Oak is a hard, strong, durable, watertight wood that comes from a tree and is from the genus Quercus. There are hundreds of species of oak, which can be classified into two broad categories: red oak and white oak. Red oak is quite porous, however, and is rarely used to make oak barrels. Nearly all barrels used for wine, beer, and spirits are made from white oak.
Before there was stainless steel, oak barrels were the typical vessel in which to store and transport beer. Oak has long been favored above other woods because its structure makes it watertight and also bendable, the latter quality being crucial for the production of barrel staves. While all oak species can impart their own flavors to beer, until recently this aspect of oak was rarely desired. At some European and English breweries, English, German, and Polish oak barrels were often used but did not impart much oak character. In some cases, the barrels were lined with pitch (a dark resin extracted from the conifer tree) which protected the beer from contamination. In Belgium, the lambic breweries have always procured used oak wine barrels from France in which to age their beer. Large oak vats were once the norm for fermentation and storage of beer in England, a fact made notorious when one of these burst in London in 1814, killing eight people.
As craft breweries became more popular, especially in the United States, many brewers have looked for new and unique ways to enhance the flavor of their beers. They quickly discovered that the use of used oak barrels that once housed bourbon, whiskey, wine, or, to a lesser extent, brandy, sherry, or port, could produce the bold flavors they sought. Bourbon and whiskey barrels are more often used for bigger styles of beer such as stout and barley wine. French oak is known for its fine mild character and is preferred by many wine producers. As a result, it tends to be much more expensive than American oak. Charred American white oak is used for the production of bourbon barrels, and by law a bourbon barrel can only be used once to produce bourbon whiskey. As a result, the American bourbon distillers produce thousands of once-used oak barrels every year as a by-product. While most of these barrels go on to second uses aging everything from Scotch whisky to high-end tequila to Tabasco sauce, an increasing number end up in the hands of American craft brewers. Bourbon barrels are by far the most common oak barrels used by brewers in the United States. The cost of these barrels has risen dramatically over the past few years due to increased interest from brewers but also from whiskey producers in countries with quickly developing economies, particularly China and India.
Before stainless steel, oak barrels were typically used to store and transport beer.
The oak itself contains a number of flavors, and those flavors differ widely depending on the oak species, the growing area, and how the wood has been treated. A range of compounds known as lactones are responsible for many of the aromas and flavors we associate with oak. These are derived from lipids within the wood and tend to impart a coconut-like flavor, especially in American oak. Bourbon barrels, therefore, tend to have plenty of producer will choose wood depending largely on its flavor profile. A prevalent secondary characteristic is the vanilla-like aromatics of the compound vanillin, an aldehyde that can be pleasant at low levels but can easily overpower other flavors. Beyond this are the aromatics of volatile phenols such as eugenol (clove-like) and 4-vinyl guiacol (cinnamon-like), and floral, fruity terpenes. Oak also contains astringent tannins, which can leach into beer over time and possibly clash with hop bitterness.
When using newly acquired used bourbon or whiskey barrels, brewers can choose to rinse them or rack the beer directly on top of any leftover liquid. Rinsing or cleaning the barrel first will lead to softer, more subtle flavors. However, it is more common for brewers to rack beer right into the barrel, and if there is a liter or two of the previous tenant still in the barrel, all the better for sanitation if that tenant was a whiskey. If the latter technique is chosen, the brewer may opt to blend some non-barrel aged beer back into the finished beer to cut the strength of the barrel aged beer and soften the overall character.
Brewers using wine barrels often use them in conjunction with the “wild” yeast Brettanomyces and bacteria such as Lactobacillus and Pediococcus.
Brettanomyces has several unique characteristics, such as the creation of a floating mat, called a pellicle, on the surface of the beer. Another is its ability to eventually consume almost any type of sugar, including the cellobiose which is created during the toasting of the oak. And a near continuous fermentation by Brettanomyces can create a constant exhaustion of carbon dioxide, thus helping to protect the beer from oxidation.
Micro-oxygenation, the slow and steady ingress of oxygen through the porous wood and into the barrel-aged beer, has many implications for beer. Brewers, as well as winemakers, may choose to use French or American oak barrels. French oak is more porous than American oak, thus allowing more oxygen diffusion through the oak itself. The micro-oxygenation of the beer has a direct effect on the acetic bacterial (acetobacter) development in sour beers. Acetobacter requires oxygen to flourish, and more oxygen means more acetic (vinegar-like) character in the beer, which may or may not be desired. Barrels that are not topped up or completely filled, leaving head space in the barrel, may also develop an acetic character.
Like winemakers, brewers must also take into consideration barrel size. Most brewers would consider a barrel that is 219 to 227 liters (57 to 59 US gal) a standard size barrel. Barrels that are smaller than 219 to 227 liters (57 to 59 US gal) can provide more oak surface area contact for the beer; however, this gives more flavor but can also lead to too much oxygen diffusion. Large oak tanks, called fouders, provide less oak surface area and can potentially protect the beer from too much oxidation. However, most brewers choose to use the more standard size barrel to provide more of an opportunity for blending. These standard size barrels are also easier to obtain.
Brewers who choose to age their beer in barrels without the use of Brettanomyces may pick up some residual oak character and wine flavor. However, the beer is at a greater risk of oxidation since there is no secondary fermentation to provide a protective barrier of carbon dioxide in the barrel and the possibility of no pellicle.
Careful consideration must be given to cleaning a barrel as this will directly affect the outcome of the beer. The most common cleaning methods are water, steam, pressure washing, scraping, chains, or less frequently, chemicals. Regardless of the brewer’s choice, how a barrel is cleaned will directly affect the beer being aged in the barrel.
New oak barrels may also be used for aging beer, but they are not common due to high costs. Oak products such as oak chips, cubes, powder, shavings, or immersible staves are useful if ersatz alternatives to new oak flavor is desired. These products have become popular in the wine industry due to their ability to be used in not only stainless steel tanks but also in barrels that have little or no oak character left after repeated uses. Some flavors that new oak will contribute are wood, vanilla, dill, spice, and toastiness. Alternative oak products can be cost effective and are easily accessible to most brewers.