Most of the investigations on the coagulation of casein by rennin has been conducted on bovine casein; however, given the differences in casein composition among species, the results obtained cannot be extrapolated to other ruminant species.
Casein, the major milk protein, exists largely in micellar form in milk. The micelle consists of a complex built up of very many casein units composed of amino acid chains. In caprine (goat) milk, s1-casein polymorphism is strongly related to the total casein content of the milk. This polymorphism is responsible for large variations in the cheesemaking properties of caprine milks. Thus, caprine milk containing high synthesis-rate s1-casein variants seems more suitable for cheesemaking because of its firmer curd and higher casein content. However, the relationship between s1-casein content and cheesemaking properties of caprine milks is still not sufficiently understood to allow definite conclusions.
The production of caprine milk constitutes an alternative to bovine milk of increasing importance. Caprine milk is mainly used in the cheese industry for the manufacture of caprine milk cheeses as well as cheeses from mixtures with other milk species. Since the rennin clotting behavior is a major factor in cheesemaking, information dealing with rennin clotting properties of caprine milk, compared to that of bovine milk, would allow the improvement of cheesemaking processes for this type of milk.
Coagulation may be considered as a three-phase system. During the primary phase the enzyme rennin splits the Phe-Met-bond of -casein causes the secondary phase of casein clotting, i.e., the aggregation of the micelles. The tertiary phase of coagulation is associated with an increase in the rigidity of the curd.
The loss of the hydrophilic glycomacropeptide should be apparent from the water relaxation rates of the proteins since the remaining caseins exhibit a rather high hydrophobicity. It has been assumed that an important decrease in hydration plays a dominant role in the mechanism of the secondary and third phase of coagulation. Similarly, one might expect the water-binding capacity of the caseins in solutions or dispersions to be affected.
We studied the hydration properties of reconstituted native and rennin-coagulated caprine caseins using oxygen-17 NMR techniques and determined the second virial coefficients of protein activity of the water transverse relaxation rates. Bovine casein was used for comparison purposes.