CuiScine: Rennet Cheese-Making

Last column, I explored the acidic chemistry of cheese-making. Most cheeses made purely with acid are sort of peculiar. Paneer, a South Asian cheese, is made solely through the acid process and is considered “non-melting”. The cheeses most people are used to encountering are created in conjunction with a substance called “rennet”.

Rennet is a combination of enzymes that is found in the gut of mammals, where it aids in the digestion of milk. In cheese-making, an enzyme in rennet called “chymosin” facilitates the creation of cheese curds through its interactions with proteins in milk.

Casein is a protein so ubiquitous in milk that it is frequently referred to as the “milk protein”. It has two portions, a polar side (called K-casein) and a nonpolar side. The polar portion enjoys hanging with the polar water molecules in milk. The nonpolar end likes to interact with the other casein molecules’ nonpolar portions. Because of this, casein molecules end up suspended in the water of milk in spherical structures called “micelles”; the nonpolar parts all huddle close together to make the center of the sphere, and the polar parts extend out from the surface of the sphere where they can touch the water.

The curdling of milk happens when these casein proteins lump together into larger structures. This can happen through the degradation of the K-casein, which will expose the hydrophobic center of the micelles. The nonpolar centers of the micelles will then lump together out of greater attraction for each other over the polar water molecules. This is what chymosin accomplishes. This plucky enzyme has but one noble duty; to cut casein on its K-casein end in such a way so as to diminish its polar properties. After this occurs, the micelles are no longer able to hang in little secluded bubbles, safely shielded by a polar surface. They adjust by joining together in one large structure, or what is more familiarly known as a cheese curd.

While chymosin is the main enzyme in rennet that is responsible for curdling milk, many enzymes in rennet will cut casein in different places, resulting in the various attributes of different cheeses. Although rennet has a less-than-regal beginning (in the stomach of a cow, generally), it is used in the creation of the richest brie and sumptuous parmagiano.