Specific Options for Producing Wines with Residual Sugar Flashcards
Explain different ways of concentrating the grape must?
This is the method used to produce many sweet and luscious styles of wine. The concentration of sugars in the grape must may be sufficient to stop the fermentation before reaching dryness; yeast struggle in very sugary environments, especially when alcohol is also present, and naturally stop fermentation even at relatively low levels of alcohol. However, the fermentation can also be stopped prematurely by chilling and/or adding SO2 and then filtering to remove yeast, giving the winemaker greater control over the balance of alcohol and sweetness.
The grape must is concentrated by the reduction of the water content in the grape, and hence this process not only concentrates sugar, but also other grape components such as acidity and flavours. The increase in acidity and flavours helps to keep these wines in balance so that sweetness does not become too dominant, and is a key reason why many of the wines made by such methods tend to be very good or outstanding in quality. The method of removing the water to concentrate the sugars can sometimes add its own flavours. For example, drying grapes off the vine can lead to flavours of dried fruits; therefore, these wines can have more complexity than those simply made by stopping the fermentation by chilling or adding a sweetening component. Because water is depleted, the volume of juice obtained from them is low. The very sugary pulp is also often hard to extract during pressing. These factors can add to the cost of production of these wines, which means they often sell for premium and super premium prices.
Methods of concentrating the grape must include the drying grapes on and off the vine, the development of noble rot and the freezing of grapes.
DRYING GRAPES ON THE VINE
As seen in Grape Development in the Vine Growth Cycle, the grape goes through four stages of ripening. If left on the vine, the grape will enter stage four of ripening in which the grape starts to shrivel. Water is lost by grape transpiration and the sugars concentrate. The flavours in the grape also continue to develop, giving very ripe flavours. For example, Pinot Gris may develop from stones fruit when ripe to tropical fruits and dried stone fruits as it becomes extra-ripe. Long, dry autumns are needed for this option to avoid the development of grey rot, which would give off-flavours. The wines produced in this way are sometimes labelled as ‘Late Harvest’ (or a local equivalent) and include Vendanges Tardives from Alsace and Spätlese from Germany or Austria.
There is another method of drying grapes while they are still attached to part of the vine. In this method, the cane of the vine is cut or broken off from the vine a short time before harvest. (The cane usually remains attached to the trellis at this point.) The grapes shrivel more quickly than they would if they were still attached to the rest of the vine. This concentrates the sugars, acids and flavours within the grapes. The reduced hang time lowers risk of grey rot and also means it is possible to obtain grapes with very high levels of sugar without extra-ripe flavours. The technique is used to make sweet wines in Jurançon, south-west France, and is also sometimes used in Australia.
DRYING GRAPES OFF THE VINE
In this method, grapes are picked and then dried. This can last from days to months depending on the extent of drying required and the speed of the drying process. Bunches of grapes may be laid out to dry in the sun in warm climates, such as southern Italy or Spain, or dried in a temperature- and humidity-controlled room in cooler climates, which gives the ability to speed up drying and avoid the development of grey rot, for example Valpolicella. (A humidity- controlled room is clearly more expensive to build and maintain than drying grapes outside.) This drying process causes water to evaporate, making the sugars, acids and flavours become more concentrated. This method is often referred to by the Italian term, passito. Wines made in this way include Recioto di Valpolicella and Vin Santo.
NOBLE ROT
This method involves the action of the fungus Botrytis cinerea. This is the same fungus that causes grey rot; however, under certain
conditions it can be used beneficially in sweet wine production. In these circumstances, it is often termed noble rot.
First, the grapes must be fully ripe before the development of the rot. Second, the grapes must be grown in a region that provides humid, misty mornings followed by sunny, dry afternoons. Damp conditions in the morning allow rot to develop on the grapes. The fungus punctures the grape skin with microscopic filaments, leaving tiny holes in the skin. The warm sunny afternoons slow the development of the rot and cause water to evaporate from the grape, concentrating its sugars, acids and flavours. The fungus can also modify some of the aroma compounds in the grape and generate its own unique flavours. Wines made from grapes affected by noble rot have distinctive honey, apricot, citrus zest, ginger and dried fruit aromas.
This method is used in the production of many premium and super premium sweet wines, including Sauternes, Beerenauslese, Trockenbeerenauslese and Tokaji. Only white grape varieties are used to make wines using this method. Although Botrytis cinerea is the cause of both noble and grey rot, the term ‘botrytis’ is frequently used as a synonym for noble rot, and the term ‘botrytised’ is often seen on sweet wine labels.
The spread of noble rot is never uniform and several pickings by hand may be needed to select the best grapes. This is an expensive process as it requires skilled labour over a prolonged period of time. This selective picking and the fact that water has evaporated from the grapes means that the volume of juice obtained is low. Furthermore, in some regions the ideal conditions for noble rot do not occur every year, making these wines relatively scarce and therefore expensive. If conditions are too damp, the fungus will develop too rapidly and cause grey rot, splitting the grapes and encouraging infections.
Furthermore, grapes infected by noble rot are not easy to process in the winery. The fungus contributes an enzyme (called laccase) that is capable of oxidising a number of components in grape must and wine, and is relatively resistant to SO2. Chilling, high doses of SO2 and use of inert gases are all options to minimise the oxidation of the must. The thick, high-sugar-content must is also difficult to press, clarify and ferment.
These wines are often matured in oak (either old or new) if relatively neutral grape varieties are used, for example Semillon. Oak can give a broader texture to these wines (the oak contributes some tannins), with new oak adding flavours such as vanilla. However, wines made from more aromatic grape varieties tend to be stored in stainless steel or concrete to retain their more pronounced varietal aromas.
FREEZING GRAPES ON THE VINE
This method requires healthy grapes to be left to hang on the vine into the late autumn or winter months. When freezing temperatures arrive, the water in the
grape pulp turns to ice. When the grapes are picked and pressed, this ice remains in the press and the sugar content of the resulting juice is concentrated. This technique is used to produce Eiswein in Germany and Austria and Icewine in Canada. These are protected terms, and winemakers must follow various regulations to be able to label their wines with these terms. For example, Canadian Icewine grapes must be harvested at −8°C (18°F) or below.
The vines used for these styles of wine need to be winter hardy and produce grapes with resilient skins that offer protection from disease and can withstand the strain of freeze–thaw cycles. The most popular grapes are Riesling and Vidal. Black grapes such as Cabernet Franc are occasionally used. These wines are often fermented and stored in stainless steel to retain the aromas and flavours of the grape variety. Oak maturation is sometimes used to add extra flavours (vanilla, clove, etc.).
The conditions for producing wine by this method are very specific. It can only be produced in a few regions where and when weather conditions allow and, even then, the unpicked grapes may become infected by disease or eaten by pests (netting against birds is an important cost in ice wine production). Freeze–thaw cycles are thought to be important for the development of typical ice wine character. The highest quality but lowest yielding grapes are picked late in the winter season, for example in late January. Similar to other wines made by the concentration of sugars, juice yield from the grapes is also very low. All of these factors mean these wines command premium or super premium prices.
A similar effect can be created by picking grapes in the autumn (at the same time as those for dry wines) and then freezing them at a winery. This is called cryoextraction. This technique can be used by winemakers in regions that would not get the typical climatic conditions required for Eiswein and Icewine. It also does not entail the risks of leaving the grapes on the vine into late autumn or winter and perhaps losing yield to disease or pests. It is not permitted in the production of Eiswein and Icewine and therefore wines made by cryoextraction cannot use these terms on the label. The method requires energy to freeze the grapes, but overall is cheaper than traditional Eiswein and Icewine production.
Explain when stopping the fermentation?
Fermentation is the process by which sugars in the grapes are converted to alcohol by yeast, and therefore stopping this process before all the sugars have been converted results in a wine with residual sugar.
The advantage of this method is that the winemaker has control over the level of sugar in the final wine. The earlier the fermentation is halted, the higher the level of residual sugar that will remain, but also the lower the level of alcohol. For example, Moscato d’Asti is produced in this way (without any concentrating of the sugar in the grapes beforehand) and contains 120–130 g/l and is 4.5–6% abv. It is more usual, however, for these wines to be off-dry to medium-sweet, for example White Zinfandel, rather than fully sweet. This process is relatively quick, simple and low risk (compared to many methods of concentrating grape sugars) and so it is often used to produce wines that are inexpensive or mid-priced. It is rare, however, to find the same intensity and/or complexity of the wines produced by concentrating the grape sugars, and they tend not to reach outstanding levels of quality. (The exception would be some fortified wines that may undergo long periods of ageing and be extremely complex.)
The most common way of interrupting the fermentation is by chilling to below 10°C (50°F) and/or adding a high dose of SO2 to inhibit the yeast. The wine is then racked off its sediment and sterile filtered to ensure fermentation does not start again at a later stage. Fortification, the addition of alcohol to kill the yeast, is also an option, but this radically changes the style of the wine. The winemaking for Fortified Wines will be covered in a separate unit.
Explain when blending in a sweetening component?
Adding a sweetening component is the easiest way of producing a wine with residual sugar. The dry wine can be stored until it is ready to be bottled and then the sweetening component blended in. The benefit of this is that dry wines are less susceptible to microbial spoilage than wines with residual sugar.
The winemaker is also able to trial, measure and add the amount of sweetener that is needed to produce the style of wine desired. This method therefore allows a high level of control, enabling high volumes of a consistent product to be made.
Also, compared to methods of concentrating the must where the volume of the wine that can be produced is reduced, adding a sweetening component will maintain or possibly slightly increase the volume of the final wine (depending upon the sweetness required) and therefore does not add to costs in this way.
These factors mean that this technique is suited to the production of high volumes of inexpensive wines. The quality of the wine being sweetened, and balance of the sweetness, will affect the quality of the final product, but because this method tends to be used for inexpensive wines, quality is likely to be acceptable to good.
Sugar, rectified concentrated grape must (RCGM) and unfermented grape juice (often referred to by the German term, Süssreserve) are common sweetening components. While unfermented grape juice will contribute a grape-juice-like character, RCGM is processed so that it just contains the sugar from the grape. It is therefore neutral (not adding any additional flavours to the wine), and smaller amounts are required to sweeten the wine. Both of these additions are inexpensive. The substance that can be used for sweetening is sometimes determined by local wine regulations. For example, the use of sugar is not permitted in the EU. Furthermore, for Protected Designation of Origin (PDO) wines, the unfermented grape juice must come from the same wine region as the wine being made.
