Two decades ago, wineries looking for a point of difference often used to print terms like “barrel select” on their labels. Today, the equivalent term is probably “wild ferment”. Wine professionals and enthusiasts are expected to understand that a wine boasting these words will be more complex and have a more authentic flavor of the terroir in which the grapes were grown. Producers of “natural” wines may disagree over how much SO2 is acceptable in a wine, but they are in absolute agreement about the need to eschew commercial yeasts.
But what is the science behind these beliefs? How wild is a wild ferment and is it fair to say that wine fermented with wild yeast is inherently superior? Are wines made with entirely cultured yeasts less authentic? How many wine drinkers have any idea how, or why, they should seek out “wild yeast”?
In favour of wild
One of the most stalwart supporters of wild ferments is Alice Feiring, author and leading natural wine advocate. “By inoculating, you are pre-selecting certain characteristics, predetermining the outcome of aroma and flavour. Natural fermentation, on the other hand, allows for the panoply of yeasts — some good and some less favourable — that all add to the complexity of the final product,” she says. “They give the wine a different voyage every year. For wines and wine drinkers that are looking for individual character of a place and time over consistency year to year, the effect of native yeasts is paramount.”
Feiring’s view is supported by one of Portugal’s top producers, Dirk Niepoort. “I don’t see any reason to use cultured yeast,” he says. “Natural wild yeast works very well indeed and we rarely have problems.” He states that terroir is not something the vines “eat” then pass on to the wine, but “a lot of little details happening during fermentation — usual deviations and ‘problems’ due to a lack of nutrients, or too much of one thing. These cause deviations and little errors, like reductions during the fermentation that in the end ‘create’ the character of the terroir.” For Niepoort, “modern, so-called winemakers” are producing “more and more ‘made’ wines instead of true wines that show where they come from”.
However, Michel Valade of the Comité Interprofessionnel du Vin de Champagne stated in an October 2004 Le Vigneron Champenois article called “Levures sélectionnées ou levures indigènes” that “terroir yeasts don’t exist”. For Valade, “selected yeast strains are simply the best yeasts among the indigenous population, nothing more, nothing less”.
Wine, he claims, is just a by-product of yeast metabolism. Yeasts that are best adapted to the specific environment take over and become the predominant micro-organisms. “But the yeast strains that multiply with the greatest ease are not necessarily the ones capable of producing the best wine. That’s why it is useful to choose strains that are both well adapted to the must to be fermented and that are endowed with interesting oenological properties. That is the principal objective behind the isolation and selection of yeast strains.”
No fan of natural wines, Valade suggests that “with wild yeast and bacteria you can produce only fermented products but not perfect and replicable wines”.
Yeasts defined
It wasn’t until the middle 19th century that fermentation was studied more closely and Louis Pasteur discovered the role of yeast in the process. It was the 20th century when yeast strains were identified, analysed and cultivated and replicated for specific intended wine characteristics.
New Zealand-based yeast researcher and professor Matthew Goddard simplifies it. “Winemakers have two choices when it comes to ferment: deliberately add a commercially purchased wine yeast (or some mix of); or not to do this, but let whatever microbes are naturally present conduct the ferment — a spontaneous ferment,” he says, adding that: “Commercial yeasts have only been available for the past 40 years or so, so by definition we have been making wine by spontaneous fermentation for a very long time.” Spontaneous ferments tend to make different wines. Goddard believes, thanks to the huge biodiversity of different types of yeast species and different strains of Saccharomyces cerevisiae. Inoculated ferments, on the other hand, are essentially “monocultures”.
Yeasts are everywhere in a vineyard, invisibly present — they measure around 0.003mm — on the surface of the grapes, the vines and soil. Apart from converting fructose into ethanol — the alcohol in wine — they also contribute to the flavour molecules. Of the 1,000 or so volatile flavour compounds in wine, approximately 400 are created by yeast, some good and some bad, but their balance affects the final wine.
According to some estimates, as many as 40 different vineyard strains — including Candida stellata, Kloeckera apiculata, Rhodotorula, and Hanseniaspora uvarum — may be present in a ferment. These yeasts may be characterful but they’re not mighty, dying off when alcohol levels reach 4% to 6%. It is the far more powerful, alcohol-tolerant, main winemaking yeast, Saccharomyces cerevisiae, that takes the ferment to completion. Antonio Palacios, professor at the University of La Rioja and one of the research scientists speaking at a recent Lallemand Oenology conference, likens the strength of S. cerevisiae over other strains to a comparison of Superman with other mortals.
The cocktail of yeasts in any ferment is made up of vineyard strains; the yeasts from the marc — the skins, stalks and pips left over after fermentation — and strains present in the winery from previous ferments.
It’s thought to be the early stages of fermentation, when the ambient yeasts work their transformative powers on the must before dying off, that has the greatest terroir-transmission effects. The temperature (native yeasts prefer it to be cooler) and length of the ferment allow more time for these flavour molecules to build and become more complex, while the minimal input of sulphur dioxide ensures their survival because the weaker native yeasts are unable to stand up to high levels of SO2.
Paul Henschke, Emeritus Fellow at the Australian Wine Research Institute (where he was formerly Principal Research Microbiologist), Affiliate Professor at the University of Adelaide and owner and winemaker of Greenhill Wines, suggests that the native yeasts’ relative weakness are actually their strength when it comes to winemaking. “Because the initial population of yeasts of a wild ferment is 10 to 100 times smaller than the inoculum added to a conventional ferment, wild fermentation can have an initial latent phase, which can last up to a week, during which the smaller initial population must multiply many times.”
While admitting that it has yet to be “formally demonstrated”, Henschke believes that this latent period constitutes a pre-fermentation maceration, during which time grape constituents can initiate chemical and biochemical interactions. These in turn can lead to the formation of complex molecules, polymers, tannins, pigments and sensory active molecules. The non-volatile flavour compounds such as polysaccharides and other molecules, which can interact with tannins and pigments that are all produced by wild yeasts, “enhance the textural and other mouthfeel properties of wine as well as wine’s aroma.” This is provided “all has gone well — the right yeasts have developed and moved on at the right time”. If all has not gone well, spoilage bacteria, acetic contamination, oxidation and other undesirables can also be produced.
Interestingly, the use of wild yeast in early fermentation may provide a layer of natural protection. Dr. Jamie Goode, author of The Science of Wine and co-author of Authentic Wine points out that the relationship between S. cerevisiae and the non-saccharomyces (native yeasts) is complex. “A single S. cerevisiae feeds more singularly on the must, whereas the mix of non-saccs feed more broadly, using different nutrients and producing different fermentation products. Perhaps the broad feeding by the native yeasts leaves less for the bad bugs to feed on.”
Semi-wild
In recent years, yeast companies such as Lallemand and CHR Hansen have developed a middle ground of cultured wild yeasts. These cultured non-Saccharomyces yeasts allow winemakers some of the influence of the native yeasts. Alternatively, winemakers may now buy mixtures of strains that attempt a simple but more reliable and predictable emulation of spontaneous ferments, though obviously without any connection at all to a vineyard site, while maintaining control over the fermentation. Wine producers like Sogrape in Portugal have gone even further, isolating yeasts from specific vineyards to create their own cultured strains to use when fermenting grapes grown in those plots.
Some winemakers choose to utilise the best of both options. Wes Pearson, who works at the Australian Wine Research Institute (AWRI) and is winemaker at McLaren Vale’s Dodgy Bros, is a typical pragmatist. “As a winemaker, I use wild yeast extensively but not exclusively. In warm regions like McLaren Vale, it is inevitable that occasionally you’re going to get high sugar musts in the winery.” He says that his concern with wild yeasts is their ability to ferment these high sugar musts to dryness. “A stuck ferment is a problem that I’d prefer to avoid. From time to time I use cultured yeasts, but I still let the ferment kick off with its wild flora, so as to reap the benefits of the complex suite of yeast fermentation bi-products from the wild yeast.” Pearson says that while he wants to “make wines that speak of the place that the grapes were grown, a red wine with 30g of sugar per litre remaining doesn’t do that.”
Paul Henschke sums up the native yeast debate succinctly. “Wild yeasts, by the very nature of their biochemical diversity, can potentially produce a wider, diverse range of flavour active molecules that contribute to the greater complexity of those wines. On the other hand, because we cannot easily predict nor manage which wild yeasts are making these wines, the results are often unpredictable and sometime unpalatable. It’s about the end goals, and the desire for predictability versus personalisation.”
