I’ve been asked by a reader to explain what reduction in wine is all about. Before I offer some sort of explanation I should confess that I flunked chemistry rather badly at school, although I do find wine chemistry rather interesting, if occasionally incomprehensible.

My first stop when a technical issue arises is the Oxford Companion to Wine (Fourth edition) by Jancis Robinson and Julia Harding (what a formidable team).

There I learned that reduction goes hand-in-hand with oxidation. It involves the transfer of electrons from the compound being oxidised to the one being reduced. An example is the reduction of acetaldehyde to ethanol which happens in the final stages of fermentation.

Wine held in a stoppered bottle or another airtight container is said to be in a “reduced” state. I had a favourite uncle who liked the good life and never worked. Toward the end, he lived in a rather reduced state.

The Oxford Companion advises that “reduction” is used as a convenient, but rather inaccurate, term to describe the formation of sulfur compounds such as hydrogen sulphide and mercaptans (thiols), which tend to form under reducing conditions. Descriptors for these include rotten eggs, garlic, struck flint, cabbage, rubber and burnt rubber.

Winemakers talk about “good sulphides” (such as struck flint, although in my view it can be overdone to the detriment of wine) and “bad sulphides” (skunk is an example). It has become fashionable to encourage the formation of so-called “good sulphides” in high-end chardonnay by reducing the exposure of must and wine to oxygen and/or using wild yeasts.

The airtight seal of screwcaps has been blamed for reduction in wine, although the Oxford Companion claims that reductive characters can appear in a wine regardless of the closure unless these characters have been removed during winemaking.

And from Dr Vinny;

“The point of reductive winemaking is to preserve fresh, fruity, vibrant notes. You probably won’t be able to tell just by tasting a wine what kind of winemaking style was employed. But you might be able to pick out “reduced” notes. These generally result from the presence of a volatile sulfur compound, or mercaptans, and can be the result of reductive winemaking. Wine needs a certain amount of oxygen to polymerise (have its molecules combine), and if it doesn’t, the reduced notes may come in.

Some wines lend themselves to reductive winemaking—syrah, for example, is more reductive than pinot noir.”

I hope this sheds a little light on a complex topic.