Milk clotting

The gel must be uniform and possess the appropriate strength in order that there should be maximum retention of casein and milk fat, as well as to minimise variation in the levels of moisture. Enzymes are preserved by ensuring that the temperature does not rise excessively and protecting the process stream from excesses of pH and oxidising agents such as the hypochlorites employed in cleaning regimes.

The most important milk clotting enzyme is chymosin, which has an optimum pH around 6.0. A shortage of calves, alongside public acceptability issues, mean that alternative source of the enzyme have been sought. The gene for chymosin has been expressed in microbes, notably A. niger, K. lactis, and E. coli K12. More than half of the world cheese market is probably now dependent on the use of such preparations.

Clotting occurs due to the hydrolysis of a single bond in k-casein, the impact of which is reduced micelle stabilising capability. The hydrolysis releases the hydrophilic N-terminal region of the molecule which in the unhydrolysed molecule serves the function of reaching out from the micelle surface into the solvent and stabilising it. Accordingly, the micelles aggregate. Enzyme activity is also important for the initial proteolysis during cheese maturation.

Cheeses differ in their optimum gel firmness. Those that have firmer gels will expel whey more slowly.

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