Soak the whole soybeans overnight using three times as much water as soybeans.
Place the soaked whole soybeans in a food processor, add enough water to just cover the soybeans and process until the beans are finely ground.
Heat a large pot containing 8 cups of water. Add the finely ground soybean mixture and cook for about 20 minutes stirring frequently. The mixture will foam but should stop foaming as it cooks. Once the cooking process is completed, the mixture should look grainy indicating the milk has separated from the fibrous part of the ground up soy.
Turn off the heat and strain the pulp from the milk. Note, the milk strained off is soy milk and can be refrigerated for use if desired.
Spread a 2-3 inch layer of fresh soy pulp (Okara) in a large baking sheet and place in a preheated oven for 25 minutes at 300F degrees. Stir the mixture after the first 10 minutes in the oven.
Pour the hot soy pulp over a spread pre-ironed towel and mix the pulp with a wooden or metal spoon until cooled to below body temperature (cool to the touch). It is important the beans are relatively dry as excess moisture can lead to unwanted bacteria and spoilage.
Transfer the beans to a clean bowl.
Add the vinegar and mix very well.
Add the starter and mix very well.
Prepare the incubation containers or plastic bags with needle-size holes poked through for ventilation.
Fill the bags about 1 inch thick with the Oncom mixture, seal and pack flat in a uniform layer. If using trays, pack the Oncom in a uniform layer 1 inch thick and over with aluminum foil with holes poked in it. There should be 1-2 inches between the foil and the beans.
Incubate at 83-86F degrees (30-31C) for 22 to 28 hours. Be sure to check the temperature every few hours during the beginning of the process--it is very important the temperature be within the correct range to encourage the starter to ferment the beans effectively.
After 16 hours of incubation, the mixture will begin producing its own heat so heat device can likely be lowered.
If excessive condensation builds up, open the bags and prop open to allow the moisture to escape.
Keep an eye on the Oncom as it nears the end of the fermentation period. It is important to stop the process before dark-colored spores appear.
The Oncom cake should be solid and white with the beans firmly connected and a mushroom-like aroma.
The Oncom can be cooked and eaten, refrigerated or frozen (do not stack in the refrigerator as the internal heat will encourage the mixture to culture further causing problems).
Tips for Making Oncom:
It's not unusual for there to be a bit of a learning curve when making Oncom. If your first batch doesn't turn out, just try again.
Use a thermometer to verify the ongoing temperature of the Oncom during fermentation (a meat thermometer with an alarm often sold for baking and for the BBQ are very useful as they will warn you if the temperature falls outside a set range).
Incubation Methods (Heat Sources) for Making Oncom:
Proper temperature is vital for the fermentation process. Ideally the Oncom should ferment at 83-86F (30-31C). Regardless of which method you choose for keeping your Oncom warm during the process, be sure to verify that the method you are using will achieve and maintain the proper temperature and check the temperature often during the process. Here are a few possible methods for maintaining the proper temperature:
Large cube-shaped dehydrator (i.e. Excalibur or TSM Dehydrators)
Oven with a low temperature setting or with just the light turned on
Cupboard with a low level heat source (like a high wattage light bulb)
Styrofoam or plastic "cooler" with warm water bottles to maintain the heat
Low-level mat such as those sold for reptiles (Pet Store) or to maintain spout seedlings (Gardening Store)--please note, these mats generally keep the tempeh about 10 degrees above the ambient temperature so they will still require a rather warm room. This is not the same as a commercial heating pad which normally run far too warm for this purpose
Incubate outside if you live in a warm climate
A warm part of your home during the summer months (ex. a non-climate controlled attic, etc.)
Removing Soybean Hulls: The easiest way to make Oncom is to use de-hulled soybeans but if they are not available you will need to remove the hulls prior to making Oncom.
Soften the soy beans either by soaking or boiling:
Soak the beans in 6 cups of water for 6-18 hours OR
Boil the beans for 15 minutes, turn off the heat and allow the beans to sit for two hours
Separate the hulls from the soy beans:
Split the beans by squeezing them with a kneading motion. The hulls will float on the water and can be skimmed off OR
Remove the beans from the water and place in a shallow baking dish. Use a potato masher to split the beans and loosen the hulls. Add the beans and hulls back to the water so the beans will sink and the hulls will float. The hulls can then be skimmed off. This process may need to be repeated several times to remove all the hulls and can take approximately 10 minutes.
Boil the now hulled beans for 30+ minutes until they are cooked through (you can then skip the alternative cooking process in the above recipe and proceed directly to drying the beans).