Oak Barrels for Wine – Part One

Oak is an important winemaking tool that can have significant impact:  influencing color, flavor, tannin profile and even the texture of wine.  Oak treatment normally occurs when wine is fermented and/or aged in barrels, but increasingly oak alternatives, chips, pellets, staves, etc., are used to add oak influence to wine in other vessels, e.g., stainless steel or plastic tanks. 

Almost all wooden barrels or wooden tanks used for wine are made of white oak.  White oak barrels influence the qualities of wine in several ways:  1) allows slow evaporation which concentrates flavor components, 2) allows low level exposure to oxygen (in the air) through the natural wood porosity which mellows tannins and modifies flavor characteristics, and 3) introduces wood-based flavors, like vanilla, spices, cocoa, smoke and tobacco notes.  There are also molecules in oak wood, known as hydrolyzable tannins, or ellagitannins, that can help protect wine from oxidation.  These molecules are derived from lignin structures in the wood and act as antioxidants, thus making oak barrels useful vessels for aging wine. 

There are two major types of oak – American and European.  The species Quercus petraea (white oak) and Quercus robur  (common oak) are found in France and other parts of Europe.  The white oak is considered superior for wine making as it has finer grain and richer flavor contributions related to vanillin, volatile lactones, tannins, phenols and volatile aldehydes, all compounds which strongly influence the aroma and flavor of a wine.  The primary French oak forests have names that are familiar to winemakers around the world:  Allier, Limousin, Nevers, Tronçais and Vosges.  Oak from each of these sources has slightly different characteristics, but a key property of French oak is the tighter grain which allows a more gradual integration of flavors into the wine. 

Most American oak barrels are made from Quercus alba, a white oak species characterized by relatively faster growth, wider grain and less wood tannin.  American oak most often comes from forests in the Eastern and Central U.S. - MO, PA, MN, WI and IN.  More recently, Quercus garryana white oak from OR has gained usage due to properties more similar to European oak.  American oak imparts stronger flavor characteristics to wine, and at a faster rate than French oak.  This is primarily due to the wider grain that allows increased contact of the wine with more “sapwood,” which usually contains more lactone flavor components than French oak. 

White oak trees used for wine barrels are typically 80-120 years old when harvested, most often in winter months when less sap flows in the trunk.  Once harvested, the tree is sawed or split into staves that are air dried for 2-4 years.  The tighter grain and less watertight nature of French oak requires coopers (barrel makers) to split the wood along the grain.  American oak is typically sawed into staves, making more efficient use of the tree.  One white oak tree can provide enough staves to produce, on average, two (French) to four (American) 225-Liter (59 gal) barrels.  Today, most of the wood waste from barrel production is used to produce oak alternatives in the form of powder, chips, pellets, etc.  Seasoning the oak staves in open air allows some bitter chemical components and wood tannins to be leached out of the staves.  Longer drying time decreases sap content and tightens grain structure, producing a better quality, albeit more expensive barrel.