Yeast bioaugmentation fosters medium-chain carboxylates and polyhydroxyalkanoates production from cheese whey

This study investigated polyhydroxyalkanoates (PHA) production from carboxylic acids obtained by yeast-assisted cheese whey (CW) fermentation. A three-stage hybrid strategy was employed. First, a lactose-fermenting yeast (Kluyveromyces marxianus) was inoculated into raw or sterilized CW to produce ethanol and/or lactic acid. Second, these intermediates were converted to carboxylic acids, with an emphasis on caproic acid, using a chain-elongating microbial culture. Batch and fermenter experiments under different operating conditions produced four distinct fermentation products with total carboxylic acid and alcohol concentrations from 4.5 to 14.1 g C/L, and a caproic acid fraction ranging from 17.8 to 72.5%. Finally, synthetic solutions simulating these complex acid mixtures were used as feedstocks for fed-batch PHA synthesis by Cupriavidus necator. The process achieved a maximum cell dry weight (CDW) of 6.54 ± 0.17 g/L with a PHA content of 44.21 ± 1.67%. The PHA contained variable proportions of polyhydroxybutyrate (PHB) and polyhydroxyvalerate (PHV), and high PHV fractions (21.91–27.56% w/w) were specifically linked to the presence of propionate and valerate in the feedstock. Yields of 0.79 ± 0.06 g CDW/g C consumed, and 0.36 ± 0.03 g PHA/g C consumed were achieved. This work investigates a novel proof-of-concept pathway for the valorization of CW into a tailored bioplastic, establishing a clear link between upstream fermentation control and the final material properties of the poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) copolymer using simulated fermentates.