No silver bullets
South Africa is considered a warm viticultural climate. As a result, grapes arriving at the cellar can have high sugar levels that can lead to alcohol levels above 14%. Depending on the rest of the wine matrix, such high alcohol concentrations can potentially be perceived as being negative for final wine quality, especially if it promotes an unwanted “hotness.” In many cases the wines are big and bold enough to support such high alcohol concentrations. However, consumer purchasing behaviour can be negatively influenced by the alcohol they see on the label.
As a result, winemakers and researchers have been searching for the Holy Grail of alcohol reduction for at least two decades. Various avenues have been explored with varying degrees of success. The bottom line is that to this date there is no one silver bullet to magically produce 2-3% alcohol less from a given amount of grape sugar. The good news is, however, that certain practices that can bring about small reductions in final alcohol concentrations, do exist. Whereas it is not worthwhile getting out of bed for a meagre 0.3% alcohol reduction, combining strategies that all bring about approximately 0.3% reduction can eventually add up to a combined effect of 1% less, or even more.
When the usual suspect offers very little joy
For years the focus of research was on Saccharomyces cerevisiae, the dominant wine yeast in fermentations and the most readily available in a commercially dried form. Unfortunately, due to S. cerevisiae’s quest through the millennia to be top of the yeast food chain, it has evolved to become a highly efficient sugar to alcohol converter. Studies by academics as well as commercial yeast companies, comparing commercial wine yeasts under the same fermentation conditions, have demonstrated that there are very little differences in sugar to alcohol conversion rates. In addition, a Winetech funded study at the Department of Viticulture and Oenology (DVO), Stellenbosch University investigated the effect of winemaking conditions (pH, temperature, YAN) on wine yeast sugar to alcohol conversion, and found it has very little effect.
It is worthwhile mentioning though, that there are some commercial S. cerevisiae yeasts that constantly produce slightly less alcohol than the rest of the pack. The reason why these yeasts produce less alcohol is because they also have alternative metabolic pathways during fermentation. This includes greater yeast biomass formation or the formation of higher amounts of other metabolic end-products, such as glycerol, acetaldehyde, pyruvate, acetic acid, malic acid and succinic acid.
Some of these “slightly lower alcohol” commercial yeast strains have been generated through non-genetic modification (GM) techniques to produce less ethanol. To compensate, their carbon metabolic flux is redirected towards the production of various organic acids, which may result in the acidification of the wine (this can be a good thing, depending on the wine). Nevertheless, the success rate in terms of alcohol reduction is condition-dependent and often limited to around 0.5% (sometimes more, sometimes less, but it’s better than nothing in some cases).
Glycerol has been a very popular target for researchers to try and obtain S. cerevisiae wine yeasts with a higher metabolic flux towards it and the accompanying lower alcohol production. Alas, no success was to be had. In most cases the yeasts also produced higher amounts of unwanted compounds such as acetic acid.
One research project, also funded by Winetech at the DVO, was indeed very successful in creating a wine yeast that produced less alcohol and no unwanted by-products. Genetic modification techniques (using no foreign DNA) were used to obtain a yeast that shifted some of its sugar metabolism towards trehalose, a storage sugar that has no organoleptic qualities. Unfortunately, this yeast will live out its days stored in a freezer in a laboratory at the Department of Viticulture and Oenology, Stellenbosch University due to the general unacceptability of the use of GM techniques in the winemaking process. The project paved the way for researchers to try and apply non-GM techniques to possibly obtain a similar result.
What about aeration?
Yeasts have two types of metabolisms, respiration (if oxygen is available) and fermentation (if oxygen is not available). S. cerevisiae almost always ferments, even if adequate amounts of oxygen is available. This is called the Crabtree effect and S. cerevisiae is described as Crabtree positive. No amount of aeration during a commercial wine fermentation will enable S. cerevisiae to respirate and produce predominantly CO2 from the sugars consumed.
Aeration during fermentation does however allow faster evaporation of alcohol, which makes it one of the strategies for lowering final alcohol levels. Anecdotal evidence from around the world has shown that open top fermenters tend to give lower final alcohol concentrations when compared to closed systems. The same goes for aerated pump-overs compared to closed system pump-overs.
The role of temperature
Alcohol is less volatile the lower the temperature. So, if one fermented with Yeast X at 20°C in one tank and 11°C in another tank, it will look as if its conversion factor miraculously increased from the warmer to the cooler tank, which of course is not the case. It’s simply a case of higher evaporation in the warmer tank. What has become a common practice over the last few years in the handling of high sugar red musts, is to ferment at “lower” temperatures (23 – 26°C) in order to prevent stuck fermentations. This is an extremely wise practice since it not only improves the alcohol tolerance of yeasts but also improves fructose utilisation. It does increase perceived sugar to alcohol conversion though, due to less alcohol evaporation.
But wait! There’s light at the end of the tunnel…
This is where non-Saccharomyces yeasts come in. Various studies around the world have demonstrated that some of the more commonly found non-Sacch wine yeasts do have lower alcohol conversions (anywhere from 0.3 – 2% less). This can be attributed to mainly two traits of these yeasts. Some of these yeasts are Crabtree negative, meaning if it has enough oxygen it will respirate (where no alcohol is formed) instead of ferment. How much this can contribute to lower alcohol production during commercial winemaking is very yeast strain dependent. The amount of aeration needed for a specific yeast to respirate might be too much and can cause oxidation of phenolic and aroma compounds in the must.
A recent study demonstrated that with controlled aeration during the first 72 hours of fermentation, immobilised Metschnikowia pulcherrima yeast cells, in a sequential fermentation trial, produced 1.38% less alcohol without adversely affecting wine quality.
The second reason why some non-Sacch yeasts can produce less alcohol from a certain amount of sugar is because they form more secondary by-products during fermentation. In general, non-Sacch yeasts tend to form more by-products during fermentation than S. cerevisiae. As with S. cerevisiae, the by-products include glycerol, acetaldehyde, pyruvate, acetic acid, malic acid and succinic acid. They can also have metabolic pathways S. cerevisiae doesn’t have, for instance Lachancea thermotolerans produces lactic acid from pyruvate, with the amount produced being strain dependent.
Non-Sacch yeasts also produce other products of oenological interest such as extracellular enzymes, that can be very low or non-existent in S. cerevisiae. This is the reason why many winemakers find spontaneous fermentations so rewarding, since it can increase the organoleptic complexity of a wine.
Spontaneous fermentations can have a dark side as well when your resident non-Sacch population happens to be high “other stuff” producers and you end up having wines high in acetic acid and acetaldehyde. Also, not necessarily a problem these days, you can just develop a fancy story how you believe in not “intervening in the natural process” and together with a provocative label the wines can sell for a premium price (or not).
Our local expertise
If you don’t want to gamble with finding out whether your resident non-Sacch population is evil or not, there is always the option of using commercial non-Sacch yeasts. Research conducted at Nietvoorbij (Dr Neil Jolly) and the DVO (Dr Evodia Setati, Dr Debra Rossouw, Prof Florian Bauer, Prof Benoit Divol), with and without Winetech funding over many years, have led to an in-depth understanding by our researchers of non-Saccharomyces wine yeasts and their behaviour in mixed culture fermentations. In the case where the research was sponsored by a commercial company, the knowledge has been transferred to the commercial company for them to better advise winemakers.
The good guys
The following non-Saccharomyces yeasts, that are also commercially available, have been demonstrated to have lower sugar to alcohol yields: Lachancea thermotolerans, Torulaspora delbrueckii and Metschnikowia pulcherrima.
Starmerella bacillaris (also known as Candida zemplinina) also demonstrates a lower sugar to alcohol ratio during alcoholic fermentation but it is not currently (at the time of writing) commercially available. It has the added advantage of being a fructophilic wine yeast. A suitable candidate for commercialisation has been identified through a Winetech funded study at Nietvoorbij and a commercial company is currently evaluating the yeast for commercial production.
It is important to realise that just as in the case of S. cerevisiae there can also be strain differences within species. In some species, the strains can be quite similar and in others very diverse. It is very important for winemakers seeking to obtain a lower alcohol conversation from a commercial non-Sacch strain to make sure that the strain they select does indeed have this trait. Many non-Sacch yeasts have been commercialised for their specific contributions to wine sensory qualities and wine stability and not necessarily for alcohol reduction.
So, here’s an idea
If it is your desire to produce wines with lower alcohols this upcoming season it is worth your while to trial the combination of a non-Sacch and S. cerevisiae yeast, both known to have a slightly lower than average alcohol yield. Combine this with increased aerated pump-overs (red) and you might just end up with lower alcohol than expected.
Please refer to the series of articles published by Dr Evodia Setati in the September to December 2020 Winetech Technical in WineLand Magazine for detailed information on five specific non-Sacch yeasts.
The author expresses her gratitude for Prof. Benoit Divol of the South African Grape and Wine Research Institute (SAGWRI), Department of Viticulture and Oenology, Stellenbosch University, for critically reviewing the article for scientific accuracy.
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