Many advances in yeast selection, identification, breeding and diversity offering, with a particular focus on wild yeasts, have been made over the past few decades. But what are the implications for winemakers?
For many winemakers, and pretty much anybody who’s involved with the miracle of alcoholic fermentation (AF), the humble Saccharomyces cerevisiae remains an important ally in the cellar. This yeast, which is most commonly associated with winemaking and is often seen as little helpers in the cellar, is also extensively used for making beer and bread.
Winemakers put huge emphasis on AF as this part of the value chain is responsible for the formation of alcohol (ethanol), one of the main products of fermentation. But other than alcohol, winemaking-associated yeasts produce a vast multitude of compounds, many of which contribute to wine aroma. The list below provides a mere peek into the variety of compounds wine yeasts produce during AF and even afterwards.
Volatile fatty acids
Organic acids such as acetic acid, lactic acid and succinic acid
Sulphur compounds such as H2S
Looking at this list it’s easy to surmise that wine yeasts are indeed industrious during AF, but not all the above-mentioned compounds are beneficial. And this is where yeast-selection programmes play an important role, namely the development of commercial yeast strains that are helpful to winemakers.
Sought-after characteristics of commercial yeasts, which are typically Saccharomyces.cerevisiae strains or hybrids thereof, include the ability to ferment high sugar musts to dryness, production of favourable aroma compounds such as esters and thiols, fermentation at low temperatures, killer activity, high glycerol production, resistance to high pressure for sparkling wine production, compatibility with malolactic fermentation (MLF), resistance to high alcohol and SO2 concentrations and the ability to convert malic acid to lactic acid. Unwanted characteristics include the excessive production of H2S, sulphur dioxide, volatile acidity and succinic acid.
Yeast selection and breeding programmes mainly focus on the traits outlined above and while it may seem a daunting task, given the odd 1 500 known yeast species on earth, microbiologists focus their efforts on the 40 grape- and wine-related species of which Saccharomyces cerevisiae is historically the most studied. Debaromyces, Zygosaccharomyces, Lachancea, Torulaspora, Cryptococcus, Schizosaccharomyces, Metschnikowia, Pichia, Kluyveromyces, Hanseniaspora, Candida and Kloeckera are but some of the wine-related yeast genera scientists have been increasingly studying over the past few decades.
Much attention has lately been given to the above-mentioned yeasts, also known as the non-Saccharomyces yeasts. They are also called wild yeasts and are favoured by some as they can impart novel characteristics to wine. Some winemakers even avoid inoculation with commercial yeast strains to allow wild yeasts to make a larger aroma contribution to wine. This practice is called wild or feral fermentation and is one way to favour the activity of wild yeasts. Other options are to inoculate must with selected commercial wild yeast strains and to use yeast hybrids. One example of a yeast hybrid is Anchor Exotics, which is a hybrid between Saccharomyces cerevisiae (commercial yeast strain) and Saccharomyces paradoxus (wild yeast strain). Hybrid yeasts are particularly popular as winemakers only need to inoculate the must once and the hybrid yeast fenotype displays positive characteristics of both parent yeasts.
“Winemakers want to get the maximum from grapes regarding flavour and the expression of the cultivar,” says Neil Jolly, a senior researcher at the ARC Infruitec-Nietvoorbij in Stellenbosch and specialist in non-Saccharomyces yeasts. “This becomes essential where the grapes are from less optimal sites. Among the many tools they can use, non-Saccharomyces yeasts together with Saccharomyces cerevisiae can add another layer of aroma, complexity and body or mouthfeel. They can also be a way to express a spontaneous-fermentation character without the dangers of a true spontaneous fermentation going wrong.”
And things can go very wrong. Stuck or sluggish fermentations, off-odours and high volatile acidity are but some of the problems that could mar wild yeast fermentations.
Morgenster cellar master Henry Kotzé is well aware of the various benefits of wild or partly wild fermentations, but is also wary of wild fermentations possibly not being able to ferment to dryness. “I never do wild-yeast-only fermentations, but sometimes opt to start a fermentation wild and then later inoculate a commercial yeast strain,” he says. “And yeast hybrids give an element of added complexity typical of wild fermentations while giving fermentation security.”
Neil says there are some interesting and beneficial wild yeast srains. “My initial work was with Metschnikowia pulcherrima and Chenin Blanc, so it will always retain my interest. One of the most interesting would be Candida zemplinina (Starmerella bacillaris) as it is a fructophilic yeast as opposed to the other commercial non-Saccharomyces strains that are glucophilic.”
Torulaspora delbrueckii is another example of such a yeast, ARC Infruitec-Nietvoorbij researcher and microbiologist Rodney Heart says. “This yeast proves useful for the production of aromatic Sauvignon Blanc wines and it also forms less volatile acidity in high-sugar musts for natural sweet and noble late harvest wines.”
MLF bacteria can have several positive or negative interactions with yeast during fermentation. Based on this and personal preference winemakers might decide to initiate MLF at the same time as AF (co-inoculation) or when AF is finished (sequential inoculation). Co-inoculation with Oenococcus oeni and/or Lactobacillus plantarum strains has been well received since its introduction to the South African market and has several advantages, says Heinrich du Plessis, another ARC Infruitec-Nietvoorbij researcher and microbiologist. “Both inoculation strategies have advantages and disadvantages. Certain yeasts and MLF bacteria combinations produce wine with more flavour and better quality when used as co-inoculation, while others produce better results when used as a sequential inoculation. With co-inoculation you do not have to wait for alcoholic fermentation to be completed, there is a decreased risk of stuck MLF due to lower ethanol or SO2 levels, and MLF generally completes in a shorter time and has a bigger impact on the sensory profile of the wines compared with sequential MLF. On the other hand the risks of co-inoculation are that yeast and MLF bacteria incompatibility can cause delayed MLF. And in nutrient-limited juice lagging or stuck alcoholic fermentations may result. Another result could be wine with a different aroma profile to what was initially expected.”
Winemakers traditionally hold AF – and ostensibly to a lesser degree MLF – in high regard as this is a winemaking step that adds numerous aroma compounds and other beneficial compounds to wine. It is also the only step during winemaking where living organisms are used to aid the winemaker. And so the saying “You are not alone” is never more apt for a winemaker than during harvest.