A flowchart on Itohiki natto manufacturing is presented in Fig. 2.
a. Selection and Washing Raw soybeans are selected through a sorting machine to remove foreign matters, broken beans, stones, and dust to select similar-sized soybeans. They are then washed to remove the dirt and dust adhered to the surface.
b. Soaking In order to assure the beans will be cooked and softened easily, it is necessary for the beans to absorb enough water. The amount of water absorbed should be
Bacillus natto starter culture
Fermentation Itohiki natto packs
2.2 times the weight of the dry beans. The cotyledons, after water absorption, should be tightly attached to each other when the bean is cut horizontally. The soaking time varies with the age of the beans, water temperature, cultivar, and size. Soaking time may be 27 to 30 hr at 2.5°C in the winter, 18-21 hr at 13°C in the spring, and 10-14 hr at 21.5°C in the summer (1). The soaking water should be of potable quality. Water with higher hardness and iron (ion) content above 5 ppm should be treated to soften the water and remove the iron ions. Cooking of beans in hard water makes the cooked beans tough, and iron ions will blacken the beans.
c. Cooking Soaked beans are drip-dried before steaming. Nowadays, most producers use pressurized steam cooking. A pressure cooker handling 100 kgs raw beans at one time is more common. Formerly, the cooking was a still process (Fig. 3). Nowadays, a rotating cooker is used. Beans are loaded in the cooker in batches with steam on. When the
steam penetrates evenly into the center of the beans and starts to come out, the venting valve is closed, and a pressure of 1-1.5 kg/cm2 (121-127°C) for 20-30 min is maintained. Then the venting valve is opened slightly to the start cooling down of the beans. With this procedure, part of the amino acids or sugars will be hydrolyzed, causing loss of nutrients. The color of cooked beans will also be darker, affecting the product quality. In practice, a pressure of 0.8-1.0 kg/cm2 for 30-40 min is a better alternative. The latter procedure will let the natto bacteria grow easily on the cooked bean.
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