MICROBES
BREW A GOOD COFFEE
Recent research has shown that when it comes to processing coffee beans,
longer fermentation times can result in better taste, contrary to conventional
wisdom. Lactic acid bacteria play an important, positive role in this process.
“A cup of coffee is the final
product of a complex chain
of operations: farming,
post-harvest processing,
roasting, and brewing,” says
principal investigator Luc De
Vuyst, MSc, PhD, Professor of
Industrial Microbiology and Food
Biotechnology, Vrije Universiteit
Brussel, Brussels, Belgium. “There
are several variants of post-harvest
processing, among which wet
processing and dry processing are
the most common.” Wet processing
- commonly used for Arabica and
specialty coffees - is the step that
includes fermentation.
“We carried out the research at
an experimental farm in Ecuador
through a multiphasic approach,
encompassing microbiological,
metabolomics, and sensory
analysis,” says Dr De Vuyst.
Fermentation was of particular
importance. During extended
fermentation, leuconostocs - a
genus of lactic acid bacteria used
in the fermentation of cabbage to
sauerkraut and in sourdough starters
- declined in favour of lactobacilli,
says Dr De Vuyst. Lactic acid
bacteria were already present before
fermentation, and these acid tolerant
lactobacilli proliferated even more
during this process.
However, it is challenging to draw a
causal link between the microbiota
and the volatile compounds in the
beans -- those compounds that
contribute to the coffee’s smell --
since many of these compounds can
be of microbial, endogenous bean
metabolism, or chemical origin,” says
Dr De Vuyst.
“However, we did see an impact
of the microbial communities, in
particular the lactic acid bacteria,”
says Dr De Vuyst. They yielded
fruity notes, and may have “had
a protective effect toward coffee
quality during fermentation because
of their acidification of the fermenting
mass, providing a stable microbial
environment and hence preventing
growth of undesirable microorganisms
that often lead to offflavours,”
he says.
“Furthermore, there is a build-up of
the fermentation-related metabolites
onto the coffee beans, which affects
the quality of the green coffee beans
and hence the sensory quality of the
coffees brewed therefrom,” says Dr
De Vuyst.
Dr De Vuyst emphasised that how
each stage of processing influences
the taste of coffee remains mostly
uncharted. “We were aware of many
different micro-organisms during wet
coffee fermentation - enterobacteria,
lactic acid bacteria, yeasts, acetic
acid bacteria, bacilli, and filamentous
fungi,” says Dr De Vuyst, but it is
still unknown how most bacteria
influence this process.
The work was a collaboration
between the Vrije Universiteit
Brussel, and the Nestlé Research
Center. “Nestlé was interested in
the fundamental aspects of coffee
processing, in particular, the postharvest
processing chain, in order to
correlate it with the roasting process
and of course the final cup quality.”