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ABSTRACT: The casein composition, salt composition, and micelle size varies substantially between milk samples of individual animals. In goats, the links between those casein characteristics are unknown and could provide useful insights into goat casein micelle structure. In this study, the casein and salt composition of 42 individual Dutch goats from 17 farms was studied and linked to casein micelle size. Micelle size, the proportions of individual caseins, and protein content were associated with one another. Milk with smaller casein micelles was higher in protein content, salt content, and proportion of αs1-CN, but lower in αs2-CN and β-CN. The higher salt content in milk with small casein micelles was mainly attributed to a higher protein content, but changes in casein composition might additionally contribute to differences in mineralization. The nonsedimentable casein content in goat milk correlated with nonsedimentable fractions of β-CN and κ-CN and was independent of micelle size. Between large and small casein micelles, goat casein micelles showed more differences in casein and salt composition than bovine micelles, indicating differences in internal structure. Nevertheless, the casein mineralization in goat milk was similar to casein mineralization in bovine milk, indicating that mineralization of casein micelles follows a general principle. These results can help to better understand how composition and micelle structure in goat milk are related to each other, which may be useful to improve processing and product properties of goat milk in the future.

The destabilization of UHT milk during its shelf life can be promoted by the residual proteolytic activity attributed to the protease AprX from Pseudomonas. To better understand the hydrolysis patterns of AprX, and to evaluate the feasibility of using low-temperature inactivation (LTI) for AprX, the release of peptides through AprX activity on milk proteins was examined using an LC-MS/MS-based peptidomic analysis. Milk samples were either directly incubated to be hydrolyzed by AprX, or preheated under LTI conditions (60 °C for 15 min) and then incubated. Peptides and parent proteins (the proteins from which the peptides originated) were identified and quantified. The peptides were mapped and the cleavage frequency of amino acids in the P1/P1′ positions was analyzed, after which the influence of LTI and the potential bitterness of the formed peptides were determined. Our results showed that a total of 2488 peptides were identified from 48 parent proteins, with the most abundant peptides originating from κ-casein and β-casein. AprX may also non-specifically hydrolyze other proteins in milk. Except for decreasing the bitterness potential in skim UHT milk, LTI did not significantly reduce the AprX-induced hydrolysis of milk proteins. Therefore, the inactivation of AprX by LTI may not be feasible in UHT milk production.