Just like with any other process, making wine results in by-products. Our story today concerns one of them. But to begin at the beginning. When you make wine, the first thing you do is crush the grapes - you break the skins and put them into a fermentation vat. The liquid separates from the skins, pips and stalks and stays at the bottom of the vat, while the skin, pips and stalk form a crust floating at the top of the vat. With red grapes, that's what gives the colour to the juice, as well as the tannins that help the wine age.

But today we're not concerned with the wine, but with that crust of stalks, pips and skins, which is termed in English the pomace. After the wine-maker has got the wine to the point that he wants, he has this by-product to deal with, and the best solution - other than using it as fertiliser - is to distil it. In Spain the resulting liquor is called aguardiente, in France it's marc, and in Italy it's grappa. It differs from other spirits therefore in its starting ingredient of pomace: whiskey, gin and vodka are grain based, brandy is distilled from white wine, calvados from cider and rum from sugar cane.

The principle of distillation is straightforward enough; you begin with a mash, a liquid in which the sugar content has been converted to alcohol as a result of the activities of yeasts - a process known as fermentation. Natural fermentation stops when the percentage of alcohol is between 16 and 17 per cent by volume, so to achieve higher alcoholic concentrations it's necessary to distil. Distillation works because alcohol is more volatile than water - that's to say if you heat a pan of wine, the alcohol will evaporate before the water. If you catch the first steam and condense it, it'll be mostly alcohol, a fact known in the East for millennia. By the fourteenth century the skill had arrived in Europe via the Arab civilisation of North Africa - we still use their words 'al cohol' for the liquid and 'al embic' for the pot-still.

There are two main kinds of distillation today, the traditional pot-still which allows the distiller control over which section of the distillate he wants to keep, and the continuous still, which as its name suggests runs continually. It's cheaper to run, is the most common commercial still, but its spirits have less congenerics, the bits that add flavour and character. The best spirits come from pot-stills.

Spirit of the Week

Grappa Cuvee 2001, Nonino.

Nonino have been making good grappa for over a century. Thirty years ago Nonino was the first distillery to make a grappa from a single grape variety - a monovine grappa. Grappa made from Moscato vines are common now, and it's interesting that the flavours of vine and grape do pass through into the distilled spirit. The Cuvee 2001 is a blend of three monovine grappas: Moscato, Ribolla Gialla and Verduzzo Friulano.

RRP €39.95 available Mitchells (Kildare Street); Bradleys (Cork); Gibneys (Malahide).