Lactate and Ethanol Chain Elongation in the Presence of Lactose: Insight into Product Selectivity and Microbiome Composition

Organic waste streams rich in carbohydrates are costly to treat; however, they can be valorized to commodity chemicals such as medium-chain carboxylic acids (MCCAs), for example, caproic acid. Simple carbohydrates are easily fermentable, providing different intermediates for competing bacterial groups, which may lead to product diversification. For that reason, it is essential to understand the impact of sugar fermentation on chain elongation (CE) in the presence of key electron donors to control the biochemical pathways for MCCAs production. The research provides an insight into the impact of co-fermentation of lactose with lactate and ethanol as electron donors on CE and process selectivity in open culture fermentation. Co-fermentation of lactose with ethanol led mostly to acetogenesis and development of the Clostridium genus, while co-fermentation of lactose with solely lactate or with both electron donors activated the propionate production and diversified the process outcome to mix short- and medium-chain carboxylates. The highest caproate production efficiency and selectivity were achieved (53 and 65%, respectively) when lactose in the presence of lactate were used; CE improved with a higher initial lactose load. Interestingly, the microbiome was highly enriched in members of the Bacillaceae family, which was not reported before.