Wine, beer, cider, spirits, sake, almost every type of fermented beverage can be aged or stored in oak barrels in order to enhance its flavor, complexity, and texture. This technique has been in practice for hundreds if not thousands of years, with different cultures using the semi-porous nature of wood in a myriad of ways, albeit for similar purposes. But what is happening during this aging process, and why has oak, above other types of wood, been the focus?
Although most types of wood are watertight (as demonstrated by its use as a boat building material for millennia), oak timber in particular has a very tight grain, meaning that its density is higher in relation to other lumber. This means that it can be used to store liquids, such as fermented beverages, that suffer from oxidation with less risk than the traditional clay amphorae used previously. That’s not to say that oak is completely impermeable; it still allows for minuscule amounts of oxygen to come into contact with the fluid stored inside. This process, also referred to as micro-oxidation, allows for the beverage to slightly oxidize over time, creating flavors of dried or cooked fruit, petrol, nuts, honey, forest floor, mushroom, or even leather, depending on the type of potent potable stored.
Additionally, when choosing to age wine in oak, the winemaker must choose between ‘new’ oak, which has not yet been used previously, or ‘neutral’ oak, meaning it has been used multiple times already. This is a critical decision, as new oak is able to impart many more flavors and aromas than oak that has been used previously. As a wine ages in a new oak barrel, it imparts certain flavor compounds, such as vanillin, to the wine, which provide the wine with aromas and flavors of vanilla, coconut, coffee, and baking spices, depending on the oak’s origin.
Neutral oak however has already imparted these flavor compounds to a previous batch of wine, and so, although still allowing for micro-oxidation, will not have such a profound effect on the stored liquid. This can be beneficial for things like aromatic white wine or cider, which naturally have a more delicate balance of flavor, and could be overwhelmed by the intense flavors of new oak.
Not all oak that has been previously used can be considered ‘neutral’, however. If an oak barrel has been used to age whiskey, port, or sherry, it can be in great demand for aging beer or spirits. The flavor imparted to the barrel by the initial fortified libation will affect whatever is stored inside next. There is an increasing trend within the United States of aging stouts in previously used whiskey barrels, and many whiskeys are aged in barrels previously used to store port or sherry. This complex life cycle of an oak barrel not only allows for fascinating flavors to be created, but also serves a financial purpose, as even a neutral oak wine barrel can cost well over $1,000.
Oak barrels used for spirits typically undergo an additional process before being filled, where the interior is either gradually heated (as done in Europe) or purposefully heated to flame (as seen in the United States), in order to blacken the inner wood. This process allows for undesired components of a distilled spirit (such as fusel alcohols) to be absorbed by the carbon in the charred wood, resulting in a smoother texture and cleaner flavor profile in the spirit.
The use of oak barrels for aging beer has been around for centuries before the invention of stainless steel vessels, and modern brewers are discovering new ways to put their own twist into the process. English “real ale”, or cask -conditioned ales, can still be found in many places across Britain. For this style of beer, fresh, uncarbonated beer is added to an oak barrel at the pub, and it finishes its fermentation in that cask, ready to be dispensed at the peak of freshness for patrons. The oak is neutral, so does not impart any flavors to the beer, but it ties current brewing with the traditions and history of the style. Belgian brewers also use oak casks to produce and age some of their beers, specifically sours, because over time the oak barrels become a natural habitat for the various microorganisms that are vital to the creation of these styles. Finally, creative American brewers are experimenting with oak for a variety of styles such as Bourbon Barrel Stouts and Vanilla Porters. This use of both new and used oak adds an exciting dimension to the American brewing scene.
Within the family of ‘oak’, there are a number of different specific styles, and each impart differing flavors and aromas depending on their place of origin and how they are treated prior to storing any liquids:
American Oak: Although technically a different species from the European species of oak listed below, American oak imparts similar characteristics as its European counterpart. The predominant difference is that American oak contains much higher levels of lactones, a chemical compound which is known to impart flavors of coconut and even dill.
French Oak: French oak is the original oak used in the aging and storage of wine in Europe. It is known to be subtler and more nuanced than American oak, and tends to impart more tannin. Often French wines are stored in massive oak barrels, such as barriques or foudres, which can traditionally hold up to 60 gallons and 1,200 gallons respectively.
Hungarian/Slavonian Oak: Oak produced in the cooler continental climates of eastern Europe tends to be significantly denser than American or French oak. This is due to the fact that trees grow more slowly in this region, resulting in a compact wood grain. This means that the stored fermented liquid extracts the chemical compounds from the oak at a lesser rate, and the effects of micro-oxidation are diminished.
Japanese Cedar: Although technically a different type of wood, Japanese Cedar or ‘Sugi’ has traditionally been used to age sake in a very similar way as European winemakers would use oak barrels. Sugi was first used in the storage and transportation of sake starting in the early 1600s, corresponding to the start of the Edo era of Japan. The barrel size is much smaller than those used in wine (on average around 19 gallons), so the wood imparts a significant amount of flavor in a short period of time. Because of this, sake aged in sugi (referred to as taru sake), is only aged in barrel for one to two weeks, and has intense, spicy flavors of ginger and cedar.