Rennet Sources

Part of Fermentation and Brewing

Rennet is the coagulant that transforms liquid milk into solid curd — the first step in making almost every hard and semi-hard cheese. Without rennet, only soft acid-set cheeses (like cottage cheese or paneer) are possible. Understanding how to obtain, prepare, and use rennet from animal and plant sources allows a self-sufficient community to make the full range of hard cheeses that can be aged for months and stored through winter.

What Rennet Does

Rennet contains proteolytic enzymes — primarily chymosin (also called rennin) in animal rennet — that act specifically on kappa-casein, a protein in milk. When kappa-casein is cleaved by chymosin, the protective coating of the micelles (fat-protein clusters suspended in milk) is destroyed. The micelles aggregate and form a gel-like network — the curd.

The result: milk that was liquid for hours becomes a solid gel within 30–60 minutes of adding rennet at the right temperature and pH. This gel can then be cut, heated, pressed, and aged into durable cheese.

Rennet Type	Active Enzyme	Source	Notes
Animal rennet	Chymosin	Stomach lining of ruminants	Most effective; best flavor development
Plant rennet	Cysteine proteases or aspartyl proteases	Various plant tissues	Works; some bitterness at high doses
Microbial rennet	Aspartyl protease	Rhizomucor miehei mold	Historically used; can cause bitterness in aged cheese
Acid coagulation	pH drop (no enzyme)	Vinegar, lemon juice, citric acid	No rennet needed; only for fresh cheeses

Animal Rennet: Preparation

Animal rennet is extracted from the fourth stomach (abomasum) of young ruminants — calves, lambs, or kids — that are still exclusively milk-fed. The abomasum produces chymosin to digest the mother’s milk; the concentration is highest in very young animals. Adult animals produce primarily pepsin rather than chymosin, which is less effective for cheesemaking.

Rennet from adult animals, or from animals that have already begun eating solid food, will be weaker and less reliable. The best rennet comes from the abomasum of an unweaned calf, lamb, or kid collected at slaughter.

Preparation Method 1: Dried Stomach Strips

1. After slaughter, remove the fourth stomach (abomasum). It is the fourth compartment in the ruminant’s stomach system — the actual true stomach with a glandular lining, smaller and smoother than the rumen.
2. Empty the stomach contents (curdled milk, which confirms the presence of active rennet).
3. Wash the stomach carefully in cold water without scrubbing.
4. Inflate with air and tie closed. Hang in a dry, well-ventilated location at room temperature (15–20°C). Dry completely over 2–4 weeks.
5. Once fully dried, cut into strips 2 cm x 5 cm. Store in a sealed dry container with salt. Will keep for years.

To use dried rennet strips:

• Soak 1 strip in 200 ml of cold, non-chlorinated, lightly salted water (1/2 tsp salt) for 30 minutes.
• Remove the strip; use the liquid as rennet.
• This solution will set 20–40 liters of milk within 30–60 minutes at 31–35°C.

Preparation Method 2: Salt-Preserved Fresh Rennet

1. Remove the abomasum from a slaughtered young animal while fresh.
2. Pour out contents (curdled milk).
3. Pack the inside of the stomach thoroughly with coarse salt. Roll up and pack in a jar with more salt.
4. Will keep refrigerated (or in a cool cellar) for 3–6 months.

To use: Rinse a piece of preserved stomach, chop finely, and soak in 100–200 ml of cold salted water for 1 hour. Strain and use the liquid.

Animal Rennet Strength Testing

Rennet strength varies by source and preparation age. Always test a new batch before committing to a full cheese:

1. Heat 100 ml of fresh milk to 32°C.
2. Add 2 ml of your rennet solution (or 1 ml if using a strong liquid rennet).
3. Note the time to coagulation — when the milk sets firm enough to cut cleanly.

Set Time	Rennet Strength Assessment
10–15 minutes	Strong; dilute 1:1 before use
15–25 minutes	Normal; use as-is
25–40 minutes	Weak; use double quantity
>60 minutes	Too weak; increase dose significantly or source new rennet

Plant-Based Rennet Sources

Plant rennets work through different enzyme types (cysteine proteases, aspartyl proteases) that also coagulate milk, though often less cleanly than chymosin. They have been used successfully across many cultures for centuries.

Fig Sap

Fresh sap from the stem and unripe fruit of fig trees (Ficus carica) contains ficin — a cysteine protease that effectively coagulates milk.

Preparation: Collect 30–50 ml of white latex sap by breaking a young fig stem or leaf. Mix immediately with warm milk (30°C). Coagulation occurs within 30–60 minutes.

Dosage: 1 ml of fresh sap per liter of milk (approximately).

Limitation: Works best for fresh cheeses consumed young. The enzymes continue acting during aging, potentially causing excessive proteolysis (breakdown of protein into amino acids), producing a bitter, soft texture in aged cheeses.

Thistle Flower Rennet

Dried stamens and pistils of cardoon (Cynara cardunculus) — the wild thistle ancestor of the artichoke — are used traditionally in Portugal (Queijo Serra da Estrela) and Spain (Torta del Casar) to make famous soft cheeses. The active enzyme is an aspartyl protease called cardosin.

Preparation:

1. Harvest thistle flowers when fully open. Remove petals; retain purple stamens and center.
2. Dry slowly at low temperature (30–40°C or in sun) until completely brittle.
3. Store in sealed dry container. Keeps for years.

To use:

1. Grind 3–5 g of dried flower material per 10 liters of milk in a mortar.
2. Steep in 150 ml of warm water (30°C) for 30 minutes.
3. Strain through cloth and add the liquid to warmed milk.
4. Coagulation occurs in 45–90 minutes at 30–32°C.

Cardoon rennet produces cheeses with a characteristically soft, almost runny texture and a slightly bitter, herbal flavor that is considered a delicacy in the regions where it is traditionally used. It is not well-suited to hard, pressed cheeses intended for long aging — favor it for soft cheeses consumed within weeks of making.

Stinging Nettle

Young stinging nettle leaves contain coagulating compounds that work weakly on milk. Best used as a supplementary coagulant with higher doses, or for very fresh soft cheeses.

Preparation: Blanch 500 g of fresh nettle tops in minimal water. Strain the cooking liquid. Add to warm milk at 30–32°C. Use 100–200 ml per liter of milk. Coagulation is slow (2–4 hours) and produces a softer, more fragile curd than animal rennet.

Other Plant Rennets

Plant	Active Part	Dosage	Notes
Greater knapweed (Centaurea scabiosa)	Flower heads	5 g dried per 10L	Traditional European use
Lady’s bedstraw (Galium verum)	Aerial parts	200 g fresh per 10L	Historically widespread
Mallow (Malva sylvestris)	Flower and leaf	100 g fresh per 10L	Weak; best for fresh cheese
Safflower (Carthamus tinctorius)	Seeds	5 g per 10L	Used in Middle East
Papaya (Carica papaya)	Fresh unripe latex	2–3 ml per 10L	Strong; use sparingly

Acid Coagulation: When No Rennet Is Available

When rennet of any type is completely unavailable, acid coagulation produces simple fresh cheeses:

• Lemon juice: 50–80 ml per liter of hot milk (82–90°C)
• Vinegar: 30–60 ml per liter of hot milk
• Citric acid: 2 g dissolved in water per liter of milk at 32°C
• Cultured whey from a previous batch: Acidic whey from a previous cheese, added at 30–50% volume to fresh milk and held at 30°C for 12–24 hours

Acid-set cheeses (paneer, ricotta, cottage cheese) have no aging potential. They are food for today, not food for winter. They do not develop the complex enzymes and rind protection that pressed, rennet-set cheeses develop during aging.

Using Rennet: Practical Cheesemaking Steps

1. Warm milk to 30–32°C (for most hard cheeses). Do not overheat — chymosin denatures above 65°C.
2. Add starter culture (acidifying bacteria) if making cultured cheese. Allow 30–60 minutes for acidification before adding rennet.
3. Add rennet. Dilute rennet in 5–10 times its volume of cold, non-chlorinated water immediately before adding to milk. Stir gently but thoroughly into the milk for 30–60 seconds.
4. Stop stirring and allow milk to set undisturbed. Clean break should occur in 30–60 minutes: when the curd pulls cleanly away from the vessel wall and a knife cut through it produces a clean, glossy face.
5. Cut curd into cubes using a long knife. Smaller cubes (1 cm) for harder cheeses with more moisture expelled; larger cubes (2 cm) for softer cheeses.
6. Cook and stir the cut curd at elevated temperature (38–52°C depending on cheese type) to firm the curds and expel more whey.
7. Drain, press, and age according to the specific cheese type.

Rennet Sources Summary

Rennet is the coagulant that transforms milk into curd for hard cheese production. Animal rennet from the abomasum of milk-fed young ruminants provides the most reliable chymosin enzyme and is obtained during normal slaughter through drying or salt-preserving the stomach lining. Plant rennets — fig sap, thistle flowers (cardoon), lady’s bedstraw — provide alternatives that work best for soft, fresh cheeses but can cause bitterness or texture problems in long-aged hard cheeses. Testing rennet strength before use allows accurate dosing. Acid coagulation without rennet produces only fresh, non-aging soft cheeses. Maintaining a supply of animal rennet through drying and storage is the most reliable long-term strategy for hard cheese production in a self-sufficient community.