Mould fermentation

What a mould contributes

A mould cannot eat. It has no mouth and no digestive system, so it does its digestion outside itself: it secretes enzymes into whatever it is growing on, waits for them to break the material down into small soluble pieces, and absorbs those.

That is precisely what makes moulds useful in food. The enzymes they flood into a substrate are exactly the ones that break starch into sugar, protein into savoury amino acids, and fat into aromatic fatty acids. A cook is essentially borrowing a fungus's digestive system and stopping the process before the fungus has finished eating.

Different moulds bring different enzyme mixtures, which is why the foods they make are so unalike. A mould strong on proteases makes savoury; one strong on lipases makes pungent and cheesy; one that produces a dense mycelium binds things together.

The moulds, and what each is for

Aspergillus oryzae — koji

Grown on rice or barley. Very strong amylases and proteases. The foundation of miso, soy sauce, sake and mirin. Contributes sweetness and savouriness rather than a mouldy flavour of its own.

Rhizopus oligosporus — tempeh

Grown through whole soya beans. Produces a dense white mycelium that binds the beans into a cake, plus a nutty, mushroomy aroma.

Penicillium roqueforti — blue cheese

Grown in the veins of a cheese, needing air, which is why blue cheeses are pierced. Strong lipases, which is where the peppery, sharp, slightly metallic flavour comes from.

Penicillium camemberti — soft rind cheese

Grown on the surface of brie and camembert. Works inward, breaking down the paste under the rind, which is why a ripe one is runny at the edge and chalky in the middle.

Moulds on cured meat

A white surface bloom is deliberately encouraged on many salamis, where it regulates drying and keeps unwanted organisms off.

How this is different from the mould on your bread

The obvious question, and it deserves a straight answer.

The moulds used in food are specific, identified, domesticated strains, selected over long periods, grown deliberately under controlled conditions of temperature and humidity, and introduced in overwhelming quantity so that they dominate before anything else can establish. The mould on a forgotten loaf is whatever happened to land there, and could be any of a great many species with very different properties.

Two things follow. First, "mould is fine in food" is not a conclusion you can draw from cheese and tempeh — the argument runs the other way round. Second, in your own kitchen, an unexpected mould on a ferment, a jar or a loaf is an unidentified organism, and the sensible response is to throw the food out rather than to reason about which species it might be. Whether a particular mould on a particular food is a hazard is a food safety judgement, not a cooking one. FoodHQ does not publish salt percentages, pH figures or keeping times for home preserving. Those are safety numbers, they are set by a food safety authority rather than by a cookery writer, and the right ones depend on what you are fermenting and where you are. In the UK the Food Standards Agency is the authority — use a recipe from them or from another body with the same standing, and treat this page as the explanation of why that recipe is shaped the way it is.

Where a cook meets it

• Every soy sauce, miso, mirin and sake in the cupboard is downstream of koji.
• Tempeh, where the mould is doing structural work as well as flavour work.
• Blue, washed-rind and bloomy-rind cheeses, where ripening is a mould slowly digesting the cheese from one direction.
• Cured salami and some hams, where a surface bloom is part of the process.
• Shio koji as a marinade, which is the most direct way a home cook uses mould enzymes deliberately.
• Doenjang and gochujang, which in traditional production rely on wild moulds colonising bean blocks rather than an inoculated culture.

Common questions