Oncom: The Forgotten Indonesian Fermented Food
I spent about three weeks in West Java back in 2019 and couldn't figure out why locals ate this dark, dense block of food for breakfast every single morning. It wasn't tempeh, though they were clearly related. After asking a few too many questions at a warung near Bandung, I learned it was called oncom, and it changed how I thought about fermentation entirely. Oncom is made from the byproducts of oil pressing — specifically peanut hulls or soybean residues that would otherwise go to waste. The magic happens when you inoculate those dried hulls with a mold called Neurospora sitophila, sometimes Actinomyces. The spores grow through the substrate over roughly two days, turning loose husks into a solid cake that tastes like earth, umami, and something slightly nutty. Indonesian street vendors still make it the old way: they wrap the mixture in banana leaves, pile it in warm corners of their stalls, and check it every few hours for texture and smell. The color tells you everything. Red oncom means the Neurospora strain produced pigment — that's the default in most of Java. White oncom is rarer and usually involves a different mold variant or sometimes a bacterial contamination that people have learned to tolerate because the flavor profile is still acceptable. I once bought a batch from a supplier in Cirebon that turned out to be white, and I almost threw it away before tasting it. It was fine, just milder. The lesson was that color isn't a quality indicator the way people assume.
Here's the thing most English-language articles about Indonesian food skip: oncom and tempeh are not interchangeable, even though both are fermented soy or peanut products. Tempeh uses Rhizopus oligosporus, a mold that forms thick white mycelium and requires soybeans as its base. Oncom uses Neurospora, grows faster, and can use peanut skins, coconut press residue, or even cassava pulp. The fermentation window for oncom is 24 to 48 hours at around 30 to 35 degrees Celsius. Tempeh takes longer and runs cooler. If you try to make oncom using tempeh starter, it won't work the same way, and if you try to make tempeh with oncom spores, the texture will be completely wrong. I learned this the hard way after ordering "authentic tempeh starter" online and getting Neurospora spores instead. There's a practical problem people encounter when trying this at home: the humidity control. Neurospora needs high moisture but not standing water. In my first attempt, I wrapped the mixture too tightly in banana leaves and the center went moldy within 12 hours — not the good kind of mold, the fuzzy black spore dots that mean contamination had already started. The workaround is simple but counterintuitive: leave the wrapping slightly loose, or use a perforated plastic bag instead of leaves if you're not in a tropical climate. The mold needs oxygen to fruit properly, and sealed environments kill it. Nutritionally, oncom is worth noting. The fermentation process increases the B vitamin content significantly compared to plain peanut or soy residue. Protein availability improves because the mold breaks down antinutrients like trypsin inhibitors. A 100 gram serving of red oncom has roughly 17 grams of protein and about 190 calories, with the rest coming from fat and carbohydrates. It's not a complete protein on its own, but paired with rice — which is what Indonesians always do — it covers the amino acid gap effectively. I tracked this for about a month during my trip and didn't feel any nutritional deficit despite eating mostly street food.
The taste is where oncom really differs from tempeh. Tempeh is mild and nutty with a firm crumb. Oncom has a sharper, almost peppery bite, especially when fried. The skin of the peanut contributes tannins that survive fermentation, giving it a slight astringency. When you cook it — typically sliced thin and pan-fried with garlic and chili, or wrapped in leaves and grilled — the exterior caramelizes quickly while the interior stays moist and dense. Indonesian cooks call this "oncom goreng" and it's a standard side dish, not some exotic delicacy. It's what your average person eats on a Tuesday. There are commercial production challenges that home fermenters don't face. Large-scale oncom makers in Bandung struggle with batch consistency because Neurospora spores degrade quickly in warm, humid conditions. A batch prepared in the dry season behaves differently from one in the rainy season. Some factories add controlled amounts of wheat bran to stabilize the growth rate, which purists argue dilutes the traditional character. I've tasted both versions and honestly couldn't tell the difference in a blind comparison, which surprised me given how strongly people defend tradition around this food. Food safety is another area worth addressing plainly. Oncom is generally safe when properly fermented, but undercooked or contaminated batches can cause gastrointestinal issues. The Neurospora strain used is non-pathogenic, but wild strains can produce unintended molds if the environment isn't clean. In 2013, a small outbreak in West Java was traced to street vendors who reused old starter material without proper monitoring. The solution was simple: train vendors on spore viability and hygiene, which the local health department eventually did. As a consumer, just make sure the oncom smells earthy and sweet, not sour or ammonia-like. If it smells wrong, don't eat it. That's not a suggestion, it's practical advice.
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If you want to try making it yourself, you need three things: peanut hulls (available from health food stores or Asian markets), Neurospora spores (sometimes sold as "oncom starter" online, though quality varies wildly), and a warm spot between 30 and 35 degrees Celsius. Mix the hulls with a bit of water until they're damp but not dripping, spread them in a shallow container, sprinkle the spores evenly, cover loosely, and wait. Check after 24 hours. If white mycelium is covering the surface and the mass is holding together when you lift it, you're good. If it's sliding apart or smells off, start over. The whole process from start to finish usually takes about two days, and the yield from one cup of hulls is roughly 200 grams of finished oncom. One edge case that caught me: if you're making oncom in a non-tropical climate, your room temperature might be too low. Neurospora slows down considerably below 28 degrees, and the fermentation can take three or four days instead of two. Longer fermentation at lower temperatures increases the risk of contamination because beneficial mold has more time to lose the race against competitors. The fix is to use a seedling heat mat set to 32 degrees, or simply place the container near a kitchen appliance that generates residual warmth. I used the top of my refrigerator and it worked fine, though I had to monitor it more closely than I would have in Bandung. Oncom is also used in dishes beyond simple frying. "Oncom manisi" is a spicy stuffed version where the oncom is mixed with chilies and vegetables, then wrapped and grilled. "Petisdaging" sometimes includes oncom as a protein component in certain regional recipes. In Sundanese cuisine specifically, it appears in rice dishes and sambals. The versatility comes from its ability to absorb flavors while maintaining structure — unlike tofu, which falls apart when overcooked, oncom holds its shape through extended simmering.
I still think about oncom more than I should. It's a food that turns waste into something nutritious through a process that's faster than tempeh, cheaper than most protein sources, and deeply embedded in everyday Indonesian cooking without receiving the international recognition that tempeh has gotten. That imbalance seems unfair to me, but it's also just the reality of how food trends work. Oncom will probably remain a regional staple unless someone exports it the way tempeh was exported in the 1970s. Until then, it's something you eat when you're in Indonesia or when you put in the effort to make it yourself.
