Heat-induced association of beta-lactoglobulin and casein micelles

Reduction in the antigenicity of beta-lactoglobulin in whole milk powder via supercritical CO2 treatment

Cow milk allergy is a common phenomenon experienced in early childhood (<5 yr of age) with an average occurrence rate of roughly 2.5%. The most prevalent allergen in cow milk is believed to be β-LG. The objective of this study was to evaluate the use of hydrophobic supercritical CO2 (ScCO2) to modify the chemical structure β-LG, thus impairing its recognition by antibodies. Whole milk powder (WMP) was selected because of its closest compositional resemblance to bovine fluid milk and its applications in reconstitution and in the beverage (infant, toddler, and adult), confectionary, bakery, and meat industries. For this study, WMP was treated with food-grade CO2 at temperatures of 50, 63, and 75°C under operating pressures of 100, 150, 200, 250, and 300 bar. Proteins in WMP were examined using SDS-PAGE, western blot, and ELISA. Orbitrap Fusion liquid chromatography-tandem MS (LC-MS/MS) and periodic staining was performed to confirm post-translational modifications in β-LG. Functional properties of WMP before and after treatment were assessed by its solubility index, oil holding capacity, emulsion capacity and stability, zeta potential, particle size, and color analysis. SDS-PAGE of treated samples yielded fuzzy bands (variable mobility of molecules due to different molecular weights results in ill-defined bands) indicative of an increase in molecular weight, presumably due to chemical change in the protein, and demonstrated a maximum of 71.13 ± 0.29% decrease in the band intensity of β-LG under treatment conditions of 75°C/300 bar for 30 min. These changes were small with samples treated with heat only. Lighter, diffused bands were observed using western blot analysis. The ELISA tests proved that ScCO2 treatment specifically and significantly affected the antigenicity of β-LG with a reduction of 42.9 ± 2.83% and 54.75 ± 2.43% at 63°C/200 bar and 75°C/300 bar, respectively. Orbitrap fusion detected the presence of fatty acids and sugar moieties bound to β-LG and the latter was confirmed by periodic staining. Functional properties of ScCO2-treated milk powder yielded a decrease in solubility index and an increase in emulsion capacity of WMP was observed under ScCO2 treatment at 75°C/300 bar, with small and insignificant changes at other treatments producing a decrease in antigenicity. Color changes were small for most samples, except at 63°C/200 bar, where a significant increase in yellowness was observed. Zeta potential and particle size measurements indicated that most changes were temperature driven. This study demonstrates 2 approaches to mitigate β-LG antigenicity via fatty acid binding and lactosylation using hydrophobic ScCO2.

Heat-induced agglomeration of water-soluble cod proteins toward gelled structures

Cod proteins (CPs) have potential applications in designing desirable gel-based products, and this study aimed to unravel their heat-induced aggregation pattern and further probe the roles in protein gels. SDS-PAGE analysis indicated that high-precipitation-coefficient aggregates (HPCAs) of CPs aggregates were composed of considerable polymers of myosin heavy chains and actin, and their low-precipitation-coefficient aggregates (LPCAs) contained myosin light chains and tropomyosin. Studies from correlation analysis between the structure and aggregation kinetics revealed that the generation of β-sheet and SS bonds were responsible for their spontaneous thermal aggregation induced by heating temperature and protein concentration, respectively. Additionally, as protein denaturation ratio increased, more and larger HPCAs were formed, which was evidenced driving the network formation of protein gels and resulting in higher storage modulus (G') values. These novel findings may be applicable to other animal proteins for better tailoring the manufacturing of muscle gel-based products.

Influence of pH on Heat-Induced Changes in Skim Milk Containing Various Levels of Micellar Calcium Phosphate

The present study investigated the effect of micellar calcium phosphate (MCP) content and pH of skim milk on heat-induced changes in skim milk. Four MCP-adjusted samples, ranging from 67 to 113% of the original MCP content, were heated (90 °C for 10 min) at different pH values (6.3, 6.6, 6.9, and 7.2), followed by determining changes in particle size, turbidity, protein distribution, and structure. The results demonstrate a strong effect of MCP level and pH on heat-induced changes in milk, with the MCP67 samples revealing the greatest thermal stability. Specifically, decreasing MCP content by 33% (MCP67) led to a smaller increase in non-sedimentable κ-casein and a lower decrease in αs2-casein concentrations after heating compared to other samples. Lower MCP content resulted in a moderate rise in the average particle size and turbidity, along with lower loading of β-turn structural component after heating at low pH (pH 6.3). Notably, MCP113 exhibited instability upon heating, with increased particle size, turbidity, and a significant decrease in non-sedimentable αs2-casein concentration, along with a slight increase in non-sedimentable κ-casein concentration. The FTIR results also revealed higher loading of intermolecular β-sheet, β-turn, and random coil structures, as well as lower loading of α-helix and β-sheet structures in MCP-enhanced skim milk samples. This suggests significant changes in the secondary structure of milk protein and greater formation of larger aggregates.

Application of Calcium Citrate in the Manufacture of Acid Rennet Cheese Produced from High-Heat-Treated Goat’s Milk from Spring and Autumn Season

The stability of milk proteins is affected by changes in the pH value of milk, the heating temperature, and the addition of calcium compounds or chelating agents, which can cause alterations in calcium distribution. The purpose of this study was to determine the potential of the use of calcium citrate to manufacture fresh acid rennet cheese from high-temperature-pasteurized goat’s milk (90 °C, 15 s) from the spring and autumn season and the effect of the calcium dose used on the physicochemical and organoleptic properties of the cheese. Autumn milk was found to be a richer source of total solids, confirming the effect of the production season on milk quality. The applied doses of calcium did not cause the denaturation of goat milk proteins and allowed pasteurization to take place at 90 °C for 15 s. The addition of calcium citrate resulted in a significant increase in the pH value of milk and cheese compared to the control sample. Adding 15 and 20 mg of Ca 100 g⁻¹ to milk as citrate had the most beneficial effect on increasing protein retention in cheese in both seasons, showing a rise from 1.33% to 2.40%. The production season significantly influenced the cheese yield. The control goat cheese from the autumn season showed a 6.85% higher yield compared to the spring cheese. An increase in cheese yield was also observed as the calcium dose of milk increased. The content of micro- and microelements in cheese was affected by the production season. The addition of calcium citrate to milk resulted in a significant increase in the calcium content of cheese—from 120.83 to 147.45 mg 100 g⁻¹ in the spring season and from 130.66 to 151.21 mg 100 g⁻¹ in the autumn season. Increasing the dose of calcium increased the hardness of cheese samples by 1.37 N in the spring and 0.90 N in the autumn. The organoleptic evaluation showed that adding calcium to milk did not significantly affect the organoleptic characteristics of goat cheese.

Microstructural, Volatile Compounds, Microbiological and Organoleptical Characteristics of Low-Fat Buffalo Milk Yogurt Enriched with Whey Protein Concentrate and Ca-Caseinate during Cold Storage

Yogurt is a popular fermented milk product across the world. Structure, taste, and odor characteristics are created by fermenting yogurt with diverse ingredients and lactic acid bacteria (LAB), which contribute the most to the acceptance and quality of yogurt. In this study, low-fat buffalo milk yogurts (LFBY) were produced with the enrichment of 1% (w/w) whey protein concentrate (WPC) and Ca-caseinate (Ca-CN). Yogurts were analyzed based on microstructural, microbiological, organoleptical properties; volatile compounds (solid-phase microextraction method associated with gas chromatography-mass spectrometry) during cold storage for 21 days. Yogurts enriched with WPC and Ca-CN had higher total solids, total protein contents, and pH values. A total of 36 volatile components were identified in all produced yogurts. Acetic acid, butanoic acid, acetaldehyde, acetoin, 2,3-butanedione, ethanol, and 1-heptanol were found in significant amounts and mainly contributed to organoleptical properties. Interestingly, the focused volatile compounds that improve taste and odor were higher in LFBY+WPC yogurt than in plain LFBY or LFBY+Ca-CN yogurt. The Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus viable counts were higher in LFBY+WPC yogurt than plain LFBY or/and LFBY+Ca-CN. Conclusively, yogurt enriched with 1% WPC exhibited the best organoleptical properties and volatile component concentrations. The microstructure of the LFBY with WPC was less compact and dense, and regular, with tiny pores and long and individualized casein filaments than the other treatments. The microstructure of the Ca-CN samples caused a compact structure and coarse than in the control yogurt.

Heat-Induced Interaction of Milk Proteins: Impact on Yoghurt Structure

Heating milk for yoghurt preparation has a significant effect on the structural properties of yoghurt. Milk heated at elevated temperature causes denaturation of whey protein, aggregation, and some case gelation. It is important to understand the mechanism involved in each state of stabilization for tailoring the final product. We review the formation of these complexes and their consequence on the physical, rheological, and microstructural properties of acid milk gels. To investigate the interactions between denatured whey protein and casein, the formation of covalent and noncovalent bonds, localization of the complexes, and their impact on ultimate gelation and final yoghurt texture are reviewed. The information regarding this fundamental mechanism will be beneficial to develop uniform quality yoghurt texture and potential interest of future research.

Effect of pH on heat-induced interactions in high-protein milk dispersions and application of fluorescence spectroscopy in characterizing these changes

This study investigated casein-whey protein interactions in high-protein milk dispersions (5% protein wt/wt) during heating at 90°C for 1.5 to 7.5 min at 3 different pH of 6.5, 6.8, and 7.0, using both conventional methods (gel electrophoresis, physicochemical properties) and fluorescence spectroscopy. Conventional methods confirmed the presence of milk protein aggregates during heating, similar to skim milk. These methods were able to help in understanding the denaturation and aggregation of milk proteins as a function of heat treatment. However, the results from the conventional methods were greatly affected by batch-to-batch variations and, therefore, differentiation could be drawn only in nonheated samples and samples heated for a longer duration. The front-face fluorescence spectroscopy was found to be a useful tool that provided additional information to conventional methods and helped in understanding differences between nonheated, low-, and high-heated samples, along with the type of sample used (derived from liquid or powder milk protein concentrates). At all pH values, tryptophan maxima in nonheated samples derived from powdered milk protein concentrates presented a blue shift in comparison to samples derived from liquid milk protein concentrates, and tryptophan maxima in heated samples presented a red shift. With the heating of the sample, Maillard emission and excitation spectra also showed increases in the peak intensities from 408 to 432 and 260 to 290 nm, respectively. As the level of denaturation increased with heating, a marked differentiation can be seen in the principal component analysis plots of tryptophan, Maillard emission, and excitation spectra, indicating that the front-face fluorescence technique has a potential to monitor and classify samples according to milk protein interactions as a function of pH and heat exposure. Overall, it can be said that the pattern of protein-protein interactions in high-protein dispersions was similar to the observation reported in skim milk systems, and fluorescence spectroscopy with chemometrics can be used as a rapid, nondestructive, and complementary method to conventional methods for following heat-induced changes.

Experimental and Modelling Study of the Denaturation of Milk Protein by Heat Treatment

Heat treatment of milk aims to inhibit the growth of microbes, extend the shelf-life of products and improve the quality of the products. Heat treatment also leads to denaturation of whey protein and the formation of whey protein-casein polymer, which has negative effects on milk product. Hence the milk heat treatment conditions should be controlled in milk processing. In this study, the denaturation degree of whey protein and the combination degree of whey protein and casein when undergoing heat treatment were also determined by using the Native-PAGE and SDS-PAGE analysis. The results showed that the denaturation degree of whey protein and the combination degree of whey protein with casein extended with the increase of the heat-treated temperature and time. The effects of the heat-treated temperature and heat-treated time on the denaturation degree of whey protein and on the combination degree of whey protein and casein were well described using the quadratic regression equation. The analysis strategy used in this study reveals an intuitive and effective measure of the denaturation degree of whey protein, and the changes of milk protein under different heat treatment conditions efficiently and accurately in the dairy industry. It can be of great significance for dairy product proteins following processing treatments applied for dairy product manufacturing.

Effect of casein to whey protein ratios on the protein interactions and coagulation properties of low-fat yogurt

In this study, we investigated the effect of casein (CN) to whey protein (WP) ratios (4:1, 3:1, 2:1, and 1:1) on gelation properties and microstructure of low-fat yogurt made with reconstituted skim milk with or without addition of whey protein concentrate. The rheological properties (storage modulus, G'; yield stress; and yield strain) of the obtained low-fat yogurt were greatly enhanced, the fermentation period was shortened, and the microstructure became more compact with smaller pores as the CN:WP ratio decreased. When CN:WP was 2:1 or 1:1, the obtained yogurt coagulum showed higher G' and greater yield stress, with more compact crosslinking and smaller pores. In addition, the more of skim milk powder was replaced by whey protein concentrate, the more disulfide bonds were formed and the greater the occurrence of hydrophobic interactions during heat treatment, which can improve the rheological properties and microstructure of low-fat yogurt.

Acid-induced gelation behavior of casein/whey protein solutions assessed by oscillatory rheology

Gelation process of acid-induced casein gels was studied using response surface method (RSM). Ratio of casein to whey proteins, incubation and heating temperatures were independent variables. Final storage modulus (G') measured 200 min after the addition of glucono-δ-lactone and the gelation time i.e. the time at which G' of gels became greater than 1 Pa were the parameters studied. Incubation temperature strongly affected both parameters. The higher the incubation temperature, the lower was the G' and the shorter the gelation time. Increased heating temperature however, increased the G' but again shortened the gelation time. Increase in G' was attributed to the formation of disulphide cross-linkages between denatured whey proteins and casein chains; whilst the latter was legitimized by considering the higher isoelectric pH of whey proteins. Maximum response (G' = 268.93 Pa) was obtained at 2.7 % w/w, 25 °C and 90 °C for casein content, incubation and heating temperatures, respectively.

Toward the understanding of the interfacial dairy fouling deposition and growth mechanisms at a stainless steel surface: a multiscale approach

The microstructures of two dairy fouling deposits obtained at a stainless steel surface after different processing times in a pilot plate heat exchanger were investigated at different scales. Electron-Probe Micro Analysis, Time-of-Flight Secondary Ion Mass Spectrometry, Atomic Force Microscopy, and X-Ray Photo-electron Spectroscopy techniques were used for this purpose. The two model fouling solutions were made by rehydrating whey protein in water containing calcium or not. Results on samples collected after 2h processing show that the microstructure of the fouling layers is completely different depending on calcium content: the layer is thin, smooth, and homogeneous in absence of calcium and on the contrary very thick and rough in presence of calcium. Analyses on substrates submitted to 1 min fouling reveal that fouling mechanisms are initiated by the deposit of unfolded proteins on the substrate and start immediately till the first seconds of exposure with no lag time. In presence of calcium, amorphous calcium carbonate nuclei are detected in addition to unfolded proteins at the interface, and it is shown that the protein precedes the deposit of calcium on the substrate. Moreover, it is evidenced that amorphous calcium carbonate particles are stabilized by the unfolded protein. They are thus more easily trapped in the steel roughnesses and contribute to accelerate the deposit buildup, offering due to their larger characteristic dimension more roughness and favorable conditions for the subsequent unfolded protein to depose.

Characterization of heat-induced changes in skim milk using asymmetrical flow field-flow fractionation coupled with multiangle laser light scattering

Separation and size measurement of protein particles are a relevant approach to monitor heat-induced changes in skim milk. Unfortunately, no method is currently available at low cost and without excessive preparation of the samples. Therefore, the present study aimed at evaluating the interest of asymmetrical flow field-flow fractionation (AFlFFF) coupled with multiangle laser light scattering (MALLS) for this purpose. Unheated and heated skim milk samples at pH 6.5 and 7.2 were prepared and comparatively analyzed using AFlFFF-MALLS, size exclusion chromatography (SEC-MALLS) and dynamic light scattering. The results showed that AFlFFF could evidence the conversion of the native whey proteins of unheated milk into heat-induced whey protein/κ-casein complexes in the serum phase of milk and possibly on the surface of the casein micelles. The pH-induced changes in the partition of the complexes between the serum and the micellar phases could also be observed. The results therefore showed the interest of AFlFFF-MALLS to monitor the heat-induced changes in particle sizes in skim milk and to separate the different protein components of unheated and heated skim milk.

Impact of food processing on the structural and allergenic properties of food allergens

This article reviews recent studies that address one of the major unanswered questions in food allergy research: what attributes of food or food proteins contribute to or enhance food allergenicity?

Use of reducing/nonreducing two-dimensional electrophoresis for the study of disulfide-mediated interactions between proteins in raw and heated bovine milk

The composition and interactions of proteins in bovine milk, and modifications resulting from milk storage and processing, are complex and incompletely understood. Analysis of the milk proteome can elucidate milk protein expression, structure, interaction, and modifications. Raw milk was analyzed by two-dimensional electrophoresis (isolelectric focusing followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis) under reducing and nonreducing, or combined, conditions, followed by mass spectrometry of separated protein spots; a small number of high-abundance proteins, that is, caseins (alpha(S1)-, alpha(S2)-, beta-, kappa-, and gamma-), beta-lactoglobulin, alpha-lactalbumin, and serum albumin, represented the vast majority of protein spots on the two-dimensional electrophoretograms of raw milk samples, but some cross-linked protein complexes (mainly homopolymers of kappa-casein and alpha(S2)-casein but also some heteropolymeric complexes) were resolved under native/unheated conditions. When skim milk was heated to 90 degrees C for up to 10 min, the level of native whey proteins decreased in parallel with an increase in disulfide-linked complexes, including very complex heteropolymers, for example, casein/whey protein polymers containing multiple species. The analysis strategy used in this study reveals numerous disulfide-mediated interactions and can be proposed to analyze reduction/oxidation of milk and dairy product proteins following processing treatments applied for processing and storage.

Effects of iodate, hydrogen peroxide and dichromate on the denaturation of whey proteins in heated milk

The effect of heating raw milk at 75 °C for 30 min and at 95 °C for 15 s in the presence of varying concentrations of three oxidizing agents on the denaturation of total and individual whey proteins was measured. At 75 °C, ±-lactalbumin could be protected against denaturation by both iodate and H202 and ²-Lactoglobulins A and B by H202 and dichromate. However, at concentrations above ∼ 15 MM, the oxidizing agents tended to increase denaturation. At 95 °C, where the extent of whey protein denaturation was much reduced in the absence of oxidizing agents, little or no protective effect was observed. It is suggested that H202 may prove a suitable alternative to iodate in reducing deposits in ultra high temperature plants.

Changes in milk on heating: viscosity measurements

Skim milk was heated at 85 °C for different holding times. As a result of such heating, whey proteins, in particular β-lactoglobulin, denatured and associated with casein micelles. This led to an increase in size of the casein micelles but also to a different interaction between them. Both these changes could be described by using a quantitative model which was developed for the viscosity of so-called adhesive hard spheres. We applied the model successfully to skim milk and were able to describe on a quantitative basis the changes due to the heat treatment of milk. It was shown that after heating the casein micelles became larger and acquired a mutual attraction. The unfolding of the whey proteins and their subsequent association with the casein micelles appeared to be responsible for these changes. How this reaction influences the fouling of heat exchangers is discussed.

Water-holding capacity and characterization of protein interactions in processed cheese

In order to predict the oozing susceptibility of heat-induced milk protein gels such as processed cheeses during storage, ultracentrifugation forces are required to accelerate the expulsion of water from the gel structure. Two predictive methods have been tested. Direct ultracentrifugation of processed cheese was used to study effects of centrifugal compression on the water-holding ability. The water release with optimal parameters (time, temperature and ultracentrifugation force) from a freshly manufactured processed cheese was correlated with visual assessment of oozing after 6 months storage. A second method was based on the susceptibility of gelled proteins to resist the dissociating action of solutions of chemical agents such as SDS, urea, EDTA and 2-mercaptoethanol. Most of the protein sedimented by ultracentrifugation in the presence of SDS represented that still in complexed form. This correlated with optimal water binding by the processed cheese. Response-surface methods, used to optimize the ultracentrifugation variables and the composition of the dissociation solutions, showed that the dissociation test was best with a dispersion of processed cheese in SDS solution (10 g/l) at a ratio of 1:6 (w/v) at 20°C, ultracentrifuged at 86000 g and 20°C for 25 min. Analysis of processed cheese cooked at 115°C and then dissociated in different solutions showed various protein–protein interactions in the gel network, whereas hydrophobic interactions were the most important stabilizers of the protein matrix of cheese cooked at higher temperature.

Effect of heat induced interaction between β-lactoglobulin and κ-casein on syneresis

The effect of preheating skim milk and artificial micelle milk on curd syneresis was studied. The inhibition of syneresis caused by heat was dependent on the presence of β-lactoglobulin (β-lg) and to a lesser extent α-lactalbumin. The degree of inhibition increased with increasing amounts of added β-lg and preheating temperature. This agrees with the hypothesis that the detrimental effect of preheating on syneresis is due to complex formation between β-lg and κ-casein. This complex appeared to be mediated via thiol–disulphide exchange and its formation appeared to interfere with the micelle–micelle interactions responsible for syneresis.

Effects of heat treatment and acidification on the dissociation of bovine casein micelles

The effects of heat treatment and subsequent acidification of milk on the distribution of proteins, Ca and Pi, between the serum and micellar phases were examined using ultracentrifugation. After heating milk at 85 °C for 10 min, and storing for 22 h at 4, 20 or 30 °C, there was a marked increase in the concentration of κ-casein in the serum. At 4 and 20 °C there was also slightly more β-casein in the serum from heat-treated milk than in that from the corresponding raw milk. The whey proteins were extensively denatured, and were almost equally distributed between the supernatants and micellar pellets. After storage for 22 h the distribution of Ca and Pibetween soluble and colloidal phases in heat-treated milk was similar to that in raw milk. After acidifying heat-treated milk by the addition of glucono-δ-lactone and storing for 22 h at 4, 20 or 30 °C there was progressive solubilization of colloidal calcium phosphate with decreasing pH, and at pH 5·0 almost all of the Ca and Piwas present in the serum. At 20 °C, and even more so at 4 °C, serum concentrations of the individual caseins increased considerably with decreasing pH, reaching maximum levels of about 25 and 40% of the total casein at pH 5·7 and 5·5 respectively, and then decreasing rapidly at lower pH. Compared with raw milk, maximum dissociation in heat-treated milks stored at 4 and 20 °C occurred at higher pH, and the overall levels of dissociation of individual caseins from the micelles were lower. At 30 °C, the concentrations of individual caseins in the serum of heat-treated milk decreased steadily as the pH was reduced, and did not show the slight increase found previously for raw milk. The role of the denatured whey proteins in interacting with κ-casein and in promoting aggregation of the micelles on acidification is discussed.

Denaturation, aggregation and heat stability of milk protein during the manufacture of skim milk powder

The effect of preheat treatment, evaporation and drying in a commercial plant on the denaturation of βlactoglobulin and α-lactalbumin, their incorporation into the casein micelle and the heat stability characteristics of the milks and powders were determined. Preheat treatments between 110 °C for 2 min and 120 °C for 3 min denatured between 80 and 91% of β-lactoglobulin and between 33 and 45% of α-lactalbumin. Evaporation increased the extent of denaturation but spray drying did not increase it further. The incorporation of α-lactalbumin and βlactoglobulin into the micelles was markedly less than the amount that denatured and was not a constant ratio to it. Heat coagulation times at 140 °C of milks, concentrates and powders diluted to the original milk concentration were measured as a function of pH. In general, the greater the collective heat treatment, the shorter the time required to achieve coagulation. Spray drying shifted the peak positions in the pH-heat coagulation time profiles. In contrast, heat coagulation times (measured at 120 °C) of concentrates and powders diluted to 20% total solids content increased with the severity of the preheat treatment. Surprisingly, spray drying markedly increased the heat coagulation times of the diluted concentrates.

Influence of concentration of milk solids on the dissociation of micellar κ-casein on heating reconstituted milk at 120°C

Skim milks were prepared from skim milk powder at several concentrations between 10 and 25% total solids and portions were pH-adjusted to between pH 6·3 and 7·1 and heated at 120°C. After ultracentrifugation (88000 g for 90 min), the supernatants were analysed using gel electrophoresis to determine the concentrations of β-lactoglobulin, α-lactalbumin and κ-casein. The dissociation of κ-casein from the micelles was dependent on both the pH and the total solids content of milk before heating. Both higher pH (in the range 6·5–7·l) and higher concentration increased the extent of dissociation. A further series of samples were heated for 2–11 min at 120°C at pH 6·55. κ-Casein dissociation increased with concentration and with heating time. It was concluded that as the milk increased in concentration, the pH at which micellar κ-casein dissociated on heating was lowered.

Estimation of casein micelles' surface energy by means of contact angle measurements

The surface energies of highly hydrated casein micelle layers isolated from variously pretreated skim milks have been determined by means of contact angle measurements. The long range Lifshitz-Van der Waals (LW) and the short range hydrogen bonding (SR) components of surface energy were determined using α-bromonaphthalene and water for contact angle measurements. Casein micelles isolated from untreated and heat treated milks showed similar surface energy values of about 63·5 mJ.m-2 with an LW component of 19·2 mJ.m-2 and an SR component of 44·3 mJ.m-2. The calculated attraction potential energy was − 0·7 mJ.m-2. Casein micelles isolated from renneted milk showed a surface energy of 33·0 mJ.m-2 with an LW component of 30·7 mJ.m-2 and an SR component of 2·3 mJ.m-2. The attraction potential energy of renneted micelles was nearly two orders of magnitude higher than those of micelles from other milks ( − 63·3 mJ.m-2). The SR component of interfacial energy accounted for 98% of this attraction potential. The importance of attractive forces in relation to casein micelle stability is discussed.

Role of kappa-casein in the association of denatured whey proteins with casein micelles in heated reconstituted skim milk

Reconstituted skim milk at pH from 6.5 to 7.1 was unheated, preheated (68 degrees C/20 min), or heated at 90 degrees C for 20-30 min. On preheating, the size of the casein micelles decreased by about 5-20 nm, with a greater effect at higher pH. The casein micelle size of the heated milk at pH 6.5 increased by about 30 nm when compared to that of the unheated or preheated milk. As the pH was increased before heating, the particle size gradually decreased so that, at pH 7.1, the size was markedly smaller than that for the unheated milk and slightly smaller than that for the preheated milk. High levels (about 85%) of denatured whey protein associated with the casein micelles at pH 6.5, and this level decreased as the pH increased so that, at pH 7.1, low levels (about 15%) were associated with the micelles. Low levels of alphaS-casein and beta-casein were found in the serum regardless of the heat treatment or the pH of the milk. At pH 6.5, low levels (about 10%) of kappa-casein were also found in the milk serum. In the unheated milk, the level of serum kappa-casein increased slightly with increasing pH; in the heated samples, the level of serum kappa-casein increased markedly and linearly with increasing pH so that, at pH 7.1, about 70% of the kappa-casein was in the serum phase. The results of this study indicate that the pH dependence of the levels of serum phase kappa-casein may be responsible for the change in distribution of the whey proteins between the colloidal and serum phases. This is the first report to demonstrate significant levels of dissociation of kappa-casein from the micelles at pH between 6.5 and 6.7, although this dissociation phenomenon is well known on heating milk at high temperatures at pH above 6.7.