The grape harvest appears to end when the tractors and trailers disappear from the roads but work continues in the wineries, hidden from view. What goes on in those wineries, be they huge cave cooperatives, magnificently equipped chateaux or humble artisanal cellars?
The basic process of making wine is common to almost all regions and quality levels but the differences are in the details.
When grapes first enter the winery they may be sorted by hand or machine to remove shrivelled, unripe, mouldy or rotten grapes as well as any leaves or snails!
White wine is made by pressing the grapes immediately to remove the pips and skins. Winemakers may use traditional vertical presses, larger horizontal ones or pneumatic bladder presses which offer a gentler, more efficient extraction of the juice, often aided by the addition of enzymes.
Often the bunches will be destemmed and crushed by machine before pressing but in some cases, notably Champagne, whole bunches are put straight into the press. The juice is left to settle in a tank to remove the sediment, called lees.
Usually this juice is chilled to prevent the fermentation from starting during settling. Some wineries may employ various techniques to reduce the amount of sediment lost or clarify the juice to a high level by adding fining agents and/or filtering.
The settled juice is then racked to the fermenting vessel. This may be a stainless steel tank, a large oak vat, several smaller oak barrels or other things like concrete eggs, clay amphora or polystyrene tanks. Each vessel leaves its imprint on the finished wine, be that the vanilla and caramel of oak or the minerality of clay.
The winemaker will also carefully control the amount of oxygen allowed to contact the juice. As little oxygen as possible for fresh, crisp whites and a small amount for more complex, richer whites.
Red wine needs contact with the skins in order to extract the colour and the tannins which characterise it.
Therefore the grapes are not pressed until after being fermented with skins and pips.
During red-wine fermentation, different techniques are used to manage the extraction of flavour and tannin from the skins. This is one of the major ways in which the winemaker designs the amount of body in the wine but also the texture and mouthfeel.
The fermenting skins rise to the top of the tank and form a “cap”. Winemakers may pump or spray juice/wine over the cap, break it up by punching it down or use clever tanks that continuously return the cap to the wine using the gas pressure built up by the ferment. Again, control of oxygen is important. More aeration will produce softer, smoother reds but may also increase the amount of volatile acidity, giving a slightly Port or vinegary aroma. A lack of aeration can lead to herbaceous, rubbery aromas.
Fermentation is the action of yeast to convert sugar into alcohol but the yeast also transforms other compounds in the grapes to give us all the wonderful flavours we find in wine.
There are many yeasts on the market that have been selected and cultured from regional strains, developed by microbiologists to emphasis particular flavours and aromas or to counteract problems with the juice, such as an acid imbalance or a high level of rot. The alternative is not to add any yeast and rely on wild yeast to do the job. This won’t give consistent results but can make more interesting, complex wines. Both wild and commercial yeasts live in wineries between vintages so each winery develops its own cultures of “wild” yeasts over the years.
The temperature at which a wine ferments will affect the end result. In general, cooler temperatures retain the fruity characters while warmer temperatures create a more ‘winey’ flavour.
Cooling may be achieved in several ways: by simply using concrete tanks which absorb the heat of ferment; placing refrigerating panels into the tank; buying tanks with inbuilt refrigeration jackets; or air-conditioning the whole winery. Some large modern wineries have completely automated refrigeration systems monitored and controlled by computer.
After between two and three weeks, the ferment will have finished. Traditionally, the yeast consumes all the sugar in the juice but certain styles of wine are made by retaining a proportion of the sugar by killing the yeast early, usually by a combination of refrigeration and sulphites but maybe by pasteurisation or sterile filtration.
Medium dry or medium sweet wines are made this way but many popular brands, especially from the New World, are designed to have between 5 and 10g or residual sugar to make them taste softer and fruitier.
So while fermentation is a pretty natural process that can be conducted in a hands-off way, there are many tools, additives and machines that can be employed to correct, manipulate or design the wine for its intended market even in those first few weeks in the winery.