Rennet-Induced Mixed Yak–Cow Casein Micelle Gels: Gelation Properties and Formation Mechanisms

Rennet-induced yak casein gels have longer gelation time but higher elastic modulus than bovine casein gels, which limits cheese processing efficiency. In this study, yak and bovine casein micelles were mixed at ratios of 100:0, 75:25, 50:50, 25:75, and 0:100 (denoted as Y100, Y75, Y50, Y25, and Y0, respectively). HPLC was used to monitor the relative peak area of released glycomacropeptide (GMP) during κ-casein hydrolysis (normalized to the 60-min maximum) to characterize rennet catalysis kinetics, and the effects on rheology, texture, and microstructure were analyzed. The results showed that the mixing ratio significantly modulated the hydrolysis rate of κ-casein, storage modulus (G′), hardness, chewiness, adhesiveness, and gel network compactness. Among them, Y75 (75% yak casein) exhibited the best performance, with G′ reaching 1709.78 Pa and a gel induction time of only 11.67 min; its hardness, chewiness, and adhesiveness were 1.46, 1.22, and 1.59 times those of pure bovine casein gel, respectively, and its microstructure showed a more uniform and dense pore distribution. In conclusion, optimizing the yak–bovine casein micelle ratio (75:25) can significantly improve gel quality while maintaining processing efficiency, providing a theoretical basis for mixed-milk cheese production, though further validation in real whole-fat milk systems is required.