 7, 13, 14). Ultrafihration systems designed for the dairy industry utilize crossflow filtration. Usually, fluid conditions near the membrane surface are quite turbulent. High fluid turbulence and high wall shear act to scour the membrane surfaces and minimize accumulation of solute near the membrane (4). Despite these favorable hydrodynamic conditions, flux decline associated with membrane fouling remains a problem during UF of dairy fluids (15, 23, 28). Membrane fouling can increase cleaning, labor, and energy costs and shorten membrane life (21, 22, 25). The objectives of this study were 1) to characterize time-dependent flux decline during whole milk UF, 2) to determine the significance of adsorption fouling in the absence of concentration polarization, and 3) to characterize the proteinaceous components of the membrane foulants.
Pasteurized whole milk was fractionated with a pilot-scale, plate and frame, uhrafiltration system to study membrane fouling and flux decline. Concentration factor was set at approximately 1.4x to simulate the first stage of a multistage UF system. Proteinaceous membrane foulant was characterized by SDS-PAGE. Distribution of proteins in the foulant was very different from distribution of proteins in milk. Whey proteins, a-lactalbumin and/3-1actoglobulin, accounted for 95% of the proteinaceous membrane foulants. Very little casein was identified as membrane foulant. The approximate amount of protein in the membrane foulant was estimated to be .6 g/m 2 of membrane area. Permeate flux studies indicated that flux decline is severe in the early stages of milk ultrafiltration and is associated with irreversible adsorption of protein on the membrane surface. A threefold difference between the water flux of clean membranes and fouled membranes was attributed to the adsorbed foulant. Identification and characterization of membrane foutants and the mechanism of their interaction with membrane surfaces should lead to the design of more efficient ultrafihration systems for the dairy industry.
MATERIALS AND METHODS Milk Ultrafiltration Trials
Seventy-six liters of fresh, unhomogenized Grade A raw whole milk were batch pasteurized at 63°C for 30 rain and cooled to 49°C just prior to UF. The milk feed stream for UF was similar to those used in whole milk retentate manufacture for cheese making (12). Ultrafiltration Conditions
INTRODUCTION
Ultrafihration (UF) has the potential to become an integral unit operation in processing dairy fluids. Research has demonstrated that milk retentates from UF can be used in manuReceived March 25, 1987. Accepted September 11, 1987. 1Kraft, Inc., 801 Waukegan Road, Glenview, IL 60025.
1988 J Dairy Sci 71:604--612
Uhrafihration was with a plate and frame UF system (Series "S", Dorr-Oliver, Inc., Stamford, CT). The system contained seven, S-10 membrane elements (.47 m R of active membrane area). Membranes were polysulfone with a nominal 10,000 molecular weight ~utoff. The membranes were characterized by the supplier as negatively charged and relatively hydrophobic. Membrane plates used in this study were actual production scale plates used in commercial UF systems. The UF system was
604
MEMBRANE FOULING DURING MILK ULTRAFILTRATION
605
RETURN BLEED CONTROL VALVE
MEMBRANE
II.
LOW
[
PERMEATE
- - -I~
~-
FILL ISO
~
HIGH PRESSURE GAUGE
THERMOMETER
--Z PUMP
PUMP
HEAT EXCHANGER
Figure 1. Schematic diagram of plate and frame ultrafiltration system (courtesy of Dorr-Oliver, Inc.).
configured in a pattern similar to the three plate system shown in Figure 1. The system has a feed pump and a recirculation pump to provide the feed pressures and linear flow velocity required for efficient UF. Ultrafiltration process conditions were a temperature of 49°C and operating pressures of 310 and 172 kPa for the inlet and outlet, respectively. These operating pressures conform to the manufacturer's recommended pressure drop per membrane plate of 15.6 kPa for milk UF. The UF system was run in total recycle (both retentate and permeate streams returned to feed tank) to maintain a constant feed composition throughout each run. Retentate and feed flow rates were adjusted initially to give a concentration factor of 1.4x to simulate the first stage of a multistage milk UF system. Under these conditions, retentate and permeate throughput was approximately 30 L/h and 15 L/h, respectively. Process time was 20 or 120 min, depending on the experiment. Analysis of Milk and Permeate
Milk samples were analyzed for fat content with an infrared milk analyzer calibrated by the
Mojonnier method (24). Fat content of permeate samples was determined by a modified Babcock method (24). Total N and nonprotein N of milk and permeate were determined by macro-Kjeldahl analysis (31). Total solids content of the samples were determined by drying 1-g samples at 100°C -+ 2°C for 5 h in a forced air oven (24). Membrane Cleaning and Clean Membrane Water Flux Determination
Prior to each milk UF trial, the system was cleaned thoroughly with 20 L of the manufacturer's recommended alkaline cleaning solution consisting of 1% sodium hydroxide, .05% sodium gluconate, .05% tetrasodium EDTA, and .04% sodium dodecyl benzene sulfonate. The cleaning solution was recirculated for 20 min at 63°C. The system was flushed completely with 20 L of deionized water at 49°C, and the clean membrane water flux was determined using an additional 20 L of fresh 49°C deionized water. Permeate ftux during water or milk UF was determined by measuring permeate flow rates and dividing by the total membrane area of the system. Clean membrane water flux data were collected at 49°C with no applied back pressure Journal of Dairy Science Vol. 71, No. 3, 1988
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(retentate bleed valve fully open). Under these conditions, inlet and outlet pressures were approximately 240 and 100 kPa, respectively. Permeate flux during milk UF was determined under the process conditions mentioned earlier and was expressed in liters per square meter per hour. Fouled Membrane Water Flux Determination
After a milk UF trial, the system was rinsed with 50 L of deionized water at 49°C for 5 rain. The water rinse was in two stages: system water flush and water recirculation. The first 10 L of water that flowed through the permeate and retentate lines were not recirculated back to the feed tank (system water flush). The remaining 40 L of deionized water were then recirculated at high linear velocity through the system at no applied back pressure for 4.5 min (water recirculation). The system water flush and high speed water recirculation were designed to remove any milk solids that were not bound tightly to the membrane surface. Fouled membrane water flux data were determined 4.5 rain into the water rinse period under the process conditions described for clean membrane water flux determinations. Adsorption Trials
Adsorption trials were conducted to determine the contribution of adsorption in the absence of concentration polarization to the process of membrane fouling. Prior to adsorption trials, seven S-10 membrane elements were cleaned and the system was shut down and disassembled. All seven membrane plates were removed from the unit and each was fitted with a stainless steel bolt, nut, and washer system to seal the permeation port of the plate. This prevented milk from coming in contact with the permeate side of the membrane. Once sealed, the seven membrane elements were submersed simultaneously into 19 L of mildly agitated, pasteurized, unhomogenized milk at 49°C. After 5 rain, membrane elements were removed from the milk and rinsed in 19 L of 49°C deionized water for 30 s. They were then placed in another 19 L of clean 49°.C water until reinsertion of all seven membrane plates back into the unit was complete (ca. 20 min). The UF unit was reassembled and the
Journal of Dairy Science Vol. 71, No. 3, 1988
fouled membrane water flux was determined as described. Sampling and Analysis of Membrane Foulants
After the system water flush (10 L), high speed water recirculation (40 L), and flux determination as described, fouled membranes from the 20-min milk UF trials were sampled. At this point, any proteins remaining on the membranes were assumed to be strongly attached. The UF pilot system was disassembled and half of one side of membrane (.017 m 2) was cut out from a membrane plate. This section of membrane was cut into 1 cm × 6-cm strips and placed into a screw-cap test tube containing 10 ml of buffer (10 mM Tris HC1 at pH 6.8, 1% SDS, 20% glycerol, and .02% bromphenol blue). The test tube was closed and placed into a boiling water bath for 5 rain to extract fully the foulants from the membrane. Pieces of membrane remaining after extraction were discarded and .0772 g of dithiothreitol was added to the solution and reboiled. Membrane foulant sample solutions were held frozen for future electrophoretic analysis. The discontinous SDS-PAGE procedures of Laemmli (16), as described by Verdi et al. (30), for milk protein analysis were used to characterize the proteinaceous fraction of membrane foulants. Twenty microliters of membrane foulant sample solutions were loaded per slot of the electrophoresis gel. RESULTS AND DISCUSSION Milk and Permeate Composition
Composition of milk and permeate are shown in Table 1. Milk composition was typical for New York state (3). Permeate composition was consistent with that reported in previous studies (8, 32). Permeate Flux
Time-dependent flux decline during whole milk UF is shown in Figure 2. If the feed (milk) gave no resistance to permeation, then the initial permeate flux should have been similar to the clean water flux. The clean membrane water flux was approximately 600 L/m 2 h whereas the flux after only 10 min of milk UF
MEMBRANE FOULING DURING MILK ULTRAFILTRATION
607
TABLE 1. Composition of milk and permeate from 20-rain milk ultraflltration trials)
Composition
% Total solids % Fat % Total N % Nonprotein N
Milk N, 11.87 3.40 .486 .025
Permeate SD .04 .07 .003 .001
Y~ 5.63
SD .07 ... .002 .001
' 1031 .026
tn=4.
was 60 L/m 2 per h. During the first 40 min of process time, flux dropped rapidly. In the later stages of milk UF, flux decline was more gradual (Figure 2). Similar results have been reported elsewhere (19, 2 3). In early stages of milk UF, adsorption fouling is probably the primary mechanism of flux decline (20). Osmotic effects and concentration polarization contribute minimally to the resistance to permeation during early stages of milk UF. In our study of milk UF, rapid flux decline in the first 40 rain accounted for over 90% of the total flux decline over the processing period. Lopez-Leiva and Matthiasson (19) attributed the early stages of flux decline to adsorption of proteins on the membranes. Adsorption of proteins onto membrane surfaces has also been reported by Ingham et al. (10). Hence, we have classified the time-dependent flux decline we observed during milk UF into two phases. Adsorption fouling is the first phase of flux decline and concentration polarization is the second phase of flux decline (Figure 2). During the second phase of flux decline compaction of the fouled Iayer may also increase resistance to permeation. Further evidence of extensive membrane fouling and flux decline in the early" stages of UF is shown in Table 2. The fourfold difference in water flux between clean membranes and membranes after 120 min of milk UF ("fouled" membranes) suggests that a tightIy bound laver acts as the primary resistance to water flux on the "fouled" membranes. Experiments were conducted to determine the role of adsorption fouling in the absence of concentration polarization. Water flux data shown in Table 2 for the adsorption trials confirm the fact that adsoprtion fouling in-
dependent of concentration polarization is an extremely important part of the milk UF membrane fouling process. After a 5-min exposure of the polysulfone membranes to milk with no permeation, adsorption of milk components on the membrane surface gave a 68% decrease (threefold) in water flux when compared with the water flux of clean membranes just prior to the adsorption trials. Furthermore, in the presence of concentration polarization (120-min UF trials) the decrease in water flux
Clean Membrane Water Flux 600
#
Concentration Polarization
Adsorption
~O4
65
I
E
x
60
¢D
~.
ss
50
45 0
20
: 40
" 60
80
100
.-
."
120
140
UF Process Time (rain)
Figure 2. Time-dependent flux decline during whole milk ultrafiltration. (n = 4, brackets are 95% confidence intervals.) Journal of Dairy Science Vol. 71, No. 3, 1988
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TONG ET AL.
TABLE 2. Water flux at 49°C of clean and fouled membranes. Water flux
Time of milk contact with membrane
n
Decrease in water flux
Fouled
Clean
(%)
(L/m 2 .h) SD
SD 5-min Adsorption 120-min UF run
5 4
583 602
f r o m t h e clean t o t h e f o u l e d m e m b r a n e s was
187 143
35.3 33.8
35.8 10.3
68 76
Electrophoretic Analysis
only slightly higher (76%). This would indicate
An example of the electrophoretic pattern
t h a t t h e m a j o r i t y o f t h e d e c l i n e in w a t e r flux d u r i n g t h e U F o f milk using p o l y s u l f o n e m e m b r a n e s is d u e to a d s o r p t i o n fouling a n d n o t c o n c e n t r a t i o n p o l a r i z a t i o n i n d u c e d fouling.
of t h e p r o t e i n a c e o u s f o u l a n t e x t r a c t e d f r o m m e m b r a n e s a f t e r 20 m i n o f milk U F is s h o w n in F i g u r e 3. Lane 1 c o n t a i n s a series o f p r o t e i n s o f k n o w n m o l e c u l a r w e i g h t as r e f e r e n c e s t a n d a r d s
1 66 kd
2
24kd Elf
(-)
BSA
q
45 kd q ~ 36kd
3
as-C:K k-(.
20kd 14kd
B LG aLA
e
(+) Figure 3. Sodium dodecyl sulfate-polyacrylamide gel electrophoretic patterns from whole milk ultrafiltration: lane 1, molecular weight standards: bovine serum albumen (66 kdal), egg albumen (45 kdal), glycerol-3phosphate (29 kdal), carbonic anhydrase (24 kdal), soybean trypsin inhibitor (20 kdal), cz-lactalbumin (14 kdal). Lane 2, milk feed for ultrafihration: bovine serum albumin (BSA), CZs-Casein (C~s-CN), 3-casein (3-CN), K-casein (K-CN), 3-1actoglobulin (3-LG), c~-lactalbumin (a-LA). Lane 3, membrane foulant after 20 min of milk ultrafiltration. Journal of Dairy Science Vol. 71, No. 3, 1988
MEMBRANE FOULING DURING MILK ULTRAFILTRATION (Dalton VII L, Sigma Chemical Co., St. Louis, MO). Lane 2 demonstrates a typical distribution of proteins from milk used in our UF trials. Bovine serum albumin, (~s-caseins (~slcasein and as2-casein), ~3-casein, K-casein, /3-1actoglobulin, ~-lactalbumin and a proteolytic fragment of /3-casein are shown clearly. Estimates of the molecular weights of the caseins in this system are higher than reference values (30). Lane 3 contains the proteinaceous membrane foulants. Surprisingly, very little intact casein was present. Of the principal caseins of milk, K-casein was the most prevalent in the foulant; however, it was in very low concentration compared with other proteins detected. More importantly, the principal foulants were identified as a-lactalbumin and/3-1actoglobulin. The densitometric quantitation of these electrophoretic patterns is shown in Table 3. Literature values for milk protein distribution are approximately 82.4% as caseins and 17.6% as whey proteins (11). The average distribution of proteins for milk used in our study was similar./3-Lactoglobulin was slightly higher than the literature values because a proteolytic fragment from the caseins (a 7-casein) comigrates with/3-1actoglobulin. In contrast, the foulant has a very different distribution of proteins - only 5% casein and
609
95% whey proteins. In addition, there are large differences in the relative percent of individual whey proteins between milk and membrane foulant. For example, a-lactalbumin represents only 3.8% of total protein in milk, whereas this protein is almost 80% of the total protein present as membrane foulant. Electrophoretic patterns of membrane foulants after 120 min of milk UF were similar to those observed after 20 min. Purified/3-1actoglobulin and a-lactalbumin foul UF membranes (10, 17). Proteolytic fragments of caseins are usually present in low concentrations in milk. This was the case for the milk used in the present study (Table 3). Such fragments are usually attributed to proteolytic damage of/3-casein by somatic cell proteases, plasmin, or bacterial proteases (1, 2, 17). Low molecular weight proteolytic fragments from caseins have been implicated in fouling during whey UF (9, 28, 29). Significant concentrations of casein fragments (7-caseins) were detected in the membrane foulants during milk UF. Further studies to determine the impact of milk quality on permeate flux during milk UF should be conducted. Comparison to Previous Studies of Membrane Foulants
Our results disagree with previous research that examined the protein components of milk
TABLE 3. Proteinaceous membrane foulants from milk ultrafiltration.
Protein I
Milk literature 2
Milk observed3
Membrane foulants 20 rain4 (%)
Total casein (CN) as,, ~s2-CN t3-CN K-CN 3'-CN Total whey protein Immunoglobulin Bovine serum albumin 3-1actoglobulin a-lactalbumin
82.4 39.9 29.5 10.5 2.5 17.6 2.3 1.3 10.2 3.8
75.4 34.8 28.9 10.2 1.5 24.6 .9 .8 18.2 4.7
SD
X
3.0 3.0 2.7 1.8 .5 ... .5 .5 2.3 .9
4.9 .3 .4 1.9 2.3 95.0 . . . 1614 78.6
SD Q
.4 .7 1.5 1.3 ... .
.
. 113 3.1
1Determined by quantitative SDS-PAGE. 2 From Jenness (11). 3Milk used in ultrafiltration trials (n = 4). 4 From m e m b r a n e after 20 rain milk ultrafiltration (n = 4). Journal of Dairy Science Vol. 71, No. 3, 1988
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TONG ET AL.
UF membrane foulants. In a previous report on UF of skimmilk in hollow fiber polysulfone membranes, Patel and Reuter (23) showed the primary foulants to be the caseins. They also indicated that these proteins appeared in the ratios typical to that in whole casein. In an earlier report, Skudder et al. (27) studied deposit formation during reverse osmosis of whole milk in tubular cellulose acetate membranes. They found the composition of the deposits to be similar to the protein distribution in milk itself. The foulant sampling procedure used in both of these studies was quite different than our procedure. In previous studies, membrane cleaning cycle solutions were collected and freeze-dried to concentrate the proteins removed from the total membrane system. Then, these freeze-dried proteins were analyzed by starch gel electrophoresis. To reconcile this apparent disagreement of our results with the reported literature, we offer the following analysis. We have estimated the amount of protein contained in the adsorbed layer from fouled membranes in our system. The total protein loaded per lane for electrophoresis was approximately 20 bLg. Based on this estimate, we have calculated the amount of adsorbed protein extracted from the fouled membrane to be .010 g protein per membrane sample section, which translates to .6 g protein/ m 2 of membrane or .28 g protein per membrane area of our entire system. F r o m these calculations we have determined the amount of free milk (i.e., milk not adsorbed on membranes) that would have been required to remain in our system prior to collection of membrane foulants to give us results for membrane foulant similar to the results using Patel and Reuter's foulant collection procedures. Assuming milk with a 3.3% protein content, we calculate that only 8.5 ml of residual milk remaining in our entire UF system could have given results similar to the previous studies. Because the hold-up volume of our system is approximately 6 L, and because there are a large number of gaskets and turnarounds in most UF systems, 8.5 ml of residual milk would not be unusual if the system were not thoroughly rinsed prior to sampling of membrane foulants or if milk components adsorbed to stainless steel, plastic, or gasket surfaces in the system. The issue of residual milk remaining Journal of Dairy Science VoL 71, No. 3, 1988
in the UF system or other surfaces was avoided in our study by a thorough water rinse of the system prior to removal of a membrane plate from the UF system and direct sampling of membranes. Two other factors may explain the differences in foulant composition between our results and those of other workers. First, in previous literature reports, UF was in small laboratory units. The hydrodynamic conditions of large UF systems are difficult to achieve in small laboratory units. Second, in Patcl and Reuter's study (23), UF was continued until permeation completely stopped. This mode of operation is not typical of commercial practice. Hence, differences in process conditions, membrane type, and system dynamics may also explain why our results disagree with the previous studies. It is unclear at this time why a-lactalbumin, and 3-1actoglobulin preferentially adsorb on membranes during milk UF. Lee and Merson (18) suggested that polymerization behavior of whey proteins can cause formation of sheets and strands of protein that attach to membrane surfaces to cause flux decline. The role of such interactions in fouling during milk UF is the subject of ongoing research in our laboratory. We have gained a better understanding of membrane fouling during milk UF that may lead to the design of more efficient UF systems for the dairy industry. In particular, adsorption fouling may be minimized by development of new membranes with different surface characteristics, whereas flux decline associated with concentration polarization can be minimized by optimization of hydrodynamic conditions near the membrane surface (26). Characterization of the fouling mechanism (adsorption or concentration polarization) causing the greatest flux decline during the initial stages of membrane fouling during milk UF will give us insight into methods for design of more efficient UF systems. CONCLUSIONS
The destructive membrane sampling procedure was a more direct and accurate method to collect membrane foulants. Hence, from this study, we have concluded that: 1) flux decline during whole milk UF occurs in two stages -- severe flux decline (adsorption fouling) and
MEMBRANE FOULING DURING MILK ULTRAFILTRATION t h e n m o r e gradual flux decline ( c o n c e n t r a t i o n polarization), 2) e l e c t r o p h o r e t i c analysis o f t h e p r o t e i n a c e o u s f r a c t i o n o f the fouied layer indicates it c o n t a i n s a p p r o x i m a t e l y .6 g prot e i n / m 2 o f m e m b r a n e area and t h a t m o s t o f this p r o t e i n is c~-lactalbumin and 13-1actoglobulin, 3) t h e ratio o f c~-lactalbumin to ~-lactoglobulin was m u c h higher in t h e f o u l a n t s (4.8:1) t h a n in milk (.25:1), w h i c h indicates selective int e r a c t i o n o f ~x-lactalbumin w i t h the m e m b r a n e surface, and 4) a d s o r p t i o n trials indicate t h a t a d s o r p t i o n fouling causes m o s t o f t h e decrease in w a t e r flux during t h e U F o f milk using polysulfone membranes. ACKNOWLEDGMENTS
We wish to a c k n o w l e d g e Dorr-Oliver, Inc., S t a m f o r d , CT for s u p p l y i n g UF e q u i p m e n t and supplies and t h e National Dairy P r o m o t i o n and R e s e a r c h Board for financial s u p p o r t . T e c h n i c a l assistance f r o m M. C h a p m a n , P. Fleming, R. R a s m u s s e n , and M. S m i t h e r s and f r o m Cornell University Dairy Plant p e r s o n n e l was e x t r e m e l y valuable. REFERENCES
1 Andrews, A. T. 1983. Proteinases in normal bovine milk and their actions on caseins. J. Dairy Res. 44:223. 2 Andrews, A. T. 1983. Breakdown of caseins in bovine milks with high somatic cell counts arising from mastitis or infusion with bacterial endotoxin. J. Dairy Res. 50:275. 3 Barbano, D. M., and J. W. Sherbon. 1984. Cheddar cheese yields in New York. J. Dairy Sci. 67: 1873. 4 Belfort, G. 1984. Membrane methods in water and wastewater treatment: an overview. Page 1 in Synthetic membrane processes: fundamentals and water applications. G. Belfort, ed. Academic Press, Inc., Orlando, FL. 5 Bundgaard, A. G., O. J. Olsen, and R. F. Madsen. 1972. Ultrafiltration and hyperfiltration of skim milk for production of various dairy products. Dairy Ind. 37:539. 6 Chapman, H. R., V. E. Bines, F. A. Glover, and P. J. Skudder. 1974. Use of milk concentrated by ultrafiltration for making hard cheese, soft cheese and yogurt. J. Soc. Dairy Technol. 27:151. 7 Covacevich, H. R., and F. V. Kosikowski. 1977. Cream cheese by ultrafiltration. J. Food Sci. 42:1362. 8 Garoutte, C. A., C. H. Amundson, and C. G. Hill, Jr. 1982. Ultrafiltration of whole milk with hollow fiber membranes. J. Food Process Eng. 5:191. 9 Hickey, M. W., and R. D. Hill. 1980. Investigation into the ultrafiltration and reverse osmosis of wheys. II. The effects of some minor whey constl-
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tuents. N.Z.J. Dairy Sci. Technol. 15:123. 10 Ingham, K. C., T. F. Busby, Y. Sahlestrom, and F. Castino. 1979. Separation of macromolecules by ultrafiltration: influence of protein adsorption, protein-protein interactions, and concentration polarization. Page 141 in Ultrafiltration membranes and applications. A. R. Cooper, ed. Plenum, New York. NY. 11 Jenness, R. 1982. Inter-species comparison of milk proteins. Page 188 in Developments in dairy chemistry - 1. P. F. Fox, ed. Appl. Sci. Publ., Essex, Engl. 12 Kosikowski, F. V. 1977. Cheese by ultrafiltration. Page 510 in Cheese and fermented milk foods. F. V. Kosikowski, ed. 2nd ed. F. V. Kosikowski and Associates, Brooktondale, NY. 13 Kosikowski, F. V., and A. R. Maters. 1983. Preparation of ice cream, skim milk and cream made from whole milk retentates. J. Dairy Sci. 66(Suppl. 1):99. (Abstr.) 14 Kosikowski, F. V., A. R. Masters, and V. V. Mistry. 1984. Cottage cheese from retentate-supplemented skim milk. J. Dairy Sci. 68:541. 15 Kuo, K., and M. Cheryan. 1983. Ultrafiltration of acid whey in a spiral-wound unit: effect of operating parameters on membrane fouling. J. Food Sci. 48:113. 16 Laemmli, U. K. 1970. Cleavage of structural proteins during the assembly of the head of bacteriophage T4. Nature 227:680. 17 Law, B. A. 1979. Reviews of the progress of dairy science: enzymes of psychotrophic bacteria and their effects on milk and milk products. J. Dairy Sci. 46:573. 18 Lee, D. N., and R. L. Merson. 1975. Examination of cottage cheese whey proteins by scanning electron microscopy: relationship to membrane fouling during ultrafiltration. J. Dairy Sci. 58: 1423. 19 Lopez-Leiva, M., and E. Matthiasson. 1981. Solute adsorption as a source of fouling in ultrafiltration. Page 299 in Fundamentals and applications of surface phenomena associated with fouling and cleaning in food processing. B. Hallstrom, D. B. Lund, and C. Tragardh, ed. Madison, Wl. 20 Matthiasson, E. 1985. Fouling in membrane filtration. Page 429 in Fouling and cleaning in food processing. D. Lund, E. Plett, and C. Sandu, Univ. Wisconsin, Madison Extension Duplicating, Madison, WI. 21 Matthiasson, E., and B. Sivik. 1980. Concentration polarization and fouling. Desalination 35:59. 22 Michaels, A. S. 1980. Fifteen years of ultrafiltration: promises and future problems of an adolescent technology. Page 1 in Ultrafiltration Membranes and Applications. A. R. Cooper, ed. Plenum Press, New York, NY. 23 Patel, R. S., and H. Reuter. t985. Deposit formation on a hollow fiber ultrafiltration membrane during concentration of skim milk. Milchwissenschaft 40:592. 24 Richardson, G. 1I. 1985. Standard methods for the examination of dairy products. 15th ed. Am. Publ.
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Health Assoc., Washington, DC. 25 Rogers, A. N. 1984. Design and operation of desalting systems based on membrane processes. Page 437 in Synthetic membrane processes: fundamentals and water applications. G. Belfort, ed. Academic Press, Inc., Orlando, FL. 26 Sivik, B., and B. Hallstrom. 1981. State of the art of fouling of membrane surfaces. Page 10 in Fundamentals and applications of surface phenomena associated with fouling and cleaning in food processing. B. Hallstrom, D. B. Lund, and C. Tragardh, ed. Madison, Wl. 27 Skudder, P. J., F. A. Glover, and M. L. Green. 1977. An examination of the factors affecting reverse osmosis of milk with special reference to deposit formation. J. Dairy Res. 44:293. 28 Tong, P. S., D. M. Barbano, and W. K. Jordan. 1985. Effect of milk coagulants on membrane
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30
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fouling and the ultrafiltration flux of Cheddar cheese whey. J. Dairy Sei. 68(Suppl. 1):58. (Abstr.) Tong, P. S., D. M. Barbano, and W. K. Jordan. 1986. Effects of treatment of calf rennet whey with a microbial rennet isolated from Mucor pusillus on ultrafiltration flux and membrane fouling. J. Dairy Sci. 69(Suppl. 1):54. (Abstr.) Verdi, R. J., D. M. Barbano, M. E. DellaValle, and G. F. Senyk. 1987. Variability in true protein, casein, nonprotein nitrogen, and proteolysis in high and low somatic cell milks. J. Dairy Sci. 70:230. Williams, S. W. 1984. Official methods of analysis of the Association of official Analytical Chemists. 14th ed. Assoc. Offic. Anal. Chem. Inc., Arlington, VA. Van, S. H., C. G. Hill, Jr., and C. H. Amundson. 1979. Ultrafiltration of whole milk. J. Dairy Sci. 62:23.
facture of frozen desserts, cheese, yogurt, and other dairy foods (5, 6, 7, 13, 14). Ultrafihration systems designed for the dairy industry utilize crossflow filtration. Usually, fluid conditions near the membrane surface are quite turbulent. High fluid turbulence and high wall shear act to scour the membrane surfaces and minimize accumulation of solute near the membrane (4). Despite these favorable hydrodynamic conditions, flux decline associated with membrane fouling remains a problem during UF of dairy fluids (15, 23, 28). Membrane fouling can increase cleaning, labor, and energy costs and shorten membrane life (21, 22, 25). The objectives of this study were 1) to characterize time-dependent flux decline during whole milk UF, 2) to determine the significance of adsorption fouling in the absence of concentration polarization, and 3) to characterize the proteinaceous components of the membrane foulants.
Pasteurized whole milk was fractionated with a pilot-scale, plate and frame, uhrafiltration system to study membrane fouling and flux decline. Concentration factor was set at approximately 1.4x to simulate the first stage of a multistage UF system. Proteinaceous membrane foulant was characterized by SDS-PAGE. Distribution of proteins in the foulant was very different from distribution of proteins in milk. Whey proteins, a-lactalbumin and/3-1actoglobulin, accounted for 95% of the proteinaceous membrane foulants. Very little casein was identified as membrane foulant. The approximate amount of protein in the membrane foulant was estimated to be .6 g/m 2 of membrane area. Permeate flux studies indicated that flux decline is severe in the early stages of milk ultrafiltration and is associated with irreversible adsorption of protein on the membrane surface. A threefold difference between the water flux of clean membranes and fouled membranes was attributed to the adsorbed foulant. Identification and characterization of membrane foutants and the mechanism of their interaction with membrane surfaces should lead to the design of more efficient ultrafihration systems for the dairy industry.
MATERIALS AND METHODS Milk Ultrafiltration Trials
Seventy-six liters of fresh, unhomogenized Grade A raw whole milk were batch pasteurized at 63°C for 30 rain and cooled to 49°C just prior to UF. The milk feed stream for UF was similar to those used in whole milk retentate manufacture for cheese making (12). Ultrafiltration Conditions
INTRODUCTION
Ultrafihration (UF) has the potential to become an integral unit operation in processing dairy fluids. Research has demonstrated that milk retentates from UF can be used in manuReceived March 25, 1987. Accepted September 11, 1987. 1Kraft, Inc., 801 Waukegan Road, Glenview, IL 60025.
1988 J Dairy Sci 71:604--612
Uhrafihration was with a plate and frame UF system (Series "S", Dorr-Oliver, Inc., Stamford, CT). The system contained seven, S-10 membrane elements (.47 m R of active membrane area). Membranes were polysulfone with a nominal 10,000 molecular weight ~utoff. The membranes were characterized by the supplier as negatively charged and relatively hydrophobic. Membrane plates used in this study were actual production scale plates used in commercial UF systems. The UF system was
604
MEMBRANE FOULING DURING MILK ULTRAFILTRATION
605
RETURN BLEED CONTROL VALVE
MEMBRANE
II.
LOW
[
PERMEATE
- - -I~
~-
FILL ISO
~
HIGH PRESSURE GAUGE
THERMOMETER
--Z PUMP
PUMP
HEAT EXCHANGER
Figure 1. Schematic diagram of plate and frame ultrafiltration system (courtesy of Dorr-Oliver, Inc.).
configured in a pattern similar to the three plate system shown in Figure 1. The system has a feed pump and a recirculation pump to provide the feed pressures and linear flow velocity required for efficient UF. Ultrafiltration process conditions were a temperature of 49°C and operating pressures of 310 and 172 kPa for the inlet and outlet, respectively. These operating pressures conform to the manufacturer's recommended pressure drop per membrane plate of 15.6 kPa for milk UF. The UF system was run in total recycle (both retentate and permeate streams returned to feed tank) to maintain a constant feed composition throughout each run. Retentate and feed flow rates were adjusted initially to give a concentration factor of 1.4x to simulate the first stage of a multistage milk UF system. Under these conditions, retentate and permeate throughput was approximately 30 L/h and 15 L/h, respectively. Process time was 20 or 120 min, depending on the experiment. Analysis of Milk and Permeate
Milk samples were analyzed for fat content with an infrared milk analyzer calibrated by the
Mojonnier method (24). Fat content of permeate samples was determined by a modified Babcock method (24). Total N and nonprotein N of milk and permeate were determined by macro-Kjeldahl analysis (31). Total solids content of the samples were determined by drying 1-g samples at 100°C -+ 2°C for 5 h in a forced air oven (24). Membrane Cleaning and Clean Membrane Water Flux Determination
Prior to each milk UF trial, the system was cleaned thoroughly with 20 L of the manufacturer's recommended alkaline cleaning solution consisting of 1% sodium hydroxide, .05% sodium gluconate, .05% tetrasodium EDTA, and .04% sodium dodecyl benzene sulfonate. The cleaning solution was recirculated for 20 min at 63°C. The system was flushed completely with 20 L of deionized water at 49°C, and the clean membrane water flux was determined using an additional 20 L of fresh 49°C deionized water. Permeate ftux during water or milk UF was determined by measuring permeate flow rates and dividing by the total membrane area of the system. Clean membrane water flux data were collected at 49°C with no applied back pressure Journal of Dairy Science Vol. 71, No. 3, 1988
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(retentate bleed valve fully open). Under these conditions, inlet and outlet pressures were approximately 240 and 100 kPa, respectively. Permeate flux during milk UF was determined under the process conditions mentioned earlier and was expressed in liters per square meter per hour. Fouled Membrane Water Flux Determination
After a milk UF trial, the system was rinsed with 50 L of deionized water at 49°C for 5 rain. The water rinse was in two stages: system water flush and water recirculation. The first 10 L of water that flowed through the permeate and retentate lines were not recirculated back to the feed tank (system water flush). The remaining 40 L of deionized water were then recirculated at high linear velocity through the system at no applied back pressure for 4.5 min (water recirculation). The system water flush and high speed water recirculation were designed to remove any milk solids that were not bound tightly to the membrane surface. Fouled membrane water flux data were determined 4.5 rain into the water rinse period under the process conditions described for clean membrane water flux determinations. Adsorption Trials
Adsorption trials were conducted to determine the contribution of adsorption in the absence of concentration polarization to the process of membrane fouling. Prior to adsorption trials, seven S-10 membrane elements were cleaned and the system was shut down and disassembled. All seven membrane plates were removed from the unit and each was fitted with a stainless steel bolt, nut, and washer system to seal the permeation port of the plate. This prevented milk from coming in contact with the permeate side of the membrane. Once sealed, the seven membrane elements were submersed simultaneously into 19 L of mildly agitated, pasteurized, unhomogenized milk at 49°C. After 5 rain, membrane elements were removed from the milk and rinsed in 19 L of 49°C deionized water for 30 s. They were then placed in another 19 L of clean 49°.C water until reinsertion of all seven membrane plates back into the unit was complete (ca. 20 min). The UF unit was reassembled and the
Journal of Dairy Science Vol. 71, No. 3, 1988
fouled membrane water flux was determined as described. Sampling and Analysis of Membrane Foulants
After the system water flush (10 L), high speed water recirculation (40 L), and flux determination as described, fouled membranes from the 20-min milk UF trials were sampled. At this point, any proteins remaining on the membranes were assumed to be strongly attached. The UF pilot system was disassembled and half of one side of membrane (.017 m 2) was cut out from a membrane plate. This section of membrane was cut into 1 cm × 6-cm strips and placed into a screw-cap test tube containing 10 ml of buffer (10 mM Tris HC1 at pH 6.8, 1% SDS, 20% glycerol, and .02% bromphenol blue). The test tube was closed and placed into a boiling water bath for 5 rain to extract fully the foulants from the membrane. Pieces of membrane remaining after extraction were discarded and .0772 g of dithiothreitol was added to the solution and reboiled. Membrane foulant sample solutions were held frozen for future electrophoretic analysis. The discontinous SDS-PAGE procedures of Laemmli (16), as described by Verdi et al. (30), for milk protein analysis were used to characterize the proteinaceous fraction of membrane foulants. Twenty microliters of membrane foulant sample solutions were loaded per slot of the electrophoresis gel. RESULTS AND DISCUSSION Milk and Permeate Composition
Composition of milk and permeate are shown in Table 1. Milk composition was typical for New York state (3). Permeate composition was consistent with that reported in previous studies (8, 32). Permeate Flux
Time-dependent flux decline during whole milk UF is shown in Figure 2. If the feed (milk) gave no resistance to permeation, then the initial permeate flux should have been similar to the clean water flux. The clean membrane water flux was approximately 600 L/m 2 h whereas the flux after only 10 min of milk UF
MEMBRANE FOULING DURING MILK ULTRAFILTRATION
607
TABLE 1. Composition of milk and permeate from 20-rain milk ultraflltration trials)
Composition
% Total solids % Fat % Total N % Nonprotein N
Milk N, 11.87 3.40 .486 .025
Permeate SD .04 .07 .003 .001
Y~ 5.63
SD .07 ... .002 .001
' 1031 .026
tn=4.
was 60 L/m 2 per h. During the first 40 min of process time, flux dropped rapidly. In the later stages of milk UF, flux decline was more gradual (Figure 2). Similar results have been reported elsewhere (19, 2 3). In early stages of milk UF, adsorption fouling is probably the primary mechanism of flux decline (20). Osmotic effects and concentration polarization contribute minimally to the resistance to permeation during early stages of milk UF. In our study of milk UF, rapid flux decline in the first 40 rain accounted for over 90% of the total flux decline over the processing period. Lopez-Leiva and Matthiasson (19) attributed the early stages of flux decline to adsorption of proteins on the membranes. Adsorption of proteins onto membrane surfaces has also been reported by Ingham et al. (10). Hence, we have classified the time-dependent flux decline we observed during milk UF into two phases. Adsorption fouling is the first phase of flux decline and concentration polarization is the second phase of flux decline (Figure 2). During the second phase of flux decline compaction of the fouled Iayer may also increase resistance to permeation. Further evidence of extensive membrane fouling and flux decline in the early" stages of UF is shown in Table 2. The fourfold difference in water flux between clean membranes and membranes after 120 min of milk UF ("fouled" membranes) suggests that a tightIy bound laver acts as the primary resistance to water flux on the "fouled" membranes. Experiments were conducted to determine the role of adsorption fouling in the absence of concentration polarization. Water flux data shown in Table 2 for the adsorption trials confirm the fact that adsoprtion fouling in-
dependent of concentration polarization is an extremely important part of the milk UF membrane fouling process. After a 5-min exposure of the polysulfone membranes to milk with no permeation, adsorption of milk components on the membrane surface gave a 68% decrease (threefold) in water flux when compared with the water flux of clean membranes just prior to the adsorption trials. Furthermore, in the presence of concentration polarization (120-min UF trials) the decrease in water flux
Clean Membrane Water Flux 600
#
Concentration Polarization
Adsorption
~O4
65
I
E
x
60
¢D
~.
ss
50
45 0
20
: 40
" 60
80
100
.-
."
120
140
UF Process Time (rain)
Figure 2. Time-dependent flux decline during whole milk ultrafiltration. (n = 4, brackets are 95% confidence intervals.) Journal of Dairy Science Vol. 71, No. 3, 1988
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TONG ET AL.
TABLE 2. Water flux at 49°C of clean and fouled membranes. Water flux
Time of milk contact with membrane
n
Decrease in water flux
Fouled
Clean
(%)
(L/m 2 .h) SD
SD 5-min Adsorption 120-min UF run
5 4
583 602
f r o m t h e clean t o t h e f o u l e d m e m b r a n e s was
187 143
35.3 33.8
35.8 10.3
68 76
Electrophoretic Analysis
only slightly higher (76%). This would indicate
An example of the electrophoretic pattern
t h a t t h e m a j o r i t y o f t h e d e c l i n e in w a t e r flux d u r i n g t h e U F o f milk using p o l y s u l f o n e m e m b r a n e s is d u e to a d s o r p t i o n fouling a n d n o t c o n c e n t r a t i o n p o l a r i z a t i o n i n d u c e d fouling.
of t h e p r o t e i n a c e o u s f o u l a n t e x t r a c t e d f r o m m e m b r a n e s a f t e r 20 m i n o f milk U F is s h o w n in F i g u r e 3. Lane 1 c o n t a i n s a series o f p r o t e i n s o f k n o w n m o l e c u l a r w e i g h t as r e f e r e n c e s t a n d a r d s
1 66 kd
2
24kd Elf
(-)
BSA
q
45 kd q ~ 36kd
3
as-C:K k-(.
20kd 14kd
B LG aLA
e
(+) Figure 3. Sodium dodecyl sulfate-polyacrylamide gel electrophoretic patterns from whole milk ultrafiltration: lane 1, molecular weight standards: bovine serum albumen (66 kdal), egg albumen (45 kdal), glycerol-3phosphate (29 kdal), carbonic anhydrase (24 kdal), soybean trypsin inhibitor (20 kdal), cz-lactalbumin (14 kdal). Lane 2, milk feed for ultrafihration: bovine serum albumin (BSA), CZs-Casein (C~s-CN), 3-casein (3-CN), K-casein (K-CN), 3-1actoglobulin (3-LG), c~-lactalbumin (a-LA). Lane 3, membrane foulant after 20 min of milk ultrafiltration. Journal of Dairy Science Vol. 71, No. 3, 1988
MEMBRANE FOULING DURING MILK ULTRAFILTRATION (Dalton VII L, Sigma Chemical Co., St. Louis, MO). Lane 2 demonstrates a typical distribution of proteins from milk used in our UF trials. Bovine serum albumin, (~s-caseins (~slcasein and as2-casein), ~3-casein, K-casein, /3-1actoglobulin, ~-lactalbumin and a proteolytic fragment of /3-casein are shown clearly. Estimates of the molecular weights of the caseins in this system are higher than reference values (30). Lane 3 contains the proteinaceous membrane foulants. Surprisingly, very little intact casein was present. Of the principal caseins of milk, K-casein was the most prevalent in the foulant; however, it was in very low concentration compared with other proteins detected. More importantly, the principal foulants were identified as a-lactalbumin and/3-1actoglobulin. The densitometric quantitation of these electrophoretic patterns is shown in Table 3. Literature values for milk protein distribution are approximately 82.4% as caseins and 17.6% as whey proteins (11). The average distribution of proteins for milk used in our study was similar./3-Lactoglobulin was slightly higher than the literature values because a proteolytic fragment from the caseins (a 7-casein) comigrates with/3-1actoglobulin. In contrast, the foulant has a very different distribution of proteins - only 5% casein and
609
95% whey proteins. In addition, there are large differences in the relative percent of individual whey proteins between milk and membrane foulant. For example, a-lactalbumin represents only 3.8% of total protein in milk, whereas this protein is almost 80% of the total protein present as membrane foulant. Electrophoretic patterns of membrane foulants after 120 min of milk UF were similar to those observed after 20 min. Purified/3-1actoglobulin and a-lactalbumin foul UF membranes (10, 17). Proteolytic fragments of caseins are usually present in low concentrations in milk. This was the case for the milk used in the present study (Table 3). Such fragments are usually attributed to proteolytic damage of/3-casein by somatic cell proteases, plasmin, or bacterial proteases (1, 2, 17). Low molecular weight proteolytic fragments from caseins have been implicated in fouling during whey UF (9, 28, 29). Significant concentrations of casein fragments (7-caseins) were detected in the membrane foulants during milk UF. Further studies to determine the impact of milk quality on permeate flux during milk UF should be conducted. Comparison to Previous Studies of Membrane Foulants
Our results disagree with previous research that examined the protein components of milk
TABLE 3. Proteinaceous membrane foulants from milk ultrafiltration.
Protein I
Milk literature 2
Milk observed3
Membrane foulants 20 rain4 (%)
Total casein (CN) as,, ~s2-CN t3-CN K-CN 3'-CN Total whey protein Immunoglobulin Bovine serum albumin 3-1actoglobulin a-lactalbumin
82.4 39.9 29.5 10.5 2.5 17.6 2.3 1.3 10.2 3.8
75.4 34.8 28.9 10.2 1.5 24.6 .9 .8 18.2 4.7
SD
X
3.0 3.0 2.7 1.8 .5 ... .5 .5 2.3 .9
4.9 .3 .4 1.9 2.3 95.0 . . . 1614 78.6
SD Q
.4 .7 1.5 1.3 ... .
.
. 113 3.1
1Determined by quantitative SDS-PAGE. 2 From Jenness (11). 3Milk used in ultrafiltration trials (n = 4). 4 From m e m b r a n e after 20 rain milk ultrafiltration (n = 4). Journal of Dairy Science Vol. 71, No. 3, 1988
610
TONG ET AL.
UF membrane foulants. In a previous report on UF of skimmilk in hollow fiber polysulfone membranes, Patel and Reuter (23) showed the primary foulants to be the caseins. They also indicated that these proteins appeared in the ratios typical to that in whole casein. In an earlier report, Skudder et al. (27) studied deposit formation during reverse osmosis of whole milk in tubular cellulose acetate membranes. They found the composition of the deposits to be similar to the protein distribution in milk itself. The foulant sampling procedure used in both of these studies was quite different than our procedure. In previous studies, membrane cleaning cycle solutions were collected and freeze-dried to concentrate the proteins removed from the total membrane system. Then, these freeze-dried proteins were analyzed by starch gel electrophoresis. To reconcile this apparent disagreement of our results with the reported literature, we offer the following analysis. We have estimated the amount of protein contained in the adsorbed layer from fouled membranes in our system. The total protein loaded per lane for electrophoresis was approximately 20 bLg. Based on this estimate, we have calculated the amount of adsorbed protein extracted from the fouled membrane to be .010 g protein per membrane sample section, which translates to .6 g protein/ m 2 of membrane or .28 g protein per membrane area of our entire system. F r o m these calculations we have determined the amount of free milk (i.e., milk not adsorbed on membranes) that would have been required to remain in our system prior to collection of membrane foulants to give us results for membrane foulant similar to the results using Patel and Reuter's foulant collection procedures. Assuming milk with a 3.3% protein content, we calculate that only 8.5 ml of residual milk remaining in our entire UF system could have given results similar to the previous studies. Because the hold-up volume of our system is approximately 6 L, and because there are a large number of gaskets and turnarounds in most UF systems, 8.5 ml of residual milk would not be unusual if the system were not thoroughly rinsed prior to sampling of membrane foulants or if milk components adsorbed to stainless steel, plastic, or gasket surfaces in the system. The issue of residual milk remaining Journal of Dairy Science VoL 71, No. 3, 1988
in the UF system or other surfaces was avoided in our study by a thorough water rinse of the system prior to removal of a membrane plate from the UF system and direct sampling of membranes. Two other factors may explain the differences in foulant composition between our results and those of other workers. First, in previous literature reports, UF was in small laboratory units. The hydrodynamic conditions of large UF systems are difficult to achieve in small laboratory units. Second, in Patcl and Reuter's study (23), UF was continued until permeation completely stopped. This mode of operation is not typical of commercial practice. Hence, differences in process conditions, membrane type, and system dynamics may also explain why our results disagree with the previous studies. It is unclear at this time why a-lactalbumin, and 3-1actoglobulin preferentially adsorb on membranes during milk UF. Lee and Merson (18) suggested that polymerization behavior of whey proteins can cause formation of sheets and strands of protein that attach to membrane surfaces to cause flux decline. The role of such interactions in fouling during milk UF is the subject of ongoing research in our laboratory. We have gained a better understanding of membrane fouling during milk UF that may lead to the design of more efficient UF systems for the dairy industry. In particular, adsorption fouling may be minimized by development of new membranes with different surface characteristics, whereas flux decline associated with concentration polarization can be minimized by optimization of hydrodynamic conditions near the membrane surface (26). Characterization of the fouling mechanism (adsorption or concentration polarization) causing the greatest flux decline during the initial stages of membrane fouling during milk UF will give us insight into methods for design of more efficient UF systems. CONCLUSIONS
The destructive membrane sampling procedure was a more direct and accurate method to collect membrane foulants. Hence, from this study, we have concluded that: 1) flux decline during whole milk UF occurs in two stages -- severe flux decline (adsorption fouling) and
MEMBRANE FOULING DURING MILK ULTRAFILTRATION t h e n m o r e gradual flux decline ( c o n c e n t r a t i o n polarization), 2) e l e c t r o p h o r e t i c analysis o f t h e p r o t e i n a c e o u s f r a c t i o n o f the fouied layer indicates it c o n t a i n s a p p r o x i m a t e l y .6 g prot e i n / m 2 o f m e m b r a n e area and t h a t m o s t o f this p r o t e i n is c~-lactalbumin and 13-1actoglobulin, 3) t h e ratio o f c~-lactalbumin to ~-lactoglobulin was m u c h higher in t h e f o u l a n t s (4.8:1) t h a n in milk (.25:1), w h i c h indicates selective int e r a c t i o n o f ~x-lactalbumin w i t h the m e m b r a n e surface, and 4) a d s o r p t i o n trials indicate t h a t a d s o r p t i o n fouling causes m o s t o f t h e decrease in w a t e r flux during t h e U F o f milk using polysulfone membranes. ACKNOWLEDGMENTS
We wish to a c k n o w l e d g e Dorr-Oliver, Inc., S t a m f o r d , CT for s u p p l y i n g UF e q u i p m e n t and supplies and t h e National Dairy P r o m o t i o n and R e s e a r c h Board for financial s u p p o r t . T e c h n i c a l assistance f r o m M. C h a p m a n , P. Fleming, R. R a s m u s s e n , and M. S m i t h e r s and f r o m Cornell University Dairy Plant p e r s o n n e l was e x t r e m e l y valuable. REFERENCES
1 Andrews, A. T. 1983. Proteinases in normal bovine milk and their actions on caseins. J. Dairy Res. 44:223. 2 Andrews, A. T. 1983. Breakdown of caseins in bovine milks with high somatic cell counts arising from mastitis or infusion with bacterial endotoxin. J. Dairy Res. 50:275. 3 Barbano, D. M., and J. W. Sherbon. 1984. Cheddar cheese yields in New York. J. Dairy Sci. 67: 1873. 4 Belfort, G. 1984. Membrane methods in water and wastewater treatment: an overview. Page 1 in Synthetic membrane processes: fundamentals and water applications. G. Belfort, ed. Academic Press, Inc., Orlando, FL. 5 Bundgaard, A. G., O. J. Olsen, and R. F. Madsen. 1972. Ultrafiltration and hyperfiltration of skim milk for production of various dairy products. Dairy Ind. 37:539. 6 Chapman, H. R., V. E. Bines, F. A. Glover, and P. J. Skudder. 1974. Use of milk concentrated by ultrafiltration for making hard cheese, soft cheese and yogurt. J. Soc. Dairy Technol. 27:151. 7 Covacevich, H. R., and F. V. Kosikowski. 1977. Cream cheese by ultrafiltration. J. Food Sci. 42:1362. 8 Garoutte, C. A., C. H. Amundson, and C. G. Hill, Jr. 1982. Ultrafiltration of whole milk with hollow fiber membranes. J. Food Process Eng. 5:191. 9 Hickey, M. W., and R. D. Hill. 1980. Investigation into the ultrafiltration and reverse osmosis of wheys. II. The effects of some minor whey constl-
61 1
tuents. N.Z.J. Dairy Sci. Technol. 15:123. 10 Ingham, K. C., T. F. Busby, Y. Sahlestrom, and F. Castino. 1979. Separation of macromolecules by ultrafiltration: influence of protein adsorption, protein-protein interactions, and concentration polarization. Page 141 in Ultrafiltration membranes and applications. A. R. Cooper, ed. Plenum, New York. NY. 11 Jenness, R. 1982. Inter-species comparison of milk proteins. Page 188 in Developments in dairy chemistry - 1. P. F. Fox, ed. Appl. Sci. Publ., Essex, Engl. 12 Kosikowski, F. V. 1977. Cheese by ultrafiltration. Page 510 in Cheese and fermented milk foods. F. V. Kosikowski, ed. 2nd ed. F. V. Kosikowski and Associates, Brooktondale, NY. 13 Kosikowski, F. V., and A. R. Maters. 1983. Preparation of ice cream, skim milk and cream made from whole milk retentates. J. Dairy Sci. 66(Suppl. 1):99. (Abstr.) 14 Kosikowski, F. V., A. R. Masters, and V. V. Mistry. 1984. Cottage cheese from retentate-supplemented skim milk. J. Dairy Sci. 68:541. 15 Kuo, K., and M. Cheryan. 1983. Ultrafiltration of acid whey in a spiral-wound unit: effect of operating parameters on membrane fouling. J. Food Sci. 48:113. 16 Laemmli, U. K. 1970. Cleavage of structural proteins during the assembly of the head of bacteriophage T4. Nature 227:680. 17 Law, B. A. 1979. Reviews of the progress of dairy science: enzymes of psychotrophic bacteria and their effects on milk and milk products. J. Dairy Sci. 46:573. 18 Lee, D. N., and R. L. Merson. 1975. Examination of cottage cheese whey proteins by scanning electron microscopy: relationship to membrane fouling during ultrafiltration. J. Dairy Sci. 58: 1423. 19 Lopez-Leiva, M., and E. Matthiasson. 1981. Solute adsorption as a source of fouling in ultrafiltration. Page 299 in Fundamentals and applications of surface phenomena associated with fouling and cleaning in food processing. B. Hallstrom, D. B. Lund, and C. Tragardh, ed. Madison, Wl. 20 Matthiasson, E. 1985. Fouling in membrane filtration. Page 429 in Fouling and cleaning in food processing. D. Lund, E. Plett, and C. Sandu, Univ. Wisconsin, Madison Extension Duplicating, Madison, WI. 21 Matthiasson, E., and B. Sivik. 1980. Concentration polarization and fouling. Desalination 35:59. 22 Michaels, A. S. 1980. Fifteen years of ultrafiltration: promises and future problems of an adolescent technology. Page 1 in Ultrafiltration Membranes and Applications. A. R. Cooper, ed. Plenum Press, New York, NY. 23 Patel, R. S., and H. Reuter. t985. Deposit formation on a hollow fiber ultrafiltration membrane during concentration of skim milk. Milchwissenschaft 40:592. 24 Richardson, G. 1I. 1985. Standard methods for the examination of dairy products. 15th ed. Am. Publ.
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Health Assoc., Washington, DC. 25 Rogers, A. N. 1984. Design and operation of desalting systems based on membrane processes. Page 437 in Synthetic membrane processes: fundamentals and water applications. G. Belfort, ed. Academic Press, Inc., Orlando, FL. 26 Sivik, B., and B. Hallstrom. 1981. State of the art of fouling of membrane surfaces. Page 10 in Fundamentals and applications of surface phenomena associated with fouling and cleaning in food processing. B. Hallstrom, D. B. Lund, and C. Tragardh, ed. Madison, Wl. 27 Skudder, P. J., F. A. Glover, and M. L. Green. 1977. An examination of the factors affecting reverse osmosis of milk with special reference to deposit formation. J. Dairy Res. 44:293. 28 Tong, P. S., D. M. Barbano, and W. K. Jordan. 1985. Effect of milk coagulants on membrane
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29
30
31
32
fouling and the ultrafiltration flux of Cheddar cheese whey. J. Dairy Sei. 68(Suppl. 1):58. (Abstr.) Tong, P. S., D. M. Barbano, and W. K. Jordan. 1986. Effects of treatment of calf rennet whey with a microbial rennet isolated from Mucor pusillus on ultrafiltration flux and membrane fouling. J. Dairy Sci. 69(Suppl. 1):54. (Abstr.) Verdi, R. J., D. M. Barbano, M. E. DellaValle, and G. F. Senyk. 1987. Variability in true protein, casein, nonprotein nitrogen, and proteolysis in high and low somatic cell milks. J. Dairy Sci. 70:230. Williams, S. W. 1984. Official methods of analysis of the Association of official Analytical Chemists. 14th ed. Assoc. Offic. Anal. Chem. Inc., Arlington, VA. Van, S. H., C. G. Hill, Jr., and C. H. Amundson. 1979. Ultrafiltration of whole milk. J. Dairy Sci. 62:23.