Untreated spent coffee grounds as an optimal feedstock for enhanced bioethanol production via simultaneous saccharification and fermentation

Abstract This study investigated the potential of spent coffee grounds (SCG) as a sustainable feedstock for bioethanol production. Using simultaneous saccharification and fermentation (SSF), the effects of different SCG pretreatments on ethanol yield, reducing sugars consumption, and yeast growth were compared. Surprisingly, untreated SCG outperformed pretreated SCG in all these aspects. Untreated SCG achieved an ethanol yield of 4.91 g L –1 , a fermentation efficiency of 81.11%, and a yeast growth of 1.86 × 10 7 cells/mL. These values were significantly higher than SCG pretreated by various methods. Untreated SCG contained higher levels of inhibitory compounds (polyphenols, furfurals, hydroxymethylfurfural) but also exhibited enhanced yeast growth. This unexpected finding suggests a positive interaction between SCG components and yeast metabolism. Polyphenols and free fatty acids had a positive effect on yeast growth. Polyphenols also correlated strongly with free fatty acids (FFA), suggesting a protective effect on the yeast in response to oxidative stress caused by certain polyphenols, thereby enhancing bioethanol production. Our findings demonstrate the untapped potential of untreated SCG as a sustainable and economically viable feedstock for bioethanol production. This approach offers a valuable alternative to traditional feedstocks, contributing to a circular economy and reducing reliance on fossil fuels.