Differences in wine quality between vineyards have long been attributed to processing techniques and seasonal variation. But research now suggests that regional differences between wines are shaped by microbes — specifically, fungi and bacteria. Cultivating certain grape microbes may actually improve wine flavor.
Without yeasts (fungi) and bacteria, wine wouldn't be possible. These hungry microorganisms break down and digest the sugar in grape juice, and this process — called fermentation — results in alcohol. In recent years, researchers have begun pinpointing specific microbes that improve the overall sensory complexity and flavor of wine. Meanwhile, other microbes have been implicated in wine spoilage.
Despite these finds, yeasts and bacteria are generally left out of the conversation when people discuss the distinctive flavors of wine. Instead, regional differences in wine quality is usually tossed up to the specifics of the fermentation process, such as the size of the container or the temperature used, or the soils in which the wine grapes grew.
"But we know from other research areas that environmental conditions can shape microbial communities," said Nicholas Bokulich, an enologist (wine scientist) at the University of California, Davis. It stands to reason, then, that the microbial communities on the surface of wine grapes could differ between vineyards, and this may influence wine quality. "But other research groups thought that this wouldn't be the case — that wine grapes all contained the same microbial communities."
So Bokulich and his colleagues set out to test if the microbial communities on wine grapes changed depending on where the grapes were growing.
The researchers began by collecting 273 grape must samples from across California in two separate vintages. Must, Bokulich explained, consists of crushed grapes that have been de-stemmed and mixed together — it represents a sample of all of the grapes from an individual vineyard block.
"We didn't actually pick the grapes because there are some issues with that," Bokulich told io9. Specifically, grapes in a single vineyard block may be exposed to different light, temperature and humidity, and therefore may all have different microbial communities. "In our minds, we thought that sampling the vineyard wouldn't give us a representative sample, so we collected must from the wineries."
The team then used next-generation DNA sequencing techniques to figure out which fungal and bacterial species were in the must samples, as well as their relative abundances. Next, they used computational methods to determine if there were any links between the microbial communities and where they came from.
Indeed, the researchers found that the fungal and bacterial communities really did differ depending on their growing region. For example, Napa Chardonnay musts were loaded with the bacterial group Firmicutes and the fungal group Eurotiomycetes. By comparison, Sonomy Chardonnay musts contained high concentrations of the fungus Botryotinia fuckeliana and Proteobacteria. They found that the grape variety also strongly influences the microbial patterns across regions and vintages.
What's more, the team discovered that local environmental conditions — wind, temperature and relative humidity, in particular — are responsible for driving the biogeographical diversity in microbial communities. Vintage also affects grape must microbiota, and this fits in with the influence of climate, given the change in climate between years.
"What we are really seeing here is that region, environmental conditions and grape varieties shape the microbial communities of the grapes that make it into the fermentation process and shape wine quality," Bokulich said.
Interestingly, the results suggest that over time, climate change may have a strong impact on the fungal and bacterial communities of wine grapes — and this may, in turn, affect the sensory qualities of wine. However, the researchers haven't modeled their results over enough regional conditions or enough years to really make such conclusions.
And though the research shows that the microbial communities do show regional differences, the scientists still need to demonstrate that this nonrandom distribution of microbes really does change grape and wine quality.
If the results hold true, the research has strong implications for improving grape and wine quality. Winemakers, for example, could possibly tailor their vineyard treatments, farming practices and wine-fermentation management to promote or discourage the growth of different fungal and bacterial communities. The work could also extend to other agricultural products, such as fresh fruits and produce, in which different microbial communities are associated with spoilage and shelf life.
For now, however, there's still a lot of work to do.
"We want to see if we can actually look at and understand the stability and changes we see in microbial patterns over time, and determine what effect they may have on wine quality," Bokulich said. "And then we can try to understand more about how the things we do in the vineyard and winery shape microbial communities."
Check out the study over in the journal PNAS.
Top image via David Mills, UC Davis. Inset image via Chuck Abbe/Flickr.