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How Rennet Works:
Vegetable Rennet contains an enzyme called chymosin, which acts on the milk proteins (caseins), in the milk, causing them to clot together. When an enzyme acts on a protein, it assists in breaking down some amino acid bonds, which form the protein, changing its structure. This is essentially what happens when milk is digested in the stomach, and why animal rennet is taken from the stomach of calves. Enzymes coagulate milk by destabilizing the casein proteins, which causes them to join together and form a stable gel. So when you add rennet to milk, at the correct temperature and pH, it will make the milk separate into curds and whey.
From a microbiological point of view, there are two phases in the formation of the curd, once rennet is added. The first is enzymatic. During this phase, the enzyme works on the k-casein, unbalancing the protein, which will then want to achieve balance by bonding to other structures, with the assistance of calcium as a catalyst. When approximately 75-80% of the casein structures have been unbalanced, the milk has destabilized to the point where a soft gel forms, trapping the fats and other silk solids within the gel.
The second stage is called aggregation. During this phase, the casein structures join together and form a curd, with the strength of the curd increasing over time.
Heat treatment of milk above 65° reduces the milk’s ability to coagulate when rennet is added. However, the addition of calcium chloride can reverse this. Treatment at above 90° renders the milk unable to coagulate, making it unsuitable for cheese making, and this is why you should not use ultra-heat treated milk.
Pre-heating milk to 65° has a beneficial effect on rennet coagulation, however, the rapid cooling of this milk after pasteurisation causes the deposition of insoluble calcium phosphate; meaning that the calcium, which is a necessary catalyst for the aggregation of the proteins in the milk to form a curd, are removed from solution, making them unavailable. Ideally, milk should be pasteurised immediately before cheese making and should not be cold stored after heat treatment. If you are using heat treated milk that has then been cold stored, such as milk from the supermarket, then you will need to add calcium chloride to the milk to make it suitable for cheese making. The addition of calcium chloride will improve the rennet activation time and give you a better curd.
For a more in depth study, and detailed scientific information on cheese we highly recommend "Fundamentals of Cheese Science" by Patrick F Fox, Timothy P Guinee, Timothy M Cogan, Paul L H McSweeney.
This book is ideal for anyone who really wants to understand the microbiology of cheese making.