Solid-state fermentation (SSF) is an advanced bioprocess technique with several advantages; however, various challenges including nutrient heterogeneity and limited mass transfer. To address these limitations, this study investigated the use of konjac sponge as an inert carrier for Bacillus subtilis in an adsorbed-carrier SSF (ACSSF) system employing loquat seed hydrolysate, and examined the effects of substrate composition, moisture content, and inoculum size, which were subsequently optimized. The results demonstrate that the adsorbed carrier system enables better contact between the microorganism and the substrate, leading to boosted mass transfer and hence Polyhydroxybutyrate (PHB) production. Under the optimized conditions (pH 7.5, 35 °C, 120 h incubation, sponge depth 15 mm, and a solid–liquid ratio of 1:30), the biomass reached 0.63 ± 0.02 g/g sponge and the PHB yield was 0.49 ± 0.01 g/g sponge using the loquat seed-derived medium.
