Trickle-bed reactor for solid-substrate fermentation : Overcoming nutrient distribution limitations in sophorolipid production

Solid-state fermentation (SSF) represents a sustainable alternative for the bioconversion of solid food waste and agricultural by-products; however, its industrial implementation is limited by mass and temperature gradients across the solid bed. To address these challenges, this study evaluates a trickle-bed reactor configuration as a means to intensify SSF through the introduction of a trickling liquid phase. This study approaches a conceptual shift from solid-state to solid-substrate fermentation (SSuF) processes to produce sophorolipids, biosurfactants synthesised by Starmerella bombicola. The effect of glucose concentration and delivery method under SSuF conditions is assessed to explore its potential for process optimisation. A Box-Behnken design was employed to assess the effects of glucose concentration, trickling time, and trickling frequency on yeast growth and product yield in 0.5 L bioreactors. Under optimised conditions (4.2 g glucose, 300 mL recirculating volume, 24 min trickling, 24 cycles/day), viable cell counts increased by 45.4 %. Different trickling operation modes were tested to assess the influence of trickling duration and timing on sophorolipid production under optimised conditions. Furthermore, the inclusion of yeast extract and urea in the trickling solution led to a 31.2 % increase in sophorolipids production compared to solid-state fermentation controls. The SSuF approach enhances nutrient distribution and enables monitoring through liquid phase analysis - offering a scalable and robust strategy for microbial fermentation processes from low-cost solid feedstocks in line with circular bioeconomy objectives.