Structural changes induced by high-pressure processing in micellar casein and milk protein concentrates

Reconstituted micellar casein concentrates and milk protein concentrates of 2.5 and 10% (wt/vol) protein concentration were subjected to high-pressure processing at pressures from 150 to 450 MPa, for 15 min, at ambient temperature. The structural changes induced in milk proteins by high-pressure processing were investigated using a range of physical, physicochemical, and chemical methods, including dynamic light scattering, rheology, mid-infrared spectroscopy, scanning electron microscopy, proteomics, and soluble mineral analyses. The experimental data clearly indicate pressure-induced changes of casein micelles, as well as denaturation of serum proteins. Calcium-binding α_S1- and α_S2-casein levels increased in the soluble phase after all pressure treatments. Pressurization up to 350 MPa also increased levels of soluble calcium and phosphorus, in all samples and concentrations, whereas treatment at 450 MPa reduced the levels of soluble Ca and P. Experimental data suggest dissociation of calcium phosphate and subsequent casein micelle destabilization as a result of pressure treatment. Treatment of 10% micellar casein concentrate and 10% milk protein concentrate samples at 450 MPa resulted in weak, physical gels, which featured aggregates of uniformly distributed, casein substructures of 15 to 20 nm in diameter. Serum proteins were significantly denatured by pressures above 250 MPa. These results provide information on pressure-induced changes in high-concentration protein systems, and may inform the development on new milk protein-based foods with novel textures and potentially high nutritional quality, of particular interest being the soft gel structures formed at high pressure levels.     

Effect of pasteurization and fat,protein, casein to serum protein ratio,and milk temperature on milk beverage color and viscosity

Our goal was to determine the effect of pasteurization-homogenization, fat and protein concentration, proportion of milk protein that is casein and serum protein, and temperature on sensory and instrumental measures of viscosity and color of milk-based beverages. A second goal was to use instrumental measures of whiteness and yellowness to predict sensory measures of whiteness and yellowness. A complete balanced 3 factor (fat, true protein, and casein as a percentage of true protein) design was applied with 3 levels of fat (0.2, 1.0 and 2.0%), 4 levels of true protein (3.00, 3.67, 4.34, and 5.00%) within each fat level, and 5 levels of casein as a percentage of true protein (CN%TP; 5, 25, 50, 75, and 80%) within each protein level for beverage formulation. Instrumental color and viscosity, and visual sensory color analyses were done on each beverage formulation. For unpasteurized beverages across 3 fat levels (0.2, 1, and 2%), changes in CN%TP had the largest effect on L values, sensory whiteness, opacity, color intensity, and yellowness, whereas changes in fat concentration had a stronger influence on a and b* values. Increasing CN%TP from 5 to 80% increased L values, sensory whiteness, and opacity, and decreased sensory color intensity and yellowness. The a and b* values increased with increasing fat concentration. For unpasteurized milk protein beverages within each fat level, variation in CN%TP dominated the changes in L values, sensory whiteness, and opacity, and decreased a and b* values, sensory color intensity, and yellowness. The effect of heat (pasteurization and homogenization) and its interaction terms had the second largest effect on color of milk protein beverages with respect to instrumental color data and sensory appearance attributes. Heat increased L values, sensory whiteness, and opacity, and decreased a and b* values, sensory color intensity, and yellowness. Increases in temperature decreased instrumental viscosity and changes in protein concentration and CN%TP had a greater effect on instrument viscosity data within each temperature (4, 20, and 50°C) than fat. Sensory perception of yellowness was not highly correlated with b* values. Multiple linear regressions of L, a, and b* values produced more robust predictions for both sensory whiteness and yellowness than simple linear regression with L and b* values alone, and may be a useful instrumental approach for quality control of sensory whiteness and yellowness of milk protein beverages.     

Ready-to-drink protein beverages: Effects of milk protein concentration and type on flavor

This study evaluated the role of protein concentration and milk protein ingredient [serum protein isolate (SPI), micellar casein concentrate (MCC), or milk protein concentrate (MPC)] on sensory properties of vanilla ready-to-drink (RTD) protein beverages. The RTD beverages were manufactured from 5 different liquid milk protein blends: 100% MCC, 100% MPC, 18:82 SPI:MCC, 50:50 SPI:MCC, and 50:50 SPI:MPC, at 2 different protein concentrations: 6.3% and 10.5% (wt/wt) protein (15 or 25 g of protein per 237 mL) with 0.5% (wt/wt) fat and 0.7% (wt/wt) lactose. Dipotassium phosphate, carrageenan, cellulose gum, sucralose, and vanilla flavor were included. Blended beverages were preheated to 60°C, homogenized (20.7 MPa), and cooled to 8°C. The beverages were then preheated to 90°C and ultrapasteurized (141°C, 3 s) by direct steam injection followed by vacuum cooling to 86°C and homogenized again (17.2 MPa first stage, 3.5 MPa second stage). Beverages were cooled to 8°C, filled into sanitized bottles, and stored at 4°C. Initial testing of RTD beverages included proximate analyses and aerobic plate count and coliform count. Volatile sulfur compounds and sensory properties were evaluated through 8-wk storage at 4°C. Astringency and sensory viscosity were higher and vanillin flavor was lower in beverages containing 10.5% protein compared with 6.3% protein, and sulfur/eggy flavor, astringency, and viscosity were higher, and sweet aromatic/vanillin flavor was lower in beverages with higher serum protein as a percentage of true protein within each protein content. Volatile compound analysis of headspace vanillin and sulfur compounds was consistent with sensory results: beverages with 50% serum protein as a percentage of true protein and 10.5% protein had the highest concentrations of sulfur volatiles and lower vanillin compared with other beverages. Sulfur volatiles and vanillin, as well as sulfur/eggy and sweet aromatic/vanillin flavors, decreased in all beverages with storage time. These results will enable manufacturers to select or optimize protein blends to better formulate RTD beverages to provide consumers with a protein beverage with high protein content and desired flavor and functional properties.     

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.     

Rheological properties of rennet gels containing milk protein concentrates

Different milk protein concentrates (MPC), with protein concentrations of 56, 70, and 90%, were dispersed in water under different treatments (hydration, shear, heat, and overnight storage at 4°C), as well as in a combination of all the treatments in a factorial design. The particle size distribution of the dispersions was then measured to determine the optimal conditions for the dispersion. Heating at 60°C for 30 min with 5 min of shear was chosen as the best condition to dissolve MPC powders. The samples were also characterized for composition, presence of protein aggregates, and ratio of calcium to protein. The total calcium present in MPC increased with increasing concentration of protein; however, the total calcium-to-protein ratio was lower in MPC90 than in MPC56 and MPC70. The level of whey protein denaturation, the presence of κ-casein-whey protein aggregates in the supernatant after centrifugation, and the amount of caseins dissociated from the micelle increased as the protein concentration in the powder increased. The total amount of casein macropeptide released was lower in samples from powders with a higher protein concentration than for MPC56 or the skim milk control. The gelation behavior of reconstituted MPC was tested in systems dispersed in water (5% protein) as well as in systems dispersed in skim milk (6% protein). The gelation time of MPC dispersions was considerably lower and the gel modulus was higher than those of reconstituted skim milk with the same protein concentration. When MPC dispersions were dialyzed against skim milk, a significant decrease in the gelation time and modulus were shown, with a complete loss of gelling functionality in MPC90 dispersed in water. This demonstrated that the ionic equilibrium was key to the functionality of MPC.     

Effects of milk heat treatment and solvent composition on physicochemical and selected functional characteristics of milk protein concentrate

Milk protein concentrate (MPC) powders (～81% protein) were made from skim milk that was heat treated at 72°C for 15 s (LHMPC) or 85°C for 30 s (MHMPC). The MPC powder was manufactured by ultrafiltration and diafiltration of skim milk at 50°C followed by spray drying. The MPC dispersions (4.02% true protein) were prepared by reconstituting the LHMPC and MHMPC powders in distilled water (LHMPC_w and MHMPC_w, respectively) or milk permeate (LHMPC_p and MHMPC_p, respectively). Increasing milk heat treatment increased the level of whey protein denaturation (from ～5 to 47% of total whey protein) and reduced the concentrations of serum protein, serum calcium, and ionic calcium. These changes were paralleled by impaired rennet-induced coagulability of the MHMPC_w and MHMPC_p dispersions and a reduction in the pH of maximum heat stability of MHMPC_p from pH 6.9 to 6.8. For both the LHMPC and MHMPC dispersions, the use of permeate instead of water enhanced ethanol stability at pH 6.6 to 7.0, impaired rennet gelation, and changed the heat coagulation time and pH profile from type A to type B. Increasing the severity of milk heat treatment during MPC manufacture and the use of permeate instead of water led to significant reductions in the viscosity of stirred yogurt prepared by starter-induced acidification of the MPC dispersions. The current study clearly highlights how the functionality of protein dispersions prepared by reconstitution of high-protein MPC powders may be modulated by the heat treatment of the skim milk during manufacture of the MPC and the composition of the solvent used for reconstitution.     

Solubility of commercial milk protein concentrates and milk protein isolates

High-protein milk protein concentrate (MPC) and milk protein isolate (MPI) powders may have lower solubility than low-protein MPC powders, but information is limited on MPC solubility. Our objectives in this study were to (1) characterize the solubility of commercially available powder types with differing protein contents such as MPC40, MPC80, and MPI obtained from various manufacturers (sources), and (2) determine if such differences could be associated with differences in mineral, protein composition, and conformational changes of the powders. To examine possible predictors of solubility as measured by percent suspension stability (%SS), mineral analysis, Fourier transform infrared (FTIR) spectroscopy, and quantitative protein analysis by HPLC was performed. After accounting for overall differences between powder types, %SS was found to be strongly associated with the calcium, magnesium, phosphorus, and sodium content of the powders. The FTIR score plots were in agreement with %SS results. A principal component analysis of FTIR spectra clustered the highly soluble MPC40 separately from the rest of samples. Furthermore, 2 highly soluble MPI samples were clustered separately from the rest of the MPC80 and MPI samples. We found that the 900 to 1,200 cm⁻¹ region exhibited the highest discriminating power, with dominant bands at 1,173 and 968 cm⁻¹, associated with phosphate vibrations. The 2 highly soluble MPI powders were observed to have lower κ-casein and α-_S1-casein contents and slightly higher whey protein contents than the other powders. The differences in the solubility of MPC and MPI were associated with a difference in mineral composition, which may be attributed to differences in processing conditions. Additional studies on the role of minerals composition on MPC80 solubility are warranted. Such a study would provide a greater understanding of factors associated with differences in solubility and can provide insight on methods to improve solubility of high-protein milk protein concentrates.     

Processing factors that influence casein and serum protein separation by microfiltration

Our objective was to demonstrate the effect of various processing factors on the performance of a microfiltration system designed to process skim milk and separate casein (CN) from serum proteins (SP). A mathematical model of a skim milk microfiltration process was developed with 3 stages plus an additional fourth finishing stage to standardize the retentate to 9% true protein (TP) and allow calculation of yield of a liquid 9% TP micellar CN concentrate (MCC) and milk SP isolate (MSPI; 90% SP on a dry basis). The model was used to predict the effect of 5 factors: 1) skim milk composition, 2) heat treatment of skim milk, 3) concentration factor (CF) and diafiltration factor (DF), 4) control of CF and DF, and 5) SP rejection by the membrane on retentate and permeate composition, SP removal, and MCC and MSPI yield. When skim milk TP concentration increased from 3.2 to 3.8%, the TP concentration in the third stage retentate increased from 7.92 to 9.40%, the yield of MCC from 1,000 kg of skim milk increased from 293 to 348 kg, and the yield of MSPI increased from 6.24 to 7.38 kg. Increased heat treatment (72.9 to 85.2°C) of skim milk caused the apparent CN as a percentage of TP content of skim milk as measured by Kjeldahl analysis to increase from 81.97 to 85.94% and the yield of MSPI decreased from 6.24 to 4.86 kg, whereas the third stage cumulative percentage SP removal decreased from 96.96 to 70.08%. A CF and DF of 2× gave a third stage retentate TP concentration of 5.38% compared with 13.13% for a CF and DF of 5×, with the third stage cumulative SP removal increasing from 88.66 to 99.47%. Variation in control of the balance between CF and DF (instead of an equal CF and DF) caused either a progressive increase or decrease in TP concentration in the retentate across stages depending on whether CF was greater than DF (increasing TP in retentate) or CF was less than DF (decreasing TP in retentate). An increased rejection of SP by the membrane from an SP removal factor of 1 to 0.6 caused a reduction in MSPI yield from 6.24 to 5.19 kg/1,000 kg of skim milk, and third stage cumulative SP removal decreased from 96.96 to 79.74%. Within the ranges of the 5 factors studied, the TP content of the third stage retentate was most strongly affected by the target CF and DF and variation in skim milk composition. Cumulative percentage SP removal was most strongly affected by the heat treatment of skim milk, the SP removal factor, and the target CF and DF. The MCC yield was most strongly affected by initial skim milk composition. Yield of MSPI was strongly affected by skim milk composition, whereas the heat treatment of milk and SP removal factor also had a large effect.     

Effects of milk protein variants on the protein composition of bovine milk

The effects of β-lactoglobulin (β-LG), β-casein (β-CN), and κ-CN variants and β-κ-CN haplotypes on the relative concentrations of the major milk proteins α-lactalbumin (α-LA), β-LG, α_S1-CN, α_S2-CN, β-CN, and κ-CN and milk production traits were estimated in the milk of 1,912 Dutch Holstein-Friesian cows. We show that in the Dutch Holstein-Friesian population, the allele frequencies have changed in the past 16 years. In addition, genetic variants and casein haplotypes have a major impact on the protein composition of milk and explain a considerable part of the genetic variation in milk protein composition. The β-LG genotype was associated with the relative concentrations of β-LG (A » B) and of α-LA, α_S1-CN, α_S2-CN, β-CN, and κ-CN (B > A) but not with any milk production trait. The β-CN genotype was associated with the relative concentrations of β-CN and α_S2-CN (A² > A¹) and of α_S1-CN and κ-CN (A¹ > A²) and with protein yield (A² > A¹). The κ-CN genotype was associated with the relative concentrations of κ-CN (B > E > A), α_S2-CN (B > A), α-LA, and α_S1-CN (A > B) and with protein percentage (B > A). Comparing the effects of casein haplotypes with the effects of single casein variants can provide better insight into what really underlies the effect of a variant on protein composition. We conclude that selection for both the β-LG genotype B and the β-κ-CN haplotype A²B will result in cows that produce milk that is more suitable for cheese production.     

Effect of milk protein standardization using different methods on the composition and yields of Cheddar cheese

Two sets of Cheddar cheese were made in which the milk protein level (%, wt/wt) was increased from 3.3 (Control A, CA) to 3.6 (set A) or from 3.3 (control B, CB) to 4.0 (set B) by the addition of phosphocasein (PC), milk protein concentrate (MPC), or freshly prepared ultrafiltered milk retentate (UFR). The cheeses were denoted CA, PCA, MPCA, and UFRA from set A, and CB, PCB, MPCB, and UFRB, from set B, respectively. The level of cheese moisture decreased significantly on increasing milk protein level from 3.3 to 3.6 or 4.0% (wt/wt), but was not affected significantly by the method of protein standardization. The percentage milk fat recovered to cheese increased significantly on increasing the level of milk protein from 3.3 to 3.6% (wt/wt) with PC, and from 3.3 to 4.0% (wt/wt) with PC, MPC, and UFR. Increasing milk protein level from 3.3 to 4.0% (wt/wt) with PC significantly increased the percentage of milk protein recovered to cheese. Actual cheese yield increased significantly with milk protein level. The yield of cheese per 100 kg of milk normalized to reference levels of fat (3.4%, wt/wt) and casein (2.53%, wt/wt) indicated no significant effects of protein content or standardization treatment on yield. However, the moisture-adjusted yield per 100 kg of milk with reference levels of fat and casein increased significantly on increasing the protein content from 3.3 to 3.6% (wt/wt) with MPC and from 3.3 to 4.0% (wt/wt) with PC, MPC, and UFR.     

Use of micellar casein concentrate and milk protein concentrate treated with transglutaminase in imitation cheese products—Unmelted texture

The amount of intact casein provided by dairy ingredients is a critical parameter in dairy-based imitation mozzarella cheese (IMC) formulation because it has a significant effect on unmelted textural parameters such as hardness. From a functionality perspective, rennet casein (RCN) is the preferred ingredient. Milk protein concentrate (MPC) and micellar casein concentrate (MCC) cannot provide the required functionality due to the higher steric stability of casein micelle. However, the use of transglutaminase (TGase) has the potential to modify the surface properties of MPC and MCC and may improve their functionality in IMC. The objective of this study was to determine the effect of TGase-treated MPC and MCC powders on the unmelted textural properties of IMC and compare them with IMC made using commercially available RCN. Additionally, we studied the degree of crosslinking by TGase in MPC and MCC retentates using capillary gel electrophoresis. Three lots of MCC and MPC retentate were produced from pasteurized skim milk via microfiltration and ultrafiltration, respectively, and randomly assigned to 1 of 3 treatments: no TGase (control); low TGase: 0.3 units/g of protein; and high TGase: 3.0 units/g of protein, followed by inactivation of enzyme (72°C for 10 min), and spray drying. Each MCC, MPC, and RCN was then used to formulate IMC that was standardized to 21% fat, 1% salt, 48% moisture, and 20% protein. The IMC were manufactured by blending, mixing, and heating ingredients (4.0 kg) in a twin-screw cooker. The capillary gel electrophoresis analysis showed extensive inter- and intramolecular crosslinking. The IMC formulation using the highest TGase level in MCC or MPC did not form an emulsion because of extensive crosslinking. In MPC with a high level of TGase, whey protein and casein crosslinking were observed. In contrast, crosslinking and hydrolysis of proteins were observed in MCC. The IMC made from MCC powder had significantly higher texture profile analysis hardness compared with the corresponding MPC powder. Further, many-to-one (multiple) comparisons using the Dunnett test showed no significant differences between IMC made using RCN and treatment powders in hardness. Our results demonstrated that TGase treatment causes crosslinking hydrolysis of MCC and MPC at higher TGase levels, and MPC and MCC have the potential to be used as ingredients in IMC applications.     

Effect of processing methods and protein content of the concentrate on the properties of milk protein concentrate with 80% protein

In recent years, a large increase in the production of milk protein concentrates (MPC) has occurred. However, compared with other types of milk powders, few studies exist on the effect of key processing parameters on powder properties. In particular, it is important to understand if key processing parameters contribute to the poor solubility observed during storage of high-protein MPC powders. Ultrafiltration (UF) and diafiltration (DF) are processing steps needed to reduce the lactose content of concentrates in the preparation of MPC with a protein content of 80% (MPC80). Evaporation is sometimes used to increase the TS content of concentrates before spray drying, and some indications exist that inclusion of this processing step may affect protein properties. In this study, MPC80 powders were manufactured by 2 types of concentration methods: membrane filtration with and without the inclusion of an evaporation step. Different concentration methods could affect the mineral content of MPC powders, as soluble salts can permeate the UF membrane, whereas no mineral loss occurs during evaporation, although a shift in calcium equilibrium toward insoluble forms may occur at high protein concentration levels. It is more desirable from an energy efficiency perspective to use higher total solids in concentrates before drying, but concerns exist about whether a higher protein content would negatively affect powder functionality. Thus, MPC80 powders were also manufactured from concentrates that had 3 different final protein concentrations (19, 21, and 23%; made from 1 UF retentate using batch recirculation evaporation, a similar concentration method). After manufacture, powders were stored for 6 mo at 30°C to help understand changes in MPC80 properties that might occur during shelf-life. Solubility and foaming properties were determined at various time points during high-temperature powder storage. Inclusion of an evaporation step, as a concentration method, resulted in MPC80 that had higher ash, total calcium, and bound calcium (of rehydrated powder) contents compared to concentration with only membrane filtration. Concentration method did not significantly affect the bulk (tapped) density, solubility, or foaming properties of the MPC powders. Powder produced from concentrate with 23% protein content exhibited a higher bulk density and powder particle size than powder produced from concentrate that had 19% protein. The solubility of MPC80 powder was not influenced by the protein content of the concentrate. The solubility of all powders significantly decreased during storage at 30°C. Higher protein concentrations in concentrates resulted in rehydrated powders that had higher viscosities (even when tested at a constant protein concentration). The protein content of the concentrate did not significantly affect foaming properties. Significant changes in the mineral content are used commercially to improve MPC80 solubility. However, although the concentration method did produce a small change in the total calcium content of experimental MPC80 samples, this modification was not sufficiently large enough (<7%) to influence powder solubility.     

Influence of milk protein concentrates with modified calcium content on enteral dairy beverage formulations: Physicochemical properties

Because of their high protein and low lactose content, milk protein concentrates (MPC) are typically used in the formulation of ready-to-drink beverages. Calcium-mediated aggregation of proteins during storage is one of the main reasons for loss of storage stability of these beverages. Control and calcium-reduced MPC [20% calcium-reduced (MPC-20) and 30% calcium-reduced (MPC-30)] were used to evaluate the physicochemical properties in this study. This study was conducted in 2 phases. In phase I, 8% protein solutions were prepared by reconstituting the 3 MPC and adjusting the pH to 7. These solutions were divided into 3 equal parts, 0, 0.15, or 0.25% sodium hexametaphosphate (SHMP) was added, and the solutions were homogenized. In phase II, enteral dairy beverage formulations containing MPC and a mixture of gums, maltodextrin, and sugar were evaluated following the same procedure used in phase I. In both phases, heat stability, apparent viscosity, and particle size were compared before and after heat treatment at 140°C for 15 s. In the absence of SHMP, MPC-20 and MPC-30 exhibited the highest heat coagulation time at 30.9 and 32.8 min, respectively, compared with the control (20.9 min). In phase II, without any addition of SHMP, MPC-20 exhibited the highest heat coagulation time of 9.3 min compared with 7.1 min for control and 6.2 min for MPC-30. An increase in apparent viscosity and a decrease in particle size of reconstituted MPC solutions in phases I and II with an increase in SHMP concentration was attributed to casein micelle dissociation caused by calcium chelation. This study highlights the potential for application of calcium-reduced MPC in dairy-based ready-to-drink and enteral nutrition beverage formulations to improve their heat stability.     

乳蛋白肽饮料微生物脱苦研究

以乳蛋白肽饮料为原料,通过苦味值、短肽得率（TCA-SNI）、总蛋白酶活三个指标筛选得到苦味值小、总蛋白酶活力高、TCA-SNI值高的复合菌种。结果表明,脱苦效果方面,复合菌种>酿酒酵母>乳酸菌,复合菌种接种量为1%、发酵培养7h,总蛋白酶活最高,达到32U/mL,TCA-SNI达到峰值34.5%,溶液苦味值为0.5。      

Protein,casein, and micellar salts in milk: Current content and historical perspectives

The protein and fat content of Dutch bulk milk has been monitored since the 1950s and has increased considerably, by 11 and 20%, respectively, whereas milk yield has more than doubled. The change in protein and fat content of milk is advantageous for the dairy industry, as these are the 2 most economically valuable constituents of milk. Increases in protein and fat content of milk have allowed increases in the yield of various products such as cheese and butter. However, for cheese and other applications where casein micelles play a crucial role in structure and stability, it is not only casein content, but also the properties of the casein micelles that determine processability. Of particular importance herein is the salt partition in milk, but it is unknown whether increased protein content has affected the milk salts and their distribution between casein micelles and milk serum. It was, therefore, the objective of this research to determine the salt composition and protein content for individual cow milk and bulk milk over a period of 1 yr and to compare these data to results obtained during the 1930s, 1950s, and 1960s in the last century. Calcium, magnesium, sodium, potassium, and phosphorus content were determined by inductively coupled plasma atomic emission spectrometry and inorganic phosphate, citrate, chloride, and sulfate content by anion-exchange chromatography in bulk milk and milk ultracentrifugate. In addition, ionic calcium and ionic magnesium concentration were determined by the Donnan membrane technique. We concluded that historical increase in milk yield and protein content in milk have resulted in correlated changes in casein content and the micellar salt fraction of milk. In addition, the essential nutrients, calcium, magnesium, and phosphorus in milk have increased the past 75 yr; therefore, the nutritional value of milk has improved.     

Effect of heat treatment and solution preparation procedure on colloidal stability of whey protein sour cherry beverage

In this study, a whey protein sour cherry beverage was prepared using whey protein concentrate, sour cherry concentrate, Angum gum, water and sugar as initial ingredients. Whey protein concentrate and gum solutions were prepared by four methods. Heat treatment of the solutions led to denaturation of proteins, a change in the solubility of proteins and sediment formation. Our results showed that denaturation of proteins made the peptide fragments of the proteins to bind the gum, thus preventing the separation of the serum.     

乳化盐强化对乳蛋白浓缩物加工性质的影响

研究乳化盐强化对乳蛋白浓缩物85（MPC85）基本成分、粒径、溶解性和表面疏水性（H0）以及蛋白分子量的影响。结果表明,柠檬酸钠（SCS）单独使用或者与焦磷酸钠（SPP）复配均能够显著改变MPC85的粒径和溶解性,且缩短了达到稳定粒径和溶解性所需要的时间（p<0.05）。其中,单独使用SCS可使得MPC85粒径由31.37μm降低至20.67μm,达到稳定粒径值的时间缩短至360 min。SCS与SPS按照不同比例使用时,随着SCS占比的增加,粒径值显著降低（p<0.05）,且溶解性由77.42%增加至81.43%,同时达到稳定溶解度的时间缩短;乳化盐可以改变蛋白构象,使得更多疏水基团暴露,从而提高H0;复配乳化盐会降低分子量>60 ku的蛋白含量,且SPP和磷酸三钠（SPS）使得MPC85形成小分子量蛋白,分子量介于κ-CN与β-LG之间。      

Short communication: Effect of pH on the heat stability of reconstituted reduced calcium milk protein concentrate dispersions

This study aimed to investigate the heat stability of dispersions from reconstituted reduced-calcium milk protein concentrate (RCMPC) with 80% protein or more. The tested RCMPC powders were produced from skim milk subjected to CO₂ treatment before and during the process of ultrafiltration. The CO₂ injection was controlled to obtain 0 (control, no CO₂ injection), 20, 30, and 40% reduction in calcium levels in the RCMPC powders. The RCMPC powders were reconstituted to 10% (wt/wt) protein in deionized water. These dispersions were tested for heat stability in a rocking oil bath at 140°C at unadjusted, 6.5, 6.7, 6.9, and 7.1 pH. Calcium ion activity (CIA) and ionic strength measurements were carried out using a Ca ion-selective electrode and conductivity meter. Unadjusted pH of the dispersions varied from 6.8 in control to 5.96 in 40% RCMPC dispersions. The CIA of unadjusted dispersions ranged from 1.31 mM in control to 2.83 mM in 40% RCMPC. Heat stability, expressed as heat coagulation time (HCT) of unadjusted dispersions decreased as the level of Ca removal in powders increased (from 13.81 min in control to 0.46 min in 40% RCMPC) and was negatively correlated with the CIA of the dispersions. For control RCMPC dispersions, the minimum and maximum heat stability were observed at dispersion pH of 6.5 and 6.9, respectively, followed by a decrease at pH 7.1 (CIA was the lowest). Dispersions from 40% RCMPC and pH 7.1 had the maximum HCT of 30.94 min among all RCMPC dispersions at all pH values. From this study, it can be concluded that improved heat stability in high protein formulation beverages subjected to UHT processing could be achieved through calcium reduction in milk protein concentrates using CO₂ injection.     

High-pressure structuring of milk protein concentrate: Effect of pH and calcium

In this study, we investigated the effect of pH and calcium on the structural properties of gels created by high-pressure processing (HPP, 600 MPa, 5°C, 3 min) of milk protein concentrate (MPC, 12.5% protein). The pH level of the MPC was varied between 6.6 and 5.1 by adding glucono-δ-lactone (GDL), and the calcium content was varied from 24 to 36 mg of Ca/g of protein by adding calcium chloride. The rheological properties and microstructure of the pressure-treated MPC were assessed. The pressurization treatments and analytical testing were conducted in triplicate. Data were analyzed statistically using one-way ANOVA with Tukey's honestly significant difference post hoc tests. A pressurization time of 3 min was sufficient to induce gel formation in MPC at pH 6.6, so it was used throughout the study. Adjusting either pH or calcium affected the structure of the HPP-created milk protein gels, likely by influencing electrostatic interactions and shifting the calcium–phosphate balance. Gels were formed after pressurization of MPC at pH above 5.3, and increasing the pH from 5.3 to 6.6 resulted in stronger gels with higher values of elastic moduli (G′). At neutral pH (6.6), adding calcium to MPC further increased G′. Scanning electron microscopy showed that reducing pH or adding calcium resulted in more porous, aggregated microstructures. These findings demonstrate the potential of HPP to create a variety of structures using MPC, facilitating a new pathway from dairy protein ingredients to novel, gel-based, high-protein foods, such as puddings or on-the-go protein bars.