Determination of the efficiency of removal of whey protein from sweet whey with ceramic microfiltration membranes

Our research objective was to measure percent removal of whey protein from separated sweet whey using 0.1-µm uniform transmembrane pressure ceramic microfiltration (MF) membranes in a sequential batch 3-stage, 3× process at 50°C. Cheddar cheese whey was centrifugally separated to remove fat at 72°C and pasteurized (72°C for 15 s), cooled to 4°C, and held overnight. Separated whey (375 kg) was heated to 50°C with a plate heat exchanger and microfiltered using a pilot-scale ceramic 0.1-µm uniform transmembrane pressure MF system in bleed-and-feed mode at 50°C in a sequential batch 3-stage (2 diafiltration stages) process to produce a 3× MF retentate and MF permeate. Feed, retentate, and permeate samples were analyzed for total nitrogen, noncasein nitrogen, and nonprotein nitrogen using the Kjeldahl method. Sodium dodecyl sulfate-PAGE analysis was also performed on the whey feeds, retentates, and permeates from each stage. A flux of 54 kg/m² per hour was achieved with 0.1-µm ceramic uniform transmembrane pressure microfiltration membranes at 50°C. About 85% of the total nitrogen in the whey feed passed though the membrane into the permeate. No passage of lactoferrin from the sweet whey feed of the MF into the MF permeate was detected. There was some passage of IgG, bovine serum albumen, glycomacropeptide, and casein proteolysis products into the permeate. β-Lactoglobulin was in higher concentration in the retentate than the permeate, indicating that it was partially blocked from passage through the ceramic MF membrane.     

Process efficiency of casein separation from milk using polymeric spiral-wound microfiltration membranes

Microfiltration is largely used to separate casein micelles from milk serum proteins (SP) to produce a casein-enriched retentate for cheese making and a permeate enriched in native SP. Skim milk microfiltration is typically performed with ceramic membranes and little information is available about the efficiency of spiral-wound (SW) membranes. We determined the effect of SW membrane pore size (0.1 and 0.2 µm) on milk protein separation in total recirculation mode with a transmembrane pressure gradient to evaluate the separation efficiency of milk proteins and energy consumption after repeated concentration and diafiltration (DF). Results obtained in total recirculation mode demonstrated that pore size diameter had no effect on the permeate flux, but a drastic loss of casein was observed in permeate for the 0.2-µm SW membrane. Concentration-DF experiments (concentration factor of 3.0× with 2 sequential DF) were performed with the optimal 0.1-µm SW membrane. We compared these results to previous data we generated with the 0.1-µm graded permeability (GP) membrane. Whereas casein rejection was similar for both membranes, SP rejection was higher for the 0.1-µm SW membrane (rejection coefficient of 0.75 to 0.79 for the 0.1-µm SW membrane versus 0.46 to 0.49 for the GP membrane). The 0.1-µm SW membrane consumed less energy (0.015–0.024 kWh/kg of permeate collected) than the GP membrane (0.077–0.143 kWh/kg of permeate collected). A techno-economic evaluation led us to conclude that the 0.1-µm SW membranes may represent a better option to concentrate casein for cheese milk; however, the GP membrane has greater permeability and its longer lifetime (about 10 yr) potentially makes it an interesting option.     

Production efficiency of micellar casein concentrate using polymeric spiral-wound microfiltration membranes

Most current research has focused on using ceramic microfiltration (MF) membranes for micellar casein concentrate production, but little research has focused on the use of polymeric spiral-wound (SW) MF membranes. A method for the production of a serum protein (SP)-reduced micellar casein concentrate using SW MF was compared with a ceramic MF membrane. Pasteurized (79°C, 18s) skim milk (1,100 kg) was microfiltered at 50°C [about 3 × concentration] using a 0.3-μm polyvinylidene fluoride spiral-wound membrane, bleed-and-feed, 3-stage process, using 2 diafiltration stages, where the retentate was diluted 1:2 with reverse osmosis water. Skim milk, permeate, and retentate were analyzed for SP content, and the reduction of SP from skim milk was determined. Theoretically, 68% of the SP content of skim milk can be removed using a single-stage 3× MF. If 2 subsequent water diafiltration stages are used, an additional 22% and 7% of the SP can be removed, respectively, giving a total SP removal of 97%. Removal of SP greater than 95% has been achieved using a 0.1-μm pore size ceramic uniform transmembrane pressure (UTP) MF membrane after a 3-stage MF with diafiltration process. One stage of MF plus 2 stages of diafiltration of 50°C skim milk using a polyvinylidene fluoride polymeric SW 0.3-μm membrane yielded a total SP reduction of only 70.3% (stages 1, 2, and 3: 38.6, 20.8, and 10.9%, respectively). The SP removal rate for the polymeric SW MF membrane was lower in all 3 stages of processing (stages 1, 2, and 3: 0.05, 0.04, and 0.03 kg/m² per hour, respectively) than that of the comparable ceramic UTP MF membrane (stages 1, 2, and 3: 0.30, 0.11, and 0.06 kg/m² per hour, respectively), indicating that SW MF is less efficient at removing SP from 50°C skim milk than the ceramic UTP system. To estimate the number of steps required for the SW system to reach 95% SP removal, the third-stage SP removal rate (27.4% of the starting material SP content) was used to extrapolate that an additional 5 water diafiltration stages would be necessary, for a total of 8 stages, to remove 95% of the SP from skim milk. The 8-plus stages necessary to remove >95% SP for the SW MF membrane would create more permeate and a lengthier process than required with ceramic membranes.     

Efficiency of serum protein removal from skim milk with ceramic and polymeric membranes at 50°C

Raw milk (2,710 kg) was separated at 4°C, the skim milk was pasteurized (72°C, 16 s), split into 3 batches, and microfiltered using pilot-scale ceramic uniform transmembrane pressure (UTP; Membralox model EP1940GL0.1μA, 0.1 μm alumina, Pall Corp., East Hills, NY), ceramic graded permeability (GP; Membralox model EP1940GL0.1μAGP1020, 0.1 μm alumina, Pall Corp.), and polymeric spiral-wound (SW; model FG7838-OS0x-S, 0.3 μm polyvinylidene fluoride, Parker-Hannifin, Process Advanced Filtration Division, Tell City, IN) membranes. There were differences in flux among ceramic UTP, ceramic GP, and polymeric SW microfiltration membranes (54.08, 71.79, and 16.21 kg/m² per hour, respectively) when processing skim milk at 50°C in a continuous bleed-and-feed 3× process. These differences in flux among the membranes would influence the amount of membrane surface area required to process a given volume of milk in a given time. Further work is needed to determine if these differences in flux are maintained over longer processing times. The true protein contents of the microfiltration permeates from UTP and GP membranes were higher than from SW membranes (0.57, 0.56, and 0.38%, respectively). Sodium-dodecyl-sulfate-PAGE gels for permeates revealed a higher casein proportion in GP and SW permeate than in UTP permeate, with the highest passage of casein through the GP membrane under the operational conditions used in this study. The slight cloudiness of the permeates produced using the GP and SW systems may have been due to the presence of a small amount of casein, which may present an obstacle in their use in applications when clarity is an important functional characteristic. More β-lactoglobulin passed through the ceramic membranes than through the polymeric membrane. The efficiency of removal of serum proteins in a continuous bleed-and-feed 3× process at 50°C was 64.40% for UTP, 61.04% for GP, and 38.62% for SW microfiltration membranes. The SW polymeric membranes had a much higher rejection of serum proteins than did the ceramic membranes, consistent with the sodium-dodecyl-sulfate PAGE data. Multiple stages and diafiltration would be required to produce a 60 to 65% serum protein reduced micellar casein concentrate with SW membranes, whereas only one stage would be needed for the ceramic membranes used in this study.     

Isolation of caseins from whey proteins by microfiltration modifying the mineral balance in skim milk

The objective of this work was to study the effect of different salts and salt concentration on the isolation of casein micelles from bovine raw skim milk by tangential flow microfiltration. Tangential flow microfiltration (0.22 μm) was conducted in a continuous process adding a modified buffer to maintain a constant initial sample volume. This buffer contained calcium chloride (CaCl₂), sodium phosphate (Na₂HPO₄), or potassium citrate (K₃C₆H₅O₇) in concentrations ranging from 0 to 100 mM. The concentrations of caseins and whey proteins retained were determined by sodium dodecyl sulfate-PAGE and analyzed using the Scion Image software (Scion Corporation, Frederick, MD). A complete isolation of caseins from whey proteins was achieved using sodium phosphate in the range of 10 to 50 mM and 20 times the initial volume of buffer added. No whey proteins were detected at 50 mM but this was at the expense of low caseins being retained. When lower sodium phosphate concentrations were used, the amount of caseins retained was higher but a small amount of whey proteins were still detected by sodium dodecyl sulfate-PAGE. Among the salts tested, calcium chloride at 50 mM and all volumes of buffer showed the higher retention of casein proteins. The highest casein:whey protein ratio was found at 30 mM CaCl₂, but no complete casein micelle isolation was achieved. Potassium citrate was the most ineffective salt because a rapid loss of caseins and whey proteins was observed at all concentrations and with low quantities of buffer added during the filtration process. Our results show the potential of altering the mineral balance in milk for isolation of casein micelles from whey proteins in a continuous tangential flow microfiltration system.     

Comparison of the behavior of two nanofiltration membranes for sweet whey demineralization

Nanofiltration is a process used to separate mineral salts from lactose, having previously removed the proteins by ultrafiltration. Both proteins and lactose can be used as raw materials to prepare a variety of products. In this paper, we studied the feasibility of demineralizing sweet whey obtained from the cheese industry of the Comunidad Valenciana (Spain) using membrane technologies. The NF200 membrane showed the highest volumetric flux and solute rejection values, whereas the DS-5 DL membrane showed the lowest values. The volumetric fluxes obtained with the NF200 and DS-5 DL membranes in these experiments with the ultra-filtered whey demonstrated significant differences between membranes. Concerning solute rejection, the highest values were obtained using the NF200 membrane. The chosen parameter to evaluate the demineralization capability was solute flux. In this way, the values obtained for chloride ion were 9.90 and 32.42 g/ (m²·h) for the NF200 and DS-5 DL membranes, respectively, with the highest demineralization rates being achieved with the DS-5 DL membrane.     

Effect of microfiltration concentration factor on serum protein removal from skim milk using spiral-wound polymeric membranes

Our objective was to determine the effect of concentration factor (CF) on the removal of serum protein (SP) from skim milk during microfiltration (MF) at 50°C using a 0.3-μm-pore-size spiral-wound (SW) polymeric polyvinylidene fluoride (PVDF) membrane. Pasteurized (72°C for 16 s) skim milk was MF (50°C) at 3 CF (1.50, 2.25, and 3.00×), each on a separate day of processing starting with skim milk. Two phases of MF were used at each CF, with an initial startup-stabilization phase (40 min in full recycle mode) to achieve the desired CF, followed by a steady-state phase (90-min feed-and-bleed with recycle) where data was collected. The experiment was replicated 3 times, and SP removal from skim milk was quantified at each CF. System pressures, flow rates, CF, and fluxes were monitored during the 90-min run. Permeate flux increased (12.8, 15.3, and 19.0 kg/m² per hour) with decreasing CF from 3.00 to 1.50×, whereas fouled water flux did not differ among CF, indicating that the effect of membrane fouling on hydraulic resistance of the membrane was similar at all CF. However, the CF used when microfiltering skim milk (50°C) with a 0.3-μm polymeric SW PVDF membrane did affect the percentage of SP removed. As CF increased from 1.50 to 3.00×, the percentage of SP removed from skim milk increased from 10.56 to 35.57%, in a single stage bleed-and-feed MF system. Percentage SP removal from skim milk was lower than the theoretical value. Rejection of SP during MF of skim milk with SW PVDF membranes was caused by fouling of the membrane, not by the membrane itself and differences in the foulant characteristic among CF influenced SP rejection more than it influenced hydraulic resistance. We hypothesize that differences in the conditions near the surface of the membrane and within the pores during the first few minutes of processing, when casein micelles pass through the membrane, influenced the rejection of SP because more pore size narrowing and plugging occurred at low CF than at high CF due to a slower rate of gel layer formation at low CF. It is possible that percentage removal of SP from skim milk at 50°C could be improved by optimization of the membrane pore size, feed solution composition and concentration, and controlling the rate of formation of the concentration polarization-derived gel layer at the surface of the membrane during the first few minutes of processing.     

Comparison of composition, sensory, and volatile components of thirty-four percent whey protein and milk serum protein concentrates

The objectives of this study were to identify and compare the composition, flavor, and volatile components of serum protein concentrate (SPC) and whey protein concentrate (WPC) containing about 34% protein made from the same milk to each other and to commercial 34% WPC from 6 different factories. The SPC and WPC were manufactured in triplicate with each pair of serum and traditional whey protein manufactured from the same lot of milk. At each replication, SPC and WPC were spray dried (SD) and freeze dried (FD) to determine the effect of the heat used in spray drying on sensory properties. A trained sensory panel documented the sensory profiles of rehydrated SD or FD powders. Volatile components were extracted by solid-phase microextraction (SPME) and solvent extraction followed by solvent-assisted flavor evaporation (SAFE) with gas chromatography-mass spectrometry and gas chromatography-olfactometry. Whey protein concentrates had higher fat content, calcium, and glycomacropeptide content than SPC. Color differences (Hunter L, a, b) were not evident between SPC and WPC powders, but when rehydrated, SPC solutions were clear, whereas WPC solutions were cloudy. No consistent differences were documented in sensory profiles of SD and FD SPC and WPC. The SD WPC had low but distinct buttery (diacetyl) and cardboard flavors, whereas the SD SPC did not. Sensory profiles of both rehydrated SD products were bland and lower in overall aroma and cardboard flavor compared with the commercial WPC. Twenty-nine aroma impact compounds were identified in the SPC and WPC. Lipid and protein oxidation products were present in both products. The SPC and WPC manufactured in this study had lower total volatiles and lower concentrations of many lipid oxidation compounds when compared with commercial WPC. Our results suggest that when SPC and WPC are manufactured under controlled conditions in a similar manner from the same milk using the same ultrafiltration equipment, there are few sensory differences but distinct compositional and physical property differences that may influence functionality. Furthermore, flavor (sensory and instrumental) properties of both pilot-scale manufactured protein powders were different from commercial powders suggesting the role of other influencing factors (e.g., milk supply, processing equipment, sanitation).     

Aroma compounds in sweet whey powder

Aroma compounds in sweet whey powder were investigated in this study. Volatiles were isolated by solvent extraction followed by solvent-assisted flavor evaporation. Fractionation was used to separate acidic from nonacidic volatiles. Gas chromatography/mass spectrometry and gas chromatography/olfactometry were used for the identification of aroma compounds. Osme methodology was applied to assess the relative importance of each aroma compound. The most aroma-intense free fatty acids detected were acetic, propanoic, butanoic, hexanoic, heptanoic, octanoic, decanoic, dodecanoic, and 9-decenoic acids. The most aroma-intense nonacidic compounds detected were hexanal, heptanal, nonanal, phenylacetaldehyde, 1-octen-3-one, methional, 2,6-dimethylpyrazine, 2,5-dimethylpyrazine, 2,3-dimethylpyrazine, 2,3,5-trimethylpyrazine, furfuryl alcohol, p-cresol, 2-acetylpyrrole, maltol, furaneol, and several lactones. This study suggested that the aroma of whey powder could comprise compounds originating from milk, compounds generated by the starter culture during cheese making, and compounds formed during the manufacturing process of whey powder.     

Survey of salty and sweet whey composition from various cheese plants in Wisconsin

Salty whey is currently underutilized in the dairy industry because of its high salt content and increased processing and disposal costs. Salty whey accounts for 2 to 5% of the total whey generated during Cheddar and other dry-salted cheese manufacture. Because relatively little information is available on salty whey, this study was conducted to determine the range of compositional components in salty whey from commercial cheese plants. Gross compositional differences in percent protein, salt, solids, and fat between sweet whey and salty whey from various dry-salted cheeses from 8 commercial plants were determined. Differences between individual whey protein compositions were determined using sodium dodecyl sulfate-PAGE. Average total solids, fat, and salt content were significantly greater in the salty whey compared with the corresponding sweet whey. True protein was reduced in salty whey although great variability existed among samples. Individual whey proteins identified included lactoferrin (Lf), BSA, immunoglobulin G, β-lactoglobulin, and α-lactalbumin. Salty whey showed an increase in Lf content and a decrease in α-lactalbumin and β-lactoglobulin content when compared with sweet whey. Salty whey may be a source of Lf, potentially increasing its value to whey processors. However, the compositional assessments showed that commercial salty whey is a highly variable waste stream.     

Comparison of functional properties of 34% and 80% whey protein and milk serum protein concentrates

This study compared the functional properties of serum protein concentrate (SPC) with whey protein concentrate (WPC) made from the same milk and with commercial WPC. The experimental SPC and WPC were produced at 34% or 80% protein from the same lot of milk. Protein contents of WPC and SPC were comparable; however, fat content was much lower in SPC compared with WPC and commercial WPC. The effect of drying methods (freeze vs. spray drying) was studied for 34% WPC and SPC. Few differences due to drying method were found in turbidity and gelation; however, drying method made a large difference in foam formation for WPC but not SPC. Between pH 3 and 7, SPC was found to have lower turbidity than WPC; however, protein solubility was similar between SPC and WPC. Foaming and gelation properties of SPC were better than those of WPC. Differences in functional properties may be explained by differences in composition and extent of denaturation or aggregation.     

Slower proteolysis in Cheddar cheese made from high-protein cheese milk is due to an elevated whey protein content

A growing number of companies within the cheese-making industry are now using high-protein (e.g., 4–5%) milks to increase cheese yield. Previous studies have suggested that cheeses made from high-protein (both casein and whey protein; WP) milks may ripen more slowly; one suggested explanation is inhibition of residual rennet activity due to elevated WP levels. We explored the use of microfiltration (MF) to concentrate milk for cheese-making, as that would allow us to concentrate the casein while varying the WP content. Our objective was to determine if reducing the level of WP in concentrated cheese milk had any impact on cheese characteristics, including ripening, texture, and nutritional profile. Three types of 5% casein standardized and pasteurized cheese milks were prepared that had various casein:true protein (CN:TP) ratios: (a) control with CN:TP 83:100, (b) 35% WP reduced, 89:100 CN:TP, and (c) 70% WP reduced, 95:100 CN:TP. Standardized milks were preacidified to pH 6.2 with dilute lactic acid during cheese-making. Composition, proteolysis, textural, rheological, and sensory properties of cheeses were monitored over a 9-mo ripening period. The lactose, total solids, total protein, and WP contents in the 5% casein concentrated milks were reduced with increasing levels of WP removal. All milks had similar casein and total calcium levels. Cheeses had similar compositions, but, as expected, lower WP levels were observed in the cheeses where WP depletion by MF was performed on the cheese milks. Cheese yield and nitrogen recoveries were highest in cheese made with the 95:100 CN:TP milk. These enhanced recoveries were due to the higher fraction of nitrogen being casein-based solids. Microfiltration depletion of WP did not affect pH, sensory attributes, or insoluble calcium content of cheese. Proteolysis (the amount of pH 4.6 soluble nitrogen) was lower in control cheeses compared with WP-reduced cheeses. During ripening, the hardness values and the temperature of the crossover point, an indicator of the melting point of the cheese, were higher in the control cheese. It was thus likely that the higher residual WP content in the control cheese inhibited proteolysis during ripening, and the lower breakdown rate resulted in its higher hardness and melting point. There were no major differences in the concentrations of key nutrients with this WP depletion method. Cheese milk concentration by MF provides the benefit of more typical ripening rates.     

Studies on physicochemical and functional properties of commercial sweet whey powders

The objective of this study was to characterize variation and interrelatedness among primary functional and compositional parameters of commercially available sweet whey powders. Samples representing different plants/processes and cheese types were assayed for foaming capacity, foam stability, pH, protein content, soluble protein, turbidity, color, particle size distribution, lipid, and moisture. Data were analyzed using principal component analysis. Foaming capacity and stability varied from 10 to 220% and 0.1 to 14 min, respectively. Protein content and solubility ranged from 8.5 to 17.6% and 3.7 to 14.1%, respectively. Lipid content of sweet whey powder varied from 0.03 to 2.00%. The two main functional properties, foaming and protein solubility, did not show significant correlation with each other. Foaming properties showed a positive correlation to particle size and L* or lightness value, and negative correlation to lipid content. Protein solubility showed positive correlation with protein content and negative correlation with turbidity of the sample. Foaming behavior, protein, and particle size attributes were the main variables responsible for grouping of samples. Sweet whey powders from the same dairy plants were grouped together. The direct or indirect significance of these relationships to processing is detailed in this study.     

Production of a high gel strength whey protein concentrate from cheese whey

In order to develop a process for the production of a whey protein concentrate (WPC) with high gel strength and water-holding capacity from cheese whey, we analyzed 10 commercially available WPC with different functional properties. Protein composition and modification were analyzed using electrophoresis, HPLC, and mass spectrometry. The analyses of the WPC revealed that the factors closely associated with gel strength and water-holding capacity were solubility and composition of the protein and the ionic environment. To maintain whey protein solubility, it is necessary to minimize heat exposure of the whey during pretreatment and processing. The presence of the caseinomacropeptide (CMP) in the WPC was found to be detrimental to gel strength and water-holding capacity. All of the commercial WPC that produced high-strength gels exhibited ionic compositions that were consistent with acidic processing to remove divalent cations with subsequent neutralization with sodium hydroxide. We have shown that ultrafiltration/diafiltration of cheese whey, adjusted to pH 2.5, through a membrane with a nominal molecular weight cut-off of 30,000 at 15 degrees C substantially reduced the level of CMP, lactose, and minerals in the whey with retention of the whey proteins. The resulting WPC formed from this process was suitable for the inclusion of sodium polyphosphate to produce superior functional properties in terms of gelation and water-holding capacity.     

Selective separation of the major whey proteins using ion exchange membranes

Synthetic microporous membranes with functional groups covalently attached were used to selectively separate β-lactoglobulin, BSA, and α-lactalbumin from rennet whey. The selectivity and membrane performance of strong (quaternary ammonium) and weak (diethylamine) ion-exchange membranes were studied using breakthrough curves, measurement of binding capacity, and protein composition of the elution fraction to determine the binding behavior of each membrane. When the weak and strong anion exchange membranes were saturated with whey, they were both selective primarily for β-lactoglobulin with less than 1% of the eluate consisting of α-lactalbumin or BSA. The binding capacity of a pure β-lactoglobulin solution was in excess of 1.5 mg/cm² of membrane. This binding capacity was reduced to approximately 1.2 mg/cm² when using a rennet whey solution (pH 6.4). This reduction in protein binding capacity can be explained by both the competitive effects of other whey proteins and the effect of ions present in whey. Using binary solution breakthrough curves and rennet whey breakthrough curves, it was shown that α-lactalbumin and BSA were displaced from the strong and weak anion exchange membranes by β-lactoglobulin. Finally, the effect of ionic strength on the binding capacity of individual proteins for each membrane was determined by comparing model protein solutions in milk permeate (pH 6.4) and a 10 mM sodium phosphate buffer (pH 6.4). Binding capacities of β-lactoglobulin, α-lactalbumin, and BSA in milk permeate were reduced by as much as 50%. This reduction in capacity coupled with the low binding capacity of current ion exchange membranes are 2 serious considerations for selectively separating complex and concentrated protein solutions.     

Astringency of bovine milk whey protein

Whey protein solutions at pH 3.5 elicited an astringent taste sensation. The astringency of whey protein isolate (WPI), the process whey protein (PWP) that was prepared by heating WPI at pH 7.0, and the process whey protein prepared at pH 3.5 (aPWP) were adjusted to pH 3.5 and evaluated by 2 sensory analyses (the threshold method and the scalar scoring method) and an instrumental analysis (taste sensor method). The taste-stimulating effects of bovine and porcine gelatin were also evaluated. The threshold value of astringency of WPI, PWP, and aPWP was 1.5, 1.0, and 0.7 mg/mL, respectively, whereas the gelatins did not give definite astringency. It was confirmed by the scalar scoring method that the astringency of these proteins increased with the increase in protein concentration, and these proteins elicited strong astringency at 10 mg/mL under acidic conditions. On the other hand, the astringency was not elicited at pH 3.5 by 2 types of gelatin. A taste sensor gave specific values for whey proteins at pH 3.5, which corresponded well to those obtained by the sensory analysis. Elicitation of astringency induced by whey protein under acidic conditions would be caused by aggregation and precipitation of protein molecules in the mouth.     

乳清中棉籽蛋白的超滤回收研究

采用超滤技术回收蛋白质沉淀后的乳清中的棉籽蛋白,考察了操作压力、超滤时间、温度、pH、蛋白质浓度等因素对超滤膜通量的影响。结果表明,在等电点处膜通量最低,故超滤操作应在偏离蛋白质等电点的条件下进行;在实验范围内膜通量随操作压力的升高而升高,呈线性关系;随着超滤时间的延长,膜通量逐渐减小,超滤进行至20~30 min后,膜通量趋于稳定;随着温度的升高,膜通量增加。     

Microfiltration of whey proteins adsorbed on yeast cells

Soluble whey proteins (WPs), adsorbed on yeast cells, were recovered by a crossflow microfiltration (MF) technique using a cellulose nitrate membrane with a pore size of 0.45 μm. The crossflow velocity was 1.5 m s^?1 with a transmembrane pressure of 200 kPa at 25 °C. A series of protein rejections occured at various pH values ranging from 2 to 8. WPs adsorbed more on to yeast cells at low pH (pH < 4) than at high pH values, probably because they were positively charged at low pH. It was also shown that permeate flux increased and Modified Membrane Fouling Index values decreased at low pH levels. When the yeast concentration was 50 g L^?1, the flux decreased five times compared with that in the absence of yeast. Protein recovery increased with increasing yeast concentrations. The highest protein recovery was found to be 85% at a yeast concentration of 50 g L^?1 at a steady state flux rate of 10^?6 m s^?1 at 25 °C. When diluted solutions of whey were used, the same rejection of protein, adsorbed on yeast cells, was achieved at ten times lower amounts of yeast cells. This technique not only provides for the recovery of protein but also may give rise to the direct use of yeast cells, which are rich in protein, in the baking industry. WPs absorbed by yeast cells can be used to produce nutritionally rich products in areas where yeasts have been already used.     

Hydrolysis of whey protein isolate using subcritical water

Hydrolyzed whey protein isolate (WPI) is used in the food industry for protein enrichment and modification of functional properties. The purpose of the study was to determine the feasibility of subcritical water hydrolysis (SWH) on WPI and to determine the temperature and reaction time effects on the degree of hydrolysis (DH) and the production of peptides and free amino acids (AAs). Effects of temperature (150 to 320 °C) and time (0 to 20 min) were initially studied with a central composite rotatable design followed by a completely randomized factorial design with temperature (250 and 300 °C) and time (0 to 50 min) as factors. SWH was conducted in an electrically heated, 100-mL batch, high pressure vessel. The DH was determined by a spectrophotometric method after derivatization. The peptide molecular weights (MWs) were analyzed by gel electrophoresis and mass spectrometry, and AAs were quantified by high-performance liquid chromotography. An interaction of temperature and time significantly affected the DH and AA concentration. As the DH increased, the accumulation of lower MW peptides also increased following SWH (and above 10% DH, the majority of peptides were <1000 Da). Hydrolysis at 300 °C for 40 min generated the highest total AA concentration, especially of lysine (8.894 mg/g WPI). Therefore, WPI was successfully hydrolyzed by subcritical water, and with adjustment of treatment parameters there is reasonable control of the end-products.