Fermented bread wastes for poly-β-hydroxybutyrate (PHB) production by purple non-sulfur bacteria

Worldwide plastic production is rising every year. Nowadays, plastics are mainly derived from petrochemical industries and are responsible for a variety of growing environmental issues including the emission of greenhouse gases, accumulation in terrestrial and marine habitats, and pollution. Bioplastics are biodegradable and made from renewable sources or biocompatible, representing an attractive alternative to conventional petroleum-derived plastics. Since the industrial expenses to produce bioplastics are much higher than for petroleum-derived plastics, one option to pursue would be to use agri-food wastes for bioplastic production. Poly-β-hydroxybutyrate (PHB) is the most well-studied member of the polyhydroxyalkanoates (PHAs), which is a family of biodegradable intracellular polyesters produced by several bacteria. Recently, PHA production from various biowastes is receiving increasing attention for industrial-scale production; in particular, bread wastes, containing an elevated percentage of carbohydrates, represent a suitable source of nutrients for microorganisms. The aim of this work was to use bread wastes as feedstock for a sustainable production of the biopolymer PHB. Fermented bread broth, obtained from previous lactic fermentation of bread wastes, was used for photofermentation by eight strains of Purple Non-Sulfur Bacteria (PNSB). The strain with the highest PHB production and growth was Rhodobacter sphaeroides strain Pisa 7, reporting 44.50 % of PHB/cell weight (w/w). The lowest PHB yields were obtained with Rhodopseudomonas palustris species. R. sphaeroides Pisa 7 was chosen to scale up the photofermentation into a photobioreactor of 5 L. This study offers a promising contribution to a circular economy, converting bread wastes into a biopolymer using a microbe-based system.