Poly-β-hydroxybutyrate Production from Bread Waste via Sequential Dark Fermentation and Photofermentation

This study explores the valorization of bread waste for poly-<i>β</i>-hydroxybutyrate (PHB) production through a combined dark fermentation (DF) and photofermentation (PF) process. DF, performed using <i>Lactobacillus amylovorus</i> DSM 20532, efficiently converted bread waste into a lactate- and acetate-rich substrate within 120 h. The resulting fermented bread broth (FBB) was enriched with essential nutrients by adding digestate from anaerobic digestion, replacing the need for chemical supplements. Six purple non-sulfur bacteria (PNSB) strains were screened for PHB production in the FBB. <i>Cereibacter johrii</i> Pisa7 demonstrated the highest PHB accumulation (50.73% w PHB/w cells), and biomass increase (+1.26 g L^-1) over 336 h, leading to its selection for scale-up. Scale-up experiments were conducted in a 5 L photobioreactor with LED lights optimized for PNSB growth. <i>C. johrii</i> Pisa7 accumulated PHB at 15.17% and 11.51% w PHB/w cells in two independent trials, corresponding to productivities of 2.03 and 0.89 mg PHB L^-1 h^-1. These results confirm the scalability of the process while maintaining competitive PHB yields. This study highlights the potential of bread waste as a low-cost carbon source for bioplastic production, contributing to a circular bioeconomy by converting food waste into sustainable materials.