Riddet Institute, Private Bag 11 222, Palmerston North 4442, New Zealand; School of Food and Advanced Technology, Massey University, Private Bag 11 222, Palmerston North 4442, New Zealand.
Riddet Institute, Private Bag 11 222, Palmerston North 4442, New Zealand; School of Food and Advanced Technology, Massey University, Private Bag 11 222, Palmerston North 4442, New Zealand.
J Dairy Sci. 2022 Feb;105(2):990-1003. doi: 10.3168/jds.2021-21177. Epub 2022 Jan 5.
Hydrolysis-induced coagulation of casein micelles by pepsin occurs during the digestion of milk. In this study, the effect of pH (6.7-5.3) and pepsin concentration (0.110-2.75 U/mL) on the hydrolysis of κ-casein and the coagulation of the casein micelles in bovine skim milk was investigated at 37°C using reverse-phase HPLC, oscillatory rheology, and confocal laser scanning microscopy. The hydrolysis of κ-casein followed a combined kinetic model of first-order hydrolysis and putative pepsin denaturation. The hydrolysis rate increased with increasing pepsin concentration at a given pH, was pH dependent, and reached a maximum at pH ∼6.0. Both the increase in pepsin concentration and decrease in pH resulted in a shorter coagulation time. The extent of κ-casein hydrolysis required for coagulation was independent of the pepsin concentration at a given pH and, because of the lower electrostatic repulsion between para-casein micelles at lower pH, decreased markedly from ∼73% to ∼33% when pH decreased from 6.3 to 5.3. In addition, the rheological properties and the microstructures of the coagulum were markedly affected by the pH and the pepsin concentration. The knowledge obtained from this study provides further understanding on the mechanism of milk coagulation, occurring at the initial stage of transiting into gastric conditions with high pH and low pepsin concentration.
胃蛋白酶作用下的酪蛋白胶束水解发生在牛奶消化过程中。本研究采用反相高效液相色谱法、振荡流变学和共聚焦激光扫描显微镜，在 37°C 下研究了 pH（6.7-5.3）和胃蛋白酶浓度（0.110-2.75 U/mL）对牛脱脂乳中κ-酪蛋白水解和酪蛋白胶束凝结的影响。κ-酪蛋白的水解遵循一级水解和假定胃蛋白酶变性的组合动力学模型。在给定 pH 下，随着胃蛋白酶浓度的增加，水解速率增加，pH 依赖性，在 pH ∼6.0 时达到最大值。胃蛋白酶浓度的增加和 pH 的降低都会导致凝结时间缩短。在给定 pH 下，凝结所需的κ-酪蛋白水解程度与胃蛋白酶浓度无关，由于较低 pH 下副酪蛋白胶束之间的静电排斥较低，当 pH 从 6.3 降低到 5.3 时，显著从约 73%降低到约 33%。此外，流变学性质和凝结物的微观结构受到 pH 和胃蛋白酶浓度的显著影响。本研究获得的知识进一步加深了对在高 pH 和低胃蛋白酶浓度下过渡到胃环境的初始阶段发生的牛奶凝结机制的理解。
