In Alcoholic Fermentation and Yeast

Some time ago I posted a comment on a site of a chap who is very pro natural fermentation. My comment was very neutral, in my opinion, but some people perceived it as pro using commercial wine yeasts. The fact that I work for a wine yeast manufacturer was like a red rag to a bull for the “naturalists” since “I have a commercial interest in people not doing natural fermentation.” True, I would like to retire comfortably, but in this case I felt I gave a very good recollection of the facts.

The pro’s and cons of both types of fermentations. You get to choose. You could always try both and see what works the best for you – that is if you still have your job the next vintage after you discovered natural is not such a good option. Natural fermentation means you utilise the yeast naturally occurring on grape skins and on your winery equipment to conduct the fermentation for you. Some people do true natural ferments, i.e. they have never introduced commercial yeasts in their cellars. Others do un-inoculated fermentations, meaning they have used (or still use for some wines) commercial yeasts.

What are the advantages of natural fermentations in all their deviations?

There are approximately 16 yeast species associated with winemaking. The yeast most commonly associated with winemaking and also called the wine yeast is Saccharomyces cerevisiae. The so-called “wild yeasts” or I believe they are called “feral yeasts” in Australia, comprise such yeasts as Kloeckera, Hanseniaspora, Pichia, Candida and Torulaspora, to name a few. When the grape skin is broken during crushing the juice is exposed to the yeasts on the skin and the yeasts will all start growing depending on the size of their populations. The strongest fermenter will eventually dominate due to the selective pressure in winemaking conditions.

The most important selective pressure is alcohol and it is usually the more alcohol tolerant S. cerevisiae that completes the fermentation. However, many yeasts can take part in the fermentation before the first 5% alcohol is formed. Each yeast will leave its unique mark in the wine. Yeasts are known to produce flavour active compounds during fermentation. Yeasts can also modulate grape flavour compounds, converting odourless compounds to very flavour intensive compounds in wine. An example of such aromas is passion fruit, grapefruit and boxwood aromas associated with Sauvignon blanc varietal character. So, the more yeasts taking part in the fermentation, the bigger the possibility of a broader range of aromas.

That is, if you have the good guys. And this is exactly where the risk associated with natural ferments comes in. The chances of you having the optimal mix of yeasts on your grapes year after year are very slim. Some people do, and they make exquisite wines. I have to add you can also only make exquisite wines with exquisite grapes. The chances that you have alcohol tolerant enough yeasts on your grapes year after year are even slimmer. Many natural fermentations lead to stuck fermentations nowadays because we just don’t pick grapes at the sugar levels we used to in the previous century. Blame it on global warming, Robert Parker, the Australian success, whatever. The reality, even Bordeaux is now picking at 24°Brix. Commercial yeasts are mostly yeasts isolated from nature that proved to be alcohol tolerant enough to complete fermentations. The risk of leaving things up to nature / fate is therefore that

  1. Your fermentation has a very good chance of getting stuck if you pick your grapes above 23°Brix.
  2. The specific aromatic profiles of the yeast mix might not be optimal for your specific style of wine and as a result you bottle un-tapped potential.

The “naturalists” feel you should bottle this untapped potential since it is a true expression of your terroir that year. Try selling a bottle of untapped potential in this economic climate, with the competition in the wine world as is. The consumer wants a wine that tastes nice now. Not a wine that has the potential to maybe taste nice in the future. So the bottom line here is that natural fermentation has the potential to be way superior to inoculating with a single commercial yeast. Some wineries are very lucky to have the ability to utilise natural ferments to help them produce exquisite wines. They are, however, in the minority. The advantages of using commercial yeasts – I can sum this up in three phrases: fermentation reliability, fermentation and style predictability, repeatability. With natural ferments you have none of this.

Oh yes and you keep your reputation and job as a winemaker. The winemaker from Chateaux Margaux summed it up very well when asked earlier this year by a group of wine journalists if he does natural ferments. His answer was “Absolutely not. I inoculated each tank with a commercial wine yeast. With the expectation the world has of Chateaux Margaux, I simply cannot risk a natural fermentation.”

The reality is that the majority of the best wines in the world are made with commercial wine yeasts. Commercial yeasts have been selected specifically because they can withstand the extreme conditions of fermentation, specifically rising alcohol levels. They are also selected based on their ability to optimise wine aroma and flavour. In the case of white winemaking your choice of yeast is critical – especially S. blanc – but this is a topic for another day.

Chances that you have the exact mix of yeasts in you S. blanc vineyard to maximise varietal expression are very slim –probably only slightly better than your chances to win the lotto. Then again if you don’t want to maximise varietal expression and bottle less of a quality wine than you could have, then I guess it is your prerogative. Personally, I would like to make the best wine I possibly could, keeping the process as natural as possible.

That brings us to what is natural and what is unnatural when it comes to winemaking? Well, that is also a whole new discussion. Using sulphuric acid to adjust your wine’s pH, is for instance “un-natural” in my opinion. I would stress about that if you feel the need to stress about the naturalness of wine. Adding commercial yeasts that originated from grapes in the first place, is not unnatural. The potential shortcoming of using only one commercial yeast to conduct the fermentation is that the wine can be less complex than a successful natural fermentation where many yeasts made a contribution to wine aroma and flavour. However, using the right commercial yeast by far outperforms “less optimal” natural ferments.

The good news is that researchers are currently tapping into the advantages of natural fermentations. A vast amount of research is being done by various institutes around the world on non-Saccharomyces yeasts. Two companies have just commercialised Torulaspora delbrueckii. By inoculating your grape juice with Torulaspora and a day later with S. cerevisiae you obtain enhanced complexity in both mouthfeel and wine aroma – so they claim. Two companies have commercialised inter-specie hybrids, meaning you stay with the genus Saccharomyces but the species can be hybrids between cerevisiae and paradoxus, bayanus, cariocanus etc. These hybrids also introduce new possibilities into the wine aroma pool that in the past were only obtained with successful natural fermentations.

The commercialisation of wild yeasts / interspecie hybrids provides winemakers with a tool to safely reap the benefits of a natural fermentation without the risks, i.e. offer them reliability, predictability and repeatability.

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