Traditionally rennet came from the curdled milk found in cows’ stomachs. It is often claimed that the method to create cheese was discovered when people first cut open a calf’s stomach to find this clotted material inside. This may be a little far fetched but we do know the Egyptians used to store things inside the stomachs of animals so perhaps this is where rennet first came to be known about and used in cheesemaking.
There are a wide variety of rennets out there, Liquid rennet, rennet powders, vegetarian rennet, rennet tablets (best avoided in my opinion) and even rennets derived from thistles!
Let’s talk about liquid rennet first; available in animal or vegetable, liquid rennet is the choice of professionals and the quality and potency (especially vegetable) has considerably improved over the last two decades. Vegetarian rennet is now not noticeably different from Animal in performance. Vegetarian microbial rennet is created through the use of particular moulds that acts similarly to the natural clotting enzyme occurring in animal rennet and is extracted from fungi - usually a species of the mould Rhizomucor.
For just one drop per litre for soft cheeses, a small bottle of rennet will last in your fridge for around 6 months and give you change out of £5.
Rennet powder works in pretty much the same way as the liquid version - you simply work out the correct dose and stir into the milk when it’s time to add it.
Rennet strength is measured in International Milk Clotting Units (IMCU) and requires getting your calculator out when working out dosages at the dairy level. Stick to a recipe’s requirements but remember old rennet can weaken and may need a few more drops to get the desired result. Use caution when adding more rennet though - too much will leave a bitter taste and can ruin the texture of the cheese you are aiming for. Instead, leave the lid on the milk pan and come back to it - rennet can take a little while to do its work.
There is a protein in the milk called casein. If you were to look at this protein under a microscope you’d see that it was made up of smaller parts called micelles. These micelles have ends known as kappas. When you add rennet to the milk it removes the kappa end which has a destabilising effect on the micelles, causing them to clump together.
This first part of the renneting process is called flocculation. It is quite easy to perform a flocculation test upon the curd during the renneting process - useful for determining how long to leave the curd before you will need to cut it. The second part is known as the hardening time - here the curd becomes more gel-like as the micelles clump together more tightly. The longer the hardening time the more the micelles join together making it more difficult to drain whey from the curd. Usually when making a soft cheese you’ll need a hard gel when it’s time to cut the curd and the opposite is true when making a hard cheese.