Rennet

The coagulant that turns milk into curd — and the one cheesemaking ingredient where the four kinds on the shelf are genuinely not interchangeable. Animal, fermentation-produced, microbial and plant rennets each set milk, then behave very differently for the next eighteen months.

What it is

Rennet is a coagulant preparation, and the enzyme doing the work in the traditional version is chymosin. What makes chymosin remarkable is how specific it is: it cleaves one bond in κ-casein — Phe105–Met106 — and largely leaves the rest of the milk’s protein alone. Cut that bond and the casein micelles lose the hairy layer holding them apart, so they aggregate into a gel.

That specificity is the whole point, and it is the axis every rennet on the market varies along. A coagulant that cuts only that bond gives a clean set and a cheese that ages predictably. A coagulant that also chews on other proteins gives you a set — and then keeps working for a year and a half.

The four you can buy

Animal rennet — extracted from the fourth stomach of a young ruminant, traditionally a calf. It is not pure chymosin: it carries pepsin and other enzymes too, in a ratio that shifts with the animal’s age and diet. A milk-fed calf yields a chymosin-rich extract; an older animal yields proportionally more pepsin, which is a blunter instrument. Excellent, traditional, and not identical bottle to bottle.

Fermentation-produced chymosin (FPC) — the calf chymosin gene expressed in a microorganism and fermented in a tank. Approved by the FDA in 1990, which made it the first bioengineered product in the US food supply, ahead of the Flavr Savr tomato and GM soy. It is in most American cheese today, usually labelled just “enzymes.” On the merits it is pure chymosin, identical every time, and does the least unwanted proteolysis during aging. It also requires no calf — while being, technically, a product of genetic engineering. There is no tidy resolution to that and I am not going to pretend otherwise.

Microbial rennet — a protease from a fungus, commonly a Rhizomucor species. This is not the same thing as FPC and the two are confused constantly, including on labels. It is not chymosin at all; it is a different enzyme that happens to clot milk. Genuinely vegetarian in the straightforward sense, and less specific — fine in a fresh cheese, a problem in one you intend to keep.

Plant rennet — the oldest alternative and the best story. Cardoon thistle (Cynara cardunculus), whose cardosins clot milk, recommended by Columella in the first century AD alongside safflower seed and fig sap. Portugal’s Serra da Estrela and Spain’s Torta del Casar are still cardoon-set as a matter of PDO law.

The bitterness rule, and the correction to it

The rule of thumb taught everywhere: less-specific coagulants do more general proteolysis, some of the short hydrophobic peptides that produces taste intensely bitter, and a wheel you keep for a year gives the enzyme all the time in the world to make them. That is why microbial rennet is a poor choice for a long-aged cheese.

But do not extend that to cardoon, because the measurements do not. The cardoon literature reports higher proteolysis and less bitterness than the simple rule predicts — so “plant coagulants are broadly proteolytic, therefore bitter” is a generalisation this collection has retired rather than repeated.

Using it

Rennet is dosed in IMCU (International Milk Clotting Units), not in spoons, and strengths differ between brands and between liquid and tablet — so match the strength your recipe assumes rather than copying its volume blindly.

Dilute in unchlorinated, cool water immediately before use and stir it in with a few gentle up-and-down strokes, then stop the milk moving. Chlorine in tap water will damage it, which is a common cause of a set that never happens.

Too much rennet is its own fault: a fast, brittle set that shatters when cut, and surplus enzyme that keeps working and turns bitter later.

Substitutes (if absolutely necessary)

• A different rennet — the honest substitute. Any of the four will set milk; the choice changes how the cheese ages, not whether it curdles.
• Acid (lemon juice, vinegar, citric acid) — makes a fresh cheese, not a substitute. An acid set at the isoelectric point gives a precipitate rather than a rennet gel: no stretch, no proper melt, no aging. Fine for ricotta, paneer or a quick mozzarella; useless for a wheel you want to keep.
• Note: nothing rescues ultra-pasteurised (UHT) milk — it cannot form a rennet gel at all, and no amount of rennet or calcium chloride will change that. Check the carton; US organic milk is very often UHT.

Where to find it

• Online: the reliable route, and where the strength is actually stated. Cheesemaking suppliers list IMCU and the type plainly.
• Local store: rennet tablets turn up in the baking or pudding aisle of some supermarkets and in Italian and Middle Eastern groceries. Strength is often vague — treat an unlabelled tablet as an unknown.
• Keep it cold and dark. Liquid rennet loses strength on the shelf; the fridge is the minimum and it does not last forever. A set that fails on good milk is very often just old rennet.