As with other fermented products, such as yoghurt, miso, natto or kambucha, a starter is required to initiate the fermentation. Without a good starter other micro-organisms will spoil the product and make it uneatable or even poisonous.
In Indonesia, tempeh starter is made by wrapping cooked soybeans inside hibiscus leaves, which naturally contains the desired Rhizopus molds, and leaving it to ferment for a few days in a warm place. It is also possible to make tempeh from a piece of very fresh live tempeh, by mincing it finely and adding it to the cooked and cooled soybeans. However, this method is not very reliable and in practice it will often result in a failed batch. Therefore, most tempeh makers, even large factories, use commercial tempeh starter with a guaranteed strength and that is free from unwanted bacteria such as Escherichia coli, Salmonella or Listeria. A good source of tempeh starter is https://www.tempeh.info
Making your own tempeh starter
If you want to make your own tempeh starter you could follow the procedure that was used by the University of Illinois.
1. Preparation of substrate
Place 1/4 cup of long grain white rice in a pint Mason jar and add 20 ml water.
Shake the jar to mix the rice and water. Cover the jar with a cotton wool cover made by placing layers of cotton wool between two layers of cheesecloth. Secure the cover by a rubber band or by replacing the metal ring. Allow to stand for a few hours with occasional swirling.
Use autoclave or pressure cooker at 15 psi and 121°C for 15 minutes. When still hot shake the jar vigorously until the rice clumps are broken loose and allow to cool to room temperature.
Make a spore suspension by aseptically adding a few ml of sterile water to an agar slant of Rhizopus oligosporus or Rhizopus oryzae. Add the spore suspension aseptically to the rice and mix well.
Incubate the jar on their sides at 30°C (86°F) for 45 days, at which time the rice will be covered with mycelia and black spores.
5. Harvesting the spores
Transfer the content of the jar into a sterile blender and blend to form a fine powder. This tempeh starter can be kept frozen to maintain the viability.
Contaminated tempeh starter
Good tempeh starter is rather expensive, but some suppliers don't take hygiene that seriously and sell product that is not pure and that may even be contaminated with pathogenic bacteria. A tainted tempeh starter was recalled in April 2012 after an outbreak of a rare type of salmonella that has sickened more than 90 people, mostly in North Carolina. Infections were spread by eating the fresh tempeh, by contact with surfaces contaminated by the product and through person-to-person contact. The FDA traced the contamination back to the Rockville Maryland-based tempeh starter distributor Tempeh Online, who imported their product directly form Indonesian. At that time, Tempeh Online operated from different websites. Before you buy tempeh starter make sure that you can identify the location and reputation of original producer.