You've opened a packet, found dark patches spreading across the surface, and you're now holding it at arm's length wondering whether it's safe to eat.
Grey and black spots on tempeh are usually the culture doing exactly what it's supposed to do, but there are signs that genuinely mean bin it, and they're worth being able to tell apart.
The quick version
Normal: white fuzzy coating, grey or black patches, firm and dry to the touch, a mushroomy or yeasty smell, possibly the faintest hint of ammonia.
Bin it: pink, yellow, green or blue colouring, a slimy or sticky surface, a mushy or crumbling texture, a strong ammonia or alcohol smell, or anything sour or rancid.
Why the black spots happen
Tempeh is made by Rhizopus, a culture that grows through cooked soya beans and binds them into a cake. The white fuzz you see is the mycelium, the culture itself. That's the product working.
When that culture matures, it does what fungi do: it produces spores. Those spores are dark, and they show up as grey or black patches, often clustered around the air holes in the packaging where there's more oxygen.
So the spots aren't contamination arriving from outside. They're the same organism, a stage further along. They can indicate the tempeh is fully mature, or that it got a little extra oxygen during fermentation. Some people think the flavour improves slightly.
What fresh tempeh should be like
Look: a firm cake with the beans held tightly together under a white coat of mycelium. The beans themselves should look like beans; if they've gone very dark, closer to coffee beans, that's a warning sign.
Feel: firm, compact and dry. It should hold its shape when you press it gently.
Smell: earthy, mushroomy, a bit like fresh sourdough. A very faint ammonia note is normal, because breaking down soya protein releases small amounts of it. Faint is the operative word.
What actually means throw it away
Colours other than white, grey or black. Pink, yellow, green or blue means something has moved in that shouldn't have.
Sliminess. A slippery or sticky film means unwanted bacteria are breaking the proteins down.
A mushy or crumbling texture. Fresh tempeh is firm. If the block has lost structure, it's past it.
A strong ammonia smell. The faint version is normal; the version that makes you pull your head back means fermentation went too far. It will taste bitter as well.
Sour, alcoholic or rancid smells. Any of these and it goes in the bin.
How to store it so this comes up less
Keep it cold — refrigerated, and preferably at or below 4°C. Once opened, transfer it to an airtight container and use it within a few days.
Tempeh freezes well, which is the useful part. Freezing halts the fermentation entirely, so a block you're not going to get to can go in the freezer and come out much the same. Defrost in the fridge rather than on the counter. Tofu behaves quite differently here — freezing and thawing changes its texture entirely.
The dates on the packet are the manufacturer's, and they're the ones to follow. Nothing above overrides a use-by date.
The short version
Tempeh is a living food and it looks like one. It develops in the packet in ways that would be expected on a block of cheddar and are entirely routine here. None of which applies to a baked tempeh snack, where the oven has long since stopped the culture doing anything at all.
Keep reading
- What Is Tempeh? A Simple Guide to Our Hero Ingredient
- Tempeh vs Tofu: What’s the Difference?
- Why We Make Snacks From Tempeh
- Why the UK Should Eat More Tempeh
- What Does Fermentation Actually Do to Tempeh?
- Is Tempeh Gluten-Free? What to Check
From tempeh to your snacking plate. Meet TEMPTÉ →
Sources and a caveat
- Rhizopus sporulation and dark pigmentation: Rhizopus — ScienceDirect Topics, noting that young sporangia are white and turn black with age, and Rhizopus oligosporus morphology
- Oxygen control during fermentation and spore masses: Tempeh — ScienceDirect Topics
- Spore morphology of the tempeh fungus: Jennessen et al., Mycologia, via PubMed
There is no single official UK standard for assessing tempeh spoilage at home, and the spoilage indicators above reflect the consistent view of tempeh producers rather than a regulatory source.
This is general guidance, not a safety guarantee. Follow the use-by date on the packet, and if something seems wrong, throw it away.