Oncom is traditionally made from okara, the by-product of soy milk 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 for better quality oncom and soymilk with less beany taste you should use dehulled soybeans.
1. To make 600g of oncom you will need:
- 300 g whole soybeans (or dehulled soybeans)
- 3 g oncom starter
- 4 ml lactic acid 80%
2. Soak the soybeans in water for 12 hours. If you are using 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 too mushy and the mold will have problems getting enough oxygen.
3. Drain the soybeans and boil them in 2 liters of water for 30 minutes. While still hot, grind the soybeans in the cooking water until the beans are finely ground. The mixture will have a milky color. Separate the okara from the soymilk with a cheesecloth. Remove as much of the liquid as possible. The resulting liquid is soy milk and can be stored in the fridge for a few days.
4. Mix the okara 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 much more susceptible to bacterial spoilage because it is more moist and has a much larger surface area than traditional soy tempeh. Place the okara in a microwaveable dish and heat for 5 minutes at 800W, stirring after 3 minutes. This heating has 2 functions: it sterilizes the okara and makes it drier.
5. Allow the okara to cool to below body temperature, add the oncom starter and mix well with a clean spoon.
6. Take 2 plastic bags 18 x 28 cm and make holes about 1 cm apart with 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 so as not to compress the okara too much and making sure that the total thickness of the beans is no more than 2 cm.
7. Place the bags in a warm place at a temperature of about 28-32°C for about 24-36 hours or until the okara is covered with a very dense white mycelium. The formation of black spores is possible. Try to keep the temperature at the lower end of the range over time.
8. When the oncom is ready you can store it in the fridge for a few days or in the freezer for a few months.