Pomegranate residues as fermentation feedstock for bioethanol production: Process design and characterization

• The biotechnological valorization of pomegranate residues (PRs) was assessed. • Extracts/hydrolysates from PRs were produced and chemically characterized. • Mixture of extracts from PRs and seeded raisins was used as fermentation substrate. • Fermentation using Saccharomyces cerevisiae was described via model fitting. • The developed bioprocess resulted in satisfactorily high bioethanol production. Pomegranate residues (PRs), namely pomegranate peels and seeds remaining after juice extraction, constitute a significant agro-industrial residue in Greece. Despite their ample content in carbon sources and bioactive compounds, PRs are largely unexploited, with their management being an increasing concern for the pomegranate processing industry. The objective of this study was the development and characterization of a bioprocess aiming at the biotechnological valorization of PRs for ethanol production. Specifically, a stepwise investigation was conducted, including (i) the production and chemical characterization of extracts/hydrolysates from PRs using distinct pretreatments not involving organic solvents or enzymatic hydrolysis; (ii) the assessment of the exploitability of a selected hydrolysate as fermentation feedstock using Saccharomyces cerevisiae ; and (iii) the advancement of the bioprocess through the joined utilization in the fermentation substrate of PRs and seeded raisins (SRs), another important agro-industrial residue in Greece. The finally developed substrate, comprising a mixture of PRs/SRs extracts at concentrations 60/40 % v/v, was used in three fermentation trials, and the fermentation process was quantitatively described using the Aiba model. The developed bioprocess resulted in satisfactorily high bioethanol production with the maximum attained concentration being 50.0 ± 0.6 g/L. The maximum specific growth rate ( µ max ) of S. cerevisiae was estimated to be 0.135 1/h and the bioethanol yield ( Y ps ), namely the amount of produced ethanol/amount of consumed reducing sugars, was estimated to be 0.423 g/g. Hence, PRs can be efficiently used in the development of a fermentation substrate for bioethanol production, providing an economic and environmentally sustainable alternative to conventional feedstocks.