Tempeh – Care For Some Mold With Those Beans?

Unlike most of the fermentation projects I play with, tempeh is not a wild ferment, at least not where I live. Indonesian in origin, traditional tempeh is made by incubating soybeans at tropical temperatures – i.e., ambient temperatures in Indonesia – until an edible white mold grows in, usually within 24 hours or so. This is the product of two species of the Rhiozopus mold, which crops up in a variety of fermentation starters, including Asian yeast balls used for making rice beer. Rhizopus oryzae and Rhizopus oligosporus perform an interesting series of functions on the beans, but what I love when I make tempeh is that well into the incubation period, the beans start to generate heat on their own as the mold develops.

In less tropical parts of the world, a starter culture of one or both Rhizopus bugs is required to make tempeh. I’ve had great success using starter purchased from TempehStarter.com, which is located in Indonesia. There are plenty of other sources online, but this pack will last me years and I haven’t seen better pricing, though shipping wasn’t especially fast.

Starter powder used to make tempeh
I admit to being hesitant about ordering a white powder from Indonesia for importation into the United States.

You’ll also probably need plastic baggies. I’ve used the snack-sized Ziploc bags with great results. I’ve also made a batch without using bags just by placing the inoculated beans on a large cookie sheet that had holes in the bottom. The surface dried out noticeably and I wasn’t as pleased with the results. Banana leaves are the traditional wrapper in Indonesia. If you use bags, poke a lot of small holes in them so the mixture can breathe.

Finally, to make tempeh, you need an incubation chamber that can hold a temperature of around 85 degrees. I use my oven, with the lamp from a clamp light (aff) inside, as well as a digital thermometer with probe (aff) for monitoring purposes. Forgive my dirty oven, but here’s a picture of the set up, which is really simple:

Lamp in oven as incubator
Lamp from a clamp light in the oven for warmth. The bulb is usually enough to hold temps around 85 F, but I’ll add a quart jar of hot water if it’s struggling.

How To Make Tempeh At Home

  1. To make tempeh, soak beans, grains, or both, overnight. Your starter packet will have instructions with the ratio of cups or grams of beans per unit of starter powder. Traditionally, soybeans are used, but I’ve made tempeh with lentils and oats. I’d like to try it with rice, but haven’t done so yet.
  2. In the morning, boil soybeans, or cook your beans or grains. For soybeans, scoop the hulls out as they float up. It’s not necessary to meticulously dehull a big batch of beans, but the traditional approach aims for as few hulls as possible.
    Important point: Don’t overdo on the cooking. The mold will grow through the tiny spaces between the solids, so if your beans or grains end up too wet and mushy, you won’t get good infiltration. Steaming may be a better option than boiling for grains and some beans. The end result should be something that’s firm but yielding, with grains or beans able to hold their shape pretty well.
  3. Drain, cool, and dry the beans. I do this by stirring the beans in a wide baking dish, but I’ve seen people point box fans at a bowl of beans and even use hair dryers. When I use soybeans, I also use paper towels to dry them as much as possible. That won’t be feasible with all substrates, but removing as much excess moisture as possible is important for mold growth. Once the beans have cooled to around body temperature, they’re ready for the next step.
  4. Inoculate the beans. Most recipes call for you to add a little bit of white vinegar here to drop the pH and give the mold a head start. I’ve never done that. I just sprinkle the spore powder in and mix it in thoroughly for several minutes.
  5. Put the beans into the bags with holes punched in them. Each package should end up around one inch thick or so.

    Packaged, inoculated beans in the oven.
    Packaged, inoculated beans going into the oven/incubation chamber.
  6. Over the next few hours, monitor the temperature. If the beans get too hot, it’ll kill the Rhizopus. If they’re too cold, you won’t get good growth. After twelve hours or so, you’ll start to see the white mycelium developing on the surface.

    Block of tempeh with obvious fungal development
    This is how you make tempeh!
  7. At this point, monitor your temperature more thoroughly, as each block will start putting off a good amount of heat. My digital thermometer has an alarm that I can set for a target temperature. If it exceeds that, it’ll chime at me and I can open the door, turn the lamp off, or whatever is required. In this picture, I have two bags of tempeh stacked with the probe between them, registering 92F.

    Blocks of tempeh registering 92F.
    The molds that create tempeh put off their own heat as they develop.
  8. Tempeh is finished when you start to see grey patches developing. I remove them from the incubator at this point, take them out of their bags, and wrap them in plastic wrap for storage in the freezer. I’ve read that dunking them in boiling water before freezing helps, but I don’t understand the mechanism of that and have never done it.

Tempeh should be eaten cooked. I like baking it with various sauces, but it goes well in sandwiches, stir fries, and plenty of other dishes. It’s a nutritional powerhouse, high in protein and relatively low in carbs, with an earthy, mushroomy type of flavor that I really like. Even more than eating it, I like to make tempeh. That phase where it’s pouring off heat of its own is fascinating for me, and I always look forward to starting a new batch.

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