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  Corkscrew Issue 23, October 2004   
Production of Sweet WinesProduction of Sweet Wines  Contents 
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Production of Sweet Wines

Genuine nectars of grapes, complex but gentle, harmonious and rich, sweet wines are an endless resource of aromas and flavors which have always enchanted wine lovers of every era

 Among the most sought wines of the world, capable of conquering the heart and soul of people, even of those who are not interested or drink wine, there are sweet wines which have always met the interest and the taste of all wine lovers in every era, from the dawn of enology up to nowadays. Noble and majestic wines, expensive and in past reserved to the ministers of cult and kings only, despite they show different colors and qualities, they all have in common the same characteristic, the one of keeping the spirit of the grape expressed in its sweetness and in its charming and enchanting aromas. The production of these wines requires a higher effort and a higher commitment to quality, a strict control which begins in the vineyard and ends in the bottle. Sweet wines are also capricious and in order to be superlative they need huge productive efforts, otherwise the finished product will be however sweet and pleasing although lacking elegance and class, in other word, of that magic touch which always surprises the senses of the ones who are capable of appreciating them and to listen to their enchanting stories.

 

Sweetness Marrying Wine

 The main organoleptic difference between “regular” wines and sweet wines consists in the quantity of sugar dissolved in wine and not transformed into alcohol by fermentation, in other words, by the so called “residual sugar”. The higher the quantity of residual sugar, the higher the sweetness. Despite in dry wines it is sometimes possible to find some residual sugar, its quantity is so low that it is not perceived by taste and therefore the wine is considered as dry. It should also remembered the organoleptic sensation of sweetness is balanced by acidity, therefore in a very crisp wine, that is with a pretty high acidity, the sweetness will be less evident, in other words, balanced. This is the case of German sweet wines - in particular Trockenbeerenauslese wines - which have pretty high quantities of residual sugar without being sickly just because of the high acidity. Even alcohol contributes to the balance of sweetness, therefore sweet wines usually have pretty high alcoholic percentages - sometimes the alcohol is also added therefore making the wine fortified - both for balancing the wine as well as for making it more stable.


Drying of the Zibibbo grapes at Carlo
Pellegrino winery to be used for the production of Passito di Pantelleria
Drying of the Zibibbo grapes at Carlo Pellegrino winery to be used for the production of Passito di Pantelleria

 As it is commonly known, alcohol in wine is produced by the fermentation of the sugar contained in the must by means of yeast. The quantity of sugar contained in the must determines the quantity of alcohol at the end of fermentation and it takes 17.5 grams of sugar per liter in order to obtain 1% of alcohol. For the production of a dry wine having an alcoholic volume of 12%, will be necessary 210 grams of sugar per liter of must. This value expresses the quantity of potential alcohol, that is the one which is obtained in case all the sugar is fermented and transformed into alcohol. In sweet wines is therefore necessary that part of the sugar is transformed (fermented) into alcohol, whereas a certain quantity must be kept in order to give sweetness to the wine. This quantity of non fermented sugar - and therefore kept - represents the residual sugar and the higher this quantity, the higher the relative perception of sweetness. In order to help to better understand this concept, let's suppose we want to make a sweet wine from a must containing 320 grams of sugar per liter. In case all the sugar should be fermented, the alcohol volume (potential alcohol) would be a slightly more than 18% (320 / 17.5 = 18.28).

 Let's suppose now that after having analyzed the quality of our must, we want to make, in order to have a balanced sweet wine, a finished product having 14% of alcohol. To do so we need 245 grams of sugar per liter to be transformed into alcohol (17,5 × 14 = 245) while leaving the rest of the sugar non fermented, that is residual. This residual sugar, in our case of 75 grams per liter (320 - 245 = 75), will be responsible for the perception of sweetness of the wine. Of course in this example we wanted to make things very easy, indeed the production of sweet wines is not made this way because yeast does not stop its action upon “command”, it continues fermenting sugar until the chemical conditions of the must allow it to do so, saved the case its activity is intentionally stopped. The most common way to interrupt the action of yeast is to add alcohol - that is to fortify the wine - therefore saving part of the sugar from fermentation. It should be remembered ethyl alcohol is a toxic substance and this is true for yeast as well that, in condition of high concentration of alcohol, generally about 16% and according to the species of yeast, it dies and therefore the fermentation is stopped.

 

A Wine Depending on Sugar

 The taste in sweet wines is determined by the quantity of residual sugar, of course these wines are not “sweetness” only, a characteristic that could be obtained in many and simple ways. Sweet wines generally offer very complex aromatic qualities, however it is sweetness to be the main quality which distinguishes them. Like we already said, the quantity of residual sugar is determined by what it is left at the end of alcoholic fermentation, at least, this is what should happen in theory. There are many ways to make the quantity of residual sugar and each one of them gives extremely different results. The quality of sweetness sensation perceived in wines is not always the same: in some wines it will seem to be more genuine and harmoniously integrated to the other gustatory qualities, in others it will be perceived the sensation of an almost “forced” sweetness, it will seem this sweetness does not belong to the wine but to something extraneous and disharmonious. Despite the fact this may sound absurd, sweetness in wine, or better the way a wine expresses its sweetness, represents a factor of primary importance in determining the quality of a sweet wine, of course, including its aromas and all the other organoleptic qualities.

 

Adding Sugar

 This method represents the most simple system - as well as the most disputable and least noble one - for the production of sweet wines and it is generally used for low quality wines. In practical terms, a dry wine produced according the normal wine making procedures is being sweetened. This method - it should be remembered - is forbidden in many wine countries of the world, whereas in other it is allowed. It should be however observed that even in countries where this is allowed, the adding of sugar to a dry wine is never done by quality producers, just because the result is very scarce and the “trick” is easily recognizable. After having produced a dry wine - and just before being bottled - it is added some sugar, it is made sure it is perfectly dissolved in the wine, then follows filtering, stabilizing and then bottling. Another method consists in adding to a dry wine a certain quantity of non fermented must - therefore rich in sugar - called sweet reserve (from the German süssreserve), properly sterilized, filtered and kept at a low temperature in order to prevent fermentation. In some fortified wines - in particular for sweet Jerez (Sherry) and in some styles of Marsala - it is added to the wine a certain quantity of concentrated must.

 

Sugar from Grape

 In the best sweet wines, sugar is never added, not even in the form of concentrated must or sweet reserve, and the effort of producers is to get out the most from the sugar contained in grapes. The best method consists in concentrating the sugar in each grape berry, a result that can be obtained in many way by subtracting water from grape juice. Finally, another common method for keeping sugar in grape juice consists in voluntarily stopping the fermentation of must by adding a certain quantity of alcohol or brandy. Each one of these methods allows the production of very different styles of wines, each one having its own characteristics and peculiarities, however they have been capable - in the course of centuries - to prove their efficiency in the production of good and high quality sweet wines. Of course these techniques alone cannot ensure a good result and high quality in wines: they simply represent one of the many quality factors that together with all the others allow the production of a great sweet wine.


 

 As it is commonly known, the development of ripeness in grapes does some chemical changes in the juice. The unripe berry is rich in acid and poor in sugar, and this conditions changes with the ripening process in which the quantity of acid diminishes whereas the sugar is increased. Therefore the riper the grape, the higher the quantity of sugar. If the grape is allowed to ripe beyond the optimal level - a condition known as overripeness - the berry begins to dry because of the loss of water with the result of obtaining a more concentrated juice and a higher quantity of sugar per liter. Using overripe grapes - that is grapes left to naturally overripe in the plant - is very common in the production of popular sweet wines, such as the ones produced in South-Western France and many Italian sweet wines. Using overripe grapes is also typical in the production of German wines - whose quality system classifies wines according to the quantity of sugar contained in grapes - and the categories of wines produced with this system are the so called Spätlese and Auslese. This system does not make use of grapes affected by the famous Botrytis Cinerea or noble rot.

 Botrytis Cinerea represents another method which allows the concentration of sugar in grape berries to be used for the production of sweet wines. Of all the methods this certainly is the most noble one, because the action of the rot does not only reduce the quantity of water in the grape juice, it also does extraordinary changes which will give wine elegant and complex aromas as well as an absolutely unique flavors. As the spores of Botrytis Cinerea - or noble rot - gets on the skin of the grape, they hole it in order to seek nourishment inside the berry necessary for its development. This operation - besides allowing the rot to get inside the berry - also favors the evaporation of water with the result of concentrating the juice. As soon as it is inside the berry, noble rot adds its flavors and its aromas to the grape juice - as well as other unknown components - which transform the sweet juice into a divine nectar. The development of Botrytis Cinerea needs particular environmental conditions - moisture in the morning in order to allow the development of the rot, sunny and dry afternoons in order to prevent its excessive development - and the skin of the grape must be thin enough in order to allow the spores to hole it and get into the berry.

 Botrytis Cinerea - despite the fact it is essential for the production of great sweet wines - represents a very risky condition for producers. When meteorological conditions are unfavorable - too much moisture or rain - its development is excessive and it irremediably compromises the grapes. Berries of botrytized grapes are very delicate and therefore harvesting is done by hand and with extreme care. Among sweet wines produced with grapes affected by Botrytis Cinerea are to be mentioned Sauternes, Barsac, Tokaji Aszú, Monbazillac, Alsatian Sélection de Grains Nobles, the so called muffati from Italy, German Beerenauslese and Trockenbeerenauslese. Because of the strong dependency on the environmental and meteorological conditions - and in particular on the risks deriving from such conditions - some producers of botrytized wines, usually Californian and Australian ones, as well as from other countries, harvest grapes, spray the spores of Botrytis Cinerea on the skin and then keep them in rooms in which are being artificially recreated environmental conditions that can favor and control the development of the rot. The result is similar - but not the same - to the wines produced with “natural” methods and this system is generally used for low quality botrytized wines.

 Other methods that allow the keeping of grape sugar include the one traditionally used in Italy for the so called passiti wines, that is produced with grapes allowed to dry in the plant, left in mats after harvesting or hung in sufficiently aerated rooms. In areas where the climate is warm and dry, the grape is usually allowed to dry in mats and left outside, whereas in humid areas it is necessary to properly protect the grapes in aerated rooms in order to prevent the formation of rot. With this system are being produced - among the many - Vinsanto, Recioto di Soave and della Valpolicella, Sagrantino Passito as well as the renowned Passito di Pantelleria. In cold areas, such as Germany, Austria and Canada, it is common practice to harvest the overripe grapes when they are frozen by low temperatures. The grapes are soon after pressed and it is obtained a must with very high concentration of sugar and acid, whereas the water, frozen inside the berry, is not extracted. With this method are produced the renowned German and Austrian Eiswein, as well as Canadian and American Ice Wines. Another method consists in interrupting the fermentation of the must by adding some alcohol or brandy, therefore keeping the non fermented sugar as residual. With this system are produced the renowned French Vin Doux Naturels - such as Banyuls and Muscats from Languedoc-Roussillon - as well as many sweet fortified wines, such as Port.

 

From Grape Juice to Wine

 The phases of the production of sweet wines are basically the same used for dry wines, both whites and reds. However in some wines the production procedures requires the adoption of specific practices in order to obtain the desired product, such as in case of sweet wines produced with grapes affected by Botrytis Cinerea. The grape juice obtained during the last phases of pressing is usually considered of lower quality and in the production of quality wines it is usually rejected or used for the production of ordinary wines. The opposite is true for grapes affected by noble rot, in which it is the final part of the pressing to produce the richest must, concentrated and sweet. The fermentation of a must rich in sugar is very slow and sometimes in those having a very high concentration of sugar the fermentation could not even start or however produce a small quantity of alcohol. This is the case of very sweet German Trockenbeerenauslese and Tokaji Eszencia wines in which the alcohol volume could also be lower than 6%. The interruption of fermentation - in order to keep sugar and therefore sweetness - can be done by adding alcohol to the must (fortification), as well as lowering the temperature, adding sulfur dioxide (SO2), treating the must with sterile filtering in order to completely eliminate yeast that would otherwise resume fermentation. Sweet wines - just like dry wines - can also be aged in casks in order to further favor their concentration as well as enriching them with the organoleptic qualities of wood.

 




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  Corkscrew Issue 23, October 2004   
Production of Sweet WinesProduction of Sweet Wines  Contents 
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