Rennet in cheese – the science: how does rennet work?

To define rennet, it is an enzyme responsible for curdling milk to separate the curds and whey and the beginning of the process to make cheese.

But for the science behind this definition of ‘how does rennet work’ read on below….

Milk is full of protein particles; most of these (>80%) are formed into the casein micelle. One part of this micelle (kappa casein: located on the extremity of the micelle) is negatively charged. This means it repels the other casein micelles, causing milk to stay in its liquid form (a bit like putting two magnets of the same pole together).

Rennet contains an enzyme (chymosin) which cuts this negatively charged kappa casein protein so that the negative end of the chain dissolves into the liquid (it will leave with the whey).

This is the primary phase of the coagulation.

With this neutralisation of the protein micelle the secondary phase can begin: the proteins no longer repel  each other and start to join together. This happens because the freshly cut kappa casein chain (now called para-kappa-casein) is sensitive to picking up minerals. Its cut end links up with phosphate and then calcium minerals present in milk to form a bridge that joins the casein micelles together (effectively creating a protein web).

Milk that is poor in calcium and phosphate (such as goats’ and cows’ milk – particular if it is not fresh) may have calcium phosphate added in to help improve this stage: this is why it can sometimes be seen on the ingredients list of cheese).

As the proteins join together (coagulate) to form the curd/junket, the web also traps in the fats and minerals.

This junket will go on to form the cheese (after other stages of manufacturer: cutting, draining, pressing, ageing).

Most of the rennet will leave with the whey: approximately 0.0000005g per kg is left in the finished cheese.