Solid-substrate Fermentation of Menaquinone 7 with Bacillus Subtilis: Comparison of Continuous Rotation with Stationary Bed Fermentation at Different Initial Moisture Levels

The Japanese fermented food Natto contains menaquinone 7 (MK7), which is known to increase bone mineral density and reduce bone fractures. Natto is traditionally produced via the solid state fermentation of soy beans by Bacillus subtilis natto. The aim of this study was to investigate the effects of static and dynamic solid state fermentation (SSF) on the production of MK7 using polenta (milled corn substrate), nixtamalized corn grits, semolina (milled wheat substrate) and soy protein granules. In addition, the effect of initial moisture content on MK7 concentration was measured for both dynamic and static SSF and all substrates. During the experiment the following process parameters were kept constant; incubation temperature (37 C), substrate wet weight (3 g), incubation time (3 days) and relative humidity (~85 %). Our initial hypothesis was that conducting dynamic fermentation should increase the production of MK7 due to enhancing the rate of microbial growth via reducing spatial temperature and moisture gradients, yielding better homogeneity. However, average MK7 yield decreased by 62.7 % (p < 0.05) when using dynamic fermentation. This result may be due to substrate lump formation, which was observed inside the bioreactor particularly at higher initial moisture levels, resulting in decreased surface area per unit volume. This reduces nutrient and oxygen availability for microbial consumption. MK7 concentration increased with initial moisture levels in static fermentation where the highest MK7 concentration was ~20% greater than commercial natto. The highest concentrations 28.26±0.11 mg/kg and 27.80±0.06 mg/kg were obtained using soy protein granules at uppermost moisture levels, 65 % and 70 %, respectively; owing to the fact that elevated rate of biochemical process due to increase in solubility of nutrients of the substrate and high degree of swelling. The highest concentration obtained by dynamic fermentation was 5.3±0.54 mg/kg at 60 % moisture. This is ~81% less than that of highest MK7 yield obtained via static fermentation.