What is 'Reduction' in Wine?

What is 'Reduction' in Wine?

The word 'reduction' crops up a lot in wine tasting notes. 'Flinty' reduction, 'smoky' reduction, 'match stick' reduction; these are all desirable and delicious, but what causes these aromas in wine? We asked our resident wine chemistry whizz, Jamie Goode, to unpack this intriguing topic.


The word 'reduction' crops up a lot in wine tasting notes. 'Flinty' reduction, 'smoky' reduction, 'match stick' reduction; these are all desirable and delicious, but what causes these aromas in wine? We asked our resident wine chemistry whizz, Jamie Goode, to unpack this intriguing topic.

One of the most interesting of all 'wine faults' goes under the name ‘reduction’. This name is actually a bit of a problem, because it has led to all sorts of misunderstandings. What we should really be calling this fault is ‘volatile sulfur compounds’ (VSCs) instead, but this is a real mouthful and sounds scary in a difficult chemistry sort of way. I’ll explain why the term ‘reduction’ is a misnomer in a bit, but first I need to mention why reduction is such an interesting wine fault.

Reduction is interesting because it isn’t always a fault, and some of these sulfur compounds, in the right context and at the right levels, are considered by many to be desirable. This infuriates wine educators and some wine judges, who want everything to be simple and binary. Instead, reduction is very much ‘it depends’.

Aromas of 'flint' or 'struck match' are commonly associated with 'reduction'

Breaking down the chemistry

The flavour impact of these VSCs varies dramatically. The most basic VSC is hydrogen sulfide, H2S, which smells of bad drains and cooked eggs. Hydrogen sulfide is often made by microbes present in our guts, and it’s a large part of the rather undesirable smell of a fart. In winemaking, it’s not uncommon for fermentations to go through a smelly stage where it all gets a bit eggy, but H2S is volatile and often blows off. It’s when it accumulates towards the end of fermentation, or is released from the yeast lees after fermentation finishes that it becomes a problem.

The H2S can then hang around long enough be converted to more complex sulfides or mercaptans, and these don’t simply blow off. These compounds also have the annoying habit of changing their form from a less smelly one to a more smelly one, and so a clean wine with a below threshold level of, say, a disulfide, can become stinky after it is bottled if that disulfide then turns into a mercaptan. Other VSC aromas include cooked cabbage, onion, gunflint, and sweet corn. Some even smell nice and fruity, like passionfruit and grapefruit. As you can see, this is a complex topic.

These VSCs are largely made by yeasts. They produce these VSCs in certain conditions, such as low nitrogen availability, or when they are stressed by sudden changes in temperature. Some yeast strains produce more of them than others. And sometimes reduction develops after bottling, especially where closures are used that don’t allow even the smallest bit of oxygen in. The chemistry involved is pretty complex, to the point that it still hasn’t fully been worked out. 

Why do we call this 'reduction'?

So if ‘reduction’ is a misnomer, why was the term originally coined? These problems frequently occur in wines at a stage where they are protected from oxygen (such as aging on lees in barrels), and reduction, chemically speaking, is the opposite of oxidation. So the association of VSCs with conditions where reductive reactions are encouraged (where oxygen is kept away) has led to the use of this name.

The good kind of 'reduction'

The reason this is such an interesting group of compounds is that some can be the source of faults in some wines, but provide positive attributes in others. Indeed, VSCs are important in the aromas of many fermented foods and drinks, such as cheese and beer, but have the paradoxical ability to be both positive and negative, depending on the context and concentration. In particular, there are a group of VSCs known as polyfunctional thiols that are a vital part of varietal character in Sauvignon Blanc, and—as more research results become available—it turns out they are involved in wine aroma for many grape varieties.

The most intriguing form of reduction in a fine wine context is that of flint/struck match/matchstick/mineral.

This is from a specific type of VSC that is one of a group of compounds called thiols or mercaptans – the terms are interchangeable. The flinty mineral character is something that works particularly well in the context of white wines, and of late there’s been a concerted effort by some winemakers to encourage this particular character in their Chardonnays. Related compounds can give attractive toasty, smoky, roasted crème brulée aromas. These can also work in red wines as well as white.

The volcanic vineyards in Tenerife express 'reduction' due to their soils, rather than winemaking alone.

'reduction' and a vineyard's 'terroir'

Some vineyards tend to give nice reductive characters to the wines that are made from them without any intervention from the winegrower. A great example here is the wines of Suertes del Marques from Oratava in Tenerife. These often show reductive notes, but it is not because Jonatan Garcia is doing something special in the cellar. He points out that he’s making the wines of Tameran, from Gran Canaria, in exactly the same way, also picking quite early, and these don’t show reductive characters. In this case, it’s the terroir having its influence. Perhaps the soils in Oratava are lower in nitrogen and so the yeasts start making VSCs. One thing interventionist winemakers like to do is to check nitrogen levels in the must and supplement them with nutrients so the yeasts are well provided for. But is this interfering with terroir expression?

Embracing 'reduction'

These days, now people realise what those complex matchstick mineral notes in Chardonnay actually are, many winemakers set out to add them to their wines. I really like them, when they are in the right context. With increasing numbers of winemakers looking to interfere less in the winemaking process, and with the growing acceptance of these sorts of complexities in wines, it’s becoming quite common to find a hint of reduction as part of the aroma of lots of different sorts of wines.