<P> Oak is used in winemaking to vary the color, flavor, tannin profile and texture of wine . It can be introduced in the form of a barrel during the fermentation or aging periods, or as free - floating chips or staves added to wine fermented in a vessel like stainless steel . Oak barrels can impart other qualities to wine through evaporation and low level exposure to oxygen . </P> <P> In early wine history, the amphora was the vessel of choice for the storage and transportation of wine . Due to the perishable nature of wood material it is difficult to trace the usage of barrels in history . The Greek historian Herodotus noted that ancient Mesopotamians used barrels made of palm wood to transport wine along the Euphrates. Palm is a difficult material to bend and fashion into barrels, however, and wine merchants in different regions experimented with different wood styles to find a better wood source . The use of oak has been prevalent in winemaking for at least two millennia, first coming into widespread use during the time of the Roman Empire . In time, winemakers discovered that beyond just storage convenience, wine kept in oak barrels took on properties that improved it by making it softer and, in some cases, better - tasting . Robert Mondavi is credited with expanding the knowledge of winemakers in the United States about the different types of oak and barrel styles through his experiments in the 1960s and' 70s . </P> <P> The porous nature of an oak barrel allows evaporation and oxygenation to occur in wine but typically not at levels that would cause oxidation or spoilage . The typical 59 - gallon (225 - liter) barrel can lose anywhere from 5 ​ ⁄ to 6 ​ ⁄ gallons (21 to 25 liters) (of mostly alcohol and water) in a year through evaporation . This allows the wine to concentrate its flavor and aroma compounds . Small amounts of oxygen are allowed to pass through the barrel and act as a softening agent upon the wine's tannins . </P> <P> The chemical properties of oak can have a profound effect on wine . Phenols within the wood interact to produce vanilla type flavors and can give the impression of tea notes or sweetness . The degree of "toast" on the barrel can also impart different properties affecting the tannin levels as well as the aggressive wood flavors . The hydrolyzable tannins present in wood, known as ellagitannins, are derived from lignin structures in the wood . They help protect the wine from oxidation and reduction . </P>

Why are red wines aged in oak barrels