The process of making cheese is quite simple. Take some milk, separate the solid portion from the liquid portion, compress then age the solids, and ta-da! You have cheese.
But now comes the tricky part. Just how do we separate out those milk solids? The answer – use a coagulation agent.
A coagulation agent is any substance that, when added to milk, causes the calcium to unite with the casein (milk protein) to create calcium caseinate, an insoluble solid, better known as curd. The curd is then separated from the liquid portion of the milk, better known as whey. The curd is then compressed and aged to create the wonderful food we know as cheese.
But what kinds of substances exist that coagulates milk in this manner. There are several.
Bacteria – will chew up the milk sugar (lactose), convert it to lactic acid, and expel it back into the milk. This lowers the pH of the milk (makes it more acidic) and creates the right conditions for the calcium to unite with the casein.
Acid –Instead of waiting for the bacteria to create acid, you could instead add some of your own. Acids used to coagulate milk include citric, acetic, and tartaric.
Rennet – the most common coagulation substance used to make cheese. Rennet is a combination of enzymes – pepsin, lipase, and chymosin, the latter being the key component. Rennet comes from the stomachs of ruminant animals – usually slaughtered cows.
Other substances – several different substances have milk-coagulating abilities. These include vegetable rennets (derived from the Rhizomucor miehei mold), thistle (first used by the Romans), and chymosin (the main component of rennet created by laboratory fermentation)
Various combinations of the above – for some cheeses, you have to first add the bacteria and give it time to lower the pH of the milk. This will prepare the milk for coagulation and multiply the coagulation effects of rennet or other coagulation substances. However you do it, if you want the affectation of coagulation, you will need to use one or more of the above if you want to make cheese.