Two-Phase Bioconversion of Greek-Yogurt Waste Into Medium-Chain Carboxylic Acid Oil via Lactic Acid Without External Electron Donor Addition

Acid whey is a Greek-yogurt waste stream and should be considered a resource to produce chemicals that can be used as biofuel precursors. Our objective was to convert acid whey into valuable medium-chain carboxylic acid (MCCA) oil with the open-culture carboxylate platform. Here, we developed a two-phase bioprocess with different anaerobic reactor microbiomes, performing thermophilic acidogenesis and mesophilic chain elongation, to produce MCCAs from acid whey via lactic acid as an intermediate. We achieved an MCCA production rate of 4.8 g chemical oxygen demand (COD) Lreactor-1 day-1 based on the volume of the chain-elongating bioreactor. The overall COD conversion efficiency for acid whey to MCCAs (C6-C9) was 53.1% with an MCCA specificity of 68.5% based on COD. n-Caproic acid (n-hexanoic acid [C6]) was the main product with a COD specificity of 49.8%. We converted a real waste stream into mainly MCCAs by chain elongation without the addition of electron donors.