Inoculated beans are put into the proper container for fermentation. This process is characteristic of natto manufacturing. Because the inoculated beans have to be put in one by one, the packaging and weighing are very labor intensive. The process has to be mechanized (Fig. 5).
In the past, the package material was rice straw. Later, thin wood slices of fir or cypress was used. Nowadays, bags with perforated inner lining of polyethylene film, or polystyrol containers are used. Another type of container is made from rice straw mat. Failure in natto manufacture is mainly due to contamination with foreign microorganisms. Inadequate sterilization of the container is one of the major causes. Therefore, proper sterilization of the containers is critical. Containers made from wood slices, rice straw, or mats should be steam-sterilized. Synthetic materials such as polystyrol cannot be heat sterilized and therefore are sterilized by fumigation with sulfur, ethylene oxide, or other government-approved sterilants. The amount of natto per container is usually 0.1 kg.
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