Tempeh starter is an ingredient that has been inoculated with specific spores so it can be added to soybeans to make the fermented food product known as tempeh. A starter must be used to ensure that the correct type of spores take root on the soybeans so they have the best possible chance of making tempeh. Under incorrect conditions, instead of properly fermenting, the soybeans could actually be overwhelmed by undesirable bacteria or molds that could cause it to spoil and become inedible. The end result of using a tempeh starter on soybeans is a block of fermented beans that can be sliced and used in cooking.
There are two varieties of fungal spores used to make tempeh starter. They are Rhizopus oligosporus and Rhizopus oryzae. Both are found in nature and occur normally in the soil where the fungus grows. Some variations have been further cultured to be more favorable toward production and are commercially available. These variations sometimes attempt to produce a more consistent product by making the color of the mold uniform, or they can help to accelerate the fermentation process under certain circumstances.
When making tempeh, there are two different ways that the starter spores can be introduced to the soybeans. The first is to use a small piece of tempeh from a successful previous batch that has been kept alive. The active spores in the starter can be transferred to a new batch, much like a bread starter. The more common way is to use a powdered tempeh starter (PTS).
Powdered tempeh starter uses a base of rice flour or ground soybeans that have been treated with the correct spores. The powder is easy to transport and can easily be added to any mixture. Another advantage is that the powdered starter can be stored for many months in a freezer to preserve the live spores.
Making tempeh directly from a tempeh starter is not necessarily an easy process and can require special attention and sometimes special equipment. For the starter to successfully thrive inside the soybeans, the amount of moisture and the temperature need to be controlled. Specifically, the spores need to ferment at 88° Fahrenheit (about 31° Celsius) for the best results; temperatures too far outside that range can cause the spores to die or not reproduce fast enough to sustain the fermentation. One thing that can be observed when working with tempeh starter is that a dark, black mold could appear on the surface of the soybeans. This dark color is actually the result of the fungus reproducing and is a sign that it is healthy.