Its presence in wine is often detected through a worrisome sign–aromas of bruised or even rotten apples. In high concentrations, there are even concerns about its effects on human health. At first glance, this should make acetaldehyde an enemy to fight at all costs for winemakers, but as with many other things in the world of grapes and fermentations, things are more complicated than that.
Whether during vinification or in the finished wines, acetaldehyde has different functions and effects in wines. During alcoholic fermentation, it appears as an intermediate product of the yeasts’ action as the sugars are being converted to alcohol. Acetaldehyde levels also change during malolactic fermentation, in conjunction with the action of malolactic bacteria.
Past the fermentation stage, the appearance of acetaldehyde occurs through ethanol oxidation, as oxygen introduced into the wine interacts with the alcohol molecules or other catalysts. Microbiology can still play a role at that stage as well, through the effects of acetic bacteria or through the action of the yeasts that form a film (often called a “veil”) over the wine, as in Sherry of the vin jaune of Jura.
Working on the chains
One of acetaldehyde’s most useful functions is to help bind tannins and anthocyanins– something it can do at any stage of vinification or aging. More precisely, it provides the chemical bridge that allows those two compounds to come together and form stable chains. This process, called polymerization, gives a wine better color stability and palate structure.
Dominik Durner, a professor of oenology at DLR Rheinpfalz in Neustadt, Germany, points out that for the acetaldehyde to play that role, it has to develop gradually in the wines. “Its formation has to be a continuous process, at a slow rate. If you have a large amount that develops quickly – say, from a large input of oxygen – you would get a lot of it that would not be able to contribute to polymerization and would just sit there.”
This is where properly-used micro-oxygenation, whether through spargers in tanks or through barrels, can play a particularly appropriate role in generating the right amount of acetaldehyde that then gets consumed through polymerization and disappears from the wine. The rate of oxygenation has to be monitored and well thought-out, however, as it is entirely possible to excessively micro-oxygenate a wine and inadvertently create excessive amounts of acetaldehyde–among other issues.
Durner also points out that when acetaldehyde is present in moderation, it can have beneficial effects beyond the polymerization of tannins and anthocyanins. “At 10 to 20 mg/liter, it helps to improve the wine’s aroma itself. It helps lift the good aromas. One of my B.Sc. student, a few years ago, did a research project involving sensory testing and he showed that pinot noir became fruitier with traces of acetaldehyde. At 100 mg/liter, however, its off flavours came through and the wine turned bad.”
At higher concentrations, acetaldehyde also heightens the alcoholic impression by creating a burning sensation on the nose. “Grappa, for instance, has high concentrations of acetaldehyde, and it gives a burning sensation, aromatically, because of that”, says Durner. The alcohol “heat”, in other words, doesn’t all come from the alcohol itself.
Take a deep breath
When acetaldehyde odors start showing up in a wine, is it too late to act? Has the wine been irreparably damaged? Not necessarily, explains Dominik Durner. In a bottled white wine that smells like browned apples a lot, it certainly isn’t good news. However, in reds going through fermentation or in the early stages of aging, it should be greeted with one thing: patience.
Winemakers could be tempted to add sulfites to the wine, since SO2 binds with acetaldehyde and thus makes the off-aromas go away. However, that also means that the acetaldehyde remains present (in a bound form, but still there) and that the SO2 is bound and therefore has no further protective effects on the wine. “In the cellar, SO2 additions shouldn’t be done immediately when high acetaldehyde contents are present. If you smell it, it doesn’t necessarily mean your wine is bad. Keep the tanks or barrels full and closed, and wait. It should react and bind, and two or three weeks later, the wine will be fruity again.”
For similar reasons, Durner also advises to wait at least two weeks after the end of malolactic fermentation to add any sulfur. Though the exact reasons aren’t fully clear, at the end of malolactic fermentation, acetaldehyde levels are higher, and that gets resolved over the following days. Rushing to add sulfites would just raise SO2 levels without having any useful effects on the wine. “Adding it too quickly will block processes that need to take place.”
There is one caveat to that general advice about patience: the later in the process, the more careful one needs to be. When acetaldehyde shows up towards the end of aging, the effects can be more permanent. By then SO2 additions would at least get rid of the off-odors, which becomes more of a worst-case scenario. Careful management of oxygen ingress, throughout the process, remains key to making sure the acetaldehyde only plays a useful and not a negative role.