Reduction: The modern curse of the wine industry

Although reduction is an issue that has long plagued the wine industry, it is still something that, perhaps somewhat surprisingly, still occurs on a regular basis

Henry Powles Cobevco

Since my return to the UK wine trade four years ago, and as a regular taster of commercial wines, I’m often surprised at how common reduction is. I frequently come across young reds and whites that should have been fruity and expressive but instead exhibit, at best, a slightly herbal aroma and, at worst, the smell rotten eggs coupled with onion skin.

If I where a customer buying off the shelf and took one of these wines home, it’s safe to say I wouldn’t be impressed. But if reduction is so common, why haven’t we worked out a way to overcome it?

Cause of reduction

Before looking at how reduction can be prevented, it is first necessary to look at the reasons why it occurs.

There has been a lot of debate over recent years that stelvin, or screw top closures, can generate a reduction in wine that corks do not. Despite this popular belief, I’m happy to say that is not actually the case. One of the wines I tasted had been packaged in a bag-in-box format, the most oxygenative packaging possible, and yet reduction had still occurred. What then, if not the closure or packaging, causes this to happen?

Reduction, or the smell of rotten egg, comes about as a result of poor fermentation management, as it is produced when yeast becomes stressed during the fermentation process. There are a number of reasons that can cause the yeast to become stressed, including extremes in fermentation temperatures, a lack of oxygen early on in the fermentation process, elevated alcohol levels or a lack of nutrients.

Prevention

It is vitally important that the yeast inoculum, if one is being used, is properly prepared, as failure to do this can cause yeast stress due to a lack of sterol, or alcohol, formation, which further reduces the ability of the yeast to survive in high ABV environments.

The nutrient levels in the grape juice should also be checked before the fermentation process takes place, as too few can cause hydrogen sulphide, the chemical compound that causes the smell of rotten eggs, to form.

As soon as fermentation and malolactic fermentation (where tart-tasting malic acid that is naturally present in grape must is converted to softer-tasting lactic acid) have finished, any small amounts of reduction need to be eliminated either through the judicial use of copper sulphate or brass fittings in the winery.

These methods prevent aldehydes, another organic compound, from reacting with hydrogen sulphide, which can result in even more pungent compounds called mercaptans. If these mercaptans are left untreated, they can react further to form disulphides, which are very hard to remove from the wine and result in a very unpleasant tasting wine.

Although it is tempting to blame reduction on the type of closure that has been used, it is important to remember that reduction only occurs within the winemaking process. Luckily, by following these few simple steps and ensuring the correct processes are in place throughout the fermentation process, the risk of reduction can be greatly reduced, resulting in a better wine for the consumers, and a better reputation for the producers.

17 March 2014