You can make rennet at home using animal stomachs, certain plants, or even fig sap. The method you choose depends on what’s available to you and what type of cheese you’re making. Each source produces enzymes that curdle milk, but they vary in strength, flavor, and ease of preparation.
Animal Rennet From Calf Stomachs
Traditional rennet comes from the fourth stomach (abomasum) of a young calf, kid, or lamb. The stomach lining contains enzymes that naturally curdle milk, since that’s exactly what these organs do during digestion. If you have access to a butchered young ruminant, you can extract these enzymes yourself.
Start by thoroughly cleaning the stomach, removing all food contents and rinsing it under cold water. The next step is salting. You can either submerge the cleaned stomach in a saturated brine solution (water with as much salt dissolved as it will hold) or pack the surface heavily with dry salt. Both methods preserve the tissue and help draw out the enzymes. After salting, hang the stomach in a cool, dry place with good airflow and let it dry completely. This can take several days to a couple of weeks depending on conditions.
Once dried, cut the stomach into small strips. To make liquid rennet, soak about a 2-inch square piece of dried stomach in a cup of lukewarm water (not hot, as heat destroys the enzymes) with a pinch of salt for 12 to 24 hours. Strain the liquid through cheesecloth. What you’re left with is your rennet solution. The strength will vary, so you’ll need to experiment with how much to add per batch of cheese, starting with small amounts and adjusting based on how quickly your milk curdles.
Cleanliness matters here. Raw animal tissue can harbor harmful bacteria, including Listeria and Salmonella. Work in a clean environment, keep everything refrigerated when not actively drying, and wash your hands and tools thoroughly throughout the process.
Thistle Rennet From Cardoon Flowers
Cardoon (a wild relative of the artichoke) has been used as a plant-based coagulant in Mediterranean cheesemaking for centuries. The dried purple flowers contain enzymes that curdle milk effectively, and they’re the most reliable plant-based rennet source available.
Harvest the flower heads when they’re fully open and dry them completely in a warm, dark place. To prepare the rennet, dissolve 2 grams of dried cardoon flowers in 50 milliliters of water. That’s roughly half a teaspoon of crushed dried flowers in about 3.5 tablespoons of water. Stir or blend the mixture, then strain it through a fine mesh or cheesecloth to remove the plant material. Use this extract at about 0.5% of the volume of your milk for cow’s milk, or closer to 1% for goat’s milk. For a practical example, that means roughly 5 milliliters of extract per liter of cow’s milk.
Thistle rennet produces slightly different results than animal rennet. Cheeses made with it tend to have a softer, creamier texture and a mildly bitter, herbaceous flavor. This is traditional in certain Portuguese and Spanish cheeses like Serra da Estrela and Torta del Casar. If you’re making a hard aged cheese, animal rennet or a commercial option will give you more predictable results.
Nettle Rennet
Stinging nettle is the most accessible option for many home cheesemakers, since it grows wild across much of North America and Europe. The coagulating power of nettle is weaker than animal rennet or cardoon, so it works best for soft, fresh cheeses rather than hard aged varieties.
You’ll need about 2 pounds of fresh stinging nettle leaves (wear gloves when harvesting). Rinse the leaves under cool water, then place them in a large pot with 4 cups of water. Add more water if needed to just cover the leaves. Bring to a light boil, reduce the heat, cover, and simmer for 30 minutes. Let it cool, then strain out all the plant material. The green liquid is your nettle rennet.
The ratio for use is generous compared to commercial rennet: 1 cup of nettle liquid per 1 gallon of warmed milk. Because the enzyme concentration is low, clotting takes longer and produces a delicate, soft curd. Don’t expect the firm curd you’d get from animal rennet.
Fig Sap as a Coagulant
Fresh sap from fig trees, particularly from wild or caprifig varieties, contains enzymes that curdle milk. This method has deep roots in southern Italy and other Mediterranean regions. There are two traditional approaches: you can either snap fresh fig branches and swirl them directly in warm milk, letting the milky sap leach out, or you can collect the sap by cutting branches and soaking them in a small amount of water to create an extract.
The direct branch method is simpler but harder to control. Sap concentration varies from tree to tree and season to season, so your results will be inconsistent until you get a feel for how much your particular tree produces. Start by swirling a freshly cut branch in your warm milk for a few minutes, then wait and observe. If the milk hasn’t begun to thicken within 30 to 45 minutes, add more sap. Fig rennet, like nettle, works best for soft cheeses.
Testing Your Rennet’s Strength
The biggest challenge with homemade rennet is that you don’t know how strong it is. Commercial rennet comes with standardized dosing instructions, but your extract could be twice as potent or half as potent as expected.
A simple home test: warm a measured amount of milk (say, one cup) to about 90°F (32°C) and add a measured amount of your rennet. Note how long it takes for the milk to form a solid curd. If it takes 30 to 45 minutes, you’re in a reasonable range for most cheese recipes. If it takes much longer, you’ll need to use more rennet per batch. If it clots in under 15 minutes, use less. Keep notes so you can replicate results.
The underlying principle is straightforward: rennet strength is inversely related to clotting time. The faster your test milk clots, the stronger your extract. Once you’ve calibrated with a small test batch, you can scale up confidently.
Storage and Shelf Life
Homemade liquid rennet is perishable. Store it in a clean glass jar in the refrigerator, where it will keep for roughly 7 to 8 months before the enzymes degrade noticeably. If you’ve made dried stomach strips instead of liquid, those last much longer, potentially years when stored in a cool, dry place or frozen.
One important rule: once you dilute your rennet in water for a specific batch of cheese, use it within 30 minutes. The enzymes begin losing effectiveness quickly once diluted. Prepare your milk first, get it to the right temperature, and only then mix your rennet with water for adding to the pot.
Plant-based rennets are even more perishable than animal rennet. Nettle rennet in particular should ideally be used the same day it’s made, or refrigerated and used within a few days. Cardoon extract keeps a bit longer when refrigerated, but fresh is always better. If you want to preserve plant rennet for longer, freeze it in ice cube trays so you can thaw individual portions as needed.