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  Corkscrew Issue 48, January 2007   
Making Wine: YeastMaking Wine: Yeast  Contents 
Issue 47, December 2006 Follow DiWineTaste on Follow DiWineTaste on TwitterIssue 49, February 2007

Making Wine: Yeast

Essential microorganisms for the fermentation process, yeasts are responsible for the transformation of the must into wine, a complex process remained mysterious for many years

 There are no reliable information about how and when fermentation was discovered, a process that for centuries has been considered a mystery, by simply observing the phenomenon and to benefit from its effects. It is believed fermentation was discovered by chance in remote times, probably observing the curious phenomenon of the “boiling” that may have happened in a cereals soup, and subsequently in other liquids showing the same behavior, including grape juice. For centuries considered as a “divine gift”, fermentation and the factors responsible for this complex chemical phenomenon, have remained one of the many unsolved mysteries, although, in a more or less empirical way, one could count on. One of the first suppositions about the fermentation phenomenon, was believing it was a “sort” of decomposition of the organic substances found in the must. Only technique and the progresses done in chemical, biological and technological fields will allow, centuries later, to understand the phenomenon of fermentation and the factors which regulate it.


Saccharomyces Cerevisiae
cells observed at the microscope
Saccharomyces Cerevisiae cells observed at the microscope

 The first step towards the understanding of the fermentation phenomenon was done by the famous French chemist Antoine Lavoisier who, in the 1700s, was successful in proving the sugar contained in the must was transformed into alcohol and carbon dioxide. In the 1800s, the French chemist Joseph Louis Gay-Lussac formulated the mathematical relationship regulating the transformation of sugar in alcohol and carbon dioxide. Thanks to the many researches done by the great Louis Pasteur, the fermentation was finally understood including the factors regulating it. In fact, it was Louis Pasteur to prove, in 1854 thanks to its countless experiments, that fermentation was produced by the activity of yeasts when they develop in an oxygenless environment. He also discovered the production of unwanted substances in wine - such as lactic acid and acetic acid - was caused by the presence of microorganisms of different nature. The contribution of Louis Pasteur in wine making and the understanding of the fermentation process have been fundamental and its job allowed the technological development of enology the way we know it today. Louis Pasteur defined the fermentation process as “a phenomenon connected to life”, as he was the first one to understand yeasts were living organisms. As for wine, Louis Pasteur defined it as «the most healthy and hygienic of beverages».

 

Yeasts

 Yeasts are microorganisms made of a single cell, classified as fungi. Researches done on these microorganisms allowed the discovery and the classification of more the one thousand different species, each having its own characteristics, although sharing the same biological principles. Yeasts are classified in two categories: aerobic yeasts and anaerobic yeasts. These two categories identify the method used by the yeast for its keeping in life. The first category uses aerobic breathing - that is they need air and oxygen for the keeping of their life - whereas the second category, in absence of oxygen, can adopt a process of anaerobic breathing, commonly known as fermentation. Anaerobic yeasts produce in fact energy from the conversion of sugar into carbon dioxide and ethyl alcohol, they therefore are the kind of yeast having an essential use in the production of alcoholic beverages, such as wine.


 

 This process is however “dangerous” for yeasts, as the production of alcohol goes on, the environment in which they are gets concentration of alcohol such to cause the death of the yeast itself. Every kind of yeast has a specific sensitivity and resistance to the concentration of alcohol: a characteristic which is carefully evaluated when choosing a yeast for the production of wine according to the quantity of alcohol wanted in the final product. Among the many kind of anaerobic yeasts, and therefore suited for the production of ethyl alcohol, in wine making are mainly used two species: Saccharomyces Cerevisiae and Saccharomyces Bayanus. Of the countless kinds of yeasts, Saccharomyces Cerevisiae certainly is the most studied one because of the countless uses for the production of beverages and foods. This kind of yeast is commonly known as brewer's yeast or baker's yeast, because it is widely used for the fermentation of beer and the leavening of bakery products, including bread.

 Another factor for the classification of yeasts is represented by shape, a factor which can be determined by the observation of these microorganisms through the microscope. Despite in the fermentation of the must are responsible many type of yeasts, the most interesting ones are those classified as apiculate and elliptic. Both Saccharomyces Cerevisiae and Saccharomyces Bayanus belong to the family of “elliptic”, whereas among the main “apiculate” yeasts of fermentation is found Kloechera Apiculata, responsible, among the other things, to the activation of fermentation which will be then completed by Saccharomyces Cerevisiae. Of these yeasts, Kloechera Apiculata is the one having the lesser resistance to the effects of alcohol: with just 3-4% of alcohol concentration its activity is stopped. Some species of Saccharomyces Cerevisiae can also support concentrations of 16-17% of ethyl alcohol, whereas the Saccharomyces Bayanus species can support even higher concentrations.

 Not all yeasts have the same yield in the production of ethyl alcohol: there are yeasts which need, for example, higher quantities of sugar in order to make the same quantity of alcohol. The Kloechera Apiculata species needs a concentration of 21-22 grams per liter of sugar to give 1% of ethyl alcohol, Saccharomyces Bayanus needs 20 grams per liter in order to make the same quantity, whereas Saccharomyces Cerevisiae needs just 17-18 grams per liter. The quantity of alcohol produced by the fermentation of the must will therefore vary according to the kind of yeast: for this reasons it is very important to do a proper selection before starting fermentation. Among the many factors regulating fermentation and the activity of yeasts, it is mentioned sulfur dioxide, whose effects greatly influence yeast's vitality and function. The use of sulfur dioxide is therefore useful for an appropriate selection of yeasts, while inactivating the less efficient and less useful kinds - such as Kloechera Apiculata - while allowing Saccharomyces Cerevisiae, more resistant to sulfur dioxide and more efficient in fermentation, to prevail and to do a better alcoholic fermentation.

 

Autochthonous Yeasts and Selected Yeasts

 Yeasts are naturally found in the air and in the surface of plants, transported by the wind, also from distant places, and from insects. In a vineyard it is therefore created an “ecosystem” in which will be naturally present many species of yeasts, some of them being useful and positive for the alcoholic fermentation, other less important and marginal, even detrimental because of their activity and for the production of unwanted substances. Yeasts are also found in the surface of grape's skins which get in contact with the juice after the grape has been crushed, therefore starting a “spontaneous” fermentation without using any other system. These yeasts are generally defined as “autochthonous”, “indigenous” or “wild”, and their use and efficiency have been widely debated since 1930s and 1940s. As yeasts greatly influence fermentation, as well as the development of aromas and wine stability, it can be said before the discovery of Louis Pasteur, the quality and the good result of a wine were determined not only by local environmental conditions and the quality of grapes, but also by the population of yeasts which were “naturally” found in a specific area.

 As the result of a good fermentation is determined by the species of yeasts found in a must, in order to improve the quality of wines, after having understood the phenomena regulating it, they started specific studies in order to improve - or better to say, select - the presence of some species in favor of others. They then started researches in laboratory in order to study the activity of specific yeasts, therefore creating selected cultures which were soon welcomed by wine producers, both for the best control over the fermentation they ensured, as well as for the better finesse and quality which could be obtained in their wines. The use of selected yeasts has later caused a strong debate on the practice of the fermenting of must, and many supported the idea it was possible to determine in advance the organoleptic characteristics of wine, greatly affecting the typical qualities of the territory. After many years of debates, studies and direct experiences, today it is believed the use of selected yeasts is indispensable for the production of quality wines.

 Despite the fermentation done with “autochthonous” yeasts, that is the ones naturally found in vineyards, can be considered “traditional” or even “romantic”, it should be noticed that yeasts are transported from a place to another by the wind or by insects, therefore the species are continuously changing, therefore risking the presence of useless species or however not efficient in the fermentation which, moreover, produce excessive quantities of unwanted substances, such as acetic acid and other elements which can compromise the stability of wine. Also for these reasons, today it is considered indispensable to do a proper selection of yeasts before starting fermentation, both by adding species with known and reliable behavior, as well as by eliminating or stopping the activity of species considered useless. In other words, the use of selected yeasts allows the production of wines with better quality, finesse and elegance and with a better stability. After many years of studies, today are mainly used the species Saccharomyces Cerevisiae for the fermentation of regular musts and Saccharomyces Bayanus for the fermentation of musts with a high content in sugar or for the production of classic method sparkling wines.

 

Effects and Use of Yeasts in Enology

 The activity and efficiency of yeasts are also determined by many factors and the product of this process is not represented by ethyl alcohol only. The activity of yeast also influences the organoleptic qualities of wine - aromas and flavors - therefore improving the finesse and the capability of clarification. Fermentation also produces substances which are not considered useful for the stability and quality of wine, such as volatile acidity, produced in variable quantities according to the species. Kloechera Apiculata yeasts produce a high quantity of volatile acidity, and some strains of Saccharomyces Cerevisiae are known to produce volatile acidity in considerable quantities. This effect further confirms the importance of using selected yeasts, whose effects and actions are well studied and known. Moreover, the varieties of selected yeasts added to the must ensure a good factor of domination on the autochthonous cultures found in the must, in order to allow a better fermentation. This latter quality - it should be remembered - is also affected by the use of sulfur dioxide that, thanks to its effects, inhibits the activity of unwanted yeasts while favoring Saccharomyces Cerevisiae which are more resistant.

 The excessive presence of Botrytis Cinerea in the grapes influence the activity of yeasts as well - frequently prolonging the time of the beginning of fermentation - because of the remarkable quantity of particular bacteria and microorganisms that, moreover, can cause faults and alterations to the wine. Excessive concentration of sugar, despite it is indispensable to anaerobic yeasts for their biologic activity, it can make the fermentation difficult, sometimes preventing the beginning or greatly prolonging the times for its completion. Among the factors influencing the activity of yeast, it is mentioned temperature. The minimum and maximum values tolerable by yeasts mainly depend on the species, however at lower temperatures the activity will be greatly reduced or stopped, whereas at higher temperatures the activity will be accelerated. High temperatures give coarser and more ordinary organoleptic results, whereas excessive high temperatures cause the death of yeasts. The advantages and the spreading of selected yeasts, have allowed the availability of many species, each of them having proper characteristics according to the type of wine to be produced and to its quality. Nowadays it is preferred the use of the so called active dried yeasts - “ADY” in short - produced in specialized laboratories and sold in freeze-dried form and shaped in small “sticks”.

 Active dried yeasts are also preferred to other types because of their ease of use. Before being added to the must, active dried yeasts need a proper and delicate operation of reactivation. The typical quantities to be added to the must, always and however before the beginning of fermentation, are generally from 10 to 20 grams per hectoliter. The quantity of yeasts is mixed with lukewarm water at a temperature of 40° C - and it is essential to make sure about the right temperature - for a volume equal to ten times the weight of yeasts. In order to facilitate the reactivation process, it can be added about 50 grams of sugar per liter of water, or - alternatively - specific products containing nutritional principles for yeasts, such as thiamine (vitamin B1) and ammonium sulfate, both available in specialized enology shops. The mixture is then well stirred and allow to rest for 30 minutes, while stirring the mixture every 10 minutes. Yeasts are then added to the must, while paying attention to carefully stir the mass and to make sure the difference of temperature does not exceed 8° C, a condition that - because of a thermal shock - can cause the death of most of yeasts.

 




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  Corkscrew Issue 48, January 2007   
Making Wine: YeastMaking Wine: Yeast  Contents 
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