Model for on-line moisture-content control during solid-state fermentation

In this study we describe a model that estimates the extracellular (nonfung al) and overall water contents of wheat grains during solid-state fermentat ion (SSF) with Aspergillus oryzae, using on-line measurements of oxygen, ca rbon dioxide, and water vapor in the gas phase. The model uses elemental ba lances to predict substrate dry matter losses from carbon dioxide measureme nts, and metabolic water production, water used in starch hydrolysis, and w ater incorporated in new biomass from oxygen measurements. Water losses cau sed by evaporation were calculated from water vapor measurements. Model par ameters were determined using an experimental membrane-based model system, which mimicked the growth of A. oryzae on the wheat grains and permitted di rect measurement of the fungal biomass dry weight and wet weight. The measu red water content of the biomass depended heavily on the moisture content o f the solid substrate and was significantly lower than the estimated values reported in the literature. The model accurately predicted the measured ov erall water content of fermenting solid substrate during fermentations perf ormed in a 1.5-L scraped drum reactor and in a 35-L horizontal paddle mixer , and is therefore considered validated. The model can be used to calculate the water addition required to control the extracellular water content in a mixed solid-state bioreactor for cultivation of A. oryzae on wheat. (C) 2 000 John Wiley & Sons, Inc.