Liquor evaporates if left open. Evaporation is the natural physical process where a liquid turns into a gas. All distilled spirits are composed primarily of ethanol (alcohol) and water. This process is inevitable because the molecules of both components constantly escape the liquid’s surface and enter the air, meaning any open container of liquor will slowly lose volume over time.
Volatility and Vapor Pressure
Alcohol evaporates more readily than water due to the intrinsic properties of the molecules. Ethanol is more volatile than water, meaning it vaporizes more easily at standard temperatures, a difference explained by their vapor pressure. Ethanol molecules are held together by weaker intermolecular forces compared to water molecules, which form a stronger network through hydrogen bonds. Because ethanol requires less energy to break these bonds, its molecules escape the liquid surface more easily. This results in ethanol having a significantly higher vapor pressure than water, leading to a faster rate of evaporation for the alcohol component; for example, ethanol’s saturated vapor pressure is about 2.7 times greater than that of water at room temperature.
The Changing Proof and Flavor Profile
Alcohol Concentration
The differential rate of evaporation causes changes in the remaining liquid’s characteristics. Since ethanol is more volatile, it evaporates faster than water, steadily decreasing the concentration of alcohol and lowering the spirit’s “proof.” The term proof is a measure of the alcohol content, with 100 proof equaling 50% alcohol by volume (ABV). As ethanol escapes, the water-to-alcohol ratio shifts, resulting in a measurable drop in ABV.
Flavor Degradation
The flavor profile also changes because aromatic compounds, known as congeners, are highly volatile. These compounds are carried away with the escaping ethanol vapor. This loss can leave the spirit tasting flat or unbalanced over time.
Environmental Factors That Influence Rate
Temperature and Air Quality
External conditions determine the speed at which evaporation occurs. Temperature is a major factor because higher heat provides more energy to the liquid molecules, increasing their kinetic energy and helping them overcome the forces holding them in the liquid state. Storing liquor in a warm environment or near a heat source dramatically accelerates the rate of loss.
Humidity and Surface Area
Air movement and humidity also play a significant role, as increased air circulation across the liquid surface constantly removes the escaped vapor. A low-humidity environment encourages evaporation, as the dry air readily accepts more vapor. Finally, the amount of liquid surface area exposed determines the total number of molecules that can escape, meaning a wide, open decanter loses volume much faster than a nearly full bottle.
Protecting Your Spirits From the Air
Protecting spirits from evaporation and flavor degradation involves controlling external factors. The most straightforward action is ensuring a tight seal immediately after pouring to minimize liquid exposure to the air. Screw caps must be fully tightened, and corked bottles should be kept upright to prevent the alcohol from degrading the cork material.
It is best to store bottles in a cool, dark place, such as a cabinet or basement, where the temperature remains consistent, ideally between 55 and 60 degrees Fahrenheit. This lower temperature reduces the liquid’s vapor pressure, slowing the escape of molecules. If a bottle is less than one-third full, the large air-to-liquid ratio increases the risk of evaporation and oxidation, so transferring the remainder to a smaller, well-sealed glass container can help preserve quality.