- ✓Almost all modern rennets are either animal-derived or fermentation-produced chymosin (FPC).
- ✓Dose by the specific rennet's declared strength, not by tradition — strengths vary widely.
- ✓The flocculation-point method is the most reliable way to pick a cut time across styles.
The types of rennet
- Animal rennet — extracted from the stomachs of unweaned calves (or lambs, kids). The traditional choice; widely used in AOC/PDO cheeses.
- Fermentation-produced chymosin (FPC) — made by fermenting yeast or bacteria carrying a cloned calf chymosin gene. Functionally almost identical to calf rennet; vegetarian.
- Microbial rennet — enzymes from moulds (Rhizomucor miehei, Rhizomucor pusillus, Cryphonectria parasitica). Cheaper, vegetarian, but often more proteolytic — can cause bitterness in aged cheese.
- Plant coagulants — from thistle (Cynara cardunculus), fig, nettle. Traditional in some Iberian cheeses; distinctive flavour, not interchangeable with rennet.
Rennet strength and dosing
Rennet strength is typically expressed in IMCU (International Milk Clotting Units). A standard liquid rennet might be 200 IMCU/ml, a double-strength 400 IMCU/ml, and a powdered rennet declared in different units entirely. Always dose to your specific product's label. A typical rule of thumb is 20–30 IMCU per litre of milk for a 30-minute set — but milder doses and longer sets are common in traditional styles, and they give different curd character.
Liquid rennet degrades. Keep it refrigerated, away from light, tightly capped. A bottle older than 12 months has usually lost a noticeable fraction of its activity. If your set times have been drifting longer, old rennet is the first suspect.
The flocculation point and the multiplier
Here's the technique that lets one recipe work across variable milk and kitchen conditions. Right after adding and stirring in the rennet, note the time. Float a small plastic dish on the surface of the milk and spin it gently every few minutes. At first it spins freely; at some point — usually 10 to 15 minutes after adding rennet — it stops spinning and drifts. That's the flocculation point.
To pick the cut time, multiply the flocculation time by a style-specific multiplier. A firm, low-moisture cheese like Parmigiano wants a multiplier of about 2 (cut early, expel lots of whey). A softer cheese like Camembert wants 5 or 6 (cut late, keep more moisture). Dutch-style semi-hards sit around 3 to 3.5. This method is astonishingly reliable — once you've calibrated the multipliers for your styles, you'll stop depending on recipe-book clock times.
The 'clean break' test
When the set is ready, slide a knife into the curd at a 30° angle, lift, and watch. A good set cleaves cleanly — whey runs clear into the gap, curd edges are sharp. A poor set tears raggedly, leaves a cloudy whey, or collapses. If it tears, wait five more minutes and retest; if it's still poor, you likely have a rennet or calcium problem.
Calcium chloride: when and why
Supermarket pasteurised milk often sets poorly because heat-treatment partially denatures whey proteins that then interfere with the coagulation process. A small dose of calcium chloride — around 1 ml of 32% solution per 4 litres — restores good setting. Raw and gently heat-treated milk rarely needs it. Too much calcium chloride makes curd harsh and bitter, so dose modestly.
Frequently asked
Can I substitute vegetarian rennet 1:1 for animal rennet?+
For FPC (fermentation-produced chymosin), essentially yes, at the same IMCU dosage. For older microbial rennets, reduce the dose slightly and don't age the cheese beyond a few months — they tend to proteolyse too aggressively for long aging.
Why didn't my milk set at all?+
In order of likelihood: old or inactive rennet; UHT-treated milk that won't coagulate cleanly; milk far below renneting temperature; raw milk from an animal with subclinical mastitis. Check rennet activity by setting a small amount of fresh milk as a control.
