Almost every distillery tour follows the same format. First, you’re led by a display of raw materials. Then, the guide takes you around the fermentation tanks and by the still. But the magical part is what comes next. Once the whiskey is collected from the still, it’s put into barrels and stored in cool, shadowy warehouses called rickhouses. The air here smells of the vanilla and oak and grain from the spirit that’s evaporated. And since most rickhouses aren’t even wired for electricity, you almost feel like you’ve stepped back in time. Whatever comes from here will taste like pure wonder.

In reality, the spell was cast long before you stepped foot into these whiskey-scented buildings. Labels, websites, and other bits of marketing work together to paint pictures about things like generations of distillers, specific grain blends, or the surface details of aging. And within those first steps of any tour, a guide spins a narrative made of half myth and half fact, incorporating widely accepted statistics like the percentage of each barrel that evaporates each year. Despite the lack of published evidence to back such information up, these whiskey standards are often repeated as fact, especially by PR reps, bartenders, and enthusiastic consumers.

The truth is, most of the research being done on whiskey, especially about how and why it ages, will never be available to the public. With revenue from whiskey sales topping $2.7 billion in 2014 in the US and projected to keep rising, producers’ hesitance to share is somewhat understandable. In many cases, the data collected could give any company a competitive advantage.

As this high-stakes competition is increasingly met by consumers demanding more product and information, a kind of arms race has developed. On the one side, producers and scientists alike are racing to discover what affects the aging process and precisely how it works. On the other, an ever-growing number of entrepreneurs is inventing new ways to try to sidestep the years previously required to produce what we would all recognize as whiskey.

Fortunately or not, both the old guard and new experimenters working the quick-aging angle are strongly against sharing their findings. And with funding for scientific research drying up every year, it’s unlikely that outside academic work will catch up any time soon. As a result, compiling a comprehensive view of what we know about whiskey aging today is largely limited to what producers are willing to share. The resulting narrative is frustratingly incomplete, but it's also part of the magic that captivated drinkers around the world in the first place.

Barreled up

Despite the luxuries of modern technology, most distilleries continue to use oak barrels to age their whiskey. As the oldest and most conventional method of aging spirits, time in a barrel is usually a legal requirement for whiskey. In fact, the definition of what may be labeled as whiskey varies by country often because of barrel aging requirements. Some types of whiskey must be aged in nothing but new barrels, while others may be stored in previously used barrels. As a result, every type of whiskey has slightly different creation parameters and a slightly different taste profile.

Many countries—including Canada and the nations of the EU, among others—require the spirit to be aged for at least three years in wood. Some others, like the US, have no minimum age requirement. But outside of those legal requirements, a lot of factors contribute to how a whiskey tastes when it comes out of the barrel. Like many other parts of spirits history, the origin of barrel aging is so steeped in folklore that the exact story is lost to history. We simply know it has been standard practice for at least a few centuries. Wherever the idea came from, it’s widely seen today as making the hooch taste better.

The barrel’s exact contribution to whiskey is at once glaringly obvious (the color and some of the flavor) and extremely subtle. Every distiller will tell you that the barrel gives the spirit all of its color. Taste-wise, experts estimate that the barrel adds as much as 60 percent of the final product’s flavor. But the exact tastes that are transmitted to the spirit also depend on the type of wood used to make the barrel.

Tom Collins, an assistant professor of wine and grape chemistry at Washington State University, has studied the effects of barrel aging on wine and more recently on spirits. In a paper published in 2014, Collins and two colleagues examined the nonvolative compounds—compounds that won’t evaporate in air—present in 63 samples of different types of American whiskey.

One of their findings was that charring the barrel before adding the whiskey releases small aromatic compounds called phenols. “In the untoasted wood, [phenols] would likely have been part of the lignin, the polymer that basically gives wood its structure,” he tells Ars. Once they’re heated, small amounts of phenols can create depth in the spirit’s flavor and give it smoky or petroleum notes.

During the toasting process, cellulose, hemicellulose, and other polymers in the wood’s cellular structure break down, Collins tells Ars. These reactions cause “sugar degradation products” that add popular caramel and vanilla flavor notes to the whiskey. “A big chunk of the character of the spirit comes from what’s extracted from the barrels and from secondary oxidative reactions that occur,” he says.

All about the environment, man

Collins' work essentially looks at the traditional aging process. Once a barrel is charred and the whiskey is distilled, the whiskey is poured into barrels for aging. From there, it sits in the warehouse, and natural, seasonal changes in temperature cause the whiskey to expand and contract. This movement forces the liquid into and out of the wood. So depending on where the distillation takes place in the world, these changes can be mild (as in Scotland) or vary wildly (like in Kentucky and Ontario).

As every whiskey distillery tour will inform you, no matter where the whiskey is aged, some will evaporate. Called the angel’s share, the exact amount of evaporation varies by place, but it's said to average out to about a two-percent annual loss worldwide. That number can be much higher in warmer climates such as in the US and Canada.

But even within a single distillery or warehouse, conditions vary to the point that they can cause fluctuations in the resulting product. If the building is tall, for example, barrels near the top tend to stay warmer. And as a result, more water than alcohol evaporates out of the barrels, leaving a whiskey that’s higher in alcohol content. Near the bottom, it’s the opposite—barrels stay cooler. To compensate in either arrangement, many producers rotate their barrels hoping to even out the effects of temperature.

The exact effects of temperature and warehouse placement on whiskey are hard to gauge. Some distilleries have built experimental warehouses to test long-standing theories on how environmental factors impact their whiskey. Buffalo Trace’s Warehouse X is perhaps the most famous of these facilities. This research laboratory has five chambers whose airflow, light exposure, and temperature can be 100 percent controlled, says Master Distiller Harlen Wheatley. “We feel that knowing why bourbon tastes the way it does gives you a kind of edge,” says Wheatley. “We’ve got five chambers [in Warehouse X]. We test variables in each of those chambers, and we’ve seen that all five of them are different.”

As one might expect, experimenting with these conditions doesn’t always yield salable whiskey. But that’s not the point, says Wheatley. “We’re basically demonstrating that when you change the environment, either artificially or naturally, you change the bourbon,” he says. “That seems like common sense, but we’re proving theories. What we’re trying to do is put some numbers on it. In other words, how much does it change the bourbon and so on?”