The de-hulled beans are then soaked in water, a seemingly simple step, but one which has very important implications for tempeh quality. It is during this steeping period (sixteen to twenty-four hours at warm ambient tempera ture), that endogenous lactic acid bacteria grow and produce acids that lower the pH.The low pH generally restricts growth of undesirable spoilage bacteria, as well as potential pathogens. It should be noted, however, that some of these undesirable bacteria can survive during the soaking step even if acid conditions are established. To promote acidification, the steep water can be acidified directly with lactic or acetic acid. Finally, potential mold inhibitors may also be extracted from the soy beans during the steeping period.
After the soaking step, the remaining hulls are removed from the beans and the dehulled beans or cotyledons are heated a second time, either by steaming or boiling (for anywhere from thirty minutes to two hours). This heat treatment enhances extraction of soluble nutrients and inactivates microorganisms that might otherwise interfere with the subsequent fermentation. This step is also necessary to denature trypsin inhibitor, a native soy protein that acts as an anti-nutritional factor.
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