"We’re not talking about reduction here, we’re talking about a wine that is reduced."
Professor Gilles Revel, one of the most eminent teachers at Bordeaux’s oenology department, has lost about half of his students attending his tasting class at this point. But he carries on regardless, trying to illuminate a point that is key to understanding not only potential faults in wine, but also the process of making fine wine in the first place.
Let’s get the simple bit out of the way. The term "reduced" is pretty much invariably referring to a wine fault. "Reduction," on the other hand, is about the process of winemaking in a reductive (i.e. oxygen-free) environment, as opposed to one that uses oxygen as part of the process.
Working out what exactly comprises good or bad reduction is, however, an area that causes plenty of confusion. Richard Bampfield MW suggests that, "it’s a semantic issue in many ways – many people simply refer to good or bad reduction."
What we can reasonably infer is that oxygen has both good and bad points when it comes to winemaking. Oxidation is what you can clearly see if you leave a glass of wine in the kitchen for a day or two and watch the color change, turning russet then brown. If a restaurant serves you a bottle of young sauvignon blanc that is already going brown even though it’s only a year old, you can pretty much guarantee that the cause is oxidation.
But using oxygen during winemaking is also a specific technique essential in the production of, for example, sherry, Madeira, certain types of port, and "vin jaune" from Jura. In the same way that reduction is "a semantic issue," these wine styles shouldn't be called oxidized, but oxidative.
The idea of reduction is essentially the opposite of oxidation. And it’s only when things are taken to an extreme – so they become reduced beyond the point of no return – that it is a fault. In the scheme of things, all "normal" fine wines are made in a reductive (read minimal oxygen) environment – and it is precisely that which allows wine to retain its fruity aromas and develop a bouquet as it ages.
Small amounts of oxygen are needed at key parts of the process (most importantly at the beginning of fermentation, usually on day two, when oxygen encourages the yeasts to multiply), and can be used to, among other things "fix tannins" during micro-oxygenation. But it can also be potentially harmful by causing, for example, the development of acetic acid bacteria that can lead to a vinegar smell.
So, efforts are made to ensure that oxygen stays away from the wine via the use of sulfur, inert gases, stainless steel tanks, and the topping up of barrels.
A reductive nose can be intriguing, such as the struck-match character often apparent on a dry white Bordeaux. Reductive aromas have even been linked by several researchers with minerality.
In contrast, the nose of a wine that is reduced (not reductive) is unpleasant – smelling of anything from burning rubber to off-cabbage. This is because both reductive and reduced smells come largely from a group of volatile sulfur-containing compounds known as mercaptans. Again, these molecules are not all bad. Some mercaptans – or thiols – are crucial to varietal character and are responsible for mineral, flinty and gunsmoke aromas – as well as those of grapefruit, passion fruit and cat’s pee.
It’s worth underlining that not all mercaptans come from the use of sulfur in winemaking, so the common idea that excessive SO2 use causes reduction is not entirely true. Making a wine in a reductive environment encourages the development of smelly sulfur compounds, as well as the more desirable fruity and/or mineral ones.
If heavily reductive (heading towards reduced) smells occur before bottling, there is still time to add oxygen to correct the fault (although not so much that it becomes oxidized). But as mercaptans are essentially unstable, it is perfectly possible that they will occur after bottling, leading to a wine that may have had a pleasantly reductive nose at bottling, but one that becomes reduced beyond return when in bottle. Whether screw-cap closures encourage this is a whole different subject, and again one that is filled with confusion).
Clare Tooley, international trade director at Direct Wines and a Master of Wine student, says: "As always with wine issues, it’s all about balance (too much/too little) and timing."
Even without understanding the science behind these two terms, you’ll know the difference: a reductive nose may have a whiff of flint to it, but if it has tipped over to reduced, you will be almost unable to hold your nose over the glass for more than a few seconds. And once you’ve experienced that, the difference between these two arcane pieces of wine vocabulary becomes crystal clear.