Are bacteria the secret to a great wine vintage?
A wine's terroir is defined by a complex formula involving climate, soil, genetics and the way the grapes are handled. But, as Viviane Richter reports, scientists have added microbes to the mix.
You wouldn’t think of bacteria and fungi as serving up cherry undertones or silky textures.
But that’s exactly where the distinct bouquet of wine linked to a certain region, known as “terroir”, may come from. A team of University of California, Davis scientists has discovered the mix of microbes in grape juice can predict the mix of chemicals that shape the flavour of a wine.
Terroir is a concept wine lovers are well familiar with. A bottle of red made from pinot noir grapes grown in Burgundy, for instance, will have a slightly different aroma, texture and taste to wine made from the same variety grown in California.
But where the characteristics denoted by terroir come from has been hotly debated.
Climate, soil composition, genetics and human practice have all been suggested to contribute, but another possibility is emerging – the microbes swimming in the grape juice, or “must”, which is fermented to make wine.
Last year, for instance, New Zealand and British scientists discovered different species of the yeast Saccharomyces cerevisiae in sauvignon blanc grapes affected the finished wine’s taste.
In this study, food microbiologist David Mills and his team analysed nearly 700 samples of chardonnay and cabernet sauvignon must and wine collected during eight stages of the wine fermentation process in two Californian wineries.
“We were curious about the ‘house microbiome,’ as it were,” said Mills.
The team used high-throughput gene sequencing to reveal that bacterial and fungal communities in the grapes and wine varied between different regions.
But the scientists also discovered the microbes present in must could predict which metabolites – chemicals that shape wine flavour and texture – were present in the finished product.
For instance, the level of the fungus Pichia guilliermondii correlated with the level of the aromatic chemical lactone in chardonnays.
It’s currently not clear whether these microbes cause, and not only correlate with, the metabolite composition of wine. But if they do, “microbial biogeography is a quantitative, definable feature of wine terroir”, the authors write.
They hope high-throughput microbial analysis could help winemakers improve their product by identifying desirable metabolites, or reduce waste by allowing species that cause spoilage to be detected early.
So is it time to ditch tradition and let the microbiologists loose on our precious barrels? Not quite.
“We don't know the relative contribution that the microbes play in the eventual flavour and sensory characteristics of the wine,” Mills admitted. He said that while winemakers may one day tweak flavours with the right microbe, bacteria and fungi, they could never completely recreate terroir.
But “this is an area that is developing fast”, he added. “That's a good thing.”
The study was published in the journal mBio.
Related reading: Is winemaking an art or science?