Cheese Rind Cultures

The four organisms you invite onto — or into — a cheese on purpose: P. candidum for white velvet, B. linens for an orange smear, P. roqueforti for blue veins, and Geotrichum, which prepares the ground for the other two and is the reason a washed rind that is still white at two weeks is on schedule.

What they are

These are not the starter cultures that acidify the milk. They are the organisms that live on the rind, or in the case of blue inside the paste, and do the ripening from there. You buy them as freeze-dried spores in small packets, dosed by the fraction of a teaspoon, and either added to the milk with the starter or sprayed onto the formed wheel.

Penicillium candidum — the bloomy rind

The white velvet on brie, Camembert and the triple-crèmes. It grows a dense white coat over a few weeks and works inwards: its enzymes break the paste down from the rind toward the centre, which is why a young one is chalky in the middle and a ripe one is spoonable — and why these cheeses are made small and flat, since a big wheel’s centre would never be reached.

⚠ Its failure mode is ammonia. P. candidum deaminates amino acids and releases ammonia. A faint ammoniac note in a very ripe bloomy is normal; a sharp one means it has gone past where you wanted it, usually from being too warm, too wet, or short of air exchange.

Brevibacterium linens — the washed rind

The orange-red bacterium behind Époisses, Taleggio, Limburger, Munster and the Trappist cheeses — responsible for both the colour and the smell, which is frankly feet. B. linens is closely related to organisms that live on human skin, which is not a coincidence.

You encourage it by washing the wheel with brine — sometimes with beer, wine or spirits in it — repeatedly over weeks. The washing keeps the surface moist and salty, which it likes and most competitors do not, and physically discourages other moulds from establishing.

⭐ It is a smear, not a fluff. A washed rind should be tacky and orange, never furry.

Geotrichum candidum — the one nobody mentions, and the one that makes the others work

A yeast-like organism used as a rind conditioner, usually alongside one of the others. Its job is to deacidify the surface — it consumes lactic acid and raises the surface pH.

🔴 That is not a refinement, it is the enabling step, and it explains the single most common washed-rind failure. B. linens wants a surface pH around 5.5, and a cheese leaves the mould nearer 4.9. It simply cannot start on a fresh cheese. So the first fortnight of “washing for B. linens” is really a yeast schedule, and the orange arrives only once the yeasts have raised the pH enough for the bacterium to take hold.

Which means a rind still white at two weeks is on schedule, and the three real causes of “it never went orange” are: no yeast in the wash, too much salt, or giving up at two weeks.

Geotrichum also gives the slightly wrinkled, brainy surface on some goat cheeses. Overdone, it produces a coat that traps moisture underneath — “toad skin.”

Penicillium roqueforti — blue

The one that goes inside. Added to the milk or the curd, it then sits dormant, because it needs oxygen and a well-knit cheese has none.

Which is why a blue is built the way it is: the curd is deliberately left open and loose rather than knitted tight, and the wheel is needled some weeks in — pierced with channels so air reaches the interior. The mould wakes up and grows along those channels, which is why the veining follows lines rather than being random. It is the one family where a leaky, badly-knit curd is the goal.

Handling, and the one rule that matters most

Store the packets dry and frozen, like any culture.

🔴 And keep blue away from everything else. P. roqueforti spores travel enthusiastically and they will colonise a cheddar on the next shelf, a brush, a cloth, or a cave you thought was clean. Use separate brushes for blues and for everything else, handle blue last, and brush away from the cave rather than over it. Blue appearing on a cheese you never inoculated is not bad luck — you carried it there.

Substitutes (if absolutely necessary)

• A ripe rind of the cheese you want — the traditional route, and it genuinely works: a scrap of rind from a good Camembert or a wedge of blue, blitzed into a little milk, carries live organisms. Expect less consistency than a packet and whatever else was living on that rind.
• Your cave’s own flora (for a natural rind) — not a substitute so much as the alternative approach: let the wheel dry and let whatever is in the air colonise it. That is a real tradition and a different cheese.
• Note: these four are not interchangeable with one another. There is no way to get an orange smear out of P. candidum, and no way to get white velvet out of B. linens.

Where to find it

• Online: cheesemaking suppliers, in small foil packets by strain. Different strains within a species behave differently — a fast, tall-growing P. candidum and a slow, flat one give visibly different rinds — so keep the strain code with your notes.
• Local store: nowhere.
• Storage: freezer, sealed, dry. A packet that has been damp is unreliable rather than merely weak, and on a rind culture that shows up three weeks later on a wheel you have already committed to.