What Does Fermentation Actually Do to Tempeh?

|Team TEMPTÉ
raw tempeh on plate

Every article about tempeh says the same three things. Protein. Fibre. Gut health. The usual headlines, all slightly boring by the fourth time you read them.

The more interesting material sits underneath, in what the culture is actually doing while the beans sit in their warm room for two days. Here's what we've found worth knowing.


The mould does some of the prep work for you

Beans and grains contain phytic acid, which binds to minerals — iron, zinc, calcium — and holds onto them. It's the reason a food can look impressive on a nutrition panel and deliver rather less in practice. It's also why traditional cooks have always soaked their pulses overnight.

Rhizopus, the culture that makes tempeh, produces an enzyme called phytase, and phytase takes phytic acid apart. A 2025 review in Applied Sciences found tempeh fermentation cut phytates by up to 65%, alongside a roughly two-thirds reduction in trypsin inhibitors — another compound in raw soya that gets in the way of protein digestion.

So the culture spends its two days quietly dismantling the parts of the bean that would otherwise work against you. It's doing the overnight soak, more thoroughly, without being asked. Researchers are still working out how much of that translates into absorption in an actual person, but the chemistry itself is well documented.


The bit your gut bacteria usually gets involved with

This one rarely gets mentioned, which is odd, because it's the objection people actually have about beans.

Soya beans contain raffinose and stachyose — oligosaccharides that humans can't break down, because we lack the enzyme. They arrive in the large intestine intact, the gut bacteria get to work, and the result is the well-known social consequence of eating a lot of pulses.

Fermentation deals with this in advance. Rhizopus secretes α-galactosidase, which breaks those sugars apart during the two days in the warm room. Studies have found sharp decreases in both raffinose and stachyose across tempeh fermentation, and work published in the Journal of the American Oil Chemists' Society found tempeh showed little to no flatus activity compared with whole soya beans.

A real, practical, unglamorous benefit that nobody puts on packaging. In other words, fermentation doesn’t just change what the bean tastes like. It changes what’s actually in it.


The whole bean stays whole

Tempeh is made from the entire soya bean, so it keeps the fibre. Tofu is made from soya milk with the pulp strained out, so it doesn't — around 7g per 100g against roughly 2g.

That's the whole argument. It doesn't need three paragraphs and we won't give it any.


Where the B12 story comes from

You'll sometimes read that tempeh is a rare plant source of vitamin B12. It's an appealing idea, and it's been repeated for decades.

The reality is more specific than that. Where B12 has been found in tempeh, it traces to incidental bacterial contamination during traditional production rather than to Rhizopus itself, which doesn't make it. Under modern hygienic production — including everything you'd buy here — that bacterium isn't part of the process. The Vegan Society's guidance is unambiguous: fortified foods and supplements are the dependable route, and tempeh isn't on the list.

Worth knowing if you're eating plant-based. Tempeh has plenty going for it without needing to claim a vitamin it doesn't reliably contain.


And the one about men's hormones

The other story that won't die, and this one is simply wrong.

The theory is that isoflavones in soya, being phytoestrogens, feminise men. It has been examined more thoroughly than almost any question in nutrition, largely because it keeps being asked.

A meta-analysis published in Reproductive Toxicology in 2021 pooled the clinical data — 41 studies, over 1,700 men — and found no effect of soya protein or isoflavone intake on total testosterone, free testosterone, oestradiol or oestrone, at any dose or duration tested. An earlier meta-analysis in Fertility and Sterility reached the same conclusion.

The myth traces back to a handful of case reports, amplified by a 2000s men's magazine culture that found the idea irresistible. The evidence settled this years ago. The story just outlived it. It's one of a few reasons soya arrived in Britain carrying a reputation it never really earned.


The short version

Tempeh is a whole food that has been improved by two days of fermentation in ways that are chemically well documented — less phytate, fewer trypsin inhibitors, practically none of the oligosaccharides that make beans hard work, and all of the fibre the bean started with.

It's not a supplement and we're not going to pretend a snack is one. We chose to make TEMPTÉ with tempeh for much simpler reasons. But it's a genuinely interesting ingredient with a couple of centuries behind it, and most of what people worry about turns out not to be true.

That seems like enough.


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