An integrated process for mixed culture production of 3-hydroxyhexanoate-rich polyhydroxyalkanoates from fruit waste

Fermented wastes have been demonstrated to be adequate sources for polyhydroxyalkanoates (PHA) production using mixed microbial cultures (MMC). In general, MMC processes are focused on the production of co-polymers containing 3-hydroxybutyrate (HB) and 3-hydroxyvalerate (HV), which limits the range of polymer properties and aplications. This study aimed at demonstrating the feasibility of producing a ter-polymer enriched in 3-hydroxyhexanoate (HHx) from a waste using a mixed-culture 3-stage process. For this purpose, each of the stages (acidogenic reactor, selection of a PHA-accumulating culture and PHA accumulation) was fine-tuned to maximise the process's yield and productivity. In the acidogenic reactor, operating conditions were manipulated to ferment fruit waste into a caproate-rich effluent (69%) with a fermentation products’ concentration (FP) of 13.0 gCOD L⁻¹. A culture enriched in a highly efficient PHA-accumulating culture capable of converting caproate into HHx was selected. This culture was used as inoculum and fed with caproate-rich effluent, achieving a maximum PHA content of 71.3%, a PHA productivity of 3.29 gCOD L⁻¹h⁻¹ and a terpolymer with a composition of 33/1/66 (HB/HV/HHx,% wt.). Batch assays were carried out to demonstrate that caproate was the precursor of HHx, and acclimatization of the biomass to caproate was an important factor in the selection step to foster HHx-containing polymer production. This paper reports unprecedented values of yield, productivity and HHx fraction for the production of a mcl-PHA using MC and a fermented waste. These results are a significant contribution towards upscaling the process to convert a waste into HHx-rich PHA.