Fermentation is the key. Under the action of yeast, many sugars can be fermented into alcohol and carbon dioxide. Apple juice can be transformed into cider. Malted cereals can become beer. Even leftover jam can start to ferment. Grape juice becomes alcoholic when the sugar in ripe grapes is transformed into alcohol and carbon dioxide in the presence of yeast, either the so-called ambient, wild or indigenous yeast that are present in the atmosphere, or more predictable specially cultured and selected commercial yeasts.
As grapes ripen they gain sugar and lose acidity. The riper the grapes, the more sugars are available to ferment into alcohol and the stronger the resulting wine, unless fermentation is stopped early and some sugar is deliberately left in the wine to make it taste sweeter.
Hotter climates tend to produce grapes with lower acidity and more sugar, which, if the fermentation is completed, will produce wines that are stronger than those from cooler regions. So, the hotter the summer, the riper the grapes and, usually, the stronger the wine. This is why wines made far from the equator tend to be lighter in alcohol. Wines from Puglia on the heel of Italy, for example, are much more potent than those produced in the far north of Italy, while the fledgling (but fast-improving) English wine industry makes wines notably high in acidity.
Once fermentation has transformed sweet grape juice into the alcoholic liquid we call wine, it maybe aged before bottling-especially if its a complex, age-worthy red. Fruity, aromatic whites are often bottled only a few months after fermentation to preserve the fruit and aroma, but more serious wines may well be aged for a further year or two before bottling to marry their different components, most often in containers of various sizes and ages made of oak, a wood that has a particular affinity with wine. The new and smaller the cask, the more oak flavour will be absorbed by the wine. The fashion today is to minimise obvious oakiness, so older, larger oak containers, or even neutral ones made of concrete, are increasingly common. Easy to clean stainless steel tanks are most commonly used for wines designed to be drunk young.