The impact of feedstock protein content on fermentative product spectrum
The consequence of these different ecological types of fermentations are important for understanding applications of mixed-culture fermentation, especially for non-aseptic or “open” bioprocesses aimed to produce economically interesting compounds. The difference in hydrogen production we observed here (Figure 1) has been reflected in a meta-study comparing different feedstocks for the production of hydrogen: food and municipal waste streams generate 32-42% less hydrogen than industrial and (pre-treated) agricultural residue waste streams (Moscoviz et al., 2018). Food waste typically contains more than 10% (w:w) of protein (Paritosh et al., 2017), while agricultural residues contain low amounts of protein, e.g. , wheat straw contains 0.6% (w:v) (Kaparaju et al., 2009). This leads to food waste fermentations being dominated by lactic acid bacteria and the secondary lactate fermentation producing no or small amounts of hydrogen gas. In contrast, fermentations of (pre-treated) agricultural residues are dominated by acetate/butyrate producing bacteria, such asClostridium species, resulting in significant amounts of hydrogen produced. This difference in performance is a direct consequence of the ecology of these two different fermentative microbial groups. Lactic acid bacteria seem to dominate environments abundant in carbohydrates and peptides where selection occurs on a maximal growth rate or maximal substrate uptake rate. The consequence of striving for a high growth rate is that the organisms have to optimise their proteome in preference for high growth rate enabling proteins, making them auxotrophic for e.g. amino acids and vitamins.
Using this ecological concept, feedstocks with readily fermentable carbohydrates and a sufficient protein content are a good target to directly produce lactic acid. Protein-poor feedstocks on the other hand are a good target to produce VFAs and hydrogen (Figure 5). In this study, we have obtained an enrichment producing only 0.11 Cmol per Cmol of lactate at the end of the batch (Figure 1). If lactic acid production from low value feedstocks is the desired bioprocess, lactate consumption has to be managed effectively. Lactate consumption can be managed by creating a selective environment which does not select for lactate consuming organisms such as Megasphaera , by using a different pH or solid retention time for example.
Here we used enrichment culture to better understand the ecological niche of lactic acid producing bacteria which showed:
  1. Lactic acid bacteria outcompete prototrophic type fermentative bacteria on high biomass specific substrate uptake rate and growth rate.
  2. This behaviour can be explained in line with the resource allocation hypothesis for protein allocation: LAB can dedicate a higher share of their proteome to catabolism, ribosomes and RNA polymerases and therefore are able to attain a significantly higher substrate uptake rate and growth rate.
  3. The anabolic efficiency of the microbial community enriched on complex medium is higher but not significantly, and only accounts for a minor possible increase in µmax
  4. Intermediately formed lactic acid is fermented to acetate, propionate, butyrate, valerate, H­2 and CO2, resulting in a different fermentation product spectrum when lactate is an intermediate fermentation product.
  5. A relatively high protein content of a feedstock can enhance the competitiveness of lactic acid bacteria, leading to lower hydrogen yield and the possibility of producing lactic acid by enrichment cultures.