Chestnut Wood Residues, with and Without Tannins, as a Potential Feedstock for PHA Bioplastic Production

The valorisation of lignocellulosic residues into bio-based feedstocks is a key strategy for advancing circular bioeconomy models. In this study, chestnut wood residues, including virgin wood (VW) and detannized wood (DT) from the tannin industry, were evaluated as substrates for polyhydroxyalkanoate (PHA) production using Cupriavidus necator. Biomass was subjected to thermo-acid hydrolysis followed by ion-exchange detoxification, yielding hydrolysates rich in organic acids (levulinic, acetic, and formic acids) and residual inhibitory compounds. Both substrates supported microbial growth and PHA accumulation, although clear differences in performance were observed. The maximum biomass concentration reached 1.26 ± 0.01 g L^-1 in VW hydrolysate and 0.40 ± 0.03 g L^-1 in DT hydrolysate. PHA production was higher in VW hydrolysate, reaching 68.51 mg L^-1 with 5.44% (w/w) accumulation, while DT hydrolysate yielded 0.21 mg L^-1 with 6.01% (w/w). The reduced biomass formation in DT hydrolysate was associated with the greater persistence of inhibitory compounds generated during thermo-acid treatment. Although the obtained PHA yields are lower than those reported for optimized lignocellulosic systems, this study demonstrates for the first time the feasibility of producing PHA from chestnut wood residues, including industrial detannized byproducts, without nutrient supplementation. These findings highlight the potential of tannin-industry waste streams as alternative feedstocks for biopolymer production, while indicating that optimization of hydrolysis conditions, detoxification efficiency, and fermentation strategy is required to improve process performance.