What you see: Black areas on your tempeh.
What it is: Spores! But not scary ones.
Eat or toss: Eat! As long as the tempeh otherwise looks fine and isn’t slimy or smelly, it should be ok to eat.
Tempeh is essentially moldy soybeans, but in a good way. During tempeh production, Rhizopus mold wraps its white filaments around the beans and releases chemicals that partially break them down. This turns a tray of beans into a unified mass that our digestive systems find easier to process than the original beans. During this fermentation process, the fungus’s work on the beans also imparts a nutty flavor and chewy texture. Finished tempeh is often sold in flat, vacuum-packed cakes near tofu in American grocery stores.
But Rhizopus is, after all, a fungus. So after all that growing and digesting, it gets to work on its next stage of life: making spores that it hopes will leave the tempeh “nest” to create new colonies. While the mold’s filaments (collectively known as the mycelium, or the fungal body) are white, its spores are black. So, it’s not uncommon to see black areas on tempeh, even tempeh purchased at the store.
“Consumers need not worry from a health viewpoint,” Bill Shurtleff, co-author of The Book of Tempeh, wrote to me in an email after I shared the images in this post with him. “Tempeh that is covered with black spores is widely used in Indonesia, where it is called Tempeh Gembus (overripe tempeh).”
So, black spores simply mean tempeh has finished fermenting and may be overripe. According to Tempehtation, a company that supplies starter cultures to tempeh makers, sporulation could also be a sign that there was too much oxygen around during fermentation. Whether it’s overripe or was exposed to too much oxygen, as long as the rest of the tempeh looks and smells fine, it’s still good to eat.
How do you know if tempeh has gone bad?
Tempeh can signal things aren’t right in a number of ways, including:
- Darkened beans. If the beans themselves look like roasted coffee beans, that’s a bad sign.
- A pink discoloration. Another warning sign that something uninvited could be present.
- Slimy, soft, sticky or mushy texture. This suggests some unwanted microbial activity. A too-long fermentation can also lead to softer tempeh.
- An intense or off odor. Quality tempeh should have an earthy, mushroomy, yeasty scent. Ammonia production is a normal part of Rhizopus fermentation and sensitive noses might detect a slight ammonia scent in properly produced tempeh. But an overwhelming ammonia scent means the fermentation went on too long. Such tempeh will also taste bitter.
Tempeh company Tempea has images of truly spoiled tempeh, as well as blackened tempeh that’s still OK, on its website. Tempehtation does as well.
How can you be certain that something problematic isn’t lurking?
Responsible tempeh makers take many steps to hold off problematic microorganisms. That includes boiling the beans early on (many microorganisms can’t handle the heat) and acidifying the beans by adding acid or lactic acid bacteria, which creates an acidic and hostile environment for problematic bacteria. This all occurs before the Rhizopus culture is added so producers can be certain they’re starting with a microbiologically clean slate.
Fresh tempeh lasts only a few days, so it is also often pasteurized, which further kills pathogens and spoilage microbes, before heading to grocery stores. Cooking at home would further diminish the risk from any bacteria that somehow managed to contaminate it.
So while it’s understandable to worry that the black is something ominous and something other than harmless Rhizopus spores, it’s extremely unlikely.
David Miller, a fungal toxins expert and distinguished research professor at Carleton University, put it this way: “If the tempeh is prepared even in a mom and pop shop by traditional methods and with good hygiene it is actually pretty safe.” He noted that the acidification step is important for nixing bacteria and that molds other than Rhizopus tend to prefer more sugary environments than tempeh. “This is why even in high risk areas,” he wrote to me, “traditional foods have evolved with lactic acid bacterial fermentations that defy the unsanitary conditions.”
Huge thanks to EatOrToss reader Dawn B. for sharing her tempeh images with us!
SOURCES:
- J. David Miller. Distinguished Research Professor. Carleton University. Email correspondence, October 2024.
- Bill Shurtleff, author of The Book of Tempeh and the History of Tempeh and other books about soy products. Co-founder of the SOYINFO CENTER.
- Black spores, the most asked question. Tempehtation. August 20, 2020. Accessed October 2024.
- Tempea FAQ. Tempea Natural Foods. Accessed October 2024.
- McIntyre, L. (editor), A. Trmčić, K. Paphitis, N. Parto and the Fermented Foods working group. (2023). Safety of fermented foods. Assessing risks in fermented food processing practices and advice on how to mitigate them. Section 3.9 Tempeh. Environmental Health Services, BC Centre for Disease Control. November 2023. Access October 2024.
- Growth of lactic acid bacteria and Rhizopus oligosporus during barley tempeh fermentation. Xin Mei Feng, Anders R.B. Eriksson, Johan Schnürer. International Journal of Food Microbiology. Volume 104, Issue 3, 25 October 2005, Pages 249-256
- Tempe (tempeh) fermentation. Environmental Health Services. BC Centre for Disease Control. November 2016. Accessed October 2024.
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