Magical Yeasts

If I was ever obliged to write a real thesis or research paper in the context of wine I think I would chose yeasts as the subject.  Perhaps for my own sanity it is as well that I have never undertaken such a task, but I have always found yeasts fascinating.  Here is a fungus on which simply everything in the glass utterly depends. 

Yeasts are sourced from many places. To some limited degree they come from the grapes in the vineyard. Each year fresh yeasts arrive though the goodness of wasps, that transfer yeasts into the grape when they bite into them. Yeast from the actual surface of the berries - mostly but not only of the Saccaromyces type - make a relatively small contribution to the overall yeast population that will involve itself in fermentation, but yeasts can be cultured from these wild vineyard yeasts. Secondly, yeast are also resident in the cellar itself - ambient in the air and on surfaces and tools - and in this capacity can be present of course over many years. Sourced from vineyard or cellar these are described as “wild” or “ambient” yeasts and can be of many types in addition to from the Saccaromyces genus - Kloeckera or Candida or Pichea for example. They can also be Brettanomyces, which can be ominous ( and quite hard to get rid of if unwelcome residents in the cellar). 

But yeasts can also be purchased in packets as ‘commercial yeasts”, which are nurtured elsewhere in large quantities and have particular properties as to the flavor or texture imparted to the wine or its ability to ferment must at high alcohol levels, which may de desirable.  Commercial yeasts of course are far more reliable in achieving the objective, but impart no local identity to the wine. Some would say they do worse - actually remove it.

It may be worth noting that when a winemaker says he uses “natural” yeast, this does not necessarily mean he does not add yeast to the must - but that what is added is not a widely available commercially manufactured yeast.  Yeasts that have been extracted from the local environment can be isolated and cultured, so they can be added to the must to initiate ad achieve fermentation. In larger wine regions, viticultural research departments of local Universities help create these. This is very common practice - Barale in Barolo for example do this. Some of these yeasts are available for selected purchase by others. These yeast of course, so the argument runs, retain the local terroir. Alternatively, what is added could be yeast from an already fermenting vat - a pied de cuve, previously inoculated with such local yeast. Simply add some already fermenting wine to kick start the fermentation. All these yeasts are described as “natural” but are actually added to the must. Those who do not add anything - relying on yeasts on the grapes themselves and ambient yeasts in the cellar to do the job, are said to use “spontaneous fermentation”. Spontaneous fermentation is much admired by some but of course comes at considerable risk.  Until you have a lot of experience, you simply don’t know what you are getting. All yeasts have some by products - esters, aldehydes and types of sulphur compounds. These are volatile compounds and can impact wine aromas especially. Some can be nasty. 

Yeasts of course turn sugar into alcohol and carbon dioxide that reaction gives off heat. The important thing to know is that all yeasts fail to perform this function under certain conditions - such as when there is too much alcohol in the solution or when there is too much Sulphur Dioxide present - but that the particular conditions that prove toxic to the yeast vary by yeast type. Critically, most yeasts actually can no longer perform the chemical transaction when in alcohol levels of much above 3.5% alcohol by volume (ABV). One type can only function when alcohol levels are in the mid range - between say 6% and 9% by ABV. The one yeast that can operate at higher ABVs is the one we are all taught to know in WSET class - the “sugar loving” yeast Saccaromyces Cerevisiae. This yeast can function until the alcohol solution gets to about 15% ABV ( as a technical matter It has been shown to be able to tolerate even higher ABV). This means that many of the yeast types in the vineyard and ambient in the cellar contribute to the alcoholic conversion only for about the first quarter of that process, after which the alcohol level is too toxic for them to continue. At that point - and likely before - Saccaromyces Cerevisiae becomes the dominant yeast, taking the fermentation from that early stage all the way to its conclusion. Saccaromyces Cerevisiae, of course could be a type of ambient yeast, influenced by the local aspect of vineyard and cellar.  It is just worth noting that the many other strains of yeast present likely contribute only at the early stage of the fermentation. So if a multitude of varying ambient yeasts are to contribute to a more precise sense of terroir, it is mostly in that early stage of fermentation that this impact occurs.  Knowing this may influence a winemakers decisions - such as not to do a long pre fermentation cold soak under a lot of sulphur. 

It is said that the yeast’s origin makes a great deal of difference. Many believe for a wine to be a true expression of terroir, there can be no commercial yeasts involved. For these observers, not using commercial yeasts is a point of real differentiation in wine quality, in the same way as not filtering or not chaptalising or not using too much new oak is to others. Natural yeasts are at the very core of a wines identity and authenticity. It is not hard to find winemakers who sign up to the importance of using natural yeasts - Jacques Selosse in Champagne is a vocal example, just to name one. There are some importers who will not import wine made with commercial yeasts - Louis Dressner is an example, I believe. There are  journalist  and critics who feel very strongly on the point - Andrew Jefford is one. Some wine stores basically will not stock wines which have seen commercial yeasts - Chambers Street Wines in New York for example. There are many instances that illustrate the strength of conviction that is held on the issue across the whole spectrum of the wine industry.

In support of the contention that the individual yeast impact the resulting wine, there is chemical evidence that shows considerable variation in the effect of yeasts when measured by the amount of by-products they produce during fermentation. The substances involved certainly suggest a relevance to the end product - including for example, glycerol and types of acid and phenols. And there ei sas ou would expect a great deal of empirical evidence. There are simply too many good makers and good tasters of wine who hold this opinion on the importance of natural yeasts to be wrong. It certainly seems to be the case that grapes from Gevrey Chambertin, for example, vinified in a cellar that has for many years only vinified grapes from Chambolle Musigny, will be influenced by those ambient yeasts in the Chambolle cellar to create a wine that is not quite the same as when those same Gevrey grapes are vinified in a cellar of long standing in Gevrey.  Empirical evidence for this comes from growers who have on occasion swapped grapes between them, of which over the years there have now been many instances. Such is the influence of wild and ambient yeasts.  And M. Millet, after 35 vintages of vinifying separate parcels of Musigny with ambient yeasts, asserted in an interview with Jasper Morris for 67 Pall Mall in June 2021 that each fermentation vat can produce a slightly different wine even though the soil from which the grapes are sourced is almost identical. He attributes these variations to the unique interaction of the yeasts on the must in each fermentation vat. Each fermentation with ambient yeasts is unique and produces subtly different outcomes.

Such a simple organism. Still not completely understood. Producing such magical results. 

Picture Credit - Melba Photo Agency / Alamy Stock Photo