Wine Microbes… Are They More Than Just Alcohol Factories?

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Could microbes be responsible for terroir? The latest research suggests they might.

Historically, winemakers were taught that wine quality and flavour came predominantly from grapes. Yeasts were simply considered to be factories converting sugars to alcohol, with little effect on wine’s sensory properties. Choices of yeast (and bacteria in the case of malolactic fermentation) were focused on ensuring robust fermentations that went through to completion and were easy to control.

In the last 10 to 15 years, however, these attitudes have changed

In the last 10 to 15 years, however, these attitudes have changed, with research and practical experience showing that both yeast and bacteria can have significant impacts on a wine’s final sensory outcome.

Winemakers now have a bewildering choice of yeasts and bacteria that are tailored for specific outcomes. For instance, yeasts are now available that produce much lower levels of stinky volatile sulfur compounds, making more consumer-friendly and marketable wines.

There are also yeasts that can enhance the specific characteristics of individual varieties such as pinot noir or sauvignon blanc. Increasingly, the choice of malolactic bacteria is also based at least in part on factors other than robust performance, with research showing that different strains can have real sensory impacts, including (at least anecdotally) in characteristics such as mouthfeel.

The continuing trend towards spontaneous ferments (ferments to which no exogenous microorganisms are added) also has its roots in the search for unique characters and style. Spontaneous ferments typically initially contain a complex mix of organisms when ethanol levels are low.

Over time, the number of organisms drops until in the final stages of fermentation there are often only one or two yeasts left.

The microbial complexity at the early stages, when ferments contain many non-traditional yeasts, can have significant flavour impacts leading to large differences in styles. It is often these differences in style which winemakers use to differentiate their wines and the place they come from.

Understanding that the unique characters of spontaneous ferments stem from non-traditional strains of yeast has led to renewed interest in developing some of these strains commercially, with a range of interspecies hybrids and non-Saccharomyces cerevisiae strains now available on the market.

These yeasts are not only being used to develop unique sensory characters in wine using reliable and reproducible fermentation processes, but in some cases are showing potential to produce wines with lower alcohol content. For example, researchers have shown that by initially using a specific non-Saccharomyces cerevisiae yeast that converts some grape sugar to biomass rather than alcohol and then finishing a ferment with more traditional strains, it is possible to produce wine with lower alcohol while retaining an acceptable sensory profile.

Understanding that the unique characters of spontaneous ferments stem from non-traditional strains of yeast has led to renewed interest in developing some of these strains commercially

It is also now recognised that it is not just through the choice of yeast that winemakers can impact flavour outcomes; it is also in the available nutrients.

For many years, the addition of yeast assimilable nitrogen (YAN) in the form of diammonium phosphate (DAP) was treated as a requirement for avoiding stuck ferments, a role in which it is very important. Research at the AWRI and other organisations has also shown, however, that the amounts of YAN available can also have a significant impact on flavours.

The old mantra of just making a standard addition to avoid a stuck ferment is receding with the realisation that high levels of YAN can also lead to negative sensory outcomes. Winemakers are increasingly using measurements of juice YAN levels to be able to target additions to achieve a ‘sweet spot’ of nutrients, which may vary between different yeasts.

Debate is continuing about the best form of nitrogen supplementation to use, with some arguing that complex nitrogen additives such as amino acids give different flavour outcomes to the more common, and usually cheaper, DAP.

Debate is continuing about the best form of nitrogen supplementation to use

The impact of microflora is not restricted to primary and secondary fermentation. In Europe and New Zealand, research groups are isolating combinations of organisms from the vineyard and soil that appear to correlate with regional differences in the final wine.

This supports the idea that regional flora, especially soil microorganisms, may have a role in shaping terroir. The soil microbiome with its significant role in vine health appears to have a major impact on fruit outcomes and the presence of flavour precursors in wine. It is these precursors which play a pivotal role in determining eventual wine quality.

The expanding knowledge on the roles of microorganisms in wine production highlights the importance of research into the microbiology of wine. Such research has given wine producers a wide range of high-value tools, increasing flexibility to achieve their vision and reflect their regional and personal uniqueness.

Eric Wilkes is group manager, Commercial Services, of The Australian Wine Research Institute.

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