Crtical Parameters for the Safe Production of Tempeh

Introduction: Tempeh is a traditional Indonesian food made from mold fermentation of a number of cereal grains. The most common mold for tempeh production is Rhizopus oligosporus, however other Rhizopus spp. can be used. Due to close proximity to soil, cereal grains commonly have low levels of Bacillus spp. Bacillus cereus is a Gram positive, endospore-forming pathogenic bacterium that is ubiquitous in soil. B. cereus is the source of “fried rice syndrome,” a form of food poisoning associated with toxin formation in improperly cooled, starchy foods.

Purpose: The purpose of this study was to establish critical processing parameters for soybean tempeh that allow confluent growth of mold while inhibiting bacterial pathogen growth.

Methods: Prior to inoculation acetic or lactic acids added to the cooking water lowered the bean pH to 5.9, 5.7, and 5.5 for acetic and 4.8, 4.6, and 4.4 for lactic. Bacillus cereus was used as the target pathogen, since it is the most likely bacterial pathogen to be present in the soybeans. Both R. oligosporus and R. oryzae were independently used as starter cultures. Tempeh was incubated at 35°C until confluent mold growth occurred.

Results: R. oligosporus fermented consistently within 24 hours. However, depending on pH and inoculum level, R. oryzae growth was slowed, or completely inhibited. B. cereus growth was inhibited at all tested pHs for lactic acid but uninhibited for all tested pHs of acetic acid.

Significance: The data suggest that B. cereus growth is independent of the fermentative mold used. If proper acidification of the beans is not reached, B. cereus growth is possible during the tempeh incubation period.