Bread waste biorefining: a low‐cost substrate for biomass production

The microbial valorization of bread waste (BW) represents an innovative biotechnological strategy for converting food waste into a renewable feedstock for the bio‐based industry, thereby supporting the principles of a circular economy. The present study focuses on exploring the potential use of BW as feedstock for biomass using a newly isolated yeast strain, Saccharomyces cerevisiae X19G2. The saccharification of BW was performed via acidic (H 2 SO 4 ) pretreatment and enzymatic hydrolysis using either commercial amylolytic enzymes ( α ‐amylase and amyloglucosidase) or lab‐scale α ‐amylase secretome of Bacillus halotolerans Gb67, yielding 118.70, 125.52 and 63.94 g L −1 of reducing sugars, respectively. The optimization of biomass production conditions was performed using a Box–Behnken design, evaluating four independent variables: KH 2 PO 4 concentration (1–7 g L −1 ), MgCl 2 concentration (0.1–1.9 g L −1 ), yeast extract concentration (0.1–4.9 g L −1 ), and inoculum size (1–9%). The optimal growth conditions were 4.75 g L −1 KH 2 PO 4 , 1.19 g L −1 MgCl 2 , 3.68 g L −1 yeast extract, and 6.73% inoculum size. Under these conditions, maximum biomass production values of 24.80, 25.88, and 15.60 g L −1 were achieved after 72 h of fermentation using acidic hydrolysate, commercial enzymatic hydrolysate (CEH), and B. haloterans Gb67 enzymatic hydrolysate (Gb67H), respectively. The highest biomass yield (0.41 g g −1 ) and volumetric production rate (0.54 g L −1 h −1 ) were obtained after 48 h of fermentation using CEH. Therefore, this study highlights the promising potential of BW hydrolysate as a sustainable feedstock for biomass production by S. cerevisiae X19G2, with potential applications in the feed industry.