How rennet works

By HomeCheeseAdam | Beginner

Oct 14

Trimming the Hairy Casein

Most non-cheesemakers will be aware of rennet, just through noticing it mentioned on the packets of cheese they buy from the supermarket. But, for the home cheese maker, rennet is an essential tool for the construction of most cheeses that we make. In this article, we explore rennet a bit more deeply.

Rennet is a coagulant; its role in cheese making is to coagulate the caseins out of the milk to for curds. What makes rennet different from coagulants used in simpler cheese making, such as lemon juice or citric acid, is that it works at a low temperature over a sustained period of time, which means we can create our cheese in a cooler environment and longer time which allows our starter cultures to create richer, deeper flavours and multiply in a habitat with a temperature more conducive to sustaining them.

The way rennet works is incredibly simple and yet, completely compelling. The beginner’s guide to milk gives an overview of the structure of milk, including the casein protein group. What you need to know to understand how rennet coagulates milk, is that there are four proteins that make up the casein group, and these four attach to each other within the milk in little groups called micelles. Micelles are naturally inclined to stick together as a solid but, clearly, they don’t, otherwise all milk would  naturally be a gel-like substance.

It helps to think of the micelles as little globes. One of the four casein proteins, called kappa-casein produces hair-like structures that rise from the surface of each micelle globe and attract water molecules to them. It is this architecture which means that each micelle orb is surrounded by water and that milk itself is naturally a liquid.

Rennet evolved in cows, sheep and goats to help unweaned calves, lambs and kids solidify mother’s milk in their stomachs. Solidified, they can take longer to digest it and, consequently, take more nutrition from it than if it remained as a liquid. Rennet is actually a very powerful enzyme called chymosin, it is so powerful that one part enzyme can coagulate 15,000 parts of milk!

Let’s get back to considering how rennet actually works. We take the chymosin from the fourth stomach of a calf and refine it (in days gone by, you would by strips of dried calf stomach to source your rennet) to a product, either liquid or tablet, that we can use with our shop-bought milk. When it is added in appropriate dilute form to our milk, the enzyme rapidly sets about removing the kappa-casein hairs from the casein micelles in the milk. When they are gone, the micelles can no longer resist their compulsion to group together, and so they come out of precipitation, coagulating the milk into more of a gel than a liquid. We cheese makers then cut that gel to form curds and whey.

Thankfully for the many vegetarian cheese makers and eaters, there are alternatives to rennet from a calf’s fourth stomach. Vegetable rennet can be sourced from plants such thistles, nettles and melon amongst others. Traditionally the enzyme (which is similar to chymosin) would be derived by creating a ‘tea’ from the plant and using it in the milk, as you might imagine, however, that makes it exceedingly difficult to control the concentration of rennet in any particular cheese batch.

The other vegetarian-friendly alternative comes from two moulds that naturally create chymosin, and in a way that can be controlled and manufactured. Later advances also mean we have the option of a genetically modified version of animal rennet, whereby cells from a calf’s stomach are extracted and the genetic code which creates the chymosin is transplanted into a yeast, which then produces chymosin exactly as a calf would. Where a vegetarian would stand on this (or a GM campaigner for that matter) I leave to them for comment.

Traditional rennet is thought by many producers to be superior to vegetable- or microbe-sourced rennet, as the latter can lend a bitter taste to a cheese matured over time. Either way, as a homemade cheese producer, there are several stores that can supply you with rennet in the form most suitable for you. And now, at least, you understand the simple brilliance of how it works.


Are you ready to try your first cheese? Each of these inexpensive cheese making kits is supplied with rennet and is so simple to follow that you’ll have made your first cheese within a week!