Evaporation also causes a loss of liquid stored in tight wooden containers, such as wine undergoing barrel maturation. water, the principal component in wines, diffuses through small pores of oak, eventually reaching the outer surface of the stave, where it evaporates into the atmosphere. alcohol also diffuses through the stave, but at a rate considerably slower than that of water.
The atmosphere in contact with the barrel stave contains many molecules of water vapour (more when humidity is high, fewer in dry conditions) but relatively few molecules of alcohol. This diffusion–evaporation process means there exists a concentration gradient of water across the barrel stave which is determined by the relative humidity. Accordingly, high humidity slows the net transfer of water from the barrel interior to the atmosphere. Whilst the rate of alcohol transfer is not affected by high humidity, alcohol loss is enhanced relative to the decreased water loss. To summarize, wine subjected to barrel maturation in a high-humidity storage cellar will decrease in alcoholic strength whereas that stored in a dry cellar will increase.