Optimization of wurster fluidized-bed parameters for coating sorghum ( Sorghum bicolor (L.) moench) seeds with an inoculant-containing polymeric formulation

Seed coating is a route for delivering biological inoculants and micronutrients to crops, but it requires controlled deposition and controlled final water content. This study aimed to determine an optimal operating window for coating forage sorghum (Sorghum bicolor (L.) Moench) seeds in a Wurster fluidized bed with a polymeric formulation containing Bacillus megaterium and Bacillus subtilis spores and cobalt and molybdenum micronutrients. A sequential experimental design assessed the effects of fluidization air flow rate, partition gap, draft tube length, and annular-zone porosity on relative mass gain, water content, germination and vigor. The Derringer–Suich desirability function was then applied to optimize mass gain and water content simultaneously. At the optimum flow rate of 82.1 m3 h−1, the models predicted a relative mass gain of 5.07% and a water content of 14.61%. Duplicate validation runs at the optimum condition confirmed these predictions (deviations of 3.0% and 0.5%) and yielded a mean process efficiency of 86.60 ± 1.71%; neither germination nor vigor differed significantly from the uncoated control. The results establish operating conditions for depositing the formulation with controlled mass gain and water content, without assessment of the viability of the bacterial component after processing.