Growth and protein content of Cupriavidus necator on organic acids derived from fermented grey starch

Grey starch (GS) is a byproduct of potato processing and is conventionally valorised as low-grade animal feed or for biogas generation. In this manuscript, we systematically investigate the mixed culture fermentation of GS to organic acids and their subsequent aerobic upgrading to microbial protein via Cupriavidus necator. We focused on protein content, growth kinetics, biomass yield on individual, and mixed organic acids (C1-C6), using both synthetic media and real fermented effluents. High-throughput cultivation in microtiter plates was employed to evaluate performance across both straight-chain forms and isoforms of the acids. C. necator demonstrated growth on all tested individual substrates, although with distinct individual behaviour. Lactate, butyrate, and hexanoate supported the highest biomass yields, reaching up to 0.24 gCDW/gCOD (grammes of cell dry weight per grammes of COD fed). Lactate enabled the highest specific growth rate (0.6 h^-1) with 29 ± 4% of protein. The maximum protein content (70 ± 11%) was observed on acetate at an initial concentration of 2 g/L. Depending on the acid, higher initial concentrations (2 and 4 g/L) led to increased cell dry weight but reduced growth rates or inhibition in some cases. Real fermented GS, primarily composed of lactate and butyrate, proved to be a viable substrate for microbial protein production. Undiluted fermented GS yielded the highest protein content (70 ± 9%), while a 1/4 dilution (2.6 gCOD/L) enabled the fastest growth (0.84 h^-1) compared to all tested fermented GS and acid concentrations. These findings highlight the potential of GS-derived organic acids as feedstock for microbial protein production for feed and food. KEY POINTS: C. necator grows on all tested individual organic acids (C1-C6). Protein production varies by acid type and initial concentration. Grey starch was converted to organic acids for microbial protein production.