Rennet-coagulation properties

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... 

Guinee, T.P., O Callaghan, D.J., Pudja, P.D., and O Brien, N., Rennet coagulation properties of retentates obtained by ultrafiltration of skim milks heated to different temperatures, Int. Dairy J., 6, 581, 1996. 

Waungana, A., Singh, H., and Bennett, R.J., Rennet coagulation properties of skim milk concentrated by ultrafiltration Effects of heat treatment and pH adjustment. Food Res. Int., 31, 645, 1998. 

Menard, O., and Gamier, B.G.F. (2005). Effect of heat treatment at alkaline pH on the rennet coagulation properties of skim milk. Lait 85, 515-526. 

Lopez, F.R. and Olano, A. 1998. Effect of high pressure combined with moderate temperature on rennet coagulation properties of milk. International Dairy Journal 8 623-627. 

Zobrist, M.R., Huppertz, T., Uniacke, T., Fox, PR, and Kelly, A.L. 2005. High pressure induced changes in the rennet coagulation properties of bovine milk. International Dairy Journal 15 655-662. 

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. 

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

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). 

The amino acid composition of some )S-lg variants is shown in Table 4.4. It is rich in sulphur amino acids which give it a high biological value of 110. It contains 2 moles of cystine and 1 mole of cysteine per monomer of 18 kDa. The cysteine is especially important since it reacts, following heat denatura-tion, with the disulphide of K-casein and significantly affects rennet coagulation and the heat stability properties of milk it is also responsible for the cooked flavour of heated milk. Some /S-lgs, e.g. porcine, do not contain a free sulphydryl group. The isoionic point of bovine j3-lgs is c. pH 5.2. 

These facts are quite favorable to the theory that pepsin, trypsin, and rennet are perhaps one and the same enzyme, the differences observed in the digestive and coagulating properties of pancreatic juice activated by varied quantities of CaClj being capable of explanation as being due to the same influences of the medium as those previously described in the chapter relative to the supposed identity of pepsin and rennet (see page 199). 

The principal component 1 (PCI) separated the acidification phases of BLG5, CREAM, BCAS, and MP clustered with negative values, from their rennet-induced coagulation phases that spread in a specific order associated with time from the left to the right of the map. These results showed that acidification and gelation of reconstituted milks induced different modifications in the fluorescence properties of Laurdan. 

The starting time for rheological measurements correspond to t = 120 min. Indeed, the rheological parameters were only recorded during the rennet-induced coagulation phase to avoid structural modifications during the acidification phase which may consequently influence the gelation process. Elastic and viscous properties of reconstituted milks... 

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. 

Removal of colloidal calcium phosphate (CCP) results in disintegration of the micelles into particles of mass 3 x 106 Da. The properties of the CCP-free system are very different from those of the normal milk system, e.g. it is sensitive to and precipitated by relatively low concentrations of Ca2 +, it is more stable to high temperatures, e.g. 140°C, and is not coagulable by rennets. Many of these properties can be restored, at least partially, by increased concentrations of calcium. 

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