I attended the Colorado Association of Vitculture and Enology seminar this last weekend. There were talks on several business aspects of the wine industry, a review of viticulture (Merlot, at ~25%, continues to be the largest cultivar in production here in Colorado), and a sensory presentation and sniffing exercise. One of the sensory faults mentioned was oxidation. This is one we've probably all experienced when leaving a bottle of wine out overnight. Oxygen isn't necessarily bad, but needs to be controlled. Apologies to casual readers, today’s blog is more technical than most.
Small amounts of oxygen are useful to stimulate the yeast when getting fermentation started (respiration), during barrel aging, and, when aging as the small amount of oxygen stored in a high quality cork can help a wine age by forming complex polymeric compounds which may produce a softer rounder wine with time. Too little oxygen is bad and can result in excessive reductive characteristics which could include aromas such as rotten egg (hydrogen sulfide), burnt rubber (mercaptans), and cooked corn (disulfides). Reductive characteristics are of particular concern to wines stored under screw cap.
In wine, oxygen is the limiting reactant in a pool of excess oxidizable substrates: phenolics, metals, enzymes, and free sulfur dioxide. When a wine is exposed to oxygen terpenes (a primary aroma component of wine) will react resulting in loss of aroma. Unless protected by sulfites, wines can brown (just as a fresh cut apple). Additionally, ethanol (the alcohol in wine) can react with oxygen to produce Acetaldehyde. Acetaldehyde is the sensory aroma that is the sign of wine oxidation (think rotten apples, or sherry which is an oxidized wine). The situation can become further complicated if wine is left for extended periods of time with excess oxygen on its surface allowing acetobacter to thrive and produce acetic acid (vinegar). Acetic acid can further react with Acetaldehyde to produce Ethyl Acetate. However, in the presence of just oxygen, the aroma of Acetaldehyde is the tell tale sign of excess oxygen exposure and is likely part of what you smell when leaving that glass, or bottle, of wine open overnight.
How does one protect a wine, either in the winery during production or as a consumer prior to consumption? Many wineries add sulfites (beyond the levels naturally produced by the yeast) to protect from browning and to react with the by-products of oxidation (sulfites are of little use in directly reacting with oxygen in wine). In the winery wine storage and transfer can have a large impact. We store all of our wines in variable capacity stainless steel tanks with lids that allow us to minimize the space between the lid and the wine itself leaving little room for oxygen to reside. Argon, an inert gas which is heavier than oxygen, is used to 'blanket' the surface of the wine and displace the oxygen. In barrel the wine is routinely 'topped-up' to replace wine that is lost via evaporation. When moving wine we can choose to either allow oxygen to contact the wine, or use inert gas to push the wine from one tank (or barrel) to another. At home, one can purchase inert gas in a can from wine shops to displace oxygen and preserve a partially filled bottle safely for many days. The vacuum stoppers act to decrease the amount of oxygen in a bottle and are fairly effective at storing a bottle overnight. Additionally, the wine can be kept at a low temperature, in your fridge, to slow the rate of reaction. Perhaps most importantly, particularly in the long term storage of unopened bottles, is constant temperature. A bottle that is exposed to increased temperature causes an expansion of the air in the bottle. If the bottle is subsequently cooled this volume of air contracts causing more air to be drawn in from the bottles surrounding. Repeated warming and cooling of a bottle thus 'pumps' air (which is 21% oxygen) from the cork.
As with many aspects of wine it is a situation of balance, the right amount of oxygen at the right time is critical. Too much oxygen, and wine loses its full potential.