Milk starts to turn into cheese once the lactic acid rises to sufficient levels to coagulate the proteins, separating the milk into curds and whey. Left to its own devices, milk would naturally turn acidic, but would also taste off. Rennet coagulates the milk before it can turn sour. In the absence of rennet, acids from citrus or vinegar will do the trick up to a point.

Although there is a vegetarian version made from a particular type of mold, traditional rennet is an enzyme obtained from the stomachs of calves or lambs before they progress from a milk-only diet.

Active between 85 and 140 degrees Fahrenheit, but at its most potent up to 105 degrees, rennet separates milk proteins into curd and whey. A less potent alternative is junket, which comprises 80 percent pepsin.

Once milk’s acidity is lowered to 4.6 pH while its temperature rises to 165 degrees F, the caseins suspended in the mixture will separate into curds. Both vinegar and citric acid are capable of dropping the acidity.

The process requires only milk, salt to taste and either distilled vinegar or fresh lemon juice. The higher the fat content in the milk, the better, although steer clear of UHT milk, which is sterilized by heating to 280 degrees Fahrenheit for 2 seconds. The process alters the milk’s protein structure, leaving it unable to separate.

  • Bring the milk to a low simmer so that it foams without burning. 
  • Remove from the heat and add lemon juice a tablespoon at a time. At the right balance, the milk will curdle.

  • Leave the milk to sit for 10 minutes, during which time it should separate completely.

  • Strain the curds through a cheesecloth laid within a colander or sieve.

  • Squeeze out any excess liquid, and lay the cheesecloth flat on a chopping board.

  • Sprinkle the curd with salt, then wrap the cheesecloth into a tight bundle in the shape of a block.

  • Press the curd block between two heavy plates and leave it to dry out for an hour.

  • Although it will keep only for a couple of days in the refrigerator, the resulting cheese is paneer, popular in Indian cuisine. The cheese is good to eat immediately, with no aging required. 

Make queso fresco, a Latin American version of paneer, by adding distilled vinegar gradually to the warm milk. The vinegar has a more neutral flavor than bright, tangy lemon juice.

Although authentic ricotta is made using leftover whey from cheese made with yogurt or rennet, you can make homemade ricotta just as you would paneer. Instead of pressing it out into a block, drain off the excess liquid instead for a wetter, smoother cheese.

Start with heavy cream, such as whipping cream, and add lemon juice to make homemade mascarpone, a cream cheese that forms the key ingredient in tiramisu. Because of the higher fat content, the temperature needs to be raised to about 190 degrees F, and the liquid won’t separate as distinctly as milk. Mascarpone doesn’t need to be squeezed and will have a consistency similar to custard.

Cottage cheese, like ricotta, is drained but not pressed. The easiest way to make it at home is to heat up milk and buttermilk with salt. Buttermilk is the residue from cream that’s been churned into butter, and turns acidic as it is allowed to ferment. As you heat it, the milk will separate into curds. The more fat in the buttermilk, the higher the temperature at which it will form curds.