Oak Barrels for Wine – Part Two

A cooper, or barrel maker, has the time-honored task of creating a liquid tight container (an oak wine barrel) from a pile of wooden staves.  The staves are heated, traditionally over an open fire but more frequently now with infrared radiant heaters or steam, until they become pliable.  The staves are then bent into the desired shape and bound together with iron rings.  The heating process “toasts” the barrel which creates a number of flavor components from wood chemicals and brings them to the surface for eventual contact with the wine.  The toasting can be light, medium, medium-plus or heavy, even charred (think Jack Daniels Whiskey barrel).  Following the traditional, hand-worked style, a cooper is typically able to construct one-to-two oak barrels per day. 

A Barrel midway through construction.

The level of toast, the type of wood and the style of cooperage, impart more or less flavor and complexity to the wine that will spend time in the oak barrel.  Lighter toasting tends to impart more oak flavors and wood tannins to wine.  Heavier toasting creates more furanic aldehydes, compounds that produce “roasted” aromas and flavors (nuts, coffee, toffee, tobacco and smoke).  Toasting also creates vanillin and the phenol compound eugenol that imparts a smoky and spicy note, similar to oil of cloves.  Many winemakers utilize barrels made from different countries, regions, cooperages and degrees of toasting to enhance the complexity of their finished wine.  The term “winemaker’s spice rack” relates to the way oak barrels are used in a winery to flavor different wines and blends.  In the U.S., Robert Mondavi is credited with important experiments in the 1960’s and '70’s on different types of oak and barrel styles that greatly expanded winemakers’ knowledge.

Through the natural porosity of wood fiber, oak barrels do two other important jobs for the winemaker:  1) allows slow evaporation of liquid and 2) low level exposure to oxygen (from the air).  Because oak wood is porous, evaporation reduces the amount of liquid wine in the barrel.  The evaporated liquid is replaced with air containing oxygen that can help mellow tannins and mature various flavor components.  Evaporation also reduces the amount of wine available - as much as 4-6 gallons of wine can be lost through evaporation each year from a typical 59 gallon (225 Liter) oak barrel.  The “angels’ share” is the common expression used to describe this evaporated, lost wine.  As liquid is lost, the resulting air space can dramatically increase wine exposure to oxygen, and perhaps cause premature oxidation or spoilage.  For this reason, winemakers constantly check their barrels and refill (a procedure called “topping”) with more wine during the aging process.  Just think, a large winery that produces 250 barrels of cabernet sauvignon per vintage (6,145 cs) will need 1,875 gal of topping wine (equal to 781 cs or another 31 barrels) in order to age the wine for 18 months before bottling and release!