Optimization of recyclable citric acid pretreatment for enhanced bioethanol production from heterogeneous starchy food wastes

<title>Abstract</title> <p> Background Sustainable conversion of starchy food waste into bioethanol represents an attractive strategy for reducing food waste while producing renewable biofuels. However, developing environmentally benign pretreatment methods that maximize sugar recovery without generating fermentation inhibitors remains a major challenge. This study optimized a mild citric acid pretreatment for enhancing fermentable sugar release and bioethanol production from diverse starchy food wastes. Results A central composite design was employed to optimize citric acid concentration (1–5%, w/v), pretreatment temperature (90–130°C), and reaction time (20–60 min). The quadratic model was highly significant (R² = 0.969), with pretreatment temperature exerting the greatest influence on reducing sugar production, followed by citric acid concentration. The optimum conditions (3% citric acid, 130°C, and 40 min) yielded 30.99 ± 1.80 g L ^− 1 reducing sugars from kitchen food waste. An indigenous yeast isolate was identified as <italic>Saccharomyces cerevisiae</italic> (ITS rRNA; GenBank accession no. PZ683636) and applied in simultaneous saccharification and fermentation. Under the optimized conditions, citric acid pretreatment combined with amylase supplementation produced 54.04 ± 1.37 g L ^− 1 ethanol within 24 h. Although sulfuric acid pretreatment generated higher reducing sugar concentrations, citric acid pretreatment achieved superior ethanol production because of markedly lower hydroxymethylfurfural formation (2.66 ± 0.03 versus 5.82 ± 0.11 g L ^− 1 ). Application of the optimized process to different starchy food wastes generated 28.58–68.49 g L ^− 1 reducing sugars and 45.28–66.74 g L ^− 1 ethanol, with damaged sorghum grains and rotten potato biomass producing the highest ethanol concentrations. Moreover, 91.94 ± 0.02% of the applied citric acid was recovered after pretreatment. Conclusions The optimized citric acid pretreatment provides an efficient and environmentally sustainable alternative to conventional mineral acid pretreatments by enhancing ethanol production while minimizing inhibitor formation. Combined with high citric acid recovery, this strategy offers a promising approach for the circular biorefinery valorization of starchy food waste into bioethanol. </p>