The manufacture of koji starts by treating the raw materials.When soybeans are used, they are first soaked for about twelve hours in several changes of water. For traditional koji-making, they are then cooked, usually with steam under pressure, for one hour—a process called puffing. For some products, soybean koji also contains wheat, which is prepared by roasting wheat kernels (or wheat flour) at a high temperature, followed by a crushing step. Rice koji for sake manufacture is prepared from polished rice (rice minus the bran) that is steeped and steamed, similar to soybean koji.
Despite the seemingly simple process involved in preparing the koji substrate, numerous innovations have been designed to enhance the digestibility of the raw soy beans and ultimately improve yield and product quality. For soy sauce, digestibility or yield is based, in large part, on the amount of total protein nitrogen that is converted to amino nitrogen during the mashing step. However, how the soy beans are initially heat processed influences the rate and extent that the soy proteins are hydrolyzed.This is because only denatured soy proteins are hy-drolyzed by fungal proteinases, whereas native proteins are not. Thus, a cooking step is essen-tial.Yields of about 82% can be achieved when a traditional cooking process (e.g., 118°C at 0.9 kg/cm2 for 45 minutes) is used. In contrast, high-temperature, short-time cooking not only enhances protein extraction, but also gives amino acid yields greater than 90%. It is now possible, for example, to perform the cooking step in less than two minutes, using temperatures and pressures above 150°C and 5 kg/cm2, respectively.
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