I'd like to spend some time talking about various methods of wine production, decisions winemakers make, and their ramifications on the final product: The wine that comes out of the bottle when you pull the cork (or twist the Stelvin, or pop the glass cap, or.. Well, whatever). I want to start this discussion with the principal player in winemaking, the true winemakers, the yeasts. Today I'm going to post some half-coherant ramblings on the differences between spontaneous fermentation and commercial yeast fermentation.
To quote (and not for the last time) the great group-wisdom conglomerate of the 21st century, Wikipedia: “Yeasts are eukaryotic micro-organisms classified in the kingdom Fungi, with the 1,500 species currently described estimated to be only 1% of all yeast species.” Fungi are, of course, that little-understood third branch of biology, not quite plant nor animal. Out of those 1,500 known species of yeast, one is of primary importance to fermentation: Saccharomyces cerevisiae. What does this little beauty do? Well, it converts the sugars existant in grape juice (or must, as is the winemaker’s term) into wonderful, intoxicating alcohol. Simple enough, yes? But even here, we run into some very complicated decisions for the winemaker. The first of these being: Spontaneous fermentation, or inoculation?
As discussed previously on this site, “spontaneous fermentation is exactly that: fermentation that occurs spontaneously, as opposed to occurring due to an inoculation of commercial yeast strains into the wine by the winemaker.” This is done for a plurality of reasons. For starters, it is the original method of winemaking; thousands of years ago, long before the development of commercial yeast strains (or even an understanding of what yeast was), fermentation just happened. This no doubt led to a somewhat mystical viewpoint on intoxication, as evidenced by the many mythical intoxication gods: Dionysus, Silenus, Ninkasi, et cetera. Secondly, many winemakers believe that it adds more complex elements to the wine; when doing spontaneous fermentation, it is actually several different species of yeast that start your fermentation. However, by the time the fermentation is complete it’s sacchararomyces cerevisiae (SC) that’s finished the job. That’s because it’s a species of yeast particularly well-suited for survival in the 3.5-4.0 pH that exists in (most) wine. Therefore, all the other species of yeast- such as Klockera, Zygosaccharomyces, et cetera- die off before the spontaneous fermentation is complete.
One of the fears with spontaneous fermentation is that the non-SC yeasts will produce off aromas in the wine. While complexity can be a good thing, it is in essence only good if it’s appealing; no consumer (or at least very few) enjoys the smell of rotting garbage in their Cabernet. Another fear is that spoilage bacteria will set in as well. When doing spontaneous fermentation winemakers must be very careful with their use of sulphur dioxide- the infamous sulfites found on the label of every bottle of wine sold in the United States.
Sulphur dioxide is a wine additive used for its antioxidaisic and antimicrobial properties. When a winemaker is doing a conventional fermentation with cultured yeast, they will generally add a certain amount of SO2 after crushing but before fermentation. The reasoning behind this is that it will suppress spoilage bacteria in the must. They will then add a mixture of cultured yeast and nitrogen-rich yeast nutrients, which will be strong enough to begin fermentation while the spoilage bacteria are still weakened. The bacteria then cannot survive in an increasingly alcohol-laden environment, and the wine (ideally) comes out clean and without spoilage imperfections.
You can see some positive and negative attributes to both spontaneous fermentation and inoculation already. However, there are many more. Another reason that some winemakers like to rely upon spontaneous fermentation is the oft-used and sometimes-maligned concept of terroir.
Spontaneous fermentation is supposedly one way for a winemaker to allow a vineyard’s terroir to express itself in his or her wines, and thus to push the wine further toward the natural end of the spectrum. The reasoning behind this idea is that the yeast involved in spontaneous fermentation is native to the vineyard; that is why the process is sometimes described as ‘native yeast fermentation.’ This is in harmony with the concept of keeping the vineyard as a closed cycle, introducing no man-made pollutants into the production of its wine. This is one of the major tenets of biodynamism, a school of thought that many works in harmony with the goal of expressing terroir.
If any winemakers are reading this, I'd be curious to learn their opinions on this subject (or anyone else's opinions, for that matter). Yeast selection is an area where a winemaker can really geek out, and I'm still trying to expand my knowledge in this area.