We Eat Mould Constantly. Why Does Tempeh Seem Different?

|Team TEMPTÉ
is tempeh mould

There's a particular pause that happens when you explain tempeh honestly.

You say it's made from soya beans. Fine. You say they're fermented. Still fine; fermentation has good press these days. Then you say the fermenting is done by a mould that grows through the beans and binds them together, and something changes in the other person's face.


An audit of the average British fridge

Brie and camembert have a white rind. That rind is built by Penicillium camemberti, a mould applied deliberately and allowed to grow across the surface until it forms a skin.

Stilton, roquefort and Danish blue are veined with Penicillium roqueforti. The veins are not a flaw, and they're not random. The spores go into the milk at the start and then sit dormant, because the mould needs oxygen and the inside of a cheese has none. So the wheel is pierced with long steel needles — a Stilton takes somewhere around sixty or seventy — and the mould wakes up and grows along the channels the needles left. The blue lines trace the holes.

Salami and chorizo carry a white bloom on the casing. That's usually Penicillium nalgiovense, sprayed on deliberately. It was identified in the early 1970s as the first mould both non-toxin-producing and practical for curing meat, and its main job is to occupy the surface so that wild moulds (which may not be so obliging) can't.

Soy sauce, miso and sake all begin with koji: rice or soya inoculated with Aspergillus oryzae and left to grow. If you've had a stir-fry with any of these sauces this month, you've eaten the output of a mould.

None of this bothers anyone. A wedge of stilton at Christmas is not regarded as an act of daring.


So why does tempeh get the look?

Familiarity, essentially. Not logic.

We've been eating Penicillium on cheese in Britain for centuries, so nobody thinks of blue cheese as mouldy. It's just blue cheese. Tempeh arrived recently enough that the mould is still visible as a category rather than absorbed into the food's identity.

There's also a presentation difference. On cheese the mould looks like part of the cheese. On tempeh the white mycelium looks like something that has happened to the beans, which reads differently even though it's the same relationship. Tofu, which involves no mould whatsoever, never provokes the same reaction — but then it goes about becoming itself in an entirely different way. 


The genuinely interesting part

Here's what almost nobody knows, and it's the thing that changed how we think about this.

These aren't wild moulds that humans decided to tolerate. They're domesticated organisms — bred, over centuries, the way wheat and dogs were bred. And in several cases we bred the danger out of them.

Aspergillus oryzae, the koji mould behind soy sauce and miso, descends from Aspergillus flavus — a serious agricultural pest and the main source of aflatoxin contamination in stored grain. Aflatoxin is a potent carcinogen. Somewhere across thousands of years of use, the domesticated line lost the ability to make it: the aflatoxin gene cluster is still sitting in the genome, but carrying deletions, frameshift mutations and substitutions, and the genes are no longer transcribed. The instructions are there. The organism can no longer read them.

Penicillium camemberti went the same way. Its wild relatives are pigmented, musty and capable of producing toxins. The domesticated line is white, sporulates less than wild Penicillium, produces no detectable mycotoxins, and makes the mushroomy notes you want in a ripening cheese.

And this can happen fast. A team at Tufts tested it directly: they took wild Penicillium strains and grew them repeatedly on cheese. After eight generations — a matter of weeks — the wild moulds had already begun losing their pigment and their toxin production, and had started producing the buttery, cheesy aromas of their domesticated relatives.

That's domestication observed in real time, in a fungus, in a laboratory, over about a month.


Tempeh's mould is the same story

Rhizopus oligosporus, the tempeh culture, belongs to the Rhizopus microsporus group — a cluster of closely related fungi whose other members include agricultural pathogens and producers of unwanted compounds. R. oligosporus is the food-fermentation member of that family.

This broad pattern is identical to koji and to the cheese moulds. A wild lineage with an awkward relative or two, and a domesticated line selected over centuries for one job: turning cooked beans into something edible, stable and worth eating.

It isn't a mould that wandered in. It's a mould that has been kept, deliberately, for at least two hundred years of written record and probably a good deal longer. It's also why a baked snack sidesteps the question altogether. By the time it reaches the bag there's nothing left to look at.


One last thing

You are, at this moment, breathing in fungal spores. Aspergillus spores drift about constantly, and researchers have estimated we can inhale a couple of hundred a day just going about ordinary life.

Mould isn't an exotic presence we occasionally invite into food. It's ambient, constant, and mostly unremarkable. What's remarkable is the handful of species humans noticed, kept, fed and shaped until they reliably made something delicious.

Tempeh's mould is one of those. It's just newer to Britain than the one on the cheeseboard.

We've written separately about which marks on a block are normal and which mean bin it.


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