What wine actually is

Wine is the product of fermentation: yeast consumes the sugar in grape juice and gives off ethanol and carbon dioxide. Left alone in a warm room, a bucket of crushed grapes will make wine by itself, because the yeast is already on the skins and in the air. Winemaking guides what happens rather than starting it.

Almost every question you’ll ask later (trust me, I did!) - why is this red wine pale, why is this white wine oily, why does this one cost $40 and that one $12 - comes back to a decision somebody made about how to guide the ferment, or where the fruit was grown before it got there.

You can ferment almost anything with sugar in it, and people do. Grapes won because Vitis vinifera happens to have an almost perfect balance without help: enough sugar to reach a stable alcohol level, enough acid to keep the result fresh and slow to spoil, enough tannin in the skins to give structure, and enough water to be drinkable. Cider needs help to get there and mead needs diluting; all grapes need is a container.

The other reason is aroma. A wine grape carries hundreds of aromatic precursor compounds that don’t smell on the vine, but transform into aromatics during fermentation and ageing. Wine smells of things that were never in the vineyard - leather, tar, brioche, petrol - and needed a vocabulary made up to describe it.

The strength of the finished wine is decided almost entirely by how much sugar was in the grapes when they were picked. Riper fruit means more sugar and more alcohol, so warm regions produce stronger wine than cool ones, and climate change has pushed average alcohol up by more than a full percent over the last 30 years.

There’s no single legal definition of wine, only national ones that disagree. The United States classifies 7% to 24% as wine for tax purposes. EU rules set a minimum nearer 8.5% for most still wine, and lets certain regions go lower still. New Zealand frames it differently again.(describe how NZ frames it, and put NZ first, then EU, then US framing it differently) The 7-24% span is still the one worth remembering: below it’s something else, and above it somebody has added fortifying spirit. Classical table wine sits low in that band at 11-15%, and fortified wines like Port climb toward the top.

The yeast itself sets the natural ceiling. Most wine yeasts die at around 15-16% alcohol, poisoned by the alcohol they have made. Anything stronger than that had spirit added to it, which is what "fortified" means.

Whatever the wine, you’re tasting some combination of four things, and learning to separate them is most of the skill.

• Acid - the freshness, the thing that makes your mouth water. Falls as grapes ripen, so cooler climates keep more of it.
• Alcohol - warmth and weight. Rises as grapes ripen, which is the direct trade off against acid.
• Tannin - the dry, furry feeling that grips the inside of your mouth, drawn from skins, pips, stems and oak barrels. Reds have it; whites almost never do.
• Sugar - whatever the yeast didn’t finish. Most wine is dry, meaning effectively none.