Mixed-culture polyhydroxyalkanoate production with variable hydroxyvalerate content from agri-food residues

Agri-food residues represent an unutilised biomass that can be valorised into high-value compounds. Polyhydroxyalkanoates (PHAs) are one such product, offering a sustainable alternative to fossil-based plastics. PHAs containing hydroxyvalerate monomers (PHBV) are more flexible and less crystalline than pure PHB, making them suitable for a broader range of applications. This study focused on producing PHBV with a targeted hydroxyvalerate monomer content (25–35%, w/w) for use in agricultural materials. Different types of feedstocks (ranging from synthetic to agri-food residue fermentation fluid) were used with mixed microbial cultures to achieve the desired hydroxyvalerate content in the stored PHA. The COD removal efficiency of the selection reactor ranged from 81.6 to 99.1% with synthetic feed, indicating effective substrate uptake, whereas agricultural fermentate resulted in lower carbon uptake (71.4–85.9%). Despite fluctuations throughout the study, the desired hydroxyvalerate monomer content was successfully obtained. The molecular weight and distribution were challenging to correlate with the different feedstocks, though they remained suitable for thermoplastic processing for most set-ups (352 to 1369 kDa). The bacterial community composition changed throughout the selection process, with the feast/famine regime favouring PHA producers such as Thauera, Paracoccus, Neomegalonema, Corynebacterium, and Flavobacterium; however, the introduction of agricultural fermentate led to a loss in speciation.