Influence of single protein fractions on goat milk coagulation ability

Milk proteins are known to be the main actors in the coagulation process, but compared to bovine milk little is known about their actual role in goat milk. For this reason, the present study aimed to analyze the effect of the single protein fractions on the clotting, curd firming, and syneresis phases of goat milk. This study is part of the GOOD-MILK project (D.M. 9367185 – 09/12/2020). The detailed milk protein profile of 820 goats belonging to 6 different breeds (Camosciata delle Alpi, Saanen, Murciano-Granadina, Maltese, Sarda, and Sarda Primitiva) was obtained using the reverse-phase high-performance liquid chromatography technique. The protein fractions were expressed quantitatively as grams per liter of milk (g/L) and qualitatively as percentage of total casein content (%CN). Traditional single-point milk coagulation properties (RCT, k20, a30, a45, a60) were obtained using the lactodynamography technique. All the measurements of curd firmness recorded with this analysis were examined using a mathematical model, which yielded new parameters of coagulation time (RCTeq), curd firming and syneresis rates (kCF and kSR), potential curd firmness (CFP), and maximum curd firmness (CFmax) at tmax time. All the coagulation traits were analyzed fitting a first linear mixed model where the fixed effects were breed, lactation stage, parity order, daily milk yield, and single protein fractions in g/L, while the flock was included as random. A second model was also applied, where milk production was substituted by the casein content and the protein fractions were expressed in %CN. The results show that the quantitative (g/L) variation of β-CN, the most abundant fraction, improves the curd firmness without affecting the clotting time. The αS1-CN improves the curd firmness as much as the β-CN even being one-third in concentration. The κ-CN, which is the smallest fraction, improves the curd firming rate and slightly the curd firmness. The αS2-CN has only a limited positive effect reducing the clotting time. When expressed qualitatively (%CN), the increasing proportion of the αS1-CN at the expense of the other caseins improves the curd firmness, while the increase of β-CN has no effect. The κ-CN exerts a similar positive effect as in g/L, while αS2-CN in %CN is detrimental for the curd firmness. In both models, the whey proteins seem to not affect the process. This information can be used as animal selection guidelines for the improvement of goat cheese production.