As noted above, the primary fermentation is mediated by growth of R. oligosporus, which can be added to the soy beans in one of several different forms. First, it can be added as a pure spore culture. Recommended strains include NRRL 2710 and DSM 1964, both isolated from Indonesian tempeh and both available from public culture collections. Like the commercial strains used for other fungal-fermented products, tempeh starter cultures should be selected based on specific phenotypic traits (Table 12-7).Alternatively, a backslop material consisting of a dried tempeh culture can be used. Finally, a third form, used in traditional tempeh manufacture, is called usar, and is made by inoculating wild Rhizopus spores onto the surface of leaves obtained from the indigenous Indonesian Hibiscus plant.After two to three days of incubation, the leaves contain a dense spore crop that can be dried and used to inoculate the soy beans. Species other than R. oligosporus may be present when wild cultures are used. Other species isolated from tempeh include Rhizopus oryzae, Rhizopus stolonifer, and Rhizopus microsporus var. chinensis.
Regardless of source or strain, tempeh cultures have a limited shelf life,as little as three to four months. This is because the Rhizopus spores enter into a dormancy phase during storage that reduces viability and germability (the ability of dormant spores to become activated and produce biomass). Moreover, even if the soy beans are inoculated with a pure spore culture of R. oligosporus,the tempeh is unlikely to be maintained in a pure state for too long, since the substrate itself will likely be contaminated with an array of different organisms. In fact, as discussed below, other microorganisms may play important nutritional and organ-oleptic roles during the tempeh fermentation.
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