Oncom is a mold-fermented cake from West Java, Indonesia, and a staple of Sundanese cooking. Like tempeh, it’s made by letting a mold grow through a damp substrate until the mycelium binds it into a firm cake – but oncom is made from by-products: the pulp left from making soy milk and tofu (okara), peanut press cake left after pressing oil, and sometimes cassava residue. Red oncom (oncom merah) is covered in orange-pink Neurospora mold; black oncom (oncom hitam) is made with Rhizopus, the tempeh mold. It’s always cooked: fried, stir-fried with chili and aromatics (tumis oncom), or wrapped in banana leaf and steamed or grilled (pepes oncom).
The Mold Behind Red Oncom
A 2024 study sequenced oncom from small producers in West Java, made from soymilk by-products, and found it dominated by Neurospora intermedia. The fungus breaks down the pectin and cellulose in okara, belongs to a genetically distinct group associated with human-made by-products, and doesn’t carry genes for known mycotoxins. It also grew on other food waste such as fruit and vegetable pomace and plant-milk pulp, making foods that tasters outside Indonesia liked (Maini Rekdal et al., 2024). Its relative Neurospora crassa is famous in science: George Beadle and Edward Tatum used it in work that won the 1958 Nobel Prize “for their discovery that genes act by regulating definite chemical events” (Nobel Prize).
Safety
- Aflatoxin and peanuts. Aflatoxins, made by some molds, contaminate oilseeds including peanuts (WHO). Peanut press cake is a by-product often stored as animal feed, and you can’t see aflatoxin. We don’t suggest making peanut-cake oncom at home; use fresh okara instead.
- Bongkrekic acid. A related Indonesian ferment, tempe bongkrek made with coconut, has caused deadly poisonings when the bacterium Burkholderia gladioli pv. cocovenenans grows instead of the mold; outbreaks have also been linked to fermented rice noodles and corn products, with 30–100% of cases fatal (Yang et al., 2026). The toxin survives cooking. Never ferment coconut this way, and discard any cake that isn’t evenly bound by healthy mycelium or smells off.
- Clean starter. In the US, a Salmonella outbreak was traced to tempeh made with a contaminated starter (Griese et al., 2013). Buy starter from a reputable supplier, steam the substrate, and cook oncom before eating.
Making It at Home
Food-grade Neurospora starter is hard to find outside Indonesia, so the recipe card uses tempeh starter (Rhizopus) on fresh okara for a black-oncom-style cake – the same method as homemade tempeh. Use okara the day it’s made (from your own soy milk or a tofu maker), squeeze it well, and steam it before inoculating. For other mold ferments, see miso and our soybean ferments guide.