What is decanting?

Decanting means pouring over to leave the sediment behind in the bottle. Aerating means bringing the wine into contact with air. Often both happen at the same time, but they are two separate aims.

With an old red wine the first purpose matters most and you want as little air as possible. With a young, closed wine the second purpose is the reason and the wine may be poured generously.

The two processes also proceed at a very different pace. According to Maurizio Ugliano of the University of Verona, evaporation sets in almost immediately, while oxygen only reacts with the wine over hours or days. What you notice in the first minutes therefore almost always comes from evaporation and not from oxidation.

Sediment forms because substances that were first dissolved precipitate over time, and that happens in two forms.

The first form is tartrate crystals: potassium bitartrate, the potassium salt of tartaric acid. Crystals form when the wine gets cold. Wineries that want to prevent this chill the wine beforehand to minus 5 to minus 10 degrees, so that the crystals already settle in the cellar. Anyone who skips that step to avoid affecting the flavour risks crystals in the bottle.

The second form is the dark deposit in old red wine: colouring matter, tannin and the polymers that arise from them, with remnants of yeast cells added. Sediment is not a fault. It mainly says something about how little intervention there has been in the cellar.

Air removes volatile sulphur compounds and at the same time lowers the alcohol content in the glass, and both change the aroma profile.

In research published in OENO One, what happens to hydrogen sulphide when wine is swirled was measured. In model wine with 4 and 8 micrograms per litre, about 70 percent disappeared. In a chenin blanc with 10 and 20 micrograms per litre it was 38 percent, and that happened largely within the first 30 seconds. The decrease was mainly due to volatilisation, not oxidation.

Alcohol evaporates too. In a study published in 2016 in the Journal of Agricultural and Food Chemistry, wine in an open glass with airflow lost about 1 percent alcohol in two hours and up to 3.2 percent in six hours. Tasters could tell aerated wine from non-aerated wine.

No, in controlled trials the effect often turns out smaller than expected, and with fragile old wines it can backfire.

Stephen Reiss compared six identical samples of a 2011 cabernet sauvignon: two from the closed bottle, two from the open bottle and two that had stood for an hour in an open bucket at 33 degrees. In a double-blind test with professionals and consumers, the tasters found no clear difference.

In the glass study from 2012 the effect of shape was small at zero minutes and larger after five and ten minutes, which shows that time does more than the vessel the wine is in. For old wine, sommeliers therefore advise decanting and pouring immediately, because the fine aromas disappear quickly.

Young, sturdy red wines and closed white wines with a sulphurous note benefit most from it. Old wines with a lot of sediment you decant only to leave the sediment behind.

• Young red wine with plenty of tannin: decant generously, let it stand and keep tasting in between.
• White wine with a closed, smoky nose: Andy Howard MW cites, among others, chardonnay from Burgundy, Australia and California in Decanter.
• Old red wine with sediment: stand the bottle upright beforehand, decant calmly and stop as soon as you see cloudiness in the neck.
• Very old, fragile wine: decant and serve immediately.

There is no fixed time that is right for everything. Taste straight after pouring and then every quarter of an hour. The moment at which a wine shows at its best differs per bottle and per vintage.