Grapes with maximum extract and sugars are required, which normally entails restricting yields. Such grapes may be picked either before, at, or after full ripeness. Twisting the stalk was once practised in Veneto but most growers now prefer to dry their grapes off the vine.
Those who pick slightly before full maturation claim there is less risk of rot, thicker skins, enhanced resistance during drying, and higher acidity, all of which favour aroma, freshness, balance, and longevity—and concentrate the grapes which remain on the vine.
Only the ripest, healthiest grapes are generally picked, which today means a pre-selection by experienced pickers (although see recioto). Healthy grapes are vital since any incipient mould or rot soon spreads during the drying process. Skins must remain intact, to which end the grapes may well be laid in small trays for transport to the winery. The bunches should be spargolo, loose rather than compact, so that air circulates around the individual berries during the all-important drying process.
Sun drying is still practised in places such as the Sicilian island of pantelleria off Tunisia, in southern puglia, and the Greek island of santorini. The process can be many times faster than drying under cover, but this can result in excessive colour, caramelized flavours, and loss of aroma, bypassing some of the microbiological transformations which are the essence of fine dried-grape wine.
Most grape drying for commercial purposes happens in a winery loft, where windows may be opened to let in plenty of air (essential against the development of rot and mould). Bunches are hung up vertically (on hooks, or on long strings), or laid out horizontally on neutral, bone-dry materials. Straw is rarely used because of its attractions for mice. Wire mesh, nylon nets, and wooden or plastic fruit boxes were 20th-century developments; more traditional cane and rush mats and bamboo racks remain popular in much of Italy. Today, in zones where dried grapes constitute an important factor in the local wine economy, purpose-built grape-drying plants are being created complete with temperature control and wind machines.
The duration of the drying process is dictated by the grape variety, the type of wine required, and microclimatic conditions during drying. Sugar-rich Greek grape varieties such as Muscat, Aleatico, and Malvasia require less time than more northern varieties. Three weeks may suffice for a Muscat whereas a Veronese variety such as garganega for a white Recioto or corvina, corvinone or Rondinella for a red Recioto or Amarone will need three to four months, in some cases, for Recioto, even up to six months. Ideal conditions include considerable currents of dry air, and humidity is such a problem in some valley sites that drying facilities are being moved to higher elevations. Excessive heat is generally regarded as negative, as is excessive cold.
The main effect of drying grapes is loss of water and the consequent concentration of sugars. The relationship between water loss and sugar gain is relatively direct so that a water loss of a third from grapes picked at 12 °baumé would result in a wine of 16% alcohol (if all the sugar were fermented out). Depending on the wine style desired, the loss of grape weight by evaporation varies between 10 and 60%, with the norm for a passito wine being somewhere in the region of 35 to 40%, so the potential alcohol is raised by just over a third.
Other components behave less predictably. The total acidity in grapes undergoing a 40% dehydration rises not by 40% but by around 25%. These and other organic substances undergo various transformations, and there may be development of certain aromas and loss of others in the process. The longer the drying period, the greater the biological change of organic substances and resultant wine quality.
noble rot may develop on the grapes during dehydration but it is not desired by most practitioners, particularly those making the drier styles of dried-grape wines such as Amarone. A further problem is insect infestation, particularly of bees, wasps, and hornets.
Crushing or pressing should ideally be as gentle as possible. Gravity, but certainly not centrifugation, may be used to clarify white must, while in the case of red wines, stems may be totally or only partially removed.
The must of raisined grapes is so concentrated that it slows fermentation, an effect accentuated in cooler climates, especially where the long drying period may mean that the grapes are crushed in midwinter and the ambient temperature is naturally low. In Italy, fermentation may therefore safely take place in wood, and may need to be started by heating or by adding specially cultured local yeast. In traditional areas, the right yeasts have been in the atmosphere for centuries. Saccharomyces uvarum begins the job in Valpolicella, according to Masi, while Saccharomyces bayanus is able to work at higher temperatures and at the alcoholic strength of 16% or more that is necessary for many Amarones.
Some producers allow the fermentation to stop and start for months or even, as in the case of the late Giuseppe Quintarelli, two to three years, and allow Nature to decide how sweet the final wine will be. Most, however, use racking and, increasingly, refrigeration to stop fermentation.
The wine is then generally racked off its lees and the lees sometimes used to enrich normal valpolicella, the process called ripasso. In Tuscany, the so-called governo process is employed.
Dried grape wines tend to be particularly high in volatile acids, a direct result of high sugar levels (accentuated if any botrytized grapes have been included). The acetic acid of such a wine may well exceed legal levels, sometimes entailing unacceptably high sulfur dioxide additions. Many argue that high levels of volatile acidity are essential to the quality of such wines, and some maintain that false ‘passito’ wines can be exposed precisely by improbably low levels of acetic acid.
Dried grape wines may be divided into two categories: those in which the fresh primary aromas are retained and those in which primary aromas are sacrificed to the development of a more complex bouquet. The former include most wines based on aromatic varieties such as Muscat, Brachetto, Aleatico, and Riesling, as well as sweet whites where the emphasis is on fruit, such as Recioto di Soave. These are subjected as far as possible to protective winemaking techniques.
Vin de paille and Vin Santo, with their rancio character, are the most notable examples of the oxidative style. Amarone and Recioto della Valpolicella of the traditional type are also treated oxidatively, the aim being to incorporate in the final tasting experience an evolution of aromas due in some measure to exposure to oxygen. Traditional Amarone and Recioto are, typically, the result of prolonged maceration, deliberately frequent racking, and ageing for years in large, old barrels. Strong, dry Valpolicella Amarone is a notable example, as is Vin Santo with the rancio character encouraged by traditionalists. Since the 1980s, there has been a movement away from such classic styles, however, and modernist Amarone and Recioto producers are aiming for more fruit-driven wines.
Italy produces more dried-grape wine than anywhere else, not least because of the dramatic increase in Amarone production, but experimentation with the technique has spread to Australia, South America, North America, and much of Europe.