An uninoculated fermentation is often referred to as “natural” or “spontaneous” fermentation that involves the sequential action of different non-Saccharomyces and a diversity of indigenous Saccharomyces yeasts. The first fermentation, looking at the 7,000 years of winemaking history, was more likely the result of serendipity than design.

Natural yeast and natural wine: a rather unnatural tale
Spontaneously, ambient yeasts fermented damaged grapes in harvesting pots, mystifying hunter-gatherers who established agriculture and the first great civilization in Mesopotamia around the Tigris-Euphrates river system and tasted wine for the first time.

However, even during those early “vintages” it was clear that, without human intervention, the result of “naturally” fermenting grapes is variable, unreliable and can be undrinkable. It did not take long before the ancients realized that the completely “natural” end result of fermenting grapes is vinegar.

There is heated argument as to whether today’s wine is of higher quality (increased flavor profile and absence of faults and taints) due to the contribution of scientific knowledge, technology and research—or whether so-called natural wine is better. There is a new-found nostalgia for the wine of yesteryear made with a minimalist approach and variable outcomes. The reality is that winemaking is both art and science and always has been.

History taught us that the best outcome for both winemaker and consumer is achieved when the wine industry harnesses what nature, human ingenuity and cutting-edge science offer in harmony with the unique “artistic” nature of wine. Here we summarize what nature’s treasure of “wild” yeasts has to offer and how inventive winemakers can use them in a scientifically controlled manner to craft wine styles that match consumer expectations in a diverse range of market segments.

Non-Saccharomyces yeasts: origin and wine flavor contribution
Non-Saccharomyces yeasts were originally seen as responsible for microbe-related problems in wine production due to their isolation from spoiled wines. Although it was known that some non-Saccharomyces yeasts could form beneficial metabolites for wine quality, this was outweighed by the high levels of volatile acidity and other negative compounds produced.

At the same time, non-Saccharomyces yeasts were considered to be sensitive to SO₂, known to be poor fermenters of grape must and intolerant to ethanol. Therefore it was accepted that those non-Saccharomyces yeasts, not initially inhibited by the SO₂, died during fermentation due to the combined toxicity of the SO₂ and alcohol.

However, research has highlighted the high numbers (106 to 108 cells/mL) and sustained presence of non-Saccharomyces yeasts in modern wine fermentations leading to revisiting the role of these yeasts in winemaking.

Newer research showed that concentrations between 50 and 100 ppm SO₂, while effective in white wine fermentations, do not prevent growth of non-Saccharomyces in red wine fermentations. Generally, SO₂ concentrations between 0 and 50 ppm have been successfully used for wild fermentation.

During crushing, the non-Saccharomyces yeasts on the grapes, on cellar equipment and in the cellar environment (air- and insect-borne) are carried over to the must. However, cellar surfaces play a smaller role than grapes as a source of non-Saccharomyces yeasts, as S. cerevisiae is the predominant yeast inhabiting such surfaces. Dominant yeasts in must after crushing should therefore be the same as those that are found on grapes.

Despite all the variables in grape harvest and wine production, the yeast species generally found on grapes and in wines are similar throughout the world. However, the proportion or yeast population profile in various regions show distinct differences.

The contribution by non-Saccharomyces yeasts to wine flavor will depend on the concentration of metabolites formed. This in turn is affected by how active the non-Saccharomyces yeasts are.

The specific environmental conditions in the must include high osmotic pressure, equimolar mixture of glucose and fructose, presence of SO₂, non-optimal growth temperature, increasing alcohol concentrations, anaerobic conditions and decreasing nutrients. All these factors play a role in determining what species can survive and grow.

The clarification of white must (centrifugation, enzyme treatments and cold settling) can reduce the initial population of yeasts.

In red must, wild yeasts are usually encouraged by cold soaking. In general, wild fermentation benefits from being initially protected from air, but once the ferment is active, aeration is advantageous for yeast activity. Similarly to inoculated fermentation, wild fermented musts containing low yeast assimilable nitrogen (YAN) will deliver wines with more complex “mineral” and “funky” styles, while nitrogen addition will produce wines with a cleaner and more fruity style.

The range of flavor compounds produced by different non-Saccharomyces yeasts includes terpenoids, esters, higher alcohols, glycerol, acetaldehyde, acetic acid and succinic acid. Although far less studied, wine color can also be affected by non-Saccharomyces yeast.

Various research articles have reported on deliberate inoculation of selected non-Saccharomyces yeasts for wine production. These included Torulaspora delbrueckii, Metschnikowia pulcherrima (Candida pulcherrima), Candida zemplinina (Candida stellata), Hanseniaspora species, Zygosaccharomyces species, Schizosaccharomyces species, Lachancea thermotolerans (formerly Kluyveromyces thermotolerans), Pichia species, Hansenula anomala, Williopsis saturnus, Candida cantarellii, Issatchenkia orientalis and Saccharomycodes ludwigii.

Combinations of more than one species of non-Saccharomyces yeasts have also been investigated. Most of these yeasts are poor fermenters, therefore S. cerevisiae (either indigenous or inoculated) is always needed to complete wine fermentation. Typically, non-Saccharomyces yeasts have been used in sequential fermentation where these yeasts are allowed to grow or ferment between one hour and 15 days before inoculation with S. cerevisiae.

Wines fermented with non-Saccharomyces yeast have shown different chemical composition and/or different flavor and aroma when compared to wines fermented only with S. cerevisiae.

Although many of these trials were conducted on a laboratory scale utilizing small volumes of grape juice, and the results may not be the same as what could be expected in larger commercial fermentations, they demonstrate the potential of non-Saccharomyces yeast to produce distinctive wines. As a result, a few non-Saccharomyces yeasts have already been commercialized.

Conclusions
The diverse array of yeast available to a winemaker through the cellar environment, in the air, on the grape or through inoculation remains a crucial element to wine production with a wide range of complex flavors and aromas. Harnessing the performance of fermentation for a desired outcome tantalizes and challenges.

Research undertaken in S. cerevisiae can make great contributions to understanding the role and uses of non-Saccharomyces yeast in spontaneous and “inoculated multi-species” ferments. The management of “mixed-ferments” is more complex than “single-species” ferments because so many things can go wrong.

Therefore, a modern approach to “multi-species” wine ferments backed by frontier science and rigorous research is essential to help winemakers achieve their primary objective of a better than 98% conversion of grape sugar to alcohol and carbon dioxide at a controlled rate and without the development of off-flavors. Therein lies wine’s magic blend of art and science.

This article is a summary of findings reported in: N. Jolly et al. “Not Your Ordinary Yeast: non-Saccharomyces Yeasts in Wine Production Uncovered,” FEMS Yeast Research (2014) 14: 215–237.