Action of rennets on casein

This subject is dealt with in Chapter 10. Suffice it to say here that K-casein is the only major casein hydrolysed by rennets during the primary phase of milk coagulation, which is the first step in the manufacture of most cheese 

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The effect produced by anti-rennet on rennet is not instantaneous. As in all the actions of enzymes, the time factor is very important. The milk, containing serum, coagulates as soon as rennet is added, because the action of rennet on casein is more rapid than that of anti-rennet on rennet. 

The gross proteolysis of casein is probably due solely to rennet and plasmin activity (O Keeffe et al. 1978). Bacterial proteases and peptides are responsible for subsequent breakdown of the large peptides produced by rennet and plasmin into successively smaller peptides and finally amino acids (O Keeffe et al. 1978). If the relative rate of proteinase activity by rennet, plasmin, and bacterial proteases exceeds that of the bacterial peptidase system, bitterness in the cheese could result. Bitter peptides can be produced from a,-,- or /3-casein by the action of rennet or the activity of bacterial proteinase on /3-casein (Visser et al. 1983). The proteolytic breakdown of /3-casein and the subsequent development of bitterness are strongly retarded by the presence of salt (Fox and Walley 1971 Stadhouders et al. 1983). The principal source of bitter peptides in Gouda cheese is 3-casein, and more particularly the C-terminal region, i.e., 3(193-209) and 3(193-207) (Visser et al. 1983). In model systems, bitter peptides are completely debittered by a peptidases system of S. cremoris (Visser et al. 1983). 

Proteins can be concentrated by isoelectric precipitation, heat precipitation, alcohol precipitation, ultrafiltration, microfiltration, and freeze concentration. Numerous references report differences in function in relation to the method of concentration of the protein. Whey protein produced by acid-heat precipitation (lactalbumin) is essentially insoluble but shows high water binding capacity and for this reason is the whey protein of choice for baking and for formulation of cereal products [89]. The use of ultrafiltration instead of acid precipitation of soy protein provides products with quite different characteristics [108], However, caseinate produced via acid precipitation is nearly as suitable for the production of imitation cheese as is rennet casein produced by the action of chymosin on skim milk [88],... 

Harding and Prucha worked, on cheese ripening at Geneva, N. Y., 1900-1910, and concluded that the digestion of toe casein of cheese during ripening is due to toe collective action of native enzymes of milk, bacterial enzymes, and rennet action. 

K-Casein was first isolated by Waugh and von Hippel (1956), who showed that this protein is responsible for the stability of the casein micelles and that its micelle-stabilizing properties are lost on renneting. Only /c-casein is hydrolyzed to a significant extent during the primary phase of rennet action. The primary cleavage site is Pheios-Metioe (Delfour et al, 1965), which is many times more susceptible to hydrolysis by acid proteinases (all commercial rennets are acid proteinases) than any other peptide bond in the milk protein system. 

The stabilizing properties of individual caseins, sodium caseinate and casein micelles are described with respect to the formation and behavior of emulsions. In particular, attempts are made to relate the properties of the emulsions (or rather the interfacial proteins) to the properties of the proteins and protein coitplexes dien they are in their solution or suspended state. In this, the stabilizing action of K-casein in the different emulsions is described an inpDrtant factor being its susceptibility to atack by rennet, which may serve as an indicator of its conformation on the interface. 

The non-coagulation of milk treated with rennet, in presence of anti-rennet, must be attributed to the formation of a combination between rennet and anti-rennet, a combination having no action on the casein. This opinion is confirmed by the fact that there exists a constant proportion between the quantity of rennet and that of serum necessary to neutralize it. This proportion is shown by the following test Put into a series of test tubes the same quantity of rennet of Imown value, and different quantities of serum. Let them stay for some time at ordinary temperature, then add to each tube lo c.c. of milk and determine the point of coagulation according to the method of Morgenroth. Repeat the same test with different quantities of rennet ... 

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