How is nattokinase produced?

Nattokinase is extracted from nattō, a food made of boiled soybeans fermented with the bacterial strain Bacillus subtilis natto.^[1] To extract nattokinase from the fermentation broth, processes such as protein salting-out, organic solvent distillation, microfiltration, and ultrafiltration are used.^[2] Because of increasing interest in nattokinase, methods of production that increase yield and stability have been explored. For example, irradiation of the bacterial strain B. subtilis natto with gamma or ultraviolet rays seems to improve its heat stability and to increase the yield of nattokinase. Furthermore, using glucose (instead of sucrose or maltose), adding soy milk, and adding certain ions such as calcium and magnesium sulfate to the fermentation medium has been shown to increase the yield of nattokinase.^[2]