Research paperRennet-induced coagulation properties of yak casein micelles: A comparison with cow casein micelles

•Rennet-induced coagulation properties of yak casein micelles gels were characterized.•Calcium phosphate and κ-casein content lead to their long coagulation time and ordered structure.•The mechanism of rennet induced gel formation for yak milk casein micelles were firstly explored.

It is essential for yak cheese processing to understand the rennet-induced coagulation properties of gel formation from casein micelles. We have previously discovered that yak milk requires a longer incubation time but forms stronger gels compared with cow milk. In this study, we are aiming to understand the rennet-induced coagulation properties of yak casein micelles comparing with cow casein micelles. Rheological analyses revealed that the gelling times of yak and cow casein micelles were 11.6 ± 0.5 and 8.7 ± 0.4 min (P < 0.05) respectively, but yak casein gel had a higher elastic modulus G′ (6.5 ± 0.2 Pa) than cow casein gel (2.5 ± 0.2 Pa; P < 0.05). This is consistent with the results obtained by micro-rheology. Confocal laser scanning microscopic images (CLSM) and cryo-scanning electron microscopic images (cryo-SEM) showed that yak casein gel was more homogeneous and had smaller pore size than cow casein gels. Yak casein micelles had higher calcium (26.00 mM), phosphate (19.90 mM) and β-casein (relative 32%) concentrations. In addition, yak casein micelles were larger (Z-average 218.6 nm) than cow casein micelles, and contained lower κ-casein (relative 13%). By comparison with cow casein micelles, yak casein micelle composition corresponding to their micellar calcium phosphate and κ-casein content may greatly contribute to the longer coagulation time and denser gel structure. An initial slower caseinomacropeptide (CMP) release rate and the slower rate of aggregation between para-casein micelles contributed to a more homogeneous yak gel network. Higher colloidal calcium phosphate is crucial for yak casein micelle aggregation and gel firmness because sufficient colloidal calcium phosphates can firmly glue sub-micelles and links casein micelles. This study provides valuable information for yak cheese production.

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