Rennet Dosage (per Litre of Milk)

1–2 mL

Liquid animal rennet (520 IMCU/mL) — Per 10 litres of milk; dilute in 20× its volume of cool, non-chlorinated water before adding

10 L

Whole milk — At 30–32°C for mesophilic cheeses; 38–42°C for thermophilic varieties

20–40 mL

Dilution water — Non-chlorinated; chlorine denatures chymosin — use filtered or boiled-and-cooled water

Ripen acidified milk to target temperature (30–32°C mesophilic, 38–42°C thermophilic). Dilute measured rennet 1:20 in cool non-chlorinated water. Add diluted rennet to the milk in one smooth pour, stirring top-to-bottom for exactly 30 seconds (no more — over-stirring breaks the forming curd matrix). Stop stirring, cover, and hold undisturbed. A firm set should be achieved in 30–45 minutes at correct temperature and acidity; the 'clean break' test (inserting a finger or knife blade and lifting) reveals a sharp, clean crack when ready.

• Always dilute rennet in non-chlorinated water — even a trace of chlorine deactivates chymosin enzymes.
• The 'flocculation time' method is more precise than a fixed timer: add rennet, stir 30 seconds, then gently spin a small floating lid on the surface — the moment it stops spinning (flocculation point) marks zero; multiply by 3–4 for soft cheeses, 4–6 for hard. This is your cut time.
• Low milk pH (below 6.2) dramatically speeds coagulation — over-acidified milk can set too fast, producing rubbery, crumbly curd. Target pH 6.4–6.6 at renneting.
• Raw milk sets faster than pasteurised because native proteases supplement chymosin; reduce dose by 10–15% to avoid over-setting.
• Store liquid rennet at 2–6°C in its original dark bottle; exposure to light and heat degrades potency within weeks. Check expiry dates before each use.
• If the curd is slow to set, do not add more rennet mid-batch — the double dose will cause severe bitterness. Instead, raise the milk temperature by 1–2°C and wait.

• Rennet tablet (Walcoren-type)

  One tablet typically sets 4–8 L of milk; potency varies by manufacturer — consult package IMCU rating and convert to equivalent mL of liquid rennet

• Vegetable rennet (thistle, fig sap)

  Higher enzyme activity is more variable; start at 0.5× liquid rennet dose and adjust; imparts floral or slightly bitter notes in aged cheeses

• Microbial rennet (Rhizomucor miehei)

  Standard substitute for vegetarian cheeses; similar dosing to liquid animal rennet but can cause bitterness in long-aged cheeses above 6 months

• Fermentation-produced chymosin (FPC)

  The dominant industrial coagulant; highly consistent at 1 mL per 10 L; virtually no off-flavour risk in aged cheese

• High-temperature milk (thermophilic cheese)

  Rennet dose stays the same but coagulation accelerates; watch carefully — a 5-minute overshoot at 42°C can make curd too firm to cut cleanly

• Milk to Curd Yield for Hard Cheese (~10:1)

  Rennet initiates the coagulation that makes yield possible; underdosing reduces recoverable solids

• Starter Culture Inoculation Rate

  Acidification from the culture directly sets the pH at which rennet is added, controlling gel firmness and cut time

• Brine Concentration for Cheese Salting (18–23% Saturated)

  Salting follows renneting in the cheesemaking sequence; brine concentration is independent of rennet dose

1. 1.Home Cheese Making — Ricki Carroll (Storey Publishing, 2002)
2. 2.Mastering Artisan Cheesemaking — Gianaclis Caldwell (Chelsea Green, 2012)
3. 3.Technology of Cheesemaking — Barry Law & A. Y. Tamime (Wiley-Blackwell, 2010)
4. 4.Cheese and Fermented Milk Foods — Frank Kosikowski & Vikram Mistry (Westport, 1997)

Confidence: high