Improving the sustainability of Gluconobacter oxydans l-sorbose production using food waste media and recyclable resting cells

Biotransformations offer the potential for sustainable and green syntheses, but a holistic assessment is required to understand their environmental impact, including biocatalyst production, the reaction itself, and downstream processes. This study focuses on the production of l-sorbose, an essential intermediate for the industrial production of vitamin C, from D-sorbitol using Gluconobacter oxydans. To improve the sustainability of the process, two approaches were considered. First, food waste effluents (apple juice from discarded apples as a carbon source, and brewer's spent yeast as a nitrogen source) were incorporated into the growth medium for G. oxydans. The effects on bacterial growth and biomass productivity were evaluated to select optimal waste-based broths, reducing water (less Water Depletion) and wastewater treatment emissions. A broth containing 50 % apple juice was observed to save between 115 and 1000 kg CO₂ · kg biomass⁻¹, depending on wastewater recalcitrance (from mild treatment to incineration). Second, we demonstrated for the first time the biotransformation of D-sorbitol to l-sorbose using resting cells in pure water, the simplest possible medium. Our results demonstrate that cells derived from food waste fermentations efficiently catalyzed the reaction, simplifying downstream processing and contributing to overall process sustainability. The resting cells were reusable across 10 cycles, retaining over 90 % of their initial activity during the first six cycles and approximately 10 % by the tenth. The cumulative productivity reached ∼180 g of l-sorbose per gram of cells. CO₂ emissions during wastewater treatment are estimated at 2–6 kg CO₂ per kg l-sorbose, providing a promising baseline for future improvements.