Soak the soybeans in water overnight (ideally 12 hours or longer).

Drain and rinse.

Add the soybeans to a large pot with boiling water and cook over medium-high heat for 20 minutes.

Add the vinegar, stir and cook for 10 to 40 more minutes or until the soybeans are soft, but not mushy. Soybeans may take from 30 to 60 minutes to cook in total.

Drain the soybeans and place them back to the pot. Cook over medium heat, stirring frequently until all the liquid has evaporated.

Allow the soybeans to cool to about 95ºF or 35ºC. I always use an instant-read thermometer, but that's up to you.

Add the tempeh starter, stir until well combined, and set aside.

For the next step, you can use a plastic zip-lock bag, which is pretty convenient for this recipe or make it the traditional way by using a banana leaf. I’ve always used a plastic bag and I use it multiple times.

If you use a zip-lock bag, perforate the bag first at one-inch interval all over. I used a wooden stick and two plastic bags. 

Then divide the soybeans between the two bags, seal them and flatten out the soybeans evenly.

To incubate the tempeh, you can use a dehydrator, an incubator, or your oven. I prefer to use a dehydrator myself, but the oven also works really well. If you’re going to use a dehydrator or an incubator, place the bags inside at a temperature between 85°F and 90°F (30-35ºC) for the next 24 to 48 hours. 

If you’re going to use the oven, transfer the two plastic bags to your oven with only the lights on for 12 hours. Then remove from the oven and keep them in a warm place for up to 48 hours.

After 12-24 hours you should start to see some white mycelium growing on the soybeans. The tempeh is ready when the entire surface is covered with dense, white mycelium (some black or gray spots are fine), as well as the spaces between the beans. 

Once the tempeh is ready, remove the bags from the dehydrator, incubator or oven and let the tempeh cool at room temperature. Then transfer it to an airtight container.

Keep it in the fridge for up to 1 week or in the freezer for up to 3 months.