Why does the alcohol content in wines vary so much?

Drinking two Australian white wines, I noticed a big difference in their alcohol readings: one was 12 per cent, the other 14 per cent. Why is this?
K.E., Springvale, Vic

Credit: Drew Aitken

A: I was asked the same question recently in a class where the wines in front of us were a Wise Sea Urchin Sauvignon Blanc Semillon (12 per cent), from Margaret River, and a D’Arenberg The Hermit Crab Viognier Marsanne (14 per cent), from McLaren Vale.

The simplest explanation is that different grape varieties need different amounts of heat and sun and need to be harvested at different ripeness levels if the wine is to have the desired flavour. Viognier is notorious for being quite fickle with regards to picking time: too early and it tastes green and lacks varietal character and palate appeal, too late and it’s flabby and oily.

On the other hand, both semillon and sauvignon blanc are pretty happy when harvested at relatively low levels of sugar. So is riesling. Their labels often declare 12 or even 11.5 per cent alcohol. Hunter Valley semillon occasionally dips as low as 10 per cent and still tastes ripe.

The amount of sugar in the grapes is usually taken as the primary indicator of ripeness. And the amount of sugar in the grape (measured in degrees Baumé) corresponds pretty closely to the amount of alcohol in the wine if it’s fermented to dryness. You might even notice the difference if you drink, say, a couple of glasses and compare how you feel afterwards. Two per cent alcohol can make a difference to how quickly you become intoxicated. So beware those big hearty reds that have 15 per cent or even more!

The accuracy of declared alcohol is another question. Australian law permits winemakers a 1.5 per cent leeway in stating alcohol content on table and sparkling wines. So, theoretically, a red labelled 14.5 per cent could actually be as low as 13 – or as high as 16! That said, I think most winemakers try to be straight with us.

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