Rennet is a set of enzymes that makes milk set into a firm, elastic curd, and it is the difference between soft spoonable cheeses and firm sliceable ones. Vegetarian microbial rennet does exactly the same job as traditional animal rennet, which is why it is what we pack in every Cheese Lab kit.
What rennet actually does
The key enzyme in rennet is chymosin. It snips a very specific protein on the surface of casein micelles, the little protein bundles floating in milk. Once snipped, the bundles stop repelling each other and knit into a three-dimensional mesh that traps fat and water: a curd. Because the mesh is enzymatic rather than acidic, it is stronger and more elastic, which is what lets you stretch mozzarella and press halloumi.
The three kinds of rennet
- Animal rennet is the traditional source, extracted from calf stomachs. Effective, but obviously not vegetarian.
- Microbial rennet is produced by fermenting a fungus, Rhizomucor miehei, which naturally makes a chymosin-like enzyme. No animal involved, fully vegetarian, and reliable at home scale. This is what The Cheese Lab uses.
- Fermentation-produced chymosin is made by microbes that have been given the chymosin recipe. Most industrial cheese is made with it.
Which cheeses need rennet, and which do not
Acid-set cheeses need no rennet at all: ricotta, paneer, mascarpone and cream cheese set with citric acid or lemon juice. Rennet earns its keep in mozzarella, burrata, halloumi, and every pressed or aged cheese. As a rule of thumb: if you can slice it, grill it or stretch it, rennet was involved.
Handling tips from the lab
Rennet is an enzyme, and enzymes are fussy about heat. Add it to milk that is warm, not hot: over about 55C the enzyme is destroyed and the milk will never set. Once added, leave the pot alone. Stirring while the mesh is forming breaks it into fragments, which is where grainy curds come from. Our Full System kit explains the why behind every step across forty recipes, and the Lab Notes cover the common rennet mishaps.
Quick answers
What is the role of rennet in cheese making?
Rennet contains enzymes that knit milk proteins into a firm, elastic curd. It is what makes stretchable and sliceable cheeses possible. Acid-set cheeses like ricotta do not need it.
Is vegetarian rennet as good as animal rennet?
Yes for home cheesemaking. Microbial rennet, fermented from the fungus Rhizomucor miehei, performs the same job and is fully vegetarian. Every Cheese Lab kit uses it.
Why did my milk not set after adding rennet?
Usually the milk was too hot, which destroys the enzyme, or it was UHT milk, which cannot set. Add rennet to warm (not hot) milk and leave it undisturbed.