Spontaan vergiste wijn. Daar komt geen fabrieksdesem aan te pas. Maar het meeste druivensap moet een handje worden geholpen bij de fermentatie. En daarvoor zijn ‘gistculturen’ ontwikkeld. Die gebruiken de meeste wijnmakers. En om te voorkomen dat de wijn daarmee te weinig complexiteit krijgt, heeft de gistindustrie weer nieuwe oplossingen bedacht. Die vergisting is onderwerp geweest van het examen Master of Wine dit jaar. Luis Alberto, die ook de website The WINEhub bestiert, heeft als student aan het Londense Institute of Masters of Wine uitvoerig antwoord op die vraag gegeven. Dat staat nu kersvers op zijn eigen website. Geen eenvoudige stof voor de buitenstaander, maar voor liefhebbers uiterst wetenswaardig. Luis had als mentor bij dit werk Patrick Farell, Master of Wine. Om de leesbaarheid van het stuk te vergemakkelijken, hebben wij het van tussenkoppen voorzien.
“Examine the advantages and disadvantages of using cultured yeasts in wine making.”
This question was on the INSTITUTE OF MASTERS OF WINE Examination of 2012 (Theory Paper 1 – Part1 – Section B)
* Disclaimer: I had an appreciable amount of help from my mentor – Patrick Farrell, MW & #winelover – who took the time to revise it and give me guidance. Thanks Pat!
Saccharomyces is the primary commercial microbe in the world and is active in the production of beer, wine, whiskey, bread and even, phramaceuticals. In the 1950’s the American wine industry worked at selecting strains that would complete fermentations as stuck fermentations were a big problem for the industry. The alcoholic fermentation by Saccharomyces cerevisiae and other yeast species is a (some would argue “the”) critical element in winemaking. The natural attributes of the grapes will be either retained or degraded, depending upon the speed, temperature and completeness of fermentation. All these factors are deeply related to the selection of the yeast that is used and much has been made about yeast strains by the commercial yeast industry, including claims of improved flavor and quality. This essay will consider the advantages and drawbacks of using cultured yeasts in converting grape sugars to alcohol, carbon dioxide and flavor compounds.
In order to better understand cultured yeasts, it’s necessary to consider their counterparts as well: spontaneous fermentations. Many good things have been said about spontaneous fermentations in the wine industry. However, there are two myths involving them that need to be addressed: The first is that all the yeasts that will work during a spontaneous fermentation come from the vineyard. The second is that indigenous yeasts (mostly of the Kloeckera, Candida and Hanseniaspora genera) are able to finish fermentation to dryness. The University of Geisenheim has recent studies that prove that the vast majority of yeasts in a spontaneous fermentation come from the winery (from last year’s fermentation) and not from the vineyard. Those studies also show that the yeasts that finish the fermentation are from the Saccharomyces genera (or strains of closely related species such as S. bayanus), as apiculate yeast species are only viable until alcohol levels reach around 4% (or must density drops to 1.070–1.060). Also, a dominant strain of Saccharomyces (based upon growth factors, genetics, timing and killer factors) will take over the fermentation and colonize the winery. This strain will finish the fermentation. The dominant strain of Saccharomyces in the vineyard will probably not wind up being the dominant one in the winery. However, there’s one possible exception to this situation: Ripe grapes are brought into a brand new winery in which there had never been a fermentation. If the grapes are allowed to spontaneously ferment, one gets a true wild fermentation.
Some advocates of spontaneous fermentation will argue that improving the quality of the wine is not their main goal. They will do it because the wine is more “natural” (less interventionist techniques) and that’s why many organic and biodynamic producers advocate this practice. So, regardless of the “musty” character that some wines can develop and the risk of a stuck fermentation, they will keep doing it. And, from a marketing standpoint, the winery will collect some extra dividends by producing a “natural product”. However, the climate is changing and the average mean temperature in many wine regions is already a couple of degrees higher than it used to be a few decades ago. The consequence is that some producers who, “philosophically” (or for marketing reasons), wanted to avoid inoculation, now are having to do so because of the higher sugar levels that are found today in the grapes. Not to mention that, while wild yeasts can play a positive role in the character of the product through ester formation, some they may also produce higher concentrations of undesired characters such as acetaldehyde, hydrogen sulﬁde, and volatile acidity.
Another misconception is that the choice between indigenous and cultured yeasts is a clear-cut decision. The simplified version defines only two options at hand: Either the winemaker can choose to let the grapes start the fermentation naturally (spontaneous fermentation) or, as most of the industry does today, winemakers can choose to inoculate the must with a commercial yeast strain. The truth, though, is that there are a few variations on the theme for both. On one hand, when dealing with a spontaneous fermentation, the winemaker can use indigenous yeasts that were successfully used previously in another fermentation and then inoculated in a new tank (the opposite can also be done – the tank is de-vatted and refilled with fresh juice). On the other hand, when dealing with cultured yeasts, the juice can be inoculated with a single strain of cultured yeasts (with a known and useful property), or with multiple strains of cultured yeasts of Saccharomyces cerevisiae such as Champagne, Prise de Mousse, Montrachet, etc; and, possibly, non-Saccharomyces. And, yet another possibility, a spontaneous beginning of the fermentation by indigenous yeasts followed by the inoculation of alcohol-resistant strains of the Saccharomyces genus after a couple of days. This is what Morgan Clendenen, the winemaker of Cold Heaven Cellars in California’s Central Coast, is doing with success in her winery. And she does so to avoid stuck fermentations while still getting the esters from the early vineyard yeast strains.
Both methods have advantages and drawbacks. With indigenous yeasts winemakers get something different every year, the character of the vintage tends to be exacerbated. (This also varies with place. If there is little climate or vintage change, then there is little change in yeast. If there is rot and wet weather at harvest, then everything changes with botrytis) It means that there will be more differentiation between vintages, but this could also be considered a drawback. Some consumers are put off by the wine’s unpredictability, while others would praise exactly this trait. It can also be said that spontaneous fermentation is a tool to add complexity to the wine. Everything else being equal, the result is a wine with more softness and finesse, combined with a higher level of aromatic complexity. This extra degree of complexity is derived not only from the array of by-products from the different native yeast strains, but also because of the fact that indigenous yeasts may take a long time to get the fermentation started (it happens because their initial populations are very small compared to a fermentation that is started with inoculated yeasts). However, a major concern when a producer doesn’t use inoculated yeasts is that the fermentation may take a long time to finish. The size of the producer is critical here. Freeing up the tank for the next batch of grapes is a top priority for a large winery, but not so much for a small producer. The biggest problem though and the reason why winemakers decide to inoculate: The risk of a fermentation getting stuck and the possible formation of off-flavors by bacterial spoilage. The risk of that happening is much higher than when the must is inoculated with cultured yeasts. An indigenous yeast fermentation that goes bad will produce a wine very low in quality and, most likely, unsellable.
Despite the large number of nice stories about wild yeasts in the wine media, the truth is that inoculating grape juice with a reliable strain of Saccharomyces cerevisiae is still the choice of the great majority of wineries. They do that for a myriad of reasons: Ask a winemaker on why that choice is made, and the ability that cultured yeasts have to very rapidly start and finish the fermentation will most likely be at the top of the list. Some will most likely mention that the production of undesirable by-products (sulfides and volatile acidity) is minimal and a “cleaner” and “fresher” fruit will be at display in the wine. Getting specific traits in the wine, such as a defined varietal aroma (through the production of specific esters), glycerol and mannoprotein production, cold tolerance (for white wines) and ethanol tolerance (for wines with alcohol potential higher than 15%); will probably be mentioned as well. With that been said, André Ostertag, a winemaker in Alsace that produces high quality wines, has been a biodynamic producer since 1998 and he never uses cultured yeasts. It comes to show that great wines can be made without inoculation as well.
Some supporters of spontaneous fermentation choose not to inoculate, because they believe that the wine made with cultured yeasts is not “natural” and “traditional”. The wine is not made the way it was made in the past, but rather based on science. The most advanced technologies and interventionist techniques are used. As Paul Draper, the winemaker at Ridge Vineyards, who has used natural yeast at Ridge for virtually 100% of the fermentations for 28 years said, “Originally, we did it because philosophically, it allows us to step out of the way and guide the development of the wine instead of thinking of ourselves as creators”. Others will argue that the wine will lack complexity, but this problem can be solved by the use of a mixture of several yeast cultures. So, aside from philosophical and marketing issues, cultured yeasts don’t seem to have any technical problems. The exception would be that some yeast strains could cause the must to reach high temperatures (in fermentations in barrel or in another type of tank with no proper temperature control). If it happens, it may lead to volatilization of the aromatic components of the wine and even lead to a stuck fermentation.
There are many winemakers that choose not to inoculate today. The payoff for these risk-takers is an increased quality of the wine if everything works to perfection. However, if what is at stake is to finish the fermentation to dryness and, as quickly as possible, then cultured yeasts are the choice for undoubtedly the greater part of the wine industry. To mitigate the problem of lack of complexity, the commercial yeast industry is bringing new solutions. In fact, this is only a complex matter because, when it comes to wine, people tend to make choices based on marketing strategies, philosophical approaches and even on mystical beliefs (some people mistakenly view it near religious terms). In the end, however, cultured yeasts are still the reliable and effective way to perform fermentations’.