Oncom is a food that mostly is produced and eaten in West Java. It is a fermented block or sheet made from the pulp of nuts, beans and other foods that have been exposed to a particular type of spore and allowed to ferment for one full day. There are two main types of oncom, black and red, with the difference sometimes being the cultures used and sometimes being an indication of the type of pulp used. Many dishes can be made from oncom, and it is very common to find it cut into thin pieces and deep fried to make a snack called krupuk or wrapped in banana leaves and roasted to make a variation of the food known as pepes. Once the pulp has been fully inoculated with the spores, it can be allowed to ferment for a longer time to make other foods, such as dage.
The material used to make oncom is the pulp of different foods that are left over after being processed for their liquids. The most common types of pulp, also called presscake, are soybean, peanut, coconut and cassava. Each imparts a slightly different texture and flavor. Most often, black oncom is made from peanut pulp and has a softer texture than other types. The pulp that is used can be gathered from industrial processing plants, or it can be made by boiling the raw ingredients, straining out the solids and grinding them.
The process of making oncom starts by washing the pulp. Once washed, it is rigorously dried to help prevent harmful mold and bacteria from taking root on the pulp. In some cases, the pulp is boiled to soften the texture or mixed with vinegar to sterilize the surface. The dry pulp is arranged into a wide, flat shape and then coated with the spores being used.
Unlike similar products, such as tempeh, spores are only placed on the outside of oncom. The inoculated pulp is placed in a shady, consistently warm location for 24 hours, during which time mold will develop on the surface of the pulp; the pulp will become very warm and start to take on a denser texture. After the 24-hour period is done, the mold will have permeated into the pulp, meaning it is ready to be eaten.
As a food, oncom has a distinctive fermented flavor. It often is cut into thin pieces that are deep fried and eaten like a snack similar to potato chips. The food also can be broken down and fried or roasted and added as a non-meat protein in rice or soup. When wrapped in banana leaves and flavored with garlic, it becomes a form of pepes.