Yeast is a very important ingredient when it comes to wine. Yeasts are single-celled fungal organisms. Without them, there would be no alcohol. Yeasts are very efficient at taking wort, (a very complex medium consisting of sugars, amino acids, peptides, proteins, vitamins, nucleic acids, ions and many other compounds), and converting it to alcohol, CO₂, and hopefully favourable flavour and aroma compounds.
Yeast selection is one of the most important decisions in the winemaking process. The yeast strain you use will determine the style of wine you create, as well as the characteristics that the wine may take on. Some winemakers prefer to use native yeasts (also called wild, or indigenous yeasts), which occur naturally in the vineyard or winery, in an effort to get a unique expression that some consider truer to the wine’s terroir, or sense of place.
Making wine with wild yeast has been a source of debate for many because of its unpredictable nature. Sometimes wild yeast is referred to as “natural” yeast. The term natural is rather ambiguous. All yeast – even cultured yeast – is natural. When grapes are harvested in the vineyard, they are covered with a myriad of biological organisms, including yeast. There are thousands of different types of wild yeast. Many do not make good wine. Along with the wild yeast are other spoilage organisms and bacteria.
Furthermore, one of the most common characteristics of indigenous yeasts is their low resistance to alcohol. Many wild types of yeast are unable to perform once alcohol levels reach 6%. The result is stuck fermentation, flabby wine with a low immune system, and a pile of unwanted residual sugar – to name just a few problems.
Most wine today is inoculated with yeast cultures, which can act more predictably. The fact is that cultured yeasts have only been in the winemaking picture within the last century. For thousands of years previously, all wine was fermented on wild (indigenous) yeast.
A favourite wine you buy from the store has a consistently reliable quality because it was inoculated with a cultured strain of yeast isolated for its desirable fermenting characteristics. Each wine yeast has its own unique set of flavours and aromas. A specific cultured yeast strain might be able to produce nice fruity aromas, a high level of alcohol, an attractive mouth texture and an ability to ferment in low temperatures or high acid.
In winemaking, one of the most important characteristics of cultured yeast is its ability to completely ferment all the sugar in grape juice and tolerate high levels of alcohol (anywhere from 8-18%). High alcohol (usually 12% or more in wine) gives wine its longevity, body and other positive features.
The most common yeast associated with winemaking is Saccharomyces Cerevisiae which has been favoured primarily due to its predictable and vigorous fermentation capabilities, tolerance of relatively high levels of alcohol and sulphur dioxide.
Until about the 1980s, the contribution of yeasts to wine production was seen as a relatively simplistic concept. Essentially, grape juice underwent a natural or a spontaneous alcoholic fermentation that, almost invariably, was dominated by strains of the yeast, Saccharomyces Cerevisiae. Consequently, pure cultures of this yeast were isolated and developed as starter cultures for conducting wine fermentations. Many other species of yeasts were known to occur in grape juice and contribute to the first stages of fermentation but, generally, they were considered to be of secondary significance or undesirable to the process.
With this technology, winemakers had established reasonably good control of the process. But fermentation is a risky process. If a ferment gets too hot, yeasts will weaken and die. If the ferment is too cold, it will not start. Or it will stop after getting underway.
Winemakers can take many steps to adjust temperature including cold soaks, tank temperature and pump overs, and adjust oxygen with pump overs and rack and return, punch down, stirring or aeration and micro-oxygenation. Exposing yeast to changes in temperature, sugar levels, as well as sulphite and nutrient levels causes them stress. If the stress is too much to bear, they may go into shock or die off.
During the last 25 years, major advances have occurred in understanding the ecology, biochemistry, physiology and molecular biology of the yeasts involved in wine production, and how these yeasts impact on wine chemistry, wine sensory properties and appeal of the final product.
Studies revealed that fermentation has been found to be much more complex than assumed. Also, the dominance of the inoculated strain of S. cerevisiae, and the metabolic impact of yeasts on wine character is much more diverse than simple fermentation of grape juice sugars.
With this greater knowledge and understanding, alcoholic fermentation is now seen as a key process where winemakers can creatively engineer wine character and value through better yeast management and can strategically tailor wines to a changing market.
The wine industry is faced with an exciting but challenging future. Having said all the above, it is easy to understand the old saying: “wine is made in the vineyard”. The phrase is not meant literally but implies that you can’t make great wine if the raw material isn’t up to scratch. Needless to say, successful winemaking with wild yeast might be spontaneous, but should not be haphazard. In fact, making good wine with wild yeast takes greater care and attention than inoculated fermentation. Wine will ferment itself, with or without us.
Selecting and developing new commercial strains, the possibilities of using yeasts other than those in the genus of Saccharomyces, the prospects for mixed culture fermentations, the interactions among different yeast strains, and yeast breeding processes to acquire desired traits and their effect on wine sensory properties are issues that remain to be fully investigated.
Another field of interest is genetic modification. It is widely available for improving yeast strains. However, due to distrust of GM foods, the wine industry currently does not allow GM products in its production process.
Thanks to developments in DNA sequencing technologies, there is an exciting future in the field of wine yeast research and development. This knowledge will enable the generation of wine yeast strains that are robust, reliable, and capable of imparting novel, desirable sensory attributes, without necessitating the use of genetic modification techniques.
Finally, in an industry that marries art and science, winemaking plays a balancing act between tradition and scientific advancement.