Coagulation

A natural chemical reaction that transforms fresh liquid milk into a solid. Traditional animal rennets are extracted from the stomachs of young ruminants. There are also vegetarian coagulants derived from stinging nettles, cardoon thistles, (cynara cardunculus), fig, papaya, and pineapple, or genetic material from molds.

Stirred into milk, rennet begins a two-step process that causes the protein in milk, which is normally dispersed in the liquid, to come together and form a matrix—what we call “curd.” To understand how this happens, try to imagine the ultra-tiny world of milk protein molecules—the casein type in particular. There are four different kinds of casein molecules in milk that attach to each other, forming protein “teams” called micelles that float throughout the milk. Milk is full of these micelle orbs. What’s remarkable is that molecules of one type of casein—kappa casein—protrude like fine hairs from the surface of each micelle sphere and attract water molecules. Without this attraction the micelles would separate from the liquid matter as solids. In other words, kappa casein makes milk proteins drinkable. 

For coagulation to happen, rennet has to perform the first step of the process, which is to cut those kappa hairs. When the surrounding layer of kappa casein breaks down, water molecules are repelled. This allows the second phase to begin, whereby all the casein molecules naturally aggregate, forming a sponge-like mass (again, curd). If caseins were not naturally inclined to attach to each other, coagulation would never happen, no matter how much rennet was added. We can thank molecular affinities, as much as rennet, for cheese as we know it.

Rennet induced or enzymatic coagulation takes about half an hour, varying with milk characteristics, temperature and recipe. Acidification continues until the lactic bacteria have ceased to function because temperature is no longer conducive, the acid level in the curds is too high, or they’ve simply run out of lactose to ferment.

Acidification also has a coagulating effect of its own. Curds formed by acid coagulation are generally less firm and elastic than those formed by rennet coagulation, they are more fragile and lead to a softer cheese with a higher moisture content. All milk bound for cheese is acidified, some by lactic fermentation (acidification) alone, with no rennet. The majority of cheeses rely on a combination of both types of coagulation.

The acid level inside a cheese is key to how it will ripen. It determines what type of micro flora can survive and thrive and how the different chemical breakdown reactions proceed toward developing texture, aroma, and flavor.

Pasteurization and heat treatment. Pasteurization kills virtually all microbes in milk, affecting the entire process of producing cheese. It necessitates recipe adjustment to reintroduce acidifying and coagulation agents and to replace the natural aroma and flavor inducing enzymes in milk.

Temperature. The higher the temperature the faster the chemical reaction. If milk is too cool it won’t proceed properly. If its too warm, the curds may become rubbery.