Rennet and Starter Cultures Explained

Once you move beyond acid-set fresh cheeses, two ingredients take over: rennet, which coagulates the milk, and starter cultures, which acidify and flavour it. Understanding what each does makes the difference between following a recipe blindly and actually controlling your cheese.

What rennet does

Rennet is a set of enzymes that causes milk proteins to clump together into a firm curd. It is what turns liquid milk into a gel you can cut, cook and press. Without rennet you can still make acid-set cheeses — ricotta, paneer — but every aged or stretched cheese needs it.

• Animal rennet — traditional, from the stomach lining of young ruminants; gives a clean, firm set
• Vegetable rennet — from plants such as cardoon thistle; suitable for vegetarians but can give a softer, sometimes bitter set
• Microbial rennet — produced by fermentation; vegetarian, widely used, reliable

For home use, liquid microbial or vegetarian rennet is the most convenient and consistent choice.

What starter cultures do

Starter cultures are selected bacteria that you add to the milk to begin fermentation. They eat lactose (milk sugar) and produce lactic acid, which:

• Lowers the pH, helping the curd form and drain
• Creates flavour and aroma as the cheese ages
• Protects the cheese by making it less hospitable to unwanted microbes

Different cultures produce different cheeses: one for a buttery Gouda, another for the tang of a sour cream, another for the eyes in Emmental.

Do you need both?

• Fresh acid cheeses — no rennet, no cultures; just acid
• Mozzarella and many soft cheeses — rennet plus a simple culture or acid
• Aged cheeses — both rennet and specific cultures, plus time and careful humidity

Handling and measuring

Rennet and cultures are sensitive. Keep them cold, use clean water for dilution, and measure carefully — too much rennet makes a bitter, rubbery cheese, and dirty equipment can ruin a batch. Follow the recipe’s doses exactly for your first attempts, and write down what you did, so you can repeat a success.

Ready to begin? Start with mozzarella, then explore the equipment you will want in our beginner’s equipment guide.