During food waste valorization to produce short-chain
fatty acids
(SCFAs), organic substrates are biotransformed step by step, but the
effects of food additives that are present at high concentrations
on these steps and targeted SCFA production have not been investigated.
Therefore,
this study aimed to address this knowledge gap by selecting capsaicin,
a widely used food additive worldwide, as a representative additive.
The results demonstrated a dose-dependent hormesis-like effect of
capsaicin on the production of SCFAs. Specifically, the presence of
4 mg of capsaicin/g of volatile solids (VSs) increased SCFAs yield
by 12.5%, while higher concentrations (8–64 mg of capsaicin/g
of VS) caused a decrement of 22.5–62.9%. Notably, low levels
of capsaicin accelerated the solubilization, hydrolysis, and acidification
processes with the exception of solubilization and protein hydrolysis,
which remained unaffected at higher levels. Additionally, the presence
of capsaicin altered the bacterial cell membrane fluidity and led
to an increase in reactive oxygen species (ROS). Slight accumulation
of ROS stimulated metabolic activities, while excessive accumulation
increased the level of microbial apoptosis. The low level of capsaicin
enhanced SCFA production by upregulating gene expression associated
with hydrolysis and SCFA bioconversion. Conversely, at high capsaicin
levels, genes encoding components of the ribosomal subunit involved
in polypeptide synthesis, cell migration, and survival regulation
were downregulated, resulting in reduced microbial cell activity and
SCFA production. This study provides valuable insights into the response
mechanisms of complex microbial processes to capsaicin at the genetic
level, specifically in SCFA production during anaerobic fermentation
of food waste, and has implications in resource utilization of food
waste (FW) containing exogenous additives.