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:
- Lactic acid bacteria outcompete prototrophic type fermentative
bacteria on high biomass specific substrate uptake rate and growth
rate.
- 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.
- 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
- 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.
- 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.