Coagulation of milk

THE EFFECTS OF THE COMPOSITION OF THE FAT GLOBULE SURFACE ON THE COAGULATION OF MILK... 

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... 

Ca2+ and Mg2 +, along with H+, play especially important roles in the stability of the caseinate system and its behaviour during milk processing, especially in the coagulation of milk by rennet, heat and ethanol. The... 

Effect of heat treatment on rennet coagulation of milk and related properties... 

Fox, P.F. (1982) Heat-induced coagulation of milk, in Developments in Dairy Chemistry, Vol. 1 Proteins (ed. P.F. Fox), Applied Science Publishers, London, pp. 189-228. 

Enzymatic coagulation of milk. The enzymatic coagulation of milk involves modification of the casein micelles via limited proteolysis by selected proteinases, called rennets, followed by calcium-induced aggregation of the rennet-altered micelles ... 

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.

Measurement of rennet coagulation time. A number of principles are used to measure the rennet coagulability of milk or the activity of rennets most measure actual coagulation, i.e. combined first and second stages, but some specifically monitor the hydrolysis of K-casein. The most commonly used methods are described below. 

Figure 10.7 (a) Hot wire sensor for objectively measuring the rennet coagulation of milk, (b) Changes in the temperature of the hot wire during the course of the rennet coagulation of milk. 

Research reports on milk stability have been published frequently since about 1919. Early studies were prompted by the need to ensure sufficient heat stability for evaporated milk to withstand heat sterilization (Sommer and Hart 1919, 1922). Between 1919 and 1960, most attention was directed to the influence of milk salts on heat stability (Miller and Sommer 1940 Pyne 1958 Pyne and McHenry 1955). It was not until the early 1960s that the importance of heating time and pH on coagulation of milk was appreciated (Rose 1961A.B). More recent work has been concerned with factors which affect the stability of milk proteins. 

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... 

Feagan, J. T., Bailey, L. F., Hehir, A. F., McLean, D. M. and Ellis, N. J. S. 1972. Coagulation of milk proteins. I. Effect of genetic variants of milk proteins on rennet coagulation and heat stability of normal milk. Aust. J. Dairy Technol. 27, 129-134. 

Pyne, G. T. 1958. The heat coagulation of milk II. Variations in sensitivity of casein to calcium ions. J. Dairy Res. 25, 467-474. 

Tumerman, L. and Webb, B. H. 1965. Coagulation of milk and protein denaturation. In Fundamentals of Dairy Chemistry. B.H. Webb and A.H. Johnson (Editors). AVI Publishing Co., Westport, Conn., pp. 506-582. 

Cheryan, M., Van Wyk, P. J., Olson, N. F. and Richardson, T. 1975A. Continuous coagulation of milk using immobilized enzymes in a fluidized-bed reactor. Biotechnoi Bioeng. 17, 585-598. 

Claesson, O. and Claesson, E. 1970. Optical measurement of the rennin coagulation of milk. XVIII Int. Dairy Congr. Proc. IE, 42. 

McMahon, D. J., Brown, R. J. and Emstrom, C. A. 1984A. Enzymic coagulation of milk casein micelles. J. Dairy Sci. 67, 745-748. 

Reuter, H. Hisserich, D. and Prokopek, D. 1981. Study on the formal kinetics of rennet coagulation of milk concentrated by ultrafiltration. Milchwissenschaft 36, 13-18. 

Further information on such a novel process, as well as the preparation of acid caseinates, electrochemical coagulation of milk proteins or electroreduction of disulfide bonds of milk proteins, using conventional or bipolar ED stacks, is given by Bazinet (2004). It is however worthy underlining that all these processes do not seem to have been used on an industrial scale yet. 

The primary (enzymatic) phase of renneting overlaps somewhat with the secondary phase of aggregation. The gel subsequently undergoes syneresis to produce curds and whey while a slow but more general proteolysis of the caseins begins, which eventually contributes substantially to the distinctive flavor and texture of cheese. The enzymatic coagulation of milk and formation of the curd has been reviewed by Dalgleish (1987). Here, attention will be confined to parts of the subject that most clearly relate to the structure and stability of bovine casein micelles. 

Milk Coagulation. The first step in cheese manufacture is the coagulation of milk. Traditionally, this coagulation step is catalyzed by the enzyme rennet. Rennet is a saline extract of the 4th stomach of calves, usually slaughtered before they are 30 days old. The principal protease in rennet is rennin. In an attempt to avoid confusion with the hormone peptide renin, the International Enzyme Nomenclature Committee has assigned the name chymosin to the protease in calf rennet. During the growth of calves, chymosin is replaced by pepsin, the acid protease of the mature stomach. 

Isoelectric coagulation of milk proteins, mainly of caseins. 

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