Tempeh Microbiology

The surface of raw soy beans contains an assortment of Gram positive and Gram negative bacteria, including Lactobacillus casei and other lactic acid bacteria; enterococci, staphy-lococci; streptococci; bacilli; Enterobacter, Klebsiella, and other coliforms.Yeasts, such as Pichia, Saccharomyces, and Candida, may also be present. During the soaking step, sucrose, stachyose, and raffinose diffuse out of the beans and into the water.Their subsequent hydrolysis by invertases and glucosidases releases glucose and fructose, which can then be used to support growth of the resident microflora. Consequently, at the end of the soaking period (twenty to twenty-four hours at 20°C),the total microflora may reach levels of 109 cfu per ml or higher. Most of the organisms isolated after soaking are lactobacilli, enterococci, and streptococci. However, the specific species that predominate appears to depend on the temperature and the pH of the soak water (i.e., in those applications where lactic or acetic acid is added to the water). Importantly, the pH values of the soak water, whether acidified or not, generally will decrease to 4.5 to 5.0 by virtue of lactic and mixed acid fermentations.

Although the manufacture of tempeh clearly depends on the growth of R. oligosporus (discussed below), the fermentation that occurs during the soaking of the soy beans is also es-sential.This is because the formation of organic acids and the decrease in pH are necessary to control pathogens, including Salmonella ty-phimurium, Yersinia enterocolitica, Staphylococcus aureus, and Clostridium botulinum, that might otherwise grow in non-acidified soy beans. Low pH also inhibits Bacillus, Enter-obacter, and other microorganisms capable of causing spoilage effects. It is important to note that even if the beans are heated prior to soaking, the lactic fermentation will still occur, although the rate and extent of the fermentation may be affected.

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