Rennet coagulation

Casein may be coagulated and recovered as rennet casein by treatment of milk with selected proteinases (rennets). However, one of the caseins, K-casein, is hydrolysed during renneting and therefore the properties of rennet casein differ fundamentally from those of acid casein. Rennet casein, which contains the colloidal calcium phosphate of milk, is insoluble in water at pH 7 but can be dissolved by adding calcium sequestering agents, usually citrates or polyphosphates. It has desirable functional properties for certain food applications, e.g. in the production of cheese analogues. 

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Food. Food-grade calcium chloride is used in cheese making to aid in rennet coagulation and to replace calcium lost in pasteurization. In the canning iadustry it is used to firm the skin of fmit such as tomatoes, cucumbers, and jalapenos. It acts as a control in many flocculation, coagulation systems (37). Food-grade calcium chloride is used in the brewing iadustry both to control the mineral salt characteristics of the water and as a basic component of certain beers (see Beer). 

Destruction of the casein micelles in the milk with subsequent precipitation of the casein can be accomplished in a number of ways. The action of heat or the action of alcohols, acids, salts and the enzyme rennet all bring about precipitation. In commercial practise the two techniques used employ either acid coagulation or rennet coagulation mechanisms. 

In the rennet coagulation process fresh skimmed milk is adjusted to a pH of six and about 40 ounces of a 10% solution of rennet are added per 100 gallons of milk. The initial reaction temperature is about 35°C and this is subsequently raised to about 60°C. The coagulation appears to take place in two stages. Firstly the calcium caseinate is converted to the insoluble calcium paracaseinate and this then coagulates. 

Casein is low in sulphur (0.8%) while the whey proteins are relatively rich (1.7%). Differences in sulphur content become more apparent if one considers the levels of individual sulphur-containing amino acids. The sulphur of casein is present mainly in methionine, with low concentrations of cysteine and cystine in fact the principal caseins contain only methionine. The whey proteins contain significant amounts of both cysteine and cystine in addition to methionine and these amino acids are responsible, in part, for many of the changes which occur in milk on heating, e.g. cooked flavour, increased rennet coagulation time (due to interaction between /Mactoglobulin and K-casein) and improved heat stability of milk pre-heated prior to sterilization. 

When heated in the presence of whey proteins, as in normal milk, K-casein and /Mactoglobulin interact to form a disulphide-linked complex which modifies many properties of the micelles, including rennet coagulability and heat stability. 

Crude enzyme preparations have been used in food processing since prehistoric times classical examples are rennets in cheesemaking and papaya leaves to tenderize meat. Added (exogenous) enzymes are attractive in food processing because they can induce specific changes, in contrast to chemical or physical methods which may cause non-specific undesirable changes. For some applications, there is no viable alternative to enzymes, e.g. rennet-coagulated cheeses, whereas in some cases, enzymes are preferred... 

The proteins can participate in sulphydryl-disulphide interchange reactions at temperatures above about 75°C at the pH of milk, but more rapidly at or above pH 7.5. Such interactions lead to the formation of disulphide-linked complexes of / -lg with K-casein, and probably as2-casein and a-la, with profound effects on the functionality of the milk protein system, such as rennet coagulation and heat stability. 

The current explanation for the maximum-minimum in the HCT-pH profile is that on heating, K-casein dissociates from the micelles at pH values below about 6.7, /Mg reduces the dissociation of K-casein, but at pH values above 6.7, it accentuates dissociation. In effect, coagulation in the pH range of minimum stability involves aggregation of K-casein-depleted micelles, in a manner somewhat analogous to rennet coagulation, although the mechanism by which the altered micelles are produced is very different. 

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

There are at least 1000 named cheese varieties, most of which have very limited production. The principal families are Cheddar, Dutch, Swiss and Pasta filata (e.g. Mozzarella), which together account for about 80% of total cheese production. All varieties can be classified into three superfamilies based on the method used to coagulate the milk, i.e. rennet coagulation (representing about 75% of total production), isoelectric (acid) coagulation and a combination of heat and acid (which represents a very minor group). 

The production of rennet-coagulated cheeses can, for convenience, be divided into two phases (1) conversion of milk to curds and (2) ripening of the curds. 

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.

Factors that affect rennet coagulation. The effect of various compositional and environmental factors on the primary and secondary phases of rennet coagulation and on the overall coagulation process are summarized in Figure 10.4. 

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 10.4 Principal factors affecting the rennet coagulation time (RCT) of milk.

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. 

While rennet-coagulated cheese curd may be consumed immediately after manufacture (and a little is), it is rather flavourless and rubbery. Consequently, rennet-coagulated cheeses are ripened (matured) for a period ranging from about 3 weeks for Mozzarella to more than 2 years for Parmesan and extra-mature Cheddar. During this period, a very complex series of biological, biochemical and chemical reactions occur through which the characteristic flavour compounds are produced and the texture altered. 

Commercial casein is usually manufactured from skim milk by precipitating the casein through acidification or rennet coagulation. Casein exists in milk as a calcium caseinate-calcium phosphate complex. When acid is added, the complex is dissociated, and at pH 4.6, the isoelectric point of casein, maximum precipitation occurs. Relatively little commercial casein is produced in the United States, but imports amounted to well over 150 million lb in 1981 (USDA 1981C). Casein is widely used in food products as a protein supplement. Industrial uses include paper coatings, glues, plastics and artificial fibers. Casein is typed according to the process used to precipitate it from milk, such as hydrochloric acid casein, sulfuric acid casein, lactic acid casein, coprecipitated casein, rennet casein, and low-viscosity casein. Differences... 

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