Studies of plasmin activity in whey

The presence of the indigenous milk alkaline proteinase, plasmin, in whey products may adversely affect the quality of food products that utilise such ingredients; factors promoting the release of plasmin into whey were therefore investigated. Acidification of pasteurised skim milk (PSM) with glucono-δ-lactone (GDL) or HCl resulted in acid whey with significantly higher (P<0.05) plasmin activity than PSM. Plasmin activity in such whey was inversely correlated with rate of acidification; thus, activity in the whey made with GDL was significantly higher (P<0.05) than in whey made with HCl. Negligible levels of plasminogen in acid whey suggested that activation of the zymogen, plasminogen, contributed to the elevated plasmin activity observed. Sweet whey, manufactured by addition of rennet to PSM, had plasmin and plasminogen-derived activities significantly lower (P<0.001) than those in PSM; the release of plasmin from renneted casein micelles into whey increased in a linear manner with cooking temperature in the range 45–65°C. However, dissociation of plasmin from casein micelles into acid whey reached a maximum at a cooking temperature of 50°C. Analysis of plasmin and plasminogen levels in ultracentrifugal supernatants of pH-adjusted milk indicated that plasmin was released from the micelles at pH>5.0, while plasminogen was released more gradually over the pH range 6.6–4.6. Overall, the technology of manufacture has significant implications for plasmin activity in whey and hence, inversely, for casein products and cheese.     

Effect of psychrotrophic bacteria and of an isolated protease from Pseudomonas fluorescens M3/6 on the plasmin system of fresh milk

In the present study, we investigated the effect of Pseudomonas spp. growth on the plasmin enzymatic system in casein and whey fractions of fresh milk. Two bacterial strains, Pseudomonas spp. SRM28A and Pseudomonas fluorescens M3/6, were inoculated at a level of approximately 10(3) cfu/ml into fresh milk and incubated at 7 degrees C for 3 d. Bacterial counts were approximately 10(8) cfu/ml by d 3. Samples collected every 24 h were treated to separate the casein from the whey fraction. Casein and whey fractions were subjected to electrophoresis to visualize protein breakdown and plasmin activity and to colorimetric assays to quantify plasmin-related activities. With psychrotrophic bacterial growth, plasmin levels in casein fractions decreased significantly and in whey fractions increased then decreased significantly. Fresh milk results were similar for the two strains and were similar to earlier results with reconstituted nonfat dry milk. A transmission electron microscopy study by immunocytochemistry showed the presence of plasmin in casein micelles and its disappearance upon microbial growth in the milk. We hypothesized that extracellular microbial proteases produced by psychrotrophic microorganisms are responsible for this effect. To confirm this, an extracellular bacterial protease was isolated from Pseudomonas fluorescens M3/6 by ammonium sulfate fractionation and ion-exchange chromatography and incubated with fresh milk. Milk samples analyzed during incubation with the protease had significantly increased plasmin and plasminogen activities in the whey fraction within 5 h of incubation, while differences in activities in the casein fraction occurred at time 7.5 h for plasmin activity and 10 h of incubation for plasminogen activity. These quantitative data were supported by plasmin activity as visualized by casein-SDS-PAGE. These results suggest that growth of the Pseudomonas strains in fresh milk, and particularly their production of extracellular proteases, may be a causative factor in the release of plasmin from the casein micelle. Such plasmin release could affect the quality of cheeses and other food products that utilize dairy ingredients.     

Invited review: Plasmin protease in milk: current knowledge and relevance to dairy industry

Plasmin is by far the predominant and most completely studied endogenous protease in bovine milk. Plasmin-induced proteolysis can have either beneficial or detrimental effects on the texture and flavor of dairy products, depending on the extent of hydrolysis and type of dairy product. In cheese, the breakdown of protein can help develop desirable flavors and texture during ripening, whereas in pasteurized milk and ultra-high-temperature milk, proteolysis causes undesirable gelation. Plasmin is part of a complex protease-protease inhibitor system in milk that consists of active and inactive forms of the enzyme, activators, and inhibitors. Considerable research has been done to isolate and characterize components of the plasmin system, determine how they interact, develop and compare quantitation methods, and determine how they are affected by cow characteristics, processing conditions, other milk components, storage conditions, and bacterial proteases. Considerable research has focused on enhancing or minimizing the activity of plasmin system components. The intent has been to control protease activity in casein and whey fractions, depending on the final food or ingredient application. Controlling the activity of plasmin has a great potential to improve dairy product quality and reduce their processing costs.     

Kinetic studies of the thermal inactivation of plasmin in acid or sweet whey

The thermal behaviour of the milk alkaline proteinase, plasmin, was studied in acid and sweet (rennet) whey; indigenous (bovine) plasmin was studied in the former system, but endogenous porcine plasmin was added in the latter, due to the very low levels of residual plasmin. The inactivation of plasmin in both systems followed first-order inactivation kinetics, which was consistent with previous observations of plasmin inactivation in milk and model milk systems. The thermal inactivation of plasmin in acid whey (D_{90 °C}=108±29 min, z=24.5±1.2 °C) was much slower than in the sweet whey system (D_{90 °C}=0.021±0.006 min, z=7.3±0.3 °C). Similarly, denaturation of β-lactoglobulin (β-lg) followed a first-order inactivation profile and this protein was also more heat stable in acid whey (D_{90 °C}=86±76 min, z=13.7±1.5 °C) than sweet whey (D_{90 °C}=0.81±0.29 min, z=9.1±0.5 °C). While it is possible that the increased heat stability of plasmin in acid whey is due to reduced sulphydryl/disulphide interchange reactions between plasmin and β-lg, it also appears that structural changes in the plasmin molecule were a significant contributory effect on the thermal stability of plasmin in this system. Increasing the pH of acid whey decreased the heat stability of plasmin. However, adjusting the pH of sweet whey had little effect on the heat stability of plasmin. Overall, severe heat treatments may be required to ensure inactivation of the enzyme in acid whey, but a balance is required between reducing the activity of plasmin and maintaining the functionality of whey proteins as food ingredients.     

Invited review: Microfiltration-derived casein and whey proteins from milk

Milk, a rich source of nutrients, can be fractionated into a wide range of components for use in foods and beverages. With advancements in filtration technologies, micellar caseins and milk-derived whey proteins are now produced from skim milk using microfiltration. Microfiltered ingredients offer unique functional and nutritional benefits that can be exploited in new product development. Microfiltration offers promise in cheesemaking, where microfiltered milk can be used for protein standardization to improve the yield and consistency of cheese and help with operation throughputs. Micellar casein concentrates and milk whey proteins could offer unique functional and flavor properties in various food applications. Consumer desires for safe, nutritious, and clean-label foods could be potential growth opportunities for these new ingredients. The application of micellar casein concentrates in protein standardization could offer a window of opportunity to US cheese makers by improving yields and throughputs in manufacturing plants.     

Compositional and structural properties of whey proteins of sweet,acid and salty whey concentrates and their respective spray dried powders

The influence of physiochemical characteristics of whey concentrates obtained by ultrafiltration of acid and salty whey streams on the surface composition, particle organisation, secondary structures and protein interactions of the respective spray dried whey powders was investigated. Their properties were compared with those of native and sweet whey. Acid whey concentrate demonstrated characteristically low surface charge, high surface hydrophobicity, high average particle size and high thiol activity compared with sweet and native whey concentrates. Salty whey concentrate was characterised by low surface hydrophobicity, high thiol activity and low average particle size. Surface characterisation of whey powders revealed protein-rich surfaces for all whey powders while those in salty whey were highly hydrophobic. Protein characteristics of native and sweet whey powders largely followed those of concentrates. In contrast, protein characteristics of the acid and salty whey powders largely changed from those of the concentrates.     

Effect of subclinical mastitis on milk plasminogen and plasmin compared with that on sodium, antitrypsin and N-acetyl-beta-D-glucosaminidase

The effect of subclinical mastitis on levels of plasminogen and plasmin in milk from cows in a high-yielding herd was investigated. Comparisons were made with levels of milk Na, antitrypsin and N-acetyl-beta-D-glucosaminidase (NAGase). In samples from mastitic quarters plasminogen activity, as measured after activation to plasmin, increased by only 21% and plasmin by 82%, while NAGase increased by 307%. Plasminogen was the only component that was normally distributed, all other components showed more or less skewed distributions. Plasmin and plasminogen were significantly related to the other components. However, plasminogen plateaued when the other components continued to increase. There was thus no further increase in plasminogen with the severity of inflammation as with the other components. Plasmin showed a similar although less pronounced tendency. Results of treatment of mastitic whey samples with acid suggested that the non-linear increase in plasmin activity was due to interaction with acid-labile proteinase inhibitors. Mastitis led to dissociation of plasminogen and plasmin from the casein micelles. The degree of activation of plasminogen was higher with casein-associated than with soluble plasminogen in both healthy and mastitic milks. Plasmin was very closely related to milk Na, which is a sensitive indicator of epithelial integrity. It is suggested that plasmin contributes to Na leakage into milk by degrading membrane proteins of the epithelial lining. Plasminogen and antitrypsin, which are both plasma proteins, were not identically affected by stage of lactation, indicating nonidentical modes of transport from plasma to milk.     

Composition and properties of whey protein concentrates from ultrafiltration

Chemical and nutrient composition and functional properties of whey protein concentrates from ultrafiltration of sweet and acid wheys were studied for potential food uses. Vitamins passed readily through the membrane; thus, vitamin content was slightly higher than in whey. Amino acid values were considerably higher, increasing in direct proportion to increases in protein. Lysine availability was not significantly affected by fractionation or by subsequent beat treatment. Since this process results in substantial removal of minerals along with the permeate, the protein to ash ratio of the protein concentrate increased. Unlike most other methods of recovering protein from whey, solubility was not adversely affected by ultrafiltration. However, protein concentrates were susceptible to heat; normal pasteurization temperatures resulted in approximately 20% denaturation. Whey protein exhibited excellent water retention. Addition of 1.5% protein to skim milk followed by heating formed a custard-like gel with sufficient body to stand alone without leakage. Approximately twice as much egg albumin was required to achieve comparable results. Whipping properties were very good when butterfat content was less than 2%. Excellent stable whips could be produced by a combination of heat and pH adjustment.     

Effects of somatic cell count and stage of lactation on the plasmin activity and cheese-making properties of ewe milk

The experiment was conducted from March to July 2002 using 5 intensively managed flocks of Southern Italy. In each flock, 2 groups of 50 ewes were created. The groups were designated LSCC (low somatic cell count [SCC]) when their milk SCC was lower than 500,000/mL and HSCC (high SCC) when their milk SCC was higher than 1,000,000/mL. Bulk milk and whey samples were analyzed for fat, total protein, lactose, casein, and whey protein contents. Renneting properties of milk were also determined. Moisture, NaCl, and nitrogen fractions were determined in fresh cheese curds. In addition, plasmin (PL) and plasminogen (PG) activities in milk and cheese were monitored. The proteolytic activity of plasmin by urea-polyacrylamide gel electrophoresis and the white blood cell (WBC) differentials were determined. The HSCC resulted in higher pH values in milk and in higher moisture and lower fat contents in fresh cheese curds. Moreover, a lower recovery of fat and whey proteins was obtained from the HSCC than from the LSCC raw milk. The crude protein and casein contents were higher in the HSCC than in the LSCC curds during early and midlactation; an opposite trend was observed in late lactation. Plasmin and PG activities underwent more marked fluctuations in the LSCC than in the HSCC curds through lactation. The results of this experiment demonstrate that the PL activity in ewe milk is markedly influenced by the SCC, although SCC is not the only parameter for predicting PL and PG evolution in ewe milk. The LSCC milk resulted in a higher proteolytic potential of Canestrato pugliese cheese curds.     

Composition and nutritional value of commercial dried whey products from feta cheese manufacture

Various commercial dried whey products—WHEYPRO20, WHEYPRO35 and WHEYPRO65 (with approximately 20, 35 and 65% protein, respectively) and LACTINA (a permeate powder)—were studied in terms of chemical composition and nutritional value. These products were produced industrially from the whey of feta cheese manufactured with ovine and caprine milk by ultrafiltration and subsequent evaporation and spray-drying. As the protein content in these products increased, the nonprotein nitrogen and fat contents increased while the lactose and ash contents decreased. Generally the concentration of minerals (Ca, P, Na, K, Cl) in these products decreased with increasing protein content. With the exception of valine and methionine + cysteine, all essential amino acids were in excess in the whey protein concentrates (WHEYPRO35 and WHEYPRO65) as compared with the Food and Agriculture Organization reference protein and with human milk protein.     

Effect of genetic potential and level of feeding on milk protein composition

Two groups of 15 multiparous cows in mid-lactation were used in a Latin square design experiment with 4-week experimental periods. The genetic milk protein concentration level was high in the first group and low in the second. Each group of cows was given in a random order three feeding levels that covered 85, 100 and 115% of energy requirements and 90, 110 and 125% of nitrogen requirements, respeetively. In both groups, increasing level of feeding induced a significant increase in milk yield (+ 2.4 kg/d between lowest and highest levels) and in protein concentration (+ 1.7 g/kg). The proportion of paracasein in total proteins was not altered by either genetics or nutrition. The proportion of casein in total proteins was slightly increased by 0.5 percentage points (P < 0.05) with the intermediate level of feeding. Plasmin and plasminogen activities were not significantly modified by the genetic milk concentration level. Plasmin activity significantly increased with nutrient supplementation, but only in animals of low genetic potential (+ 21% between low and high levels, P < 0.01). Casein composition was not significantly altered by the genetics or level of nutrition. Over the whole range of individual measurements taken (n = 90), the relationships between casein or paracasein and total protein concentrations were linear and very narrow (R2 = 0.92 and 0.95, respectively). The proportion of casein or paracasein in total proteins significantly decreased as plasmin activity increased.     

Milk plasmin, N-acetyl-beta-D-glucosaminidase, and antitrypsin as determinants of bacterial replication rates in whey

Quarter milk samples were collected monthly on a selected herd of 80 Ayrshire cows having a high frequency of subclinical mastitis. Analysis of bacterial growth rates in milk showed that whey prepared from infected or inflamed quarters stimulated bacterial growth. Milk N-acetyl-beta-D-glucosaminidase, antitrypsin, and plasmin activities all showed positive correlations with bacterial replication rates (Staphylococcus aureus and Escherichia coli) in respective whey samples as determined by a turbidometric micro-technique. Increased bacterial replication rates in mastitic whey represent an increased yield of the key nutrients for bacteria. Bacterial growth enhancement can be partly explained by proteose-peptone originating from plasmin activation and casein degradation. However, as multiple regression analysis showed that a combination of the predictor variables: antitrypsin, N-acetyl-beta-D-glucosaminidase and plasmin explained enhancement of bacterial growth better than plasmin alone, other factors connected with inflammation should be sought when searching for growth-enhancing factors in whey. Milk plasmin activities showed increasing activities toward end of lactation (before drying off) as well as during later lactation (age of cow in years minus 2). Bacterial replication was enhanced in parallel with these changes in plasmin activities.     

Influence of skimmed milk concentrate replacement by dry dairy products in a low fat set‐type yoghurt model system. I: Use of whey protein concentrates,milk protein concentrates and skimmed milk powder

Ten commercial samples of dry dairy products used for protein fortification in a low fat yoghurt model system at industrial scale were studied. The products employed were whey protein concentratres, milk protein concentrates, skimmed milk concentrates and skimmed milk powder which originated from different countries. The gross chemical composition of these dried products were determined, including polyacrylamide gel electrophoresis (SDS‐PAGE) and isoelectric focusing of the proteins, and minerals such as Na, Ca, K and Mg. Yoghurts were formulated using a skim milk concentrated as a milk base enriched with different dry dairy products up to a 43 g kg⁻¹ protein content. Replacement percentage of skim milk concentrated by dry dairy products in the mix was between 1.49 and 3.77%. Yoghurts enriched with milk protein concentrates did not show significantly different viscosity (35.12 Pa s) and syneresis index (591.4 g kg⁻¹) than the two control yoghurts obtained only from skimmed milk concentrates (35.6 Pa s and 565.7 g kg⁻¹) and skimmed milk powder (32.77 Pa s and 551.5 g kg⁻¹), respectively. Yoghurt fortified with the whey protein concentrates, however, was less firm (22.59 Pa s) and had less syneresis index (216 g kg⁻¹) than control yoghurts. Therefore, whey protein concentrates may be useful for drinking yoghurt production. © 1999 Society of Chemical Industry     

Functional and technological aspects of whey powder and whey protein products

Commercial whey powder, whey protein concentrates and whey protein isolates (WPIs) were evaluated for certain functional properties and for their application in full‐fat and nonfat yoghurts. The functional properties of whey products varied, and the highest functionality was recorded in samples with high protein levels. Whey powder had the lowest foaming performance and emulsifying capacity, while WPIs possessed the best functional properties of all the other samples. Curd tension (CT), viscosity and syneresis were improved in yoghurts made using fortified cow's milk or reconstituted skim milk with any whey products, while whey powder had no impact on CT.     

Effect of partial whey protein depletion during membrane filtration on thermal stability of milk concentrates

Membrane filtration technologies are widespread unit operations in the dairy industry, often employed to obtain ingredients with tailored processing functionalities. The objective of this work was to better understand the effect of partial removal of whey proteins by microfiltration (MF) on the heat stability of the fresh concentrates. The micellar casein concentrates were compared with control concentrates obtained using ultrafiltration (UF). Pasteurized milk was microfiltered (80 kDa polysulfone membrane) or ultrafiltered (30 kDa cellulose membrane) without diafiltration (i.e., no addition of water) to 2× and 4× concentration, based on volume reduction. The final concentrates showed no differences in pH, casein micelle size, or mineral concentration in the serum phase. The micellar casein retentates (obtained by MF) showed a 20 and 40% decrease in whey protein concentration compared with the corresponding UF milk protein concentrates for 2× and 4× concentration, respectively. The heat coagulation time decreased with increasing protein concentration, regardless of the treatment; however, MF retentates showed a higher thermal stability than the corresponding UF controls. The average diameter for casein micelles increased after heating in UF but not MF concentrates. The turbidity (measured by light scattering) increased after heating, but to a higher extent for UF retentates than for MF retentates at the same protein concentration. It was concluded that the reduced amount of whey protein in the MF retentates caused a significant increase in the heat stability compared with the corresponding UF retentates. This difference was not due to ionic composition differences or pH, but to the type and amount of complexes formed in the serum phase.     

Use of acid whey protein concentrate as an ingredient in nonfat cup set-style yogurt

Acid whey resulting from the production of soft cheeses is a disposal problem for the dairy industry. Few uses have been found for acid whey because of its high ash content, low pH, and high organic acid content. The objective of this study was to explore the potential of recovery of whey protein from cottage cheese acid whey for use in yogurt. Cottage cheese acid whey and Cheddar cheese whey were produced from standard cottage cheese and Cheddar cheese-making procedures, respectively. The whey was separated and pasteurized by high temperature, short time pasteurization and stored at 4°C. Food-grade ammonium hydroxide was used to neutralize the acid whey to a pH of 6.4. The whey was heated to 50°C and concentrated using ultrafiltration and diafiltration with 11 polyethersulfone cartridge membrane filters (10,000-kDa cutoff) to 25% total solids and 80% protein. Skim milk was concentrated to 6% total protein. Nonfat, unflavored set-style yogurts (6.0 ± 0.1% protein, 15 ± 1.0% solids) were made from skim milk with added acid whey protein concentrate, skim milk with added sweet whey protein concentrate, or skim milk concentrate. Yogurt mixes were standardized to lactose and fat of 6.50% and 0.10%, respectively. Yogurt was fermented at 43°C to pH 4.6 and stored at 4°C. The experiment was replicated in triplicate. Titratable acidity, pH, whey separation, color, and gel strength were measured weekly in yogurts through 8 wk. Trained panel profiling was conducted on 0, 14, 28, and 56 d. Fat-free yogurts produced with added neutralized fresh liquid acid whey protein concentrate had flavor attributes similar those with added fresh liquid sweet whey protein but had lower gel strength attributes, which translated to differences in trained panel texture attributes and lower consumer liking scores for fat-free yogurt made with added acid whey protein ingredient. Difference in pH was the main contributor to texture differences, as higher pH in acid whey protein yogurts changed gel structure formation and water-holding capacity of the yogurt gel. In a second part of the study, the yogurt mix was reformulated to address texture differences. The reformulated yogurt mix at 2% milkfat and using a lower level of sweet and acid whey ingredient performed at parity with control yogurts in consumer sensory trials. Fresh liquid acid whey protein concentrates from cottage cheese manufacture can be used as a liquid protein ingredient source for manufacture of yogurt in the same factory.     

Effect of high-pressure treatment at various temperatures on indigenous proteolytic enzymes and whey protein denaturation in bovine milk

The objective of the present study was to determine the effect of high pressure (HP) processing (200, 450 and 650 MPa) at various temperatures (20, 40 and 55 degrees C) on the total plasmin plus plasminogen-derived activity (PL), plasminogen activator(s) (PA) and cathepsin D activities and on denaturation of major whey proteins in bovine milk. Data indicated that transfer of both PL and PA from the casein micelles to milk serum occurred at all pressures utilized at room temperature (20 degrees C). In addition to the transfer of PL and PA from micelles, there were reductions in activities of PL (16-18%) and PA (38-62%) for the pressures 450 and 650 MPa, at room temperature. There were synergistic negative effects between pressure and temperature on residual PL activity at 450 and 650 MPa and on residual PA activity only at 450 MPa. Cathepsin D activity in the acid whey from HP-treated milk was in general baroresistant at room temperature. The residual activity of cathepsin D decreased significantly at 650 MPa and 40 degrees C and at the pressures 450 and 650 MPa at 55 degrees C. Synergistic negative effects on the amount of native beta-lactoglobulin were observed at 450 and 650 MPa and on the amount of native alpha-lactalbumin at 650 MPa. There were significant correlations between enzymatic activities (PL, PA and cathepsin D) and the residual native beta-lactoglobulin and alpha-lactalbumin in bovine milk. In conclusion, HP significantly affected the activity of indigenous proteolytic enzymes and whey protein denaturation in bovine milk. Reduction in activity of indigenous enzymes (PL, PA and cathepsin D) and transfer of PL and PA from the casein to milk serum induced by HP is expected to have a profound effect on cheese yield, proteolysis during cheese ripening and quality of UHT milk during storage.     

Detection of adulteration of pasteurised milk with whey by determination of the casein-bound phosphorus and protein nitrogen content

A new method to detect the adulteration of pasteurised milk with whey is described. The method is based on the determination of casein-bound phosphorus (Pcas) and the protein nitrogen content in milk. By using a mean Pcas content (0.85 g P/100 g casein), a Kjeldahl factor of 6.34 and protein N, the relative casein-N content (casein N/protein N x 100) can be estimated. Over a period of one year the variation in the phosphorus factor of dried skimmed milk samples amounted to 2.9%. In order to calculate the percentage of added whey, a standard curve has been prepared in which the relative casein content is plotted against the corresponding whey percentage. The results of 26 analyses on genuine pasteurised milk (Pcas = 21.7 mg/100 ml; casein N/protein N = 80.9%) and on 25 whey samples were used for the standard curve. In laboratory-made blends of pasteurised milk and cheese whey (5%, 10%, 20% and 30%), the estimated percentage of whey varied between -1.2% and +2.9% from the actual value. The analysis of nine samples from the Brazilian market showed that four samples were clearly adulterated with cheese whey. Sweet and/or acid whey addition will be detected by the proposed method.     

Valuation of milk composition and genotype in cheddar cheese production using an optimization model of cheese and whey production

A mass balance optimization model was developed to determine the value of the κ-casein genotype and milk composition in Cheddar cheese and whey production. Inputs were milk, nonfat dry milk, cream, condensed skim milk, and starter and salt. The products produced were Cheddar cheese, fat-reduced whey, cream, whey cream, casein fines, demineralized whey, 34% dried whey protein, 80% dried whey protein, lactose powder, and cow feed. The costs and prices used were based on market data from March 2004 and affected the results. Inputs were separated into components consisting of whey protein, ash, casein, fat, water, and lactose and were then distributed to products through specific constraints and retention equations. A unique 2-step optimization procedure was developed to ensure that the final composition of fat-reduced whey was correct. The model was evaluated for milk compositions ranging from 1.62 to 3.59% casein, 0.41 to 1.14% whey protein, 1.89 to 5.97% fat, and 4.06 to 5.64% lactose. The κ casein genotype was represented by different retentions of milk components in Cheddar cheese and ranged from 0.715 to 0.7411 kg of casein in cheese/kg of casein in milk and from 0.7795 to 0.9210 kg of fat in cheese/kg of fat in milk. Milk composition had a greater effect on Cheddar cheese production and profit than did genotype. Cheese production was significantly different and ranged from 9,846 kg with a high-casein milk composition to 6,834 kg with a high-fat milk composition per 100,000 kg of milk. Profit (per 100,000 kg of milk) was significantly different, ranging from $70,586 for a high-fat milk composition to $16,490 for a low-fat milk composition. However, cheese production was not significantly different, and profit was significant only for the lowest profit ($40,602) with the κ-casein genotype. Results from this model analysis showed that the optimization model is useful for determining costs and prices for cheese plant inputs and products, and that it can be used to evaluate the economic value of milk components to optimize cheese plant profits.     

Plasmin activity, β-lactoglobulin denaturation and proteolysis in high pressure treated milk

The effects of high pressure (HP) on plasmin activity, β-lactoglobulin denaturation and proteolysis during subsequent storage of HP treated milk, were studied. Fresh raw milk samples were exposed to a range of pressures from 50 to 800 MPa, for times of 1, 10 or 30 min, at 20°C. Residual plasmin activity and whey protein denaturation were measured immediately post HP-treatment. Indices of proteolysis were measured during post-HP storage. Treatment at pressures >300 MPa resulted in extensive β-lactoglobulin denaturation. Plasmin activity decreased in milk treated at pressures 400 MPa; the loss of activity was not well correlated with β-lactoglobulin denaturation. Compared to raw milk, treatment at 50 MPa had little effect on proteolysis during storage of treated milk measured as increases in pH 4.6-soluble N and liberation of proteose peptones, but at pressures of 300–400 MPa, proteolysis was increased relative to raw milk. After pressurisation >500 MPa, proteolysis during storage of milk was less than that observed in raw milk. Overall, HP influenced proteolysis in milk in a way which is different from that produced by heat, in terms of subsequent susceptibility of casein to proteolysis during storage or incubation. In particular, HP treatment at pressures of 300–500 MPa can increase proteolysis in milk, possibly through changes in micelle structure facilitating increased availability of substrate bonds to plasmin, which has implications for products prepared from milk thus treated.