To improve performance, we replaced the gravity settler with a hollow-fiber membrane module to retain more biomass in the bioreactor. At the end of period A-I, however, this estimated selectivity had been considerably lower at 41.3%, because not all lactose was converted while galactose was barely converted ( Figure 2). The selectivity, which was estimated by the LG-into-lactic acid conversion efficiency, had reached 70.7% (in mmol C) at the end of period A-II ( Table S3). Reducing the amount of possible microbial pathways in each of the two microbiomes was required and resulted, for the first time, in the bioconversion of a real waste stream into primarily MCCA via lactic acid as the intermediate without the addition of external electron donors.įor the phase A bioreactor, we found that almost all lactose and galactose (LG) was converted to lactic acid at the end of the operating period ( Table S3). The novelty of our bioconversion system lies in phasing the microbiomes into different operating conditions (i.e., temperatures) by placing two existing bioconversion schemes in series. We converted the complex substrate into largely lactic acid in the first phase (thermophilic phase A at 50☌) by lactic acid production and then the lactic acid into MCCAs in the second phase (mesophilic phase B at 30☌) by chain elongation. We then pivoted to a temperature-phased bioreactor system ( Figure 1). However, preliminary experiments with a single bioreactor resulted in a low specificity as estimated by the production ratio of MCCAs compared with all produced carboxylic acids (hereafter referred to as the MCCA-to-CA production ratio).
Because acid whey contains sugars in addition to lactic acid, we hypothesized that it should be possible to convert acid whey into MCCAs within a single reactor microbiome. Here, our objective was to convert the acid-whey waste stream from Greek-yogurt production into MCCAs without external electron donors by following a trial-and-error strategy to prove the concept within an n = 1 study. However, capital costs should be further reduced during scale-up. Due to biological chain elongation, the oil-like MCCAs were extracted with relatively low energy consumption. MCCAs can be precursors for biofuels or chemicals or can be used as green antimicrobials or livestock feed additives.
Here, we showed that acid whey was converted into valuable medium-chain carboxylic acids (MCCAs), such as n-caproic acid ( n-hexanoic acid) and n-caprylic acid ( n-octanoic acid), without addition of external electron acceptors. Until now, no other products could be produced with microbiomes from this waste stream. However, the revenue from methane has been relatively low. This whey and other waste streams have been successfully converted into methane gas by anaerobic digesters with open cultures of microbial consortia (microbiomes). Acid whey is an example waste stream and is produced by the Greek-yogurt industry in large volumes. Waste streams can be renewable feedstocks to produce biofuels and chemicals.