LAUSR

In this study, a vegetable waste stream that predominantly contains onions was subjected to KOH pretreatment with the main goal of finding a condition that maximizes the yield of fermentable sugars while minimizing the yield of fermentation inhibitors. Temperature (50–150 °C) and KOH loading (0–7 wt.%) were considered using an orthogonal central composite response surface design with eight center points. The results indicated that the pretreatment was highly affected by the waste composition, particularly organic acids and proteins. The organic acids separated the pretreatment design space into an acid- and alkali-catalyzed region through KOH neutralization. On the other hand, the protein–sugar interaction (through the Maillard reaction) was significant at elevated temperatures. An optimum sugar yield of ~ 0.46 g/g dry vegetable waste was obtained at 150 °C, 3.5 wt.% KOH, and 30 min. The pretreatment also resulted in an organic acid yield of ~ 0.36 g/g dry vegetable waste, with no detectable furans. The sugars and organic acids in the pretreated biomass could serve as platform chemicals for the development of an urban-based biorefinery, where waste biomass resources from different sectors are used to produce high-value commodity products. This study highlights the importance and possible role of biomass composition (other than cellulose, hemicellulose, and lignin) during alkaline pretreatment.