Coagulation
During this step, the milk turns into a solid curd. Rennet is usually added to create this reaction which can take 30 minutes to 24 hours. The minimum amount of rennet that causes the desired reaction should be used. It should be diluted in some cool bottled water prior to stirring it into the milk. It should be combined well, but do not stir for more than a couple of minutes. If substituting goat or ewe milk for cow milk in a recipe, decrease the rennet by 20%. The pot temperature should be regulated and kept constant during the renneting process, and the pot should be covered and left undisturbed.
Some cheeses are coagulated without the use of rennet (or with the use of very small amounts). In these cheeses the coagulation process is produced by lactic bacteria turning lactose into lactic acid. Most cheeses, however, do call for the use of rennet to compliment and speed this process.
Another form of coagulation is direct acid addition to the milk. Vinegar, lemon juice or citric acid may be added to the milk to force coagulation in heat-treated milk.