Cheese Cultures (Mesophilic & Thermophilic)

The bacteria that acidify the milk — and the single most common way to wreck a cheese before it ever reaches the mould is to use the wrong family. Mesophilic cultures die in a hot cook; thermophilic ones barely wake up in a cool one.

What it is

A cheese culture is a preparation of lactic acid bacteria whose job is to turn the milk’s lactose into lactic acid. Everything downstream depends on that: the pH falls, calcium leaves the casein network on a schedule, the rennet works in the window it wants, the curd expels whey at the rate the recipe assumes, and the finished cheese is acidic enough to be safe and to taste like itself.

They also do a second job that gets less attention — the enzymes they leave behind carry on breaking down protein and fat for the whole of the aging, which is where a great deal of the flavour comes from.

The two families, and why it matters

Mesophilic — “middle-loving.” Comfortable roughly from room temperature up to the high eighties Fahrenheit, and killed by a hot cook. These are the cultures for cheddar, gouda, most blues, the bloomy rinds and nearly every fresh cheese.

Thermophilic — “heat-loving.” Built for recipes that cook the curd hot, and the family behind the Italian and alpine cheeses: Parmesan and the granas, Emmental, Gruyère, Comté, provolone and the pasta filata cheeses.

The classic failure is using a mesophilic culture in a hot-cooked recipe. The acid starts, then stalls partway through the make, because you killed your own starter in the vat. Match the culture to the cook temperature before you match anything else.

Some cultures are heterofermentative — they produce carbon dioxide as well as acid, which is what you want for the small irregular openings in a havarti and emphatically not what you want in a close-textured wheel.

Mother cultures and direct-vat inoculation

DVI (direct-vat inoculation) powders are dosed straight into the milk. They are consistent, keep well frozen, and are what most home recipes assume.

A mother culture is a small batch of milk you culture yourself, then use to inoculate future batches. It is cheaper per make and it is the traditional method, but it wants care: it can drift, it can pick up contaminants, and it has to be refreshed on a schedule.

Handling

Cultures are alive, and the two things that kill them are heat and moisture. Store the packets dry and frozen, take out only what you need, and close the bag before it can pick up condensation. A sluggish acidification from the very start is usually an old or moisture-damaged culture rather than an under-dose.

Sprinkle the powder over the milk’s surface and let it rehydrate for a minute or two before stirring — dumping it in a clump leaves you inoculating one small part of the vat.

Substitutes (if absolutely necessary)

Cultured buttermilk (for mesophilic) — a workable improvisation, since it carries mesophilic organisms. The blend is not the one your recipe assumes and the results wander batch to batch, but it will acidify milk.
Plain live yoghurt (for thermophilic) — the same trick at the hot end; yoghurt cultures are thermophilic. Expect a yoghurt-ish note.
Acid (citric, vinegar, lemon) — not a substitute. It gets the pH down and produces none of the flavour a culture develops over months. Right for a direct-acid fresh cheese, wrong for anything aged.
Note: cultures are not interchangeable within a family either — a gas-producing blend and a close-textured one give different cheeses from identical milk.

Where to find it

Online: the only reliable source for named blends, and worth it. Cheesemaking suppliers sell single-use packets and bulk tins, with the organisms listed.
Local store: nowhere, in practice — which is why the buttermilk and yoghurt improvisations exist.
Storage: freezer, dry, sealed. Bought in bulk and kept properly, a tin lasts a long time; left in a warm cupboard it is dead before you notice.