Rennet is the enzyme that sets milk into curd, and the best source is the fourth stomach of an unweaned calf or kid, with fig sap and thistle flowers as plant alternatives. One stomach supplies rennet for hundreds of gallons of milk, but only unweaned young animals supply it, so a small herd’s rennet supply can fall well short of its cheese plans and every batch of homemade rennet must be tested for strength before use.
Part of Dairy Production at Scale
Rennet sourcing is the planning and preparation of the coagulating enzyme for cheese: where it comes from, how much a community can obtain, and how to calibrate it.
Rennet Supply Is Set By The Calving Calendar
The parent article gives the sources: the abomasum of an unweaned calf or kid, sliced, salt-dried and soaked in salt water to extract rennet; vegetable rennet from fig sap, thistle flowers (Cynara cardunculus) and nettles; and acid coagulation, which works for fresh cheese but not for aged hard cheese. The detailed preparation is in Rennet Sources. This article concerns the question the household article does not ask: whether a community can obtain enough.
Worked Example: Supply Against Demand
The inputs: Cheese Basics says one stomach makes rennet for 50-100 batches of 10 liters. That is 500-1,000 liters, or 132-264 US gallons. Two further assumptions come from general knowledge and not from the wiki: each cow calves once a year, and about half the calves are male.
- Suppose cheese milk is 5 gallons per day for a year, which is 5 × 365 = 1,825 gallons.
- Stomachs needed: 1,825 ÷ 264 = 7 at the best strength and 1,825 ÷ 132 = 14 at the lowest.
- Supply from a herd of 4 cows: 4 calves per year, about 2 male. If only young unweaned males are used, that is about 2 stomachs.
By the wiki’s own figures a 5-gallon-a-day cheese program needs several times the stomachs a small herd supplies. The remedies are to use rennet more sparingly, to keep and dry every stomach (dried strips store for years per Rennet Sources), to cull more kid and lamb stomachs where those animals exist, and to use plant rennet for part of the output.
What You Need
- Fresh abomasum from an unweaned animal, coarse salt, and a clean place to dry the stomach
- For plant rennet: fig sap or dried thistle flowers, warm water
- A thermometer, a clean cup and a clock, to calibrate each batch
Method: Calibrate Before You Rely On It
Commercial liquid rennet at the parent article’s dose is 1/4 teaspoon per 2 gallons. Homemade rennet has no fixed strength. Rennet Sources reports that one strip soaked in 200 ml of salted water sets 20-40 liters, which is a very different dose.
Step 1: Test on 1 liter. Warm 1 liter of milk to 31-35°C, add a measured dose of your rennet, stir for a minute and leave still.
Step 2: Time the clean break. The target in the parent article is 45-60 minutes. If it takes much longer, the rennet is weak or the milk too cold or too hot. As a rule of thumb the setting time shortens in proportion to the rennet added, so doubling the dose roughly halves it (an approximation from general knowledge, not a wiki figure).
Step 3: Scale to the batch. Dose is proportional to milk volume, not to the vat. With the 200 ml strip extract setting 20-40 liters, a 5-gallon batch of 19 liters needs 19 ÷ 40 × 200 = 95 ml at best strength, or 19 ÷ 20 × 200 = 190 ml at the weakest.
Step 4: Record it. Write the strength and the date on the container.
Common Mistakes
- Using adult-animal stomachs. Adults produce mostly pepsin, which is less effective at setting milk.
- Expecting plant rennet to behave like animal rennet. It is less predictable and some leave a bitter note.
- Skipping the test batch. An unknown strength wastes a full vat if it fails.
- Chlorinated water for soaking. It weakens the enzyme; use clean unchlorinated water.
Safety
Butchering and raw stomach contents
Stomach contents and raw tissue carry bacteria; wear gloves, wash knives and hands, and dry the stomach thoroughly. The brucellosis risk from handling animal birth materials is covered in Zoonotic Diseases. Milk set with rennet from a stomach of unknown origin should still be pasteurized before setting.
What’s Next
- Learn next: Cultures and acidification — the other half of coagulation
- Combine with: Semi-hard and hard cheeses — where the rennet is used