Brett + bacteria = worse, or better

Microbiology has gotten a lot wrong studying yeast and bacteria. We’ve assumed, until quite recently, that if a microbe doesn’t grow in a dish it’s not there. And that a microbe is either on/live/growing or off/dead. And that we can study microbes in isolation — “pure culture” — away from other species in little sterile dishes and expect them to behave normally. In all fairness, microbiologists have sometimes seen these as a problems, but have mostly just gone on this way, writing books about what we think we know.

DNA detection and sequencing technology is showing just how many bugs don’t grow in dishes — “high throughput” technology can document (theoretically) all of the species in a drop of [insert favorite liquid here]. That’s pretty routine these days. And we’re slowly beginning to study how mixtures of microbes — you know, the way they live in the wild — behave in the lab. Wine was a bit ahead of the curve here: microbial enologists have been studying the goings-on of spontaneous and mixed fermentations since the late 1980’s.*

Usually, mixed-microbe studies are about what grows where together. Occasionally, you can predict something more specific with a bit of logic and some scratch paper. That, plus a little knowledge of yeast and bacteria metabolism, leads to an interesting hypothesis: some malolactic fermentation bacteria should make Brett smell worse.

Brettanomyces bruxellensis (aka “Brett,” aka barnyard-stench spoilage yeast) creates its signature aroma by converting hydroxycinnamic acids (HCAs) naturally present in wine to smelly volatile phenols. This is a two-step process. First, an enzyme (a decarboxylase) converts HCA to a vinylphenol. Second, a different enzyme (a reductase) converts the vinylphenol to the volatile ethylphenol, including the Brett signature 4-EP and 4-EG.

But before that can happen, Brett has to be able to get to the HCAs. Many of the HCAs in wine are chemically bound to tartaric acid. Brett can’t use them if they’re bound. The HCA-tartaric acid bond spontaneously and slowly breaks, giving off free HCAs for Brett to use, but there’s theoretically a much bigger pool of pre-stink molecules that need only lose their acid first.

Some lactic acid bacteria — like the ones that commonly perform the malolactic fermentation (MLF) so important to most reds and a lot of white wines — can enzymatically split HCAs from tartaric acid. In theory, that should mean that some (but not all) MLF bacteria are Brett enablers. Wine + bacteria + Brett = worse smell than wine + Brett alone.

Building on previous research, a team at Oregon State University has made that more than a theory. Their recent paper (currently pre-press in AJEV) shows that some commercially available MLF strains make more HCAs available than others, AND that leads to Brett making more 4-EP and 4-EG,

The team only experimented with one strain of Brettanomyces, and they obviously couldn’t test anywhere near all of the MLF strains on the market, but this (plus the multiple studies that have come before it supporting the effects of lactic acid bacteria on HCAs) is strong evidence indeed that winemakers buying commercial bacteria for MLF may have better and worse choices if they’re worried about Brett.


*A good open-access (no paywall) example of this kind of research is Granchi and company’s 1999 study here.


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