Converting food waste into high-value medium chain fatty acids and long chain alcohols via chain elongation with an internally produced electron donor

Abstract

To improve the economic gain of anaerobic fermentation, this study proposes a novel chain elongation-based biotechnology of converting food waste (FW) into medium chain fatty acids (MCFAs) and long chain alcohols (LCA). These two fermentative products are more profitable due to their higher energy density and easier separation compared to short-chain fatty acids (SCFAs). The necessity and the cost for externally dosing an electron donor (ED) for chain elongation were eliminated by inoculating different yeast levels (2.25 × 10⁷, 3.37 × 10⁷, 6.74 × 10⁷, and 8.99 × 10⁷ cells per FW ml Saccharomyces cerevisiae) into the fermenters. In situ ethanol production was then facilitated and was subsequently utilized to produce MCFAs. Overall, MCFA formation was enhanced by yeast inoculation compared to the control. 533 mg of COD per L MCFAs were generated in the batch test with 2.25 × 10⁷ cells per FW ml S. cerevisiae. Higher but similar MCFA yields (∼1500 mg COD per L) were obtained in batch reactors with higher yeast inoculations. The greater endogenous ethanol and SCFA supply, enriched chain elongating organisms, along with the promoted CE pathway based on the increasing gene abundance jointly contributed to the higher MCFA production from FW with yeast inoculation. More interestingly, a high accumulative butanol level (1228 mg COD per L) was observed under 8.99 × 10⁷ cells per FW ml S. cerevisiae inoculation. The more abundant genes encoding the acetone–butanol–ethanol platform and the lower conversion from butanol to caproate likely induced such a high butanol yield according to metagenomics analysis.