In the process the milk in cheese becomes something completely unlike milk, but cheese has its own interesting and delicious properties. Cheese making is a long and involved process that makes use of bacteria, enzymes and naturally formed acids to solidify milk proteins and fat and preserve them. Once turned into cheese, milk can be stored for months or years. The main preservatives that give cheese its longevity are salt and acids. The basic steps in cheese making go something like this for most common cheeses like cheddar.
First milk is inoculated with lactic acid bacteria and rennet .
The rennet contains enzymes that modify proteins in milk. Specifically, rennet contains rennin, an enzyme that converts a common protein in milk called caseinogen into casein , which does not dissolve in water. The casein precipitates out as a gel-like substance and we see it as curd . The casein gel also captures most of the fat and calcium from the milk. The curds and whey are allowed to soak until the lactic acid bacteria create a lactic acid concentration that is just right.