Casein rennet coagulation

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. 

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. 

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. 

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. 

The primary step in the manufacture of most cheese varieties and rennet casein involves coagulation of the casein micelles to form a gel. Coagulation... 

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. 

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... 

The casein micelles become surrounded by whey proteins and cannot interact with one another, thus reducing whey syneresis. This results in a soft curd that retains more moisture. The yield of cheese is increased due to the incorporation of whey proteins and the higher moisture content. Overheated milk requires longer rennet coagulation times. If milk is heated for 30 min at 75° C, it will not clot at all (Ustu-nol and Brown 1985). 

The effects of homogenization on milk components have been summarized by Walstra and Jenness (1984) and Harper (1976). Homogenization disrupts fat globules and results in an increase in fat surface area (about 4-10 times). Casein micelles adsorb on the fat surface and constitute part of the fat globule membrane. The curd tension of milk is thus lowered. Walstra and Jenness (1984) have described the effect of homogenization on rennet coagulation. 

The proteolytic systems of psychrotrophic bacteria selectively attack /3- and as-caseins (Cousin and Marth 1977A), whereas whey proteins are relatively unaffected. Growth of psychrotrophic bacteria in milk results in decreased stability of casein, as measured by rennet coagulation time and heat stability (Cousin and Marth 1977B). Growth of psychrotrophs in milk also causes an increased rate of acid production by starter cultures as a result of increased quantities of readily available nitrogen compounds (Cousin and Marth 1977C.D). 

These are compounds obtained by chemical or enzymatic methods and are divided into primary and secondary derivatives, depending on the extent of change that has taken place. Primary derivatives are slightly modified and are insoluble in water rennet-coagulated casein is an example of a primary derivative. Secondary derivatives are more extensively changed and include proteoses, peptones, and peptides. The difference between these breakdown products is in size and solubility. All are soluble in water and... 

The stability of the caseinate particles in milk can be measured by a test such as the heat stability test, rennet coagulation test, or alcohol stability test. Addition of various phosphates—especially polyphosphates, which are effective calcium complexing agents—can increase the caseinate stability of milk. Addition of calcium ions has the opposite effect and decreases the stability of milk. Calcium is bound by polyphosphates in the form of a chelate, as shown in Figure 5-3. 

Casein enrichment of cheese milk significantly improves the rennet coagulability and the productivity of cheese plants, especially those producing hard cheeses [100]. For instance, the rennet coagulation time of a 3% native PPCN solution is reduced by 53% compared to that of raw milk, and the gel fimmess after 30 min is increased by more than 50% (Pierre et al., 1992, cited in Ref. [100]). 

MUk coagulation depends on a number of factors, such as the kinetics of the enzyme reaction, the concentration, and the state of the proteins, especially casein, the balance of minerals, especially calcium, and pH [101]. Most of these factors are directly influenced by UF or MF processing. Caron et al. [101] compared the rennet coagulation properties of nfilk enriched with a regular ultrafiltered retentate powder (RUF) to milk enriched with a diafiltered MF (DMF) retentate powder. RUF was prepared by concentrating skim milk to concentration factor of 5 by UF, while DMF was prepared from skim nfilk concentrated... 

Heat treatment of milk above 60 °C, which promotes whey protein denaturation and its complexation with K-casein at normal milk pH (6.6), also affects renneting properties. An increase in rennet coagulation time and a decrease in gel firmness were observed with increased heat treatment of milk (Menard and Gamier, 2005). Ultra-high temperature (UHT) treated milk failed to coagulate completely but the coagulation properties were restored by threefold concentration of the UHT milk (McMahon ef al., 1993). 

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