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

Abstract

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.

Medium chain carboxylate production from cassava wastes pretreated by ensiling

“…The concentration of lactic acid and ethanol decreased progressively and they were completely used up in both silages after 28 days, an indication that the CE process followed both the lactic acid and ethanol utilization pathways. Contrary to the report of Duber et al 41 . where the simultaneous use of lactic acid and ethanol as EDs favored the production of odd‐numbered MCCs, those carboxylic acids were only produced in small quantities in the present study, an indication that product formation depends to a very large extent on the microbial community and nature of substrate involved in the fermentation process.…”

“…The concentration of lactic acid and ethanol decreased progressively and they were completely used up in both silages after 28 days, an indication that the CE process followed both the lactic acid and ethanol utilization pathways. Contrary to the report of Duber et al 41 where the simultaneous use of lactic acid and ethanol as EDs favored the production of odd-numbered MCCs, those carboxylic acids were only produced in small quantities in the present study, an indication that product formation depends to a very large extent on the microbial community and nature of substrate involved in the fermentation process. The production of higher concentrations of even-numbered MCCs than odd-numbered MCCs in the present study is consistent with the report of Wu et al 42 Clearly, the presence of butyric acid in silage F enhanced the formation of MCCs during fermentation, probably because the acid, which is also an ED, 43 is better utilized for microbial CE than lactic acid or ethanol.…”

Lactate-mediated medium-chain fatty acid production from expired dairy and beverage waste

“…The relatively high selectivity of caproate (∼85%) calculated by dividing the concentration of electrons in the formed caproate by the net consumed electrons from lactate in Period CE-VI was comparable to previous studies [ 15 , [39] , [40] , [41] ], leading to an 8.6-fold higher extraction rate for caproate (256 ± 15 vs. 30 ± 5 mmol C m −2 d −1 for butyrate) from the chain elongation reactor using a continuous membrane-liquid extraction unit [ 7 , 12 ]. This could further enhance the selectivity of longer-chain fatty acids after chain elongation, which could later reduce the cost of MCCA purification.…”

Microbial Recycling of Polylactic Acid Food Packaging Waste into Carboxylates via Hydrolysis and Mixed-Culture Fermentation

“…Lactate-based chain elongation processes were originally reported to be favored in mildly acidic conditions. Under slightly higher pH conditions (>6), lactate may undergo other fermentation pathways that lead to propionate production as discussed earlier [ 55 ]. Interestingly, pH dynamics during organic waste primary fermentation processes combined with lactate-based chain elongation processes can also be exploited by fed-batch operation [ 24 ].…”