Mechanistic elucidation of Rhizopus stolonifer-fermented orange peel in enhancing Saccharomyces cerevisiae growth and multi-stress tolerance: process optimization, metabolomics, and pathway analysis

<i>Saccharomyces cerevisiae</i>, a pivotal organism in ethanol fermentation, frequently encounters challenges in high-stress industrial settings due to ethanol toxicity, acidic conditions, and thermal stress. This study aimed to optimize the fermentation of orange peel to enhance bioactive compounds and evaluate its impact on the stress tolerance of <i>S. cerevisiae</i>. The optimized conditions led to an increase in the total flavonoid content of the fermented orange peel (FOP). Supplementation with FOP significantly enhanced yeast tolerance; viability exceeded that of the control under 20% ethanol, improved at pH 2.8, and cell counts increased after 24 h at 40 °C. Experiments conducted in a 15-l fermenter demonstrated that FOP significantly promotes sugar consumption and alcohol production. Further mechanistic analysis revealed that hesperidin and rutin in FOP both exhibit potential to enhance yeast tolerance. This approach not only offers a cost-effective method for valorizing orange waste but also enhances yeast productivity and biofuel yield.