Corynebacterium glutamicum single-cell protein produced with fermented spent coffee grounds extract supports L-lactate and acetate production by lactic acid bacteria and bifidobacteria

Spent coffee grounds (SCG) contain substantial organic matter, but much of it is not readily accessible for microbial cultivation. This study evaluated fermented spent coffee grounds (FSCG) extract as a nutrient source for Corynebacterium glutamicum single-cell protein (SCP) production and assessed the resulting biomass as a substrate for L-lactate and acetate production by lactic acid bacteria and bifidobacteria. SCG were subjected to solid-state fermentation, and the FSCG extract produced by 15 days of once-daily stirring followed by 30 days of static incubation was selected for C. glutamicum -SCP production. At 5% (w/v), the selected extract yielded 4.91 g/L of recovered SCP; the corresponding values were 4.76 g/L in minimal medium and 4.85 g/L in complex medium. FSCG extract-derived and complex medium-derived C. glutamicum -SCP contained acid-hydrolyzable rhamnose, arabinose/galactose, mannose, and glucose, together with alkali-extractable glycogen. Both SCP preparations supported strain-dependent L-lactate and acetate production by lactic acid bacteria and bifidobacteria. In a 22-strain mixed culture, acetate production reached 1.83 mg/mL with complex medium-derived C. glutamicum -SCP and 2.28 mg/mL with FSCG extract-derived C. glutamicum -SCP. Both values were significantly higher than that obtained under the glucose-containing condition, and acetate production was significantly higher with FSCG extract-derived C. glutamicum -SCP than with complex medium-derived C. glutamicum -SCP. These findings provide proof of concept for integrating SCG valorization with C. glutamicum -SCP production and demonstrate that the resulting SCP supports subsequent organic acid formation by lactic acid bacteria and bifidobacteria.