Oncom is traditionally made from okara, the waste product of soymilk production. Making oncom is a more difficult than making tempeh, mainly because the particle size and moisture content of the okara can vary. You can make oncom from whole soybeans, but to obtain better quality oncom and soymilk with less beany taste you should use dehulled soybeans.
1. To make 600g of oncom you need:
- 300 g whole soybeans (or dehulled soybeans)
- 3 g oncom starter
- 4 ml lactic acid 80%
2. Soak the soybeans in water during 12 hours. If use whole soybeans you have to split them with a kneading motion and remove most of the hulls. If you don’t remove the hulls the final oncom cake will be to pulpy and the mold will have problems getting enough oxygen.
3. Drain the soybeans and boil them in 2 liter water for 30 minutes. While still hot, process the soybeans in the cooking water until the beans are finely ground. Then the mixture will have a milky color. Separate the okara from the soymilk through a cheese cloth. Remove as much liquid as possible. The resulting liquid is soymilk and can be kept in the fridge for a few days.
4. Mix the okara very well with the lactic acid. It is important to use lactic acid to prevent spoilage of the oncom by unwanted and pathogenic bacteria. Oncom is a lot more prone to bacterial spoilage because it is more moist and has a lot more surface area that traditional soybean tempeh. Place the okara in a microwavable dish and heat for 5 min at 800W, stir after 3 min. This heating has 2 functions: it sterilizes the okara and makes the okara drier.
5. Allow the okara to cool down to below body temperature add tempeh starter and mix very well with a clean spoon.
6. Take 2 plastic bags 18 x 28 cm and perforate them with holes at a distance of about 1 cm by a thick but sharp needle. This will allow the Rhizopus mold to breathe. Put the okara in the two bags and seal them. Gently press them flat, using as little pressure as possible as not to compact the okara too much and making sure that the total thickness of the beans is max 2 cm.
7. Place the bags in a warm place with a temperature of about 28-32 °C (82-90 °F) for about 24-36 hours, or until the okara is covered with a very dense white mycelium. The formation is black spores is possible. Try to keep the temperature at the lower end of the range as the time wears on.
8. When the oncom is ready you can store it for a few days in the fridge or a few months in the freezer.