Coffee silverskin fermentation in a single step for sustainable production of carbohydrate-based mixture rich in pectic oligosaccharides

A competitive single-step fermentation using Bacillus subtilis 3610 was developed to convert coffee silverskin (CSS) into an oligosaccharide-rich carbohydrate mixture, mainly composed by pectic oligosaccharides (POS). CSS is an abundant by-product of the coffee industry, and POS are widely reported as emerging prebiotics, motivating sustainable production processes from agro-industrial by-products. In this study, OS were produced using CSS and CSS-derived pellets (CP) via a single-step fermentation leveraging the native pectinolytic activity of B. subtilis 3610. CP consistently outperformed CSS, achieving higher yields of reducing sugars at the optimal time (Y RSₘₐₓ ). After optimisation, a Y RSₘₐₓ of 162 ± 7 mg·g⁻¹ was obtained at 8 h, 45°C and pH 7.0, with 10 g·L⁻¹ CP. The combined hydrolysis of CP with commercial xylanase and pectinase led to a synergetic activity potentially occurring during fermentation, where xylanase may disrupt cell walls to release pectin, while pectinase hydrolyses it into POS. Under optimal conditions, single-step fermentation achieved a significantly higher Y RSmax than the combined enzymatic hydrolysis (141 ± 6 mg·g⁻¹ at 12 h, 1 U·mL −1 per enzyme). In aerated 3.2 L bioreactors, CP fermentation showed comparable performance to shaken-flask assays (Y RS ₘₐₓ 162 ± 2 vs. 162 ± 7 mg·g⁻¹; Pₘₐₓ 9.3 ± 0.1 vs. 9 ± 1 mg·g −1 ·h −1 , respectively). The carbohydrate-based mixture obtained at optimal time contained a higher content of POS (uronic acids 78.8%mol) with a neutral xyloglucan-derived fraction. This study reinforces single-step fermentation as a simplified and competitive process for carbohydrate-based mixtures that avoid enzyme purchase.