Wine connoisseurs, or oenophiles, possess a seemingly endless vocabulary for describing their tipples of choice. To the uninitiated, it may sound like they are describing an entire gourmet meal, or even a good friend, but this is not just make-believe: those in the know can sometimes pinpoint not just the country or region a wine came from, but the exact vineyard. How can this be possible?
While we cannot hope to distinguish every single chemical in a wine using just our five senses, experienced wine tasters can still perceive hundreds of different compounds that contribute to the drink’s unique flavour and aroma. These compounds are a result of enzymatic and microbial activities during the alcoholic fermentation process, in which yeasts and other microbes convert sugar into ethanol and carbon dioxide.
Getting the mixture right
Winemakers often exert control over the fermentation process by culturing specifically selected yeast strains that they add to their wine in order to achieve certain flavours. However, wild yeasts are also present everywhere in a vineyard, living in the soil and on the processing equipment that turns grapes into ‘must’, the unfiltered, freshly-pressed grape juice that is the precursor of wine. Yeast spores are even found in the air around vines. These wild yeasts all end up in the final product and are called ‘autochthonous’ yeasts, meaning that they are a part of the local environment.
Autochthonous yeasts can impart unique, desirable traits on wines, but as their impact is harder to control detrimental effects can also arise. The effects of local autochthonous microbes on wine are considered to be part of the terroir effect. Many experts credit the effect for the subtle nuances it introduces into wines, and some lament the use of commercially available yeast starter cultures that dilute the terroir effect, make different wines more similar and rob them of their individual “personality”.
Dr Roberto Foschino, a food scientist and microbiologist of the University of Milan, set out to investigate what determines the subtle variations between wines: are they affected more by the year the wine was produced or by terroir, the local impact of the region and vineyard of origin? He and his colleagues journeyed to the Italian wine-growing regions of Franciacorta and Oltrepò Pavese to study the microbial biodiversity of a number of vineyards and cellars over the course of three growing seasons.
To get a complete picture of the autochthonous yeast populations, Dr Foschino and his colleagues sampled the air, must and base wine (the non-sparkling precursor to sparkling wine). To his knowledge, their study is the first to observe the presence of Saccharomyces cerevisiae, a widespread yeast essential for winemaking, in vineyard air.
The researchers isolated over 500 yeast samples from vineyard air, must and base wine and found a total of 31 species of yeast. Must contained the highest species diversity since only the more alcohol-tolerant species survive the fermentation process that turns the liquid into base wine. Furthermore, the winemakers only allowed the team to sample the very top layers of the base wine to avoid disturbing the liquid.
Terroir or vintage?
Dr Foschino and his team made two unexpected findings. Firstly, no strain of the same microbial species was discovered in the same vineyard in consecutive years, suggesting that autochthonous microbial communities vary between years. If no strain is isolated in the same vineyard and cellar over consecutive years, then the microbiota cannot confer a consistent flavour to wines from different years. The effects of terroir and autochthonous microbes may be considered negligible because yeast communities are not specific to geographic areas.
The researchers also found that there was overlap in the species found in samples taken in one year from vineyards up to 100 km apart. The implication is that the vintage – the year in which a wine was made – has an important effect on its flavour. While microbial species and strain varied from year to year in each vineyard, the researchers isolated similar microbes across vineyards in each year of their study.
It appears that the fluctuations of microbial populations over time cause changes in the flavours and characters of wines from different years: the “vintage effect” not only exists but is also more relevant than the “terroir effect”. Oenologists and winemakers may therefore be keen to understand what drives the gradual changes in microbial fauna – microbes may be indicative of environmental changes relevant to their work. Dr Foschino and his team plan to investigate this in the near future, using wild grape varieties in more remote locations to avoid any contamination with artificial microbial cultures.
Vigentini, I., De Lorenzis, G., Fabrizio, V., Valdetara, F., Faccincani, M., Panont, C., Picozzi, C., Imazio, S., Failla, O., & Foschino, R. (2014). The vintage effect overcomes the terroir effect: a three year survey on the wine yeast biodiversity in Franciacorta and Oltrepo Pavese, two northern Italian vine-growing areas Microbiology, 161 (Pt_2), 362-373 DOI: 10.1099/mic.0.000004