UHT milk contains multiple forms of αS1-casein that undergo degradative changes during storage

Milk proteins are susceptible to chemical changes during processing and storage. We used proteomic tools to analyse bovine α_S1-casein in UHT milk. 2-D gels of freshly processed milk α_S1-casein was presented as five or more spots due to genetic polymorphism and variable phosphorylation. MS analysis after phosphopeptide enrichment allowed discrimination between phosphorylation states and genetic variants. We identified a new alternatively-spliced isoform with a deletion of exon 17, producing a new C-terminal sequence, K¹⁶⁴SQVNSEGLHSYGL¹⁷⁷, with a novel phosphorylation site at S¹⁷⁴. Storage of UHT milk at elevated temperatures produced additional, more acidic α_S1-casein spots on the gels and decreased the resolution of minor forms. MS analysis indicated that non-enzymatic deamidation and loss of the N-terminal dipeptide were the major contributors to the changing spot pattern. These results highlight the important role of storage temperature in the stability of milk proteins and the utility of proteomic techniques for analysis of proteins in food.     

Comparison of pH-dependent sonodisruption of re-assembled casein micelles by 35 and 130 kHz ultrasounds

Sonodisruption behavior of re-assembled casein micelles was compared at two ultrasound frequencies (35 and 130 kHz) by turbidity measurement and laser-diffraction based particle size analysis. Sonochemical ultrasound (130 kHz) was more effective than power ultrasound (35 kHz) in micelle disruption. This was attributed to the higher strain rates generated upon implosion of cavities, as well as the liberation of more free radicals to the surrounding medium. The higher the pH of solution, the more effective was the ultrasonic disruption due to a looser expanded assembly of particles at higher pH values. Sonochemical ultrasound decreased the consistency coefficient of casein solutions and increased their flow index except at a pH value of 6.35, while power ultrasound did not affect the flow behavior of solutions across the whole pH range.     

Rehydration process of milk protein concentrate powder monitored by static light scattering

Static light scattering (SLS) was applied to monitor the rehydration process of milk protein concentrate (MPC) powder. The size distribution and volume concentration of the suspended powder particles were measured to quantify the dissolution kinetics of MPC powder. The results obtained showed that the low solubility index reported for MPC85 (85% protein) powder at room temperature was the consequence of slow dissolution kinetics rather than the presence of a large amount of insoluble material in the rehydrated powder. The rehydration process of MPC85 powder occurs in two overlapping steps: the disruption of agglomerated particles into primary powder particles and, simultaneously, the release of material from the powder particles into the surrounding aqueous phase. The latter process appeared to be the rate-limiting step of dissolution of MPC85 and was accelerated by an increase of the solvent temperature.     

Proteolysis in model Portuguese cheeses: Effects of rennet and starter culture

To shed further light onto the mechanisms of proteolysis that prevail throughout ripening of Portuguese cheeses, model cheeses were manufactured from bovine milk, following as much as possible traditional manufacture practices – using either animal or plant rennet. The individual role upon proteolysis of two (wild) strains of lactic acid bacteria – viz. Lactococcus lactis and Lactobacillus brevis, which are normally found to high viable numbers in said cheeses, was also considered, either as single or mixed cultures. Our experimental results confirmed the influence of rennet on the proteolysis extent, but not on proteolysis depth. On the other hand, the aforementioned strains clearly improved release of medium- and small-sized peptides, and contributed as well to the free amino acid pool in cheese.     

酶工程技术在开发活性多肽方面的研究进展

概述了酶工程技术在开发生物活性肽方面的研究进展，着重介绍了利用酶工程技术生产大豆多肽、玉米肽、酪蛋白磷酸肽等方面的研究现状。     

A possible structure of the casein micelle based on high-resolution field-emission scanning electron microscopy

New electron micrographs, produced using the technique of Field Emission Scanning Electron Microscopy, showing the details of the micellar surface, are presented. The images show the micellar surfaces without any coating, and suggest that the surface of the micelle may have a much more complex structure than has previously been demonstrated. Although there appear to be no spherical subunits (submicelles), there is evidence for the organization of the caseins into tubular structures within the micelle. The surface is not smooth, and contains gaps between the substructures. The observations are discussed in terms of published models of micellar structure, where is it suggested how the depiction of the micellar surface can be used to explain certain factors of its reactivity and behaviour.     

酪朊胶粘贴啤酒商标耐水性的研究

本文对酪朊胶粘贴的啤酒商标的耐水性进行了较系统的研究。试验证明其耐水性与酚朊胶组分、贴标时胶的用量、贴标质量、贴标后放置的时间以及商标本身的质量等各因素有关．在具有较好耐水性的酪朊胶的基础上保持一定的上胶量（78.5cm2珠啤普通标，250mg左右），胶上得均匀，标贴得平整，贴标后放置十天左右，再在约5℃冰水中冷冻，商标就能在72小时内不自行脱落．     

胰蛋白酶水解酪蛋白进程研究

研究了碱性务件下胰蛋白酶对酪蛋白的水解，用甲醛固定法对蛋白质的水解度进行测定，并用水解度（DH）表征其反应历程。分析了底物浓度、酶浓度、时间、pH和温度对DH、水解速度的影响。     

Casein glycomacropeptide pH-driven self-assembly and gelation upon heating

Casein glycomacropeptide (CMP) found in cheese whey is a C-terminal hydrophilic glycopeptide released from κ-casein by the action of chymosin during cheese making. In a previous work a self-assembly model for CMP at room temperature was proposed, involving a first step of hydrophobic assembly followed by a second step of electrostatic interactions which occurs below pH 4.5. The objective of the present work was to study, by dynamic light scattering (DLS), the effect of heating (35–85 °C) on the pH-driven CMP self-assembly and its impact on the dynamics of CMP gelation. The concentration of CMP was 3% w/w for DLS and 12% w/w for rheological measurements. The solutions at pH 4.5 and 6.5 did not show any change in the particle size distributions upon heating. In contrast the solutions at pH lower than 4.5 that showed electrostatic self-assembly at room temperature were affected by heating. The mean diameter of assembled CMP increased by decreasing pH. For all solutions with pH lower than 4.5, the particle size did not change on cooling, suggesting that the assembled CMP forms formed during heating were stable. The gel point determined as G′–G″ crossover, occurred in all systems at 70 °C, but at different times. The rate of self-assembly determined by DLS as well as the rate of gelation increased with increasing temperature and decreasing pH from 4 to 2. Increasing temperature and decreasing pH, the first step of CMP self-assembly by hydrophobic interactions is speed out. All the self-assembled structures and the gels formed at different temperatures were pH-reversible but did not revert to the initial size (monomer) but to associated forms that correspond mainly to CMP dimers.     

羧甲基茯苓多糖对酪蛋白pH稳定性与生物活性的调控与机制研究

目的? 探究羧甲基茯苓多糖以不同比例与酪蛋白复配时对酪蛋白稳定性、结构与理化性质以及生物活性的影响。方法? 通过羧甲基衍生化,提升茯苓多糖的溶解性并表征其结构;以不同比例将羧甲基茯苓多糖与酪蛋白进行物理共混,测定复合物在不同pH值下的浊度、zeta电位、蛋白质溶解性、相图;通过激光共聚焦显微镜和旋转流变仪比较不同酪蛋白样品在酸性条件下的聚集特性与理化特性;通过自由基清除能力与非酶糖基化产物的抑制能力比较不同酪蛋白样品的生物活性。结果? 本研究所得羧甲基茯苓多糖的分子量为2.43×105 g/mol,通过取代度（0.84）和红外光谱中的羧甲基特征吸收峰可以确定羧甲基多糖的成功制备。当羧甲基茯苓多糖与酪蛋白共混后,能够有效提高酪蛋白的pH稳定性,显著影响了酪蛋白在酸性条件下的浊度、提高蛋白溶解性,这可能是由于多糖-蛋白质间静电互作力影响了复合物分子的表面电荷分布。其中,当二者以5: 1（w/w）复合时,羧甲基茯苓多糖表现出最好的稳定效果。随后的结构与理化性质研究表明,引入多糖,特别当二者以5: 1（w/w）复合时,复合物具有较低的聚集程度、表观粘度与模量,表明此时复合物的凝胶网络结构具有更好的结构稳定性。生物活性结果表明,复合物,特别以5: 1（w/w）复合时,表现出最强的抗氧化活性和非酶糖基化产物的抑制能力。结论 本研究所得羧甲基茯苓多糖-酪蛋白复合物在酸性环境下具有更好的结构稳定性与生物活性,为稳定的酸性乳制品的开发与加工提供理论基础和技术支持。