How much rennet per litre of milk?

The dosing table

These amounts are for our liquid calf rennet. Count on roughly 20 drops per millilitre. Since no two drops are exactly the same size, it is worth checking the first time with a small oral syringe (the kind without a needle) marked in tenths of a millilitre.

With a 50 ml bottle you can make cheese from 250 to 500 litres of milk. If you make a 2 litre fresh cheese every week, it lasts a very long time, as long as the bottle is stored properly.

Where in the range you land

The range of 1 to 2 ml exists because not every milk and not every cheese needs the same. Three things decide it.

The milk

• Sheep's milk: very rich in protein, sets fast and firm. Lower end of the range.
• Goat's milk: gives a softer, more fragile curd. Middle of the range, and cut it gently.
• Very fresh cow's milk or raw milk: middle of the range.
• Pasteurised cow's milk from the supermarket: the heat treatment makes it a little slower. Upper end of the range.
• UHT milk: not suitable, however much rennet you use.

The cheese

For a fresh cheese you want a soft curd that holds plenty of moisture: lower end to middle. For a hard cheese that you cut finely, stir, warm and press, you need a firmer curd: upper end. How that works is in How to make hard cheese.

The temperature

Rennet works best between 32 and 35°C. Below that it is slow and the curd stays weak. Above that it becomes unpredictable. A good kitchen thermometer is the best thing you can buy for cheese making, better than a spare bottle of rennet.

Three worked examples

• A fresh cheese from 2 litres of pasteurised whole milk: start with 6 drops, right in the middle. If the curd has not set after 45 minutes, use 8 next time. The full method is in How to make fresh cheese.
• Fresh goat's cheese from 3 litres of goat's milk: 8 to 9 drops, just below the middle, and cut very gently. See the recipe Fresh goat's cheese.
• A hard cheese from 10 litres of fresh cow's milk: about 1.5 to 2 ml. Measure it with a syringe and dilute it in a quarter of a cup of boiled and cooled water.

How to add the rennet properly

How you add the rennet matters almost as much as how much.

Signs of too much rennet

• The milk sets in less than 20 minutes.
• The curd is very firm, almost rubbery, and squeaks.
• The cheese tastes bitter, especially after a few days.
• The texture is tough instead of creamy.

The fix is simple: less next time. The cheese is still perfectly good to eat, fried in a pan, say, or crumbled into a salad with a punchy dressing.

Signs of too little rennet

• After 45 minutes the milk is still liquid, or only as thick as drinking yoghurt.
• At the clean break test, the curd falls apart in flakes.
• The whey is white and milky instead of clear and yellowish-green.
• Your cheese turns out small and very wet.

If that happens, wait ten to fifteen minutes before doing anything. If the milk is still liquid, the problem is often the milk itself: with UHT milk, no dose will help. Check your thermometer too, and the date on the bottle.

The clean break

This simple test tells you when the curd is ready. Slide a clean knife into the curd at an angle and lift it slowly, as if you were opening a seam. If the curd splits with clean edges and clear, yellowish-green whey gathers in the gap, it is time to cut. If it breaks raggedly or the whey is white, give it a little longer. With practice one glance is enough: tilt the pot slightly and a ready curd pulls away cleanly from the side.

What rennet actually is

Rennet is one of the oldest tools in cheese making. Calf rennet comes from the stomach of young calves, hence the name. The enzyme it contains makes the milk protein draw together into a firm, smooth mass, the curd, and the whey separates out.

Rennet is a coagulant, not a live culture. It does not sour the milk; it only sets it. Unlike milk curdled with lemon juice or vinegar, a rennet curd is not crumbly but smooth and sliceable. That is why rennet is the base of nearly all classic cheeses, from a soft fresh cheese to a well-aged farmhouse wheel. Because it is an animal product, cheese made with calf rennet is not vegetarian.

Storing rennet properly

• Cold: always in the fridge, at about 4°C, ideally not in the door, where the temperature jumps around.
• Dark and tightly closed: light and air slowly weaken the enzyme.
• Clean dosing: never put a used spoon into the bottle; always use the dropper or a clean syringe.
• The date: check the best before date on the bottle. Older rennet works more slowly, so you may need a little more.

Frequently asked questions

Our liquid calf rennet comes in a 50 ml bottle for £13 and sets 250 to 500 litres of milk. If you are just starting, a fresh cheese is the best first step, and everything about milk, equipment and types of cheese is on the cheese making page.