Production and aging of wine in the small oak cooperage

Wines & Vines, March, 2001 by Richard H. Graff, Andre Tchelistcheff

The importance of the first three items, first emphasized by Pasteur, has been forcefully demonstrated by Ribereau-Gayon [3], whose experiments with 60-gallon oak barrels gave the following results: in good cellars which are cold and humid, the evaporation loss can be kept to 1% or 2%; in ordinary wineries it is 4% or 5%; and in poorly designed wineries which are too warm, too dry, and/or too much exposed to air currents, the loss can be as high as 9%.

The evaporation loss is also greatly influenced by the type and thickness of the wood.

Another very important physical effect of small cooperage is temperature control during white wine fermentation. With proper cellar temperatures, artificial cooling of fermenting musts is never necessary, as there is sufficient surface area for the heat to dissipate by itself.

Physico-chemical. The very slow oxygenation of the wine caused by the gradual penetration of air into the barrel results in what we choose to call physico-chemical processes. These are closely associated with redox potential and the extraction and development of tannic and related compounds from the wood.

Next to the interior surface of the barrel a layer develops with a high concentration of tannoids and tannic compounds extracted from the wood by the wine. This extraction is facilitated by the small amounts of oxygen present which act as a solvent and react with the tannoids. The compounds which form at the interior surface are slowly diffused because of their different densities and continually being replaced by new wine.

In addition to the tannins, which contribute to the color stability of red wines and aid in the clarification of white wines, there is a certain amount of color, gummo-resinous substances and some bitter compounds. These substances are minimized in properly treated wood, and their relative proportions vary according to the origin of the oak. There are certain aromatic substances also extracted from the wood, such as cinamaldehyde and vanillin, which contribute to the quality of the wine's bouquet.

There are many slow reactions not necessarily related to the wood itself which proceed more readily in the presence of the small amounts of oxygen admitted by the wood, e.g., the formation of aldehydes, carboxyacids and acetals.

That the oxygenation is an essential feature of these extractions and reactions is demonstrated by the failure of attempts to produce artificial aging by means of oak chips, which can obviously give the proper surface-to-volume ratio in a large stainless steel tank of wine.

The use of oak chips cannot possibly produce the same conditions as exist along the interior surface of an oak barrel. Although some oakiness will be extracted from the chips, the absence of the proper amount of oxygen will preclude the extraction of some of the important compounds and the formation of others. The result will be a lack of proper balance in the wine. The introduction of air into the wine along with the oak chips does not work either, because it is not possible to restrict the oxygen to the area of high tannoid concentration along the surface of the wood. The oxygen, therefore, instead of reacting with the compounds of the wood, produces its well-known undesirable effects on the wine, such as browning.