Modelling of heat stability and heat‐induced aggregation of casein micelles in concentrated skim milk using a Weibullian model

Heat stability is known to be limited in concentrated skim milk leading to severe coagulation and sediment formation during storage. It has often been found to be in conflict with the extension of the shelf life by thermal treatment. A Weibullian model was used to describe the course of coagulation of casein micelles in concentrated skim milk of different total solids and at different heating temperatures. A maximum heating temperature–time–total solids relationship was calculated based on a certain maximum allowance of sediment formation. Optimal heat treatment conditions for concentrated skim milk of different total solids content could be defined.     

Kinetics of heat-induced interactions among whey proteins and casein micelles in sheep skim milk and aggregation of the casein micelles

The interactions among the proteins in sheep skim milk (SSM) during heat treatments (67.5–90°C for 0.5–30 min) were characterized by the kinetics of the denaturation of the whey proteins and of the association of the denatured whey proteins with casein micelles, and changes in the size and structure of casein micelles. The relationship between the size of the casein micelles and the association of whey proteins with the casein micelles is discussed. The level of denaturation and association with the casein micelles for β-lactoglobulin (β-LG) and α-lactalbumin (α-LA) increased with increasing heating temperature and time; the rates of denaturation and association with the casein micelles were markedly higher for β-LG than for α-LA in the temperature range 80 to 90°C; the Arrhenius critical temperature was 80°C for the denaturation of both β-LG and α-LA. The casein micelle size increased by 7 to 120 nm, depending on the heating temperature and the holding time. For instance, the micelle size (about 293 nm) of SSM heated at 90°C for 30 min increased by about 70% compared with that (about 174.6 nm) of unheated SSM. The casein micelle size increased slowly by a maximum of about 65 nm until the level of association of the denatured whey proteins with casein micelles reached 95%, and then increased markedly by a maximum of about 120 nm when the association level was greater than about 95%. The marked increases in casein micelle size in heated SSM were due to aggregation of the casein micelles. Aggregation of the casein micelles and association of whey protein with the micelles occurred simultaneously in SSM during heating.     

Reaction behaviour of highly concentrated whey protein isolate under defined heat treatments

For better development of extrusion processes, more information about the reaction behaviour of highly concentrated whey protein isolate (WPI) at elevated temperatures is necessary. Therefore, temperature and time sweep analyses were performed under defined extrusion-like conditions using a closed-cavity rheometer. WPI samples taken after various temperature–time treatments were analysed by size-exclusion chromatography and fluorescence spectroscopy. Together with the inline rheological data, G′ and G″, information about the reaction behaviour such as onset temperature of aggregation and degradation were obtained. Aggregation was observed during the isothermal time sweep measurements for all temperatures applied (80–160 °C). High temperature treatments (T ≥ 120 °C) resulted in non-disulphide covalent cross-links and degradation reactions. The results showed that non-disulphide covalent cross-links include isopeptides and Maillard reaction products, and that their formation is accelerated with increasing temperature.     

Controlling denaturation and aggregation of whey proteins during thermal processing by modifying temperature and calcium concentration

Whey protein isolate solutions (8.00 g protein/100 g; pH 6.8) were treated for 2 min at 72, 85 or 85 °C with 2.2 mM added calcium Ca to produce four whey protein systems: unheated control (WPI‐UH), heated at 72 °C (WPI‐H72), heated at 85 °C (WPI‐H85) or heated at 85 °C with added Ca (WPI‐H85Ca). Total levels of whey protein denaturation increased with increasing temperature, while the extent of aggregation increased with the addition of Ca, contributing to differences in viscosity. Significant changes in Ca ion concentration, turbidity and colour on heating of WPI‐H85Ca, compared to WPI‐UH, demonstrated the role of Ca in whey protein aggregation.     

Influence of calcium-binding salts on heat stability and fouling of whey protein isolate dispersions

The effect of the calcium-binding salts (CBS), trisodium citrate (TSC), tripotassium citrate (TPC) and disodium hydrogen phosphate (DSHP) at concentrations of 1–45 mm on the heat stability and fouling of whey protein isolate (WPI) dispersions (3%, w/v, protein) was investigated. The WPI dispersions were assessed for heat stability in an oil bath at 95 °C for 30 min, viscosity changes during simulated high-temperature short-time (HTST) and fouling behaviour using a lab-scale fouling rig. Adding CBS at levels of 5–30 mm for TSC and TPC and 25–35 mm for DSHP improved thermal stability of WPI dispersions by decreasing the ionic calcium (Ca²⁺) concentration; however, lower or higher concentrations destabilised the systems on heating. Adding CBS improved heat transfer during thermal processing, and resulted in lower viscosity and fouling. This study demonstrates that adding CBS is an effective means of increasing WPI protein stability during HTST thermal processing.     

Relationship between functional properties and aggregation changes of whey protein induced by high pressure microfluidization

Aggregation changes of whey protein induced by high-pressure microfluidization (HPM) treatment have been investigated in relation with their functional properties. Whey protein was treated with HPM under pressure from 40 to 160 MPa. Functional properties (solubility, foaming, and emulsifying properties) of whey protein concentrate (WPC) ultrafiltered from fluid whey were evaluated. The results showed significant modifications in the solubility (30% to 59%) and foaming properties (20% to 65%) of WPC with increasing pressure. However, emulsifying property of WPC treated at different pressures was significantly worse than untreated sample. To better understand the mechanism of the modification by HPM, the HPM-induced aggregation changes were examined using particle size distribution, scanning electron microscopy, and hydrophobicity. It was indicated that HPM induced 2 kinds of aggregation changes on WPC: deaggregation and reaggregation of WPC, which resulted in the changes of functional properties of WPC modified by HPM.     

Yak milk whey protein denaturation and casein micelle disaggregation/aggregation at different pH and temperature

At the natural pH of yak milk (pH 6.6), a low level (<30%) of κ-casein (κ-CN) was found in the serum phase after heating at 95 °C for 30 min, indicating that as much as 70% of the β-lactoglobulin (β-Lg) and κ-CN complexes is associated with the micelle colloidal particles. The β-Lg and κ-CN levels increased from 13.2% and 2.6% at pH 6.0 to 35.2% and 60.1% at pH 7.0, respectively, when yak milk was heated at 95 °C for 30 min. At pH 6.0–6.4, the denatured whey proteins were associated with the caseins in the colloidal phase, resulting in milk gelation upon heating. The distribution of β-Lg and κ-CN complexes increased in the serum phase, demonstrated by the increasing levels of both β-Lg and κ-CN with increasing pH; at high pH (6.6–7.0), large proportions of β-Lg and α-lactalbumin were lost, presumably forming complexes in the colloidal phase.     

Modulation of rheological properties by heat-induced aggregation of whey protein solution

Heat-induced protein aggregation at low protein concentrations generally leads to higher viscosities. We here report that aggregated protein can yield weaker gels than those from native protein at the same concentration. Aggregated protein was produced by heating a solution of whey protein isolate (WPI) at 3% and 9% w/w. The higher protein concentration resulted in a larger aggregate size and a higher intrinsic viscosity. The protein fraction in native WPI had the smallest size and the lowest intrinsic viscosity. The same trend was observed for the shear viscosity after concentrating the suspensions containing aggregates to around 15% w/w. Suspensions containing aggregates that were produced from a higher concentration possessed a higher viscosity. After reheating the concentrated suspensions, the suspension from the 9% w/w aggregate system produced the weakest gel, followed by the one from 3% w/w, while the native WPI yielded the strongest gel. Reactivity of the aggregates was also an important factor that influenced the resulting gel properties. We conclude that aggregation of whey protein solution is a feasible route to manipulate the gel strength of concentrated protein systems, without having to alter the concentration of the protein.     

Expanded functionality of modified whey protein dispersions after transglutaminase catalysis

The functionality of whey dispersions, prepared with a modified whey protein concentrate (mWPC) ingredient, was significantly altered after cross-linking with microbial transglutaminase (TGase) upon pH adjustment to 8. Test TGase-mWPC solutions, pH 8, gelled faster than control mWPC dispersions, as measured in real time; whereas, the gelling temperature of pretreated TGase-mWPC samples (37 °C, 2.5 h) increased from 67.8 to 74.8 °C with a minimal change in gel strength. Prolonged prior incubation with the enzyme (37 °C, 20 h) raised the gel strength in both control mWPC and TGase-mWPC dispersions, though these values were approximately 2.7 times lower in TGase-mWPC samples. Furthermore, the gelling temperature was raised by 9 °C after extensive polymerization. The water holding capacity was not impacted by enzymatic processing while emulsions prepared with TGase-mWPC dispersions proved very stable with no evidence of phase separation during storage at room temperature for 1 mo. Moreover, the apparent viscosity of TGase-mWPC emulsions exhibited a 10-fold increase compared to nonenzyme-treated mWPC samples. The particle size was nearly 11 μm in covalently linked TGase-mWPC test fractions compared with 8 μm in nonpolymerized mWPC dispersions. Ultimately, the functional characteristics of TGase-mWPC ingredients may be designed to deliver superior performance, especially with regard to improving heat and emulsion stability.     

Kinetics of heat-induced whey protein denaturation and aggregation in skim milks with adjusted whey protein concentration

Microfiltration and ultrafiltration were used to manufacture skim milks with an increased or reduced concentration of whey proteins, while keeping the casein and milk salts concentrations constant. The skim milks were heated on a pilot-scale UHT plant at 80, 90 and 120 degrees C. The heat-induced denaturation and aggregation of beta-lactoglobulin (beta-lg), alpha-lactalbumin (alpha-la) and bovine serum albumin (BSA) were quantified by polyacrylamide gel electrophoresis. Apparent rate constants and reaction orders were calculated for beta-lg, alpha-la and BSA denaturation. Rates of beta-lg, alpha-la and BSA denaturation increased with increasing whey protein concentration. The rate of alpha-la and BSA denaturation was affected to a greater extent than beta-lg by the change in whey protein concentration. After heating at 120 degrees C for 160 s, the concentration of beta-lg and alpha-la associated with the casein micelles increased as the initial concentration of whey proteins increased.     

Thermal aggregation of whey proteins in the presence of buttermilk concentrate

The objective of this study was to assess the impact of adding buttermilk concentrate to the denaturation and microparticulation process of cheese whey protein concentrate. For this purpose, the two concentrates were mixed and co-denatured (pH 4.6, 90 °C,) and homogenized. The presence of buttermilk significantly increased aggregation yield and decreased water-holding capacity of aggregates up to a buttermilk:whey protein ratio of 75:25. Modification of rheological properties suggests that denatured whey protein interacted with casein. A thiol blocker, N-ethylmaleimide, was added before heating to measure the role of disulphide bond formation in the aggregation process. Results showed that both covalent and non-covalent interactions were involved in aggregate formation. Ultrasound treatment was applied during denaturation process and was shown to influence aggregate formation. It appeared that under increased turbulence and cavitation conditions, aggregation yield was increased and water-holding capacity decreased.     

Functional improvement of milk whey proteins induced by high hydrostatic pressure

High pressure is emerging as a new processing technology that produces particular changes in the molecular structure of proteins and thus gives rise to new properties inaccessible via conventional methods of protein modification. This review deals with the main effects of high hydrostatic pressure on the physicochemical characteristics of milk whey proteins and how modifications in their structural properties contribute to functionality. In this paper the mechanism underlying pressure-induced changes in ss-lactoglobulin, a-lactabumin, and bovine serum albumin is explained, and related to functional properties such as gel-forming ability, emulsifying activity, or foaming capacity. The possibility of using high pressures to favor chemical reactions of proteins with other food components, such as carbohydrates, to produce novel molecules with new food uses is also considered.     

大豆乳清蛋白的热变性和热聚集的研究

通过SDS-PAGE电泳、差示扫描量热仪(DSC)和体积排阻色谱(SEC-HPLC)研究了大豆乳清蛋白(WSP)的热变性和热聚集.结果表明:大豆乳清蛋白的变性温度为70.6℃和89.4℃,对应于胰蛋白酶抑制剂和大豆凝集素的热变性温度;1%(w/v)的大豆乳清蛋白经80℃和100℃加热30min,会生成分子量约为270万Da的可溶性聚集体.     

Association of denatured whey proteins with casein micelles in heated reconstituted skim milk and its effect on casein micelle size

When skim milk at pH 6.55 was heated (75 to 100 degrees C for up to 60 min), the casein micelle size, as monitored by photon correlation spectroscopy, was found to increase during the initial stages of heating and tended to plateau on prolonged heating. At any particular temperature, the casein micelle size increased with longer holding times, and, at any particular holding time, the casein micelle size increased with increasing temperature. The maximum increase in casein micelle size was about 30-35 nm. The changes in casein micelle size were poorly correlated with the level of whey protein denaturation. However, the changes in casein micelle size were highly correlated with the levels of denatured whey proteins that were associated with the casein micelles. The rate of association of the denatured whey proteins with the casein micelles was considerably slower than the rate of denaturation of the whey proteins. Removal of the whey proteins from the skim milk resulted in only small changes in casein micelle size during heating. Re-addition of beta-lactoglobulin to the whey-protein-depleted milk caused the casein micelle size to increase markedly on heat treatment. The changes in casein micelle size induced by the heat treatment of skim milk may be a consequence of the whey proteins associating with the casein micelles. However, these associated whey proteins would need to occlude a large amount of serum to account for the particle size changes. Separate experiments showed that the viscosity changes of heated milk and the estimated volume fraction changes were consistent with the particle size changes observed. Further studies are needed to determine whether the changes in size are due to the specific association of whey proteins with the micelles or whether a low level of aggregation of the casein micelles accompanies this association behaviour. Preliminary studies indicated lower levels of denatured whey proteins associated with the casein micelles and smaller changes in casein micelle size occurred as the pH of the milk was increased from pH 6.5 to pH 6.7.     

Effect of heat treatment and casein to whey protein ratio of skim milk on graininess and roughness of stirred yoghurt

The aim of this work was to study how heat treatment and casein (CN) to whey protein (WP) ratio of skim milk affect physical characteristics of stirred yoghurt. Different heat treatments (95 °C/256 s, 110 °C/180 s and 130 °C/80 s) were applied to the yoghurt milk with the CN to WP ratios of 1.5:1, 2:1, 3:1 and 4:1. Physical properties, including graininess and roughness, of stirred yoghurt were determined during storage at 4 °C for 15 days. Visual roughness, number of grains, perimeter of grains, storage modulus, and yield stress decreased, when heating temperature or CN to WP ratio increased.     

酪蛋白水解物替代乳清蛋白的加工性能评价研究

利用复合蛋白酶水解酪蛋白制备适度及深度水解酪蛋白产品,测定酪蛋白水解物的加工性能。结果表明,经过酶解后,适度水解酪蛋白溶解度接近90%,深度水解酪蛋白溶解度接近100%,显著高于酪蛋白和乳清蛋白。此外,适度水解酪蛋白吸油性、起泡性分别约为乳清蛋白的3倍和1.5倍。深度水解酪蛋白在起泡性和乳化性上也显著高于乳清蛋白。可见,两款酪蛋白水解物在起泡性、乳化性、吸油性、溶解性等方面均在一定程度上优于乳清蛋白,可广泛替代乳清蛋白在食品工业中大规模应用。      

酪蛋白与乳清蛋白比例对酸奶凝胶性质的影响

研究了乳中酪蛋白和乳清蛋白比例对凝固型酸奶流变学特性和微观结构的影响,结果表明,固定蛋白质质量分数、降低酪蛋白和乳清蛋白的比例,可以明显提高酸奶凝胶的质量.乳中蛋白质质量分数一致时,酸奶凝胶的硬度、黏度、持水力随着酪蛋白和乳清蛋白比例的减小而增大,凝胶网络结构变得更规则、致密,孔隙更小.在低蛋白质质量分数下,降低乳中酪...