Optimizing the treatment and valorization of spent coffee grounds (SCGs) through the integration of sequential pretreatment steps, enzymatic hydrolysis, and selective fermentation

This study aimed to optimize the simultaneous production of D-mannose and bioethanol from spent coffee grounds (SCGs) using an integrated process involving pretreatment, enzymatic hydrolysis, and selective fermentation. Sequential alkali, bleaching, and NaOH‑hydrogen peroxide pretreatment steps increased the total sugar content to 74.1 %, with mannose and glucose reaching 45.2 % and 27.2 %, respectively, while effectively removing lignin, as confirmed by colorimetric and solid-state nuclear magnetic resonance analyses. Enzymatic hydrolysis with cellulase and mannanase produced high concentrations of glucose (15.3 mg/mL) and mannose (10.6 mg/mL). To optimize fermentation, a mutant strain of <i>Pichia stipitis</i> (MH05) was developed that selectively consumed glucose, leaving 79.9 % of mannose unconsumed after 48 h. This approach resulted in both efficient ethanol production and significant mannose retention, highlighting the potential of SCGs for generating high-value bioproducts and promoting waste valorization and the circular bioeconomy.