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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