Making Wine: Alcoholic Fermentation

 After having crushed the grapes, obtained the must and having done the proper corrections, it has now come the time to let our grape juice begin the journey which - by means of a complex chemical process - will transform it into wine. Alcoholic fermentation - or primary fermentation - is a fundamental process for the production of wine. Its role is not only limited to the transformation of sugar into alcohol and carbon dioxide - as well as other byproducts - indeed, it is what defines, or better to say, reveals, the organoleptic qualities of the grape. In fact, all the potential qualities of a wine are, like to say, hidden in the grape: they will be revealed or they will disappear forever by means of alcoholic fermentation and during the other phases of vinification. The role of alcoholic fermentation is fundamental for the development of aromatic and gustatory qualities of a wine, it is not by chance it is said “primary aromas are hidden in the grape, but it is fermentation which will reveal them”.

Steel tanks for fermentation

 Fermentation is a natural process allowing the transformation of grape juice - the must - into wine. Alcoholic fermentation, or primary fermentation, can be considered as a chain reaction of chemical phenomena. During these processes, the sugar contained in the grape juice is converted by yeast's enzymes into ethyl alcohol and carbon dioxide. According to the type of yeast, from the conversion of sugar is obtained about 50% of alcohol, 45% of carbon dioxide, 3% of glycerol and 2% of other byproducts. This complex phenomenon, unknown for centuries, produces other substances as well, considered as “byproducts” and which play an essential role in the aromatic and gustatory qualities of wine. Of these products, the most important ones are acetaldehyde, acetic acid, ethyl acetate, glycerol and other types of alcohol. These “secondary” products are in fact essential for the development of any wine's aromatic qualities and, depending on how the alcoholic fermentation was done, aromas can get refined or spoiled.

 Fermentation is an extremely complex process which is the result of more than consecutive reactions caused the yeast's enzymes, the unicellular microorganisms which use the sugar found in the must in order to ensure their growth and multiplication. The most common type of yeast used in enology is Saccharomyces Cerevisiae, also used for the fermentation of beer and the leavening of bread. The entire process is done in anaerobic conditions, that is without oxygen, therefore yeast produces energy by converting sugar. In the initial phases of fermentation, yeasts do an aerobiotic respiration, that is they use the oxygen found in the must, therefore transforming the sugar into water and carbon dioxide. When in the must there is no more oxygen, the real fermentation begins - a process done in anaerobic conditions - during which the yeast produces energy by the oxidation of sugar and transforming it into ethyl alcohol, carbon dioxide and other secondary products.

 The quality and quantity of primary and secondary substances produced during fermentation, basically depend on the conditions happening during this process. It is extremely important for the must to be in the best possible condition before the fermentation begins, in particular, it is essential it is not oxidized. For this reason, it is very important the fermentation of the must starts as soon as possible, soon after the normal operations of stabilization, decanting and clarification. During this phase the must is in fact particularly sensitive to the attacks of bacteria, spoilage caused by microbial activities and oxidation. For this reason, it is essential the period of pre-fermentation should be as short as possible and the must to be properly protected by adding sulfur dioxide. This practice allows in fact to do all the operations of decanting and clarification of the must without taking the risk of spoilage.

 Moreover, it is of fundamental importance the fermentation is done according the typical times for this process, a time which - according to the type of must, the operations done for its stabilization and the quality of the grapes - can be from 5 to 15 days. An excessively slow fermentation can in fact favor the development of secondary substances responsible for “ordinary” aromas and taste as well as an excessive quantity of volatile acidity. On the contrary, in case the fermentation is too fast, this will cause an excessive increasing of temperature and a loss of aromas - which will be lost with the huge quantity of carbon dioxide released during the process - with the result of obtaining an ordinary wine having no pleasing organoleptic qualities. Temperature plays a fundamental role for fermentation. According to the type of wine to be made, it is essential the temperature is kept constant within a specific range, as we will see later.

 A too low temperature - usually lower than 15° C (59° F) - inhibits in fact the start and the process of fermentation, while increasing the risks of oxidation or, in case of red wines, an insufficient extraction of color. Too high temperatures will cause a too rapid fermentation with the risk of obtaining a coarse wine and pretty ordinary, lacking of any quality of finesse. An excessively high temperature causes the interruption of fermentation as well as the death of yeast. Temperature is also important during the adding of selected yeast to the must. It is important the temperature of selected yeast is as equal as possible to the one of the must to which they will be added, as an excessive difference of temperature can cause the death of most of the yeast, therefore making useless this operation. Finally, it is good to remember selected yeast must be added to the must before the beginning of the fermentation process.

 For the fermentation of white wines it will be used a must from which will be eliminated the skins soon after having crushed the grapes. In order to have a better fermentation and a more stable wine, it will also be appropriate to clarify and decant the must in order to eliminate any solid substance before starting fermentation, as already mentioned in past articles published in DiWineTaste. The primary goal of fermentation of the must destined to the production of white wine is to keep finesse and quality of aromas, a result which is mainly obtained with a scrupulous control of the temperature and by keeping it from 16° and 20° C (60°-68° F). This range of temperatures will in fact allow the optimal development of aromas, a slow transformation of sugar and an excellent production of alcohol. Lower temperatures will make fermentation difficult and can also stop the whole process, therefore favoring the oxidation of the must.

 Higher temperatures will make fermentation excessively active, with the result of losing most of refined aromas, therefore obtaining a pretty coarse and ordinary wine. For this reason, during the fermentation of white wines, it will be scrupulously controlled the temperature by making sure it does not go outside the correct range. Fermentation is a process developing heat, therefore it is very likely the temperature easily raises above 20° C (68° F), in particular during mild seasons. The best method to cool the fermentation tank down, in case there are no automatic systems for the control of temperature, is to run cold water along the sides of the tank. In case temperature gets too low, it can be used the same method by using lukewarm water. Another method consists in heating the room where are found the fermentation tanks. It is not recommended to warm the must by plunging electric heaters or similar systems, because this can cause the excessive heating of the must in contact with the heater therefore developing “burnt” tastes.

 For the fermentation of red wines will be used a must in which will be allowed skins to macerate, an essential operation allowing the extraction of color. It will be the extraction of color and polyphenolic substances, one of the primary goals in the alcoholic fermentation of red wines. Coloring substances contained in the skins can be easily extracted at higher temperatures, and the higher the temperature, the better the extraction of coloring substances and polyphenolic substances, in other words, a red wine fermented at a higher temperature will have a fuller structure and body. Also in this case it is of fundamental importance to control temperature, by making sure it does not go outside the optimal range, which for red wines is from 25° and 30° C (77°-86° F). This temperature allows the production of red wines with good quality, a good extraction of color and polyphenolic substances, as well as a good balance between these two qualities.

 Temperatures lower than 20° C (68° F) will make difficult the extraction of coloring substances from the skin, therefore it is always recommended to not allow temperature to get so low. Temperatures from 20° and 25° C (68°-77° F) will allow a moderate extraction of coloring substances, however not enough for the optimal extraction of polyphenolic substances, with the result of obtaining a light wine and with a light body. Temperatures included in this range are recommended for light red wines, not destined to aging and to be consumed within the year. Temperature higher than 30° C (86° F) make wines rich in polyphenolic substances as well as robust, however the fermentation could cause the development of taste with a pronounced “herbaceous” quality and, in case it is not properly controlled, this can also interrupt the fermentation process. Such high temperatures are sometimes useful for the fermentation of grapes having little coloring substances, such as Pinot Noir. In this specific case, after the first racking, that is the separation of the skins from the wine, it is recommended to complete fermentation at a temperature of about 20° C (68° F), in order to allow an optimal development of aromas and the best condition for malolactic fermentation.

 The normal conditions for wine home making do not always allow scrupulous controls on fermentation because specific laboratory tools are not usually available. However, also in wine home making it is indispensable to properly control the process of fermentation in order to ensure the best possible result. Of all the controls usually done on fermentation, two of them are considered to be of fundamental importance: the control of temperature and the control of density. These two controls are indispensable and they will be done on a daily basis, carefully writing down the results in a record, in order to get a trend of the fermentation and the changes of the two values according to time. The control of temperature is very easy to do: it will be enough to detect it from the fermentation mass by using a thermometer. In case temperature is outside the optimal range for the wine to be made, it will be necessary to properly correct it by using the methods discussed above.

 The control of density is very simple to do as well and it allows the verification of the good progressing of fermentation and, in particular, to realize when it is interrupted before the normal end. The control of density can be easily done by using a normal densimeter, the same used for the control of grape's ripeness. During fermentation, the density of the must progressively diminishes, until reaching a value from 0.990 to 0.995. Values greater than 1 mean the presence of sugar, therefore the fermentation is not completed yet. In case density gets stabilized on a value greater than 1, it means the fermentation is stopped and it is then necessary to restart it as soon as possible. The most common cause is a variation of temperature and therefore it will be necessary to take proper action. This phenomenon is properly detected by controlling the density as, after the fermentation is stopped, carbon dioxide continues to develop from the tank - just like the normal progressing of fermentation - and when it completely stops it is usually to late and no remedy is useful.