The hydrodynamics of spouting various grain particles, including brown rice, lentils, soybean, and cracked corn, was studied in gas-solid spouted bed bioreactors. The grains with various sizes, shapes, and moisture contents suitable for solid state fermentation (SSF) were studied for their spoutability under SSF conditions. In general, increasing the moisture content reduced particle spoutability due to increased particle size and stickiness, even though the particle density was also reduced. Solid state fermentation can dramatically change particle size and density of the solid substrate, which greatly affect the spoutable bed height. In general, the fountain height and particle annular speed in the spouted bed increased linearly with increasing the airflow rate or spouting air velocity. Under appropriate spouting conditions, the particle mixing time was relatively short, less than I minute; and therefore, the spouted bed bioreactor can be considered as a well mixed reactor. The minimum spouting velocity increased with increasing moisture content, bed height, and air inlet tube diameter. A new correlation between the minimum spouting velocity U., and particle properties (mainly size and density) and reactor geometry (air inlet tube diameter, reactor column diameter, bed height) was obtained using the experimental data. The new correlation is a significant improvement over existing correlations, including Mathur and Gishler, and Uemaki equations, and can predict U-ms fairly well with an average deviation of 4.6%.