Fermented soybeans are soybeans transformed by bacteria or molds into foods that range from sticky whole beans to salty, long-aged pastes. Across East and Southeast Asia, three families of microbes do the work: Bacillus bacteria (natto, cheonggukjang), koji molds such as Aspergillus oryzae (miso, soy sauce) and wild-mold starters (doenjang), and Rhizopus molds (tempeh). Eastern Nepal’s kinema belongs to the Bacillus family. Fermentation breaks down protein into savory amino acids, changes texture and flavor, and converts soy isoflavones into their aglycone forms (do Prado et al., 2022).
The Main Fermented Soybean Foods
| Food | Origin | Microbes | What it’s like | Our guide |
|---|---|---|---|---|
| Natto | Japan | Bacillus subtilis | Whole beans, sticky strings, strong aroma; about a day to ferment | Natto recipe |
| Cheonggukjang | Korea | Bacillus | Quick 2–3 day ferment, mashed into a pungent stew base | Cheonggukjang |
| Miso | Japan | Koji (Aspergillus oryzae) | Salty paste aged weeks to years; white to red | Homemade miso |
| Doenjang | Korea | Wild molds and Bacillus on meju blocks | Chunky, earthy paste aged months to years | Doenjang |
| Tempeh | Indonesia | Rhizopus mold | Firm cake of whole beans bound by mold, mild and nutty; 1–2 days | Homemade tempeh |
| Sufu | China | Molds, then brine | Fermented tofu cubes, creamy and cheese-like | Sufu |
Others include Chinese fermented black beans (douchi) and chili bean paste (doubanjiang), Indonesian tauco, and soy sauce.
Nutrition: What the Numbers Say
From Japan’s official food composition tables, per 100 g:
- Natto: 16.5 g protein, 90 mg calcium, 3.3 mg iron and 600 µg vitamin K, most of it menaquinone-7 made by the bacteria (MEXT).
- Tempeh: 15.8 g protein, 70 mg calcium, 2.4 mg iron and just 11 µg vitamin K; no vitamin B12 was found (MEXT).
- Miso: a seasoning, not a protein food – from about 6.1 g of salt per 100 g in sweet white miso to 13.0 g in red salty miso (MEXT; red). Doenjang is similarly salty; use both by the spoonful.
Research on fermented soy links it to potential benefits for cholesterol, blood sugar and inflammation, but much of it is from lab, animal or small human studies (do Prado et al., 2022). Treat fermented soybeans as nourishing, flavorful foods rather than remedies.
Natto, Vitamin K and Warfarin
Because natto is so rich in vitamin K, it can interfere with the blood thinner warfarin. In a Japanese study, eating natto raised blood levels of menaquinone-7 within 5 hours and kept them raised for 48 hours (Homma et al., 2006). If you take warfarin, don’t add natto (or change how much you eat) without talking to your doctor. This doesn’t apply in the same way to tempeh or miso, which contain far less vitamin K.
Live Cultures
- Natto is eaten uncooked and contains large numbers of live Bacillus subtilis.
- Unpasteurized miso and doenjang can contain live microbes; stir miso into soup off the boil to keep them.
- Tempeh is always cooked, and soy sauce is usually pasteurized, so neither is a source of live cultures.
Which to Try First
- Miso – the easiest flavor; soups, dressings and glazes.
- Tempeh – mild and meaty; fry, grill or crumble it.
- Doenjang – in doenjang-jjigae (soybean paste stew).
- Natto – the strongest; start with a small amount stirred with soy sauce and mustard over rice.
For other fermented legumes – including non-soy ferments – see fermented beans, West Africa’s Bacillus-fermented dawadawa and Indonesia’s oncom.
