Oncom: Indonesia’s Fermented Peanut Cake Made by a Nobel Prize-Winning Organism

Most people who care seriously about fermented foods have heard of tempeh. The soybean cake from Central Java has made its way into Western health food stores, recipe books, and vegan cooking with remarkable success. Almost nobody outside West Java has heard of oncom. This is a genuine loss.

Oncom is tempeh’s Sundanese cousin — a mold-fermented cake from the same Indonesian fermentation tradition, but made from peanut press cake instead of soybeans, fermented with a different organism, and producing a flavor profile that’s simultaneously more subtle and more complex. The red-orange variety gets its color from a mold called Neurospora crassa, which happens to be one of the most important organisms in the history of molecular biology — the subject of Nobel Prize-winning research. The fact that Sundanese home cooks were using it to make delicious, nutritious food for centuries before scientists figured out its genetic structure is exactly the kind of thing that makes food history extraordinary.

The Sundanese Food Culture and Where Oncom Fits

The Sundanese people are the second-largest ethnic group in Indonesia, concentrated in West Java — the region encompassing Bandung, Bogor, Tasikmalaya, Sukabumi, and Cirebon. Sundanese cuisine (masakan Sunda) is distinct from Javanese cuisine in its emphasis on raw vegetables (lalapan), peanut-based sauces, and what food anthropologists call “fermentation intelligence” — a sophisticated understanding of how to transform agricultural by-products into nutrition.

Oncom appears in Sundanese cooking as a versatile fermented protein source — fried as a main dish, crumbled into sauces, wrapped in banana leaf and steamed, or incorporated into traditional salads. In rural West Java, where peanut oil production and tofu-making have been household industries for centuries, oncom was the elegant solution to the waste problem: what do you do with the dry cake remaining after pressing peanuts for oil? What do you do with the soybean dregs left after making tofu? You ferment them into oncom, which is more nutritious, more flavorful, and more digestible than the raw by-products.

This sustainability logic predates the modern zero-waste food movement by hundreds of years. Oncom production was never a compromise; it was sophisticated circular food economy. The fermentation transforms low-value agricultural residue into high-value fermented food with superior protein digestibility, enhanced vitamin content, and complex flavor. Contemporary food systems designers would recognize it as brilliant.

The Nobel Prize Connection You Weren’t Expecting

Neurospora crassa — the organism responsible for red oncom’s distinctive orange-pink color — has a peculiar distinction in the history of science: it was the organism used by George Wells Beadle and Edward Lawrie Tatum in the experiments that won them the 1958 Nobel Prize in Physiology or Medicine.

Beadle and Tatum’s work in the early 1940s demonstrated that each gene controls the synthesis of one specific enzyme — the “one gene, one enzyme” hypothesis that became one of the foundational principles of molecular biology. They chose Neurospora crassa because it’s genetically simple, grows quickly, and produces easily observable phenotypic changes when genes are mutated. Their ability to create and study biochemically deficient mutants of this mold established the direct connection between genes and metabolic pathways, reshaping biology as a discipline.

The characteristic orange color of Neurospora — caused by carotenoid pigment synthesis by a gene cluster on chromosome I — was one of the earliest genetic markers used in this research. When you look at a block of red oncom and see its salmon-orange surface, you’re looking at the same carotenoid expression that helped unlock the secrets of genetic inheritance. Sundanese cooks had already been eating the product of this organism’s carotenoid synthesis for generations before Beadle and Tatum gave it a Nobel Prize.

In food use, the Neurospora strain isolated for oncom production (often identified as Neurospora intermedia var. oncomensis) is a food-adapted variant that produces fewer of the potentially harmful secondary metabolites found in wild strains. Food-grade Neurospora crassa cultures have been selected and maintained by Indonesian fermenters over generations for reliable, safe performance in the oncom substrate.

Red Oncom vs Black Oncom

Two distinct types of oncom exist, differentiated by the fermenting organism and the substrate.

Red oncom (oncom merah) uses Neurospora intermedia or Neurospora crassa as the primary mold. The substrate is peanut press cake (bungkil kacang) — the dry, oil-depleted cake remaining after peanuts are pressed for oil. Sometimes tofu lees (the solids remaining after straining soy milk) are added. The finished oncom has a distinctive salmon-to-orange surface color (from the mold’s carotenoids), a nutty flavor inherited from the peanut base, and a tender, slightly crumbly texture. Red oncom has a milder flavor than black oncom and is considered the “traditional” variety by most Sundanese.

Black oncom (oncom hitam) uses Rhizopus oligosporus — the same mold used to make tempeh — as the primary fermenter. The substrate is typically soybean dregs (okara, the insoluble soy fiber and protein remaining after extracting soy milk) or cassava dregs (the residue from cassava starch production). Black oncom has a darker surface (gray-to-black from the Rhizopus spores), denser texture, and stronger, more tempeh-like flavor. Black oncom is more common in areas with active tofu and cassava industries and is considered more robust and savory.

For home fermenters, black oncom is the more accessible starting point because Rhizopus oligosporus (tempeh starter) is widely available online and from fermentation suppliers. Red oncom requires sourcing Neurospora crassa starter, which is available from specialty suppliers (Cultures for Health, GEM Cultures) but less commonly stocked than tempeh starter.

The Fermentation Science

Both types of oncom undergo solid-state fermentation — the mold grows through a solid substrate, binding it into a cohesive cake with mycelium, just as Rhizopus does in tempeh. This is fundamentally different from liquid fermentations like sauerkraut or kefir; the mold’s physical mycelium network is the structural element that makes the cake hold together and creates its characteristic texture.

For red oncom, the Neurospora mycelium begins germinating from spores within 8–12 hours at 28–30°C. By 18–24 hours, white mycelium is visible on the surface, beginning to bind the peanut press cake particles together. Between 24 and 36 hours, the mycelium matures and the characteristic orange carotenoids are synthesized and appear at the surface. The interior remains softer and more moist than the surface; the surface, where oxygen exposure is greatest, shows the densest mycelial growth and the deepest color.

Neurospora produces several enzymes during this fermentation that transform the substrate:

• Proteases: break down peanut proteins into peptides and amino acids, improving digestibility and creating the savory depth of flavor
• Lipases: partially break down peanut fats, releasing free fatty acids that contribute to flavor complexity
• Phytase: degrades phytic acid (the anti-nutrient in peanuts that binds minerals like zinc and iron), dramatically improving mineral bioavailability
• Carotenoid synthesis enzymes: produce beta-carotene precursors and other carotenoids (the orange color also indicates genuine nutritional value)
• Riboflavin synthesis: Neurospora is exceptional at producing riboflavin (vitamin B2) — one of the richest natural sources, which is why oncom provides meaningful B2 content

Why Oncom’s Nutritional Profile Is Remarkable

The combination of substrate (peanuts or soybeans, inherently high in protein, fat, and minerals) and Neurospora fermentation creates a nutritional profile that rivals any fermented food.

Protein content runs 15–22% by weight in red oncom made from peanut press cake, with protein digestibility significantly higher than raw peanuts due to the protease enzyme activity — studies suggest digestibility improvements of 20–30% compared to unfermented peanut press cake. The phytate reduction from Neurospora’s phytase means that minerals — particularly zinc, iron, and magnesium — present in peanuts are actually absorbed by the body rather than bound and excreted.

Riboflavin (vitamin B2) content is among the highest of any naturally produced food. Niacin (vitamin B3) is also produced during Neurospora fermentation. Some studies on traditionally prepared oncom have found detectable B12 — unusual for a plant-based fermented food — though this likely comes from bacterial contamination of the culture rather than Neurospora itself.

The sustainability dimension amplifies the nutritional argument. Peanut press cake and tofu lees are waste products that would otherwise be discarded or used as animal feed. Oncom fermentation captures the residual nutrients in these substrates and makes them bioavailable to humans. Per kilogram of peanuts processed for oil, the press cake represents approximately 40–45% of the original weight and retains most of the protein. Without oncom, that protein is lost to the human food chain. With oncom, it becomes a staple food.

How Oncom Is Used in Sundanese Cooking

Oncom’s versatility in Sundanese cooking is impressive. It can be fried, steamed, grilled, or eaten raw (fresh oncom, consumed the day it’s made, has a softer texture suitable for raw applications). Its mild flavor makes it a chameleon that absorbs spices and aromatics readily.

Oncom goreng (fried oncom): sliced into 1cm pieces and pan-fried in coconut or palm oil until the surface is golden and crispy. Often eaten with rice and a sambal. The frying develops Maillard browning that adds nutty depth to the already-nutty peanut base.

Tumis oncom (stir-fried oncom with aromatics): crumbled oncom stir-fried with shallots, garlic, chilies, galangal, and often kecap manis (sweet soy sauce). This is one of the most common Sundanese home preparations — quick, economical, and deeply flavorful.

Karedok: a signature Sundanese raw vegetable salad dressed with a sauce containing oncom blended with peanuts, palm sugar, galangal, kencur (aromatic ginger), and chilies. The oncom contributes an earthy fermented note to the peanut sauce that distinguishes karedok from other Indonesian peanut-dressed salads.

Pepes oncom: fresh oncom mixed with aromatics, wrapped in banana leaf, and steamed or grilled directly over charcoal. The banana leaf imparts a subtle vegetal note; the charcoal grilling creates smokiness. This is considered a prestige preparation for celebrations.

Outside Indonesia, oncom’s closest cultural analogy in the West is firm tofu — it can be used in similar contexts. But the flavor is more complex, the nutritional profile is different, and the fermentation character gives it a depth that fresh tofu lacks.

Making Oncom at Home

What You’ll Need

For black oncom (more accessible): tempeh starter (Rhizopus oligosporus), available from online fermentation suppliers, health food stores, and Asian grocery stores. For red oncom: Neurospora crassa food-grade starter from specialty fermentation suppliers (Cultures for Health, GEM Cultures). Both require an incubation environment at 28–30°C for 24–36 hours.

Ingredients (Makes one 20x15cm cake)

• 400g peanut flour (blanched, ground peanuts) — available at Asian grocery stores or make your own from roasted peanuts processed in a food processor until fine crumbs form; do not over-process into peanut butter
• 100g cassava flour (optional, for structure; omit for pure peanut oncom)
• 1/2 tsp tempeh starter (Rhizopus oligosporus for black oncom) or Neurospora crassa starter for red oncom
• Water — approximately 150ml, enough to bring substrate to 50–55% moisture
• 1/2 tsp white wine vinegar (to slightly acidify substrate, inhibiting unwanted organisms while Neurospora or Rhizopus establish)

Equipment

• Mixing bowl
• Banana leaves, parchment paper, or perforated plastic bags — for wrapping the oncom cakes during incubation (banana leaf imparts traditional flavor)
• Incubation environment — oven with just the light on, dehydrator set to 30°C, or a proofing box; consistent 28–30°C is essential

Step 1: Prepare the Substrate

Mix peanut flour and cassava flour (if using) in a bowl. Add water gradually, mixing until the mixture holds together when squeezed but crumbles easily when pressed — like damp sand. Add the vinegar and mix again. The moisture is critical: too dry and the mold won’t grow through the substrate; too wet and unwanted bacteria outcompete the mold.

Step 2: Cool and Inoculate

If your peanut flour was briefly heated or roasted (which helps sterilize the substrate), allow it to cool to below 35°C before adding starter. Sprinkle the starter powder evenly over the substrate and mix thoroughly with clean hands, ensuring even distribution. Inoculation consistency is the single most important factor in oncom quality.

Step 3: Shape and Wrap

Shape the inoculated substrate into cakes roughly 2–3cm thick. Wrap each cake in a banana leaf (cut to size), folded like a parcel, or place in a perforated plastic bag (punch small holes every 2cm to allow air circulation). The wrapping holds the cake together while allowing the mold’s oxygen consumption to be balanced with ambient oxygen supply.

Step 4: Incubate at 28–30°C for 24–36 hours

Place wrapped cakes in your incubation environment. Check after 18 hours. You should see white mycelium beginning to appear through the wrapper. By 24 hours, the surface should be visibly colonized. For black oncom, 24 hours is usually sufficient. For red oncom with Neurospora, continue to 30–36 hours until the orange carotenoid color develops at the surface.

If temperature exceeds 35°C during peak mycelial activity (which produces heat), briefly ventilate the incubation environment. Overheating is the most common oncom failure mode — excessive heat kills the mold mid-fermentation, resulting in a wet, unbound substrate.

Step 5: Verify and Store

Finished oncom should be bound together by dense mycelium — it should hold together as a cake when unwrapped and handled. The aroma should be pleasant: nutty, slightly earthy, mildly fermented. Refrigerated oncom keeps 3–5 days; it will continue to develop flavor and eventually over-ferment if kept too long. Freeze slices for up to 3 months for longer storage.

Troubleshooting

Oncom didn’t bind — substrate crumbles after incubation

Either the starter was old and inactive, the substrate was too dry, or the temperature was too low. Check starter freshness (perform a test inoculation on a small amount of cooked grain), add slightly more moisture next batch, and verify your incubation temperature with a thermometer.

Black spots or unusual colors

Black spots that appear after 36+ hours of incubation in Rhizopus fermentation are normal — the Rhizopus spores mature from white to dark gray-black. This is not mold contamination; it’s the natural life cycle. If you see green (Penicillium) or pink slimy patches (bacterial contamination), discard the batch and clean equipment thoroughly.

Strong ammonia smell

Over-fermentation — the oncom has been incubating too long or at too high a temperature. The protein breakdown has gone too far, producing ammonia as a by-product. This is the same process as over-fermented tempeh. Consume immediately or discard; cooking may reduce the ammonia note, but the flavor will be compromised.

Frequently Asked Questions

What does oncom taste like?

Mild, nutty (from the peanut base), earthy with a clean fermented note. Less assertive than tempeh — the flavor is genuinely subtle rather than aggressively fermented. Red oncom has a slightly sweet, almost roasted-peanut quality; black oncom is more savory and complex, closer to tempeh in character. When cooked, oncom takes on additional Maillard browning that gives it a deeper nutty character.

Is oncom safe to eat?

Yes, when made from food-grade Neurospora crassa cultures. Wild Neurospora strains can produce mycotoxins, which is why sourcing from reputable fermentation suppliers matters. Traditionally made oncom in Indonesia uses strains that have been selected over generations for food safety. For home production, use a certified food-grade Neurospora starter, not a wild Neurospora culture collected from the environment.

Is oncom the same as tempeh?

They’re related but distinct. Both are Indonesian mold-fermented cakes. Tempeh typically uses soybeans as the substrate and Rhizopus oligosporus as the fermenting mold, producing a firm, white-surfaced cake with a strong mushroomy flavor. Red oncom uses peanut press cake as the substrate and Neurospora crassa as the fermenting mold, producing an orange-surfaced cake with a milder, nuttier flavor. Black oncom uses Rhizopus (same as tempeh) but on a different substrate (tofu or cassava lees). The flavor profiles and culinary applications are different enough that they’re not interchangeable.

Where can I buy oncom outside Indonesia?

Rarely. Indonesian grocery stores in major Western cities occasionally carry frozen oncom, and some Indonesian restaurants serve dishes containing it. For most cooks outside Indonesia, making it at home is the realistic path. The ingredients (peanut flour, tempeh starter) are available from fermentation supply companies and Asian grocery stores. Neurospora crassa starter for red oncom is the harder-to-find item but is available from specialty fermentation suppliers.

Can I use roasted peanuts instead of peanut press cake?

Yes — this is the most practical substitution for home cooks who can’t source peanut press cake. Process roasted, blanched peanuts in a food processor until they reach a fine, crumbly texture (stop before they become peanut butter). Spread the ground peanuts on a baking sheet and bake at 160°C for 10 minutes to reduce moisture and inhibit competing organisms. Cool completely before inoculating. The flavor will be slightly richer and more roasted than traditional peanut press cake oncom, which is not a bad thing.

The Quiet Brilliance of Oncom

Oncom doesn’t shout. It doesn’t have the punchy intensity of kimchi or the assertive fungal depth of a good aged miso. It works quietly, in the background of Sundanese cooking, providing protein, nutrition, and subtle fermented complexity to dishes that might otherwise be monotonous.

There’s something instructive about the way Sundanese food culture treats oncom: not as a health food or a superfood or a fermentation project, but simply as food — the right ingredient for certain dishes, made at home or from the market, eaten several times a week, unremarkable in the best possible sense. The Nobel Prize connection is a curiosity. The sustainability story is important. But mostly oncom is just good food, made from things that would otherwise be wasted, by a mold that knows what it’s doing.

Start with the black oncom version using tempeh starter. Master that process before moving to red oncom with Neurospora. Use it wherever you’d use tempeh, and then go further: make karedok with the peanut sauce, wrap some in banana leaf and grill it, crumble it into a stir-fry with chilies and galangal. The Sundanese have been doing exactly that for centuries. It turns out they knew what they were doing.