Tempeh is a cultured soy product rich in protein and beneficial enzymes. A homemade tempeh incubator can be any kind of box that will hold culturing tempeh at a temperature of about 30 degrees Celsius, or 86 degrees Fahrenheit. Unless you live in a place where the weather stays at around that temperature (such as Indonesia, where tempeh first originated) you will also need some kind of heat source.
An old refrigerator makes an ideal homemade tempeh incubator. An old refrigerator provides a sealed, insulated box that seals well to keep out outside microbes and dirt, and has smooth surfaces on the inside so you can clean it easily and thoroughly. In addition, it comes with built-in shelves for the tempeh you will be incubating.
The Heat Source
A homemade tempeh incubator needs a heat source to maintain its temperature. A lightbulb can usually provide enough heat, provided the incubator is well insulated and sealed. Position the light bulb in the center of the bottom shelf to allow for even heat distribution. Most older refrigerators have a hole in the back for ventilation; you can use this opening to run the power cord for your light bulb. If there is no cord-sized opening available, you will need to drill one. You can also experiment with the existing light bulb in the refrigerator. By turning off the refrigeration control but plugging in the unit you can keep the lightbulb lit, providing some heat. Monitor the temperature to make sure that the existing lightbulb provides enough heat to culture tempeh.
You will need to monitor and control the temperature inside your homemade tempeh incubator. Use a thermometer to check that you are maintaining a temperature of about 86 degrees Fahrenheit inside the unit. Move the thermometer to different shelves to check whether the temperature is consistent in different parts of the incubator. Using a thermometer with a remote temperature reading will enable you to monitor the incubation temperature without changing it by opening the door of the unit. If the unit is too cold, make it warmer by using a more powerful light bulb. If your tempeh incubator is too warm, make it cooler by using a less powerful light bulb.