Chymosin casein micelles

The most abundant milk protein is casein, of which there are several different kinds, usually designated a-, (1-, and K-casein. The different caseins relate to small differences in their amino acid sequences. Casein micelles in milk have diameters less than 300 nm. Disruption of the casein micelles occurs during the preparation of cheese. Lactic acid increases the acidity of the milk until the micelles crosslink and a curd develops. The liquid portion, known as whey, containing water, lactose and some protein, is removed. Addition of the enzyme rennet (chymosin) speeds up the process by hydrolysing a specific peptide bond in K-casein. This opens up the casein and encourages further cross-linking. 

The Daily Industiy. The first step in cheese manufacture is the coagulation of milk. Coagulation can be divided into two distinct phases, enzymatic and the non-enzymatic. In the primary enzymatic phase a proteol ic enzyme such as chymosin (rennet), or less effectively, pepsin, carries out an extremely specific and limited proteolysis, cleaving a phenylalanine-methionine bond of /c-casein, making the casein micelle metastabie. In the second, non-enzymatic phase, the... 

Figure 10.3 Schematic representation of the rennet coagulation of milk, (a) Casein micelles with intact K-casein layer being attacked by chymosin (Q (b) micelles partially denuded of K-casein (c) extensively denuded micelles in the process of aggregation (d) release of macropeptides ( ) and changes in relative viscosity (0) during the course of rennet coagulation.

Most current models put K-casein on the outer casein micelle surface (Heth and Swaisgood 1982 McMahon and Brown 1984A Shahani 1974). This allows the possibility that heat-induced coagulation of milk is the result of serum proteins interacting with K-casein on the micelle surface and with each other to interconnect micelles. The observation that chymosin cannot release macropeptides from K-casein in heated milk (Morrissey 1969 Shalabi and Wheelock 1976,1977) suggests that... 

Shalabi, S. I. and Wheelock, J. V. 1976. The role of a-lactalbumin in the primary phase of chymosin action on heated casein micelles. J. Dairy Res. 43, 331-335. 

Reddy, D., Payens, T. A. and Brown, R. J. 1986. Effect of pepstatin on the chymosin-triggered coagulation of casein micelles. J. Dairy Sci. 69 (Suppl. 1), 72. 

Historically, ideas of casein micelle structure and stability have evolved in tandem. In the earlier literature, discussions of micellar stability drew on the classical ideas of the stability of hydrophobic colloids. More recently, the hairy micelle model has focused attention more on the hydrophilic nature of the micelle and steric stabilization mechanisms. According to the hairy micelle model, the C-terminal macropeptides of some of the K-casein project from the surface of the micelle to form a hydrophilic and negatively charged diffuse outer layer, which causes the micelles to repel one another on close approach. Aggregation of micelles can only occur when the hairs are removed enzymatically, e.g., by chymosin (EC 3.4.23.4) in the renneting of milk, or when the micelle structure is so disrupted that the hairy layer is destroyed, e.g., by heating or acidification, or when the dispersion medium becomes a poor solvent for the hairs, e.g., by addition of ethanol. 

Casein micelles are remarkably stable structures. Milk may be boiled, sometimes for several hours, without coagulating the micelles. Also, the addition of CaCU to milk does not precipitate the micelles up to concentrations greatly in excess of that required to precipitate purified whole casein. On the other hand, micelles rapidly flocculate after treatment with chymosin, at or above room temperature, and casein... 

The stomach environment is acidic as a result of HC1 secretion by the parietal cells. The acidic pH serves to denature many proteins, thus making them susceptible to proteolysis. The chief cells of the stomach produce pepsinogen, which is activated to pepsin by the HC1 (see Table 20.3). The optimum pH of peptic activity is around 2, and pepsin is inactivated at neutrality. Another stomach enzyme is rennin or chymosin, which is present in infants but not in adults. It removes a glycopeptide from milk-K-casein, disrupting the casein micelle and promoting milk protein coagulation and digestion. 

Acidification to the isolectric pH of casein using lactic acid bacteria or food-grade acids/acidogens, at 20 40 C. and resultant slow quiescent aggregation of the sensitized casein micelles e.g., for cream cheese. [A combination of acidification and rennet-hydrolysis (a smaller quantity of rennet than for rennet-curd cheeses, e.g., 5-100 versus 900-1000 chymosin units per 100 L milk) is normally used for low-fat acid-curd cheeses such as Quark and related varieties (Schulz-Collins and Senge, 2004)]... 

Chymosin is secreted by the abomasum in the form of an intially inactive precursor, prochymosin, which is converted autocatalytically to chymosin (see below, p. 176). Many reviews on the properties of the zymogen and the enzyme have been published. One of the most recent, by Foltmann (J), also contains references to earlier reviews. The action of chymosin on K-casein is primarily responsible for the milk-clotting process. The numerous studies of this reaction have been reviewed recently by Mackinlay and Wake (32). Because of the number of diflFer-ent milk proteins and the complexity of their interactions, especially in the casein micelle, the full details of the coagulation process are not yet understood. 

Dalgleish, D.G. (1979). Proteolysis and aggregation of casein micelles treated with immobilized or soluble chymosin. J. Dairy Res. 46, 653-661. 

Walstra, P., Bloomfield, V.A., Wei, G.J., and Jenness, R. (1981). Effect of chymosin action on the hydrodynamic diameter of casein micelles. Biochim. Biophys. Acta 669, 258-259. 

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