The subject of natural wines certainly is among the most trendy ones in the world of the beverage of Bacchus. It is very debated, sometimes talking nonsense, fighting ridiculous ideological wars, there are factions of wine lovers and taster who will not have a wine in case it does not have the imprimatur in the label or in words it is a “natural wine”. What a natural wine is, as a matter of fact, no one can give an answer or define it clearly. In general terms - without any pretension of defining it - a natural wine is produced with the essential intervention of man, both in vineyard and in the winery, while limiting, or even avoid, the use of substance of chemical origin, by favoring - at the same time - natural practices and methods. Many forget, like to say, the only final and natural process about grape juice, even in case the intervention of man is limited to the essential, is to become vinegar. Allowing a must to ferment without interfering in any way - that is like “Nature” wants - means allowing bacteria and yeast to do the job Nature assigned to them, with the unavoidable result of making vinegar.

 Avoiding the development of vinegar during the production of wine requires the indispensable intervention of man who, by using technology and the control of chemical-biological processes, avoids the contamination and development of bacteria, as well as faults. The explicit goal - and certainly noble - of the production of natural wines is producing a beverage capable of expressing the potential and quality of a territory, considered as a whole of many and different factors. Among these elements, besides grapes, territory and micro climate, there is also yeast, sometimes a controversial element, naturally found in the air and which settles on the skin of grapes. Yeast, just like all the other living organisms and beings, fight a “war” for survival, with the result the most weak or minority organisms are being overwhelmed by the stronger species which become the dominant one. The same happens - as a matter of fact - during the fermentation of wine: an unrelenting struggle in which the stronger species win therefore becoming the dominant ones responsible for the effects of fermentation.

 Yeast, during fermentation, does not “demolish” sugar only, transforming it into ethyl alcohol and other substances, including carbon dioxide and sulfur dioxide. During fermentation, yeast, as a consequence of its activity, also gives wine specific organoleptic qualities, and can modify - even drastically - the olfactory and gustatory qualities of the beverage. The organoleptic qualities produced during fermentation are defined “secondary”, whereas the ones defined as “primary” are about the specific qualities of grapes and “tertiary” ones are about the result of the aging process. The role of yeast in the development of aromas in a wine is however fundamental and, it is not by chance, it is said primary aromas are hidden in the grape and it is the fermentation which reveals them”. Every kind and species of yeast, as well as bacteria, during their activity develop different aromas and flavors and not all the times these “organoleptic sensations” are considered positive for the wine.

Saccharomyces Cerevisiae: the most used yeast species used for the fermentation of wine

 The “colony” of yeast and bacteria getting in contact with the must and that will transform it into wine, is not easily determined, to be precise, is determined only by a specific laboratory analysis. On the skin of grapes are naturally present the very same yeast and bacteria found in the area where the vineyard is being cultivated. As the grape is crushed, essential operation in order to get the must, these yeast and bacteria get in contact with grape juice, therefore starting the fermentation process. Every yeast species, besides giving wine specific organoleptic qualities, also need proper conditions for its biological activity, among them, there is temperature. Thanks to the research on fermentation and on biological activity of yeast - research which had a fundamental contribution with the work of Louis Pasteur - the control of temperature during fermentation allows, as a matter of fact, the full activity of some yeast and the inhibition of others.

 The main yeast strain responsible for the fermentation of the must is the famous Saccharomyces Cerevisiae, the same used for the fermentation of beer. It is not the only yeast strain responsible for the production of wine, but - as a matter of fact - it is the one mainly favored during fermentation. This particular yeast is the same selected in laboratories for their so called “selected yeast”. Saccharomyces Cerevisiae is also found in the so called “indigenous yeast”, as well as other yeast and bacteria, not always useful for a stable and useful fermentation according to an enological point of view. Using the so called “indigenous yeast” or “autochthonous”, certainly means giving wine the “autochthonous” qualities of a place, but also taking the risk of developing a colony of unknown bacteria and yeast and not always producing a decent result.

 For this reason, producers who use the “indigenous” yeast - that is the one Nature and all the other environmental factors existing in their vineyard deposit on the skin of grapes - rarely take the risk of using it “as such”. Saved the case they are lucky of cultivating a vineyard in a territory blessed by the good fate of Mother Nature, in which are found “positive” yeast and bacteria for enological purposes, in general terms a sample of the indigenous yeast is sent to a laboratory in order to select it and to “get rid of” unwanted yeast and bacteria. It should also be said that in the winery is however worked a “natural” selection of the indigenous yeast during the control of fermentation. One of the most common natural techniques in winery is the control of fermentation temperature. This technique - which certainly is useful and accepted as essential in order to get a fine wine - as a matter or fact, works a fierce selection of the colony of yeast found in the must, while inhibiting the activity of the ones which cannot “live” at certain temperatures.

 It should also be noticed that the common technique of sanitizing the must before starting fermentation, by adding sulfur dioxide, works a preliminary selection of yeast. Sulfur dioxide literally poisons some strain of yeast, all considered negative for quality fermentation, a treatment to which survives the stronger and more resistant strains only. The rooms in which the fermentation takes place are naturally populated by yeast, not only the ones found in the air, but also the ones developed and multiplied during past fermentations. Also the yeast found in the winery will therefore pollute the must, by adding themselves to the ones already present, a phenomenon which can be considered as a further selection. The unavoidable consequence of these factors, besides technical and environmental conditions, is to work a selection on indigenous yeast. The final result is therefore strongly affected by the skill of the wine maker in controlling the fermentation process, as well as “living” in a privileged environmental context.

 Wine, of course, is not the simple result of fermentation done by yeast, it is however undeniable this fundamental process strongly affects the organoleptic qualities of the final product. In order to get positive organoleptic qualities from fermentation, although identifying a place and a terroir, it is however indispensable this process is scrupulously controlled. For this reason, many wine producers prefer using a selected yeast, capable of making fermentation more reliable, controllable and stable. The use of selected yeast also produces a specific and typical organoleptic result, therefore causing the unavoidable homologation of wine's organoleptic qualities. In other words, a wine produced with selected yeast can be easily recognized by a quick olfactory examination, as their aromatic characteristics are common to all the wines in which it has been used in fermentation.

 There are many types of selected yeast and laboratories producing it, frequently offer cultures and strains suited for the production of many types of wine. The main producers of selected yeast can offer strains suited for the production of white wines, aromatic white wines, red and rosé wines, sparkling and sweet wines, as to mention the most common cases. Every strain of yeast has specific qualities which are not only related to the specific organoleptic characteristic it will give to the wine. To better clear this concept, let's consider, for example, a sweet wine. One of the characteristics of these wines is the high alcohol by volume, frequently higher than 15%, therefore it is indispensable the yeast used for its fermentation can survive and “work” also with high quantities of alcohol. In these cases it is in fact preferred the use of Saccharomyces Bayanus, capable of tolerating a higher quantity of alcohol than the famous Saccharomyces Cerevisiae. Both the two yeast belong to the Saccharomyces genus, however they produce different fermentation results, by giving the wine specific organoleptic qualities.

 If it is true the organoleptic contribution of selected yeast is pretty predictable, the one of indigenous yeast is quite unpredictable. The unforeseeability of indigenous yeast - autochthonous or non selected - must however be considered both in a positive and negative way. In case the control of fermentation is not properly done, as well as in presence of a diversified and uncertain “colony” of yeast and bacteria, the result could be a wine with pretty poor organoleptic qualities, also characterized by evident faults. Moreover, the fermentation done with indigenous yeast can be considered as a sort of lottery: in case negative yeast and bacteria become dominant, the result will certainly be negative. By using the indigenous yeast, and in case the fermentation is mainly done by positive yeast for quality wine making, the final result will however and in any case be determined also by the organoleptic influence of the other minority yeast and bacteria.

 This does not mean the influence of other minority yeast and bacteria necessarily is negative: most of the times their contribution on wine's organoleptic qualities is absolutely positive, giving a greater complexity and sensorial diversification. The use of indigenous yeast certainly is more difficult than the selected one: this is the main reason for which most of producers prefer not to use it. Selected yeast ensures a quicker start of fermentation, a better control, higher stability, a higher warranty on the final result, including aromas and flavors. On the other hand, the huge variability of the colony making the so called indigenous yeast, makes it very unpredictable, despite some sanitizing and wine making practices work - as a matter of fact - a considerable selection on its components.

 The population of yeast and bacteria making the indigenous yeast must however be considered as the expression of a specific territory, just like all the other environmental factors, as it directly affects the organoleptic qualities of wine. According to this point of view, the use of selected yeast hides these typical and territorial qualities, by giving homologated aromas and flavors which can be replicated everywhere. The influence of yeast - both selected and indigenous - on the organoleptic qualities of wine is not however destined to mark the wine forever. With time and aging, the organoleptic qualities given by yeast to the wine tend to attenuate, while allowing the genuine character of the wine to emerge, as well its tertiary qualities. The impact of yeast on wine is in fact very strong and dominant after the end of fermentation and during the youth of wine, progressively attenuating its influence with time.