Bourbon barrels in a warehouse

Bourbon is aged in rick houses. The Master Distiller decides when each barrel is ready

Will you be toasting the New Year with a glass of bourbon?  Judging by the growth of the bourbon industry, many people seem to enjoy Kentucky’s signature whiskey. The bourbon industry is booming, with new distilleries and warehouses being built and old, even abandoned, distilleries being revived.

As a forester, when I think of bourbon, I think of white oak. Here for your holiday reading is the story of bourbon, essence of white oak.

Bourbon and its flavors
Bourbon is a product defined by federal law. It is produced in the United States, though not necessarily in Kentucky. It is made from any grains but must be at least 51% corn.  After fermentation of the mash, bourbon is distilled to no more than 80% alcohol by volume, aged at no more than 62.5% alcohol and bottled at 49% alcohols or higher.  There is no minimum aging period except for straight bourbon, which is aged for at least 2 years and has no additives. Bourbon must be aged in new, charred oak barrels.

Mash fermenting in open tanks

The mash is fermented in open tanks made from baldcypress wood

Bourbon is one of the most complex beverages known. Although most distilleries use traditional methods of making bourbon, there is increasing interest in how bourbon acquires its flavors. Some distilleries, especially new ones, are studying bourbon chemistry in hopes of accelerating the aging process. For a startup distiller, waiting many years to produce a marketable product is hard on the bottom line

The flavor of bourbon comes from the grain bill (the combination of grain ingredients), the oak wood of the barrel, and the combination of time and temperature in the rick house (the warehouse in which barrels are aged).  Of these, the oak wood is the most important.  Freshly distilled white dog, the distilled whiskey before it goes in the barrel, has complex taste but very little smell.

cis-3-Methyl-4-octanolide, also called whiskey lactone or quercus lactone. ChemSpider

cis-3-Methyl-4-octanolide, also called whiskey lactone or quercus lactone. ChemSpider

Bourbon contains hundreds of compounds that may contribute to flavor. Over 350 of these compounds are volatile, meaning that they evaporate at room temperature and may contribute to flavor. Many of these volatiles have been identified and they include molecules like (E)-βdamascenone,  δ-nonalactone,  vanillin, cis-3-Methyl-4-octanolide also called whiskey lactone, and eugenol.  Nearly all these chemicals have one thing in common – they are natural compounds present in oak wood.  It is the wood that gives bourbon its characteristic color and flavor.  Wood is a highly variable, complex material, whose chemistry depends on where the tree grew, what kinds of stresses it experienced and all the events in its long life. When you take a sip of bourbon, you are tasting the entire history of an oak tree. Some distilleries are now experimenting with barrels made from individual trees from known locations to see what effect the tree’s site and history have on bourbon flavor.

The barrel

Oak wood

Red oak (upper) has open vessels; white oak (lower) has vessels sealed with tyloses (T) that prevent leakage. Photos by Elisabeth Wheeler, Peter Gasson courtesy Inside Wood

For early settlers in Kentucky, the barrel was the most useful form of storage and shipping.  Every town had a cooperage, where skilled coopers cut wood into staves and assembled them into barrels.  The village cooperage was as important to daily life as the village smithy.

The cooper had to understand differences among trees. Oak was the most important source of raw material for barrels, boxes and crates.  A barrel made of red oak would not hold liquids but would allow exchange of air. A barrel made of white oak would hold liquids but allow little exchange of air.  The difference between red oak and white oak seems minor but was critical: all oaks have large vessels, pipes that allow the flow of water in the living tree, but in white oak, the vessels become plugged with tyloses that stops them from conducting water. Fill a white oak barrel with liquid and the tyloses will keep the liquid inside the barrel. Fill a red oak barrel with liquid and it will leak out, but dry goods like grain will stay dry and aerated.  The oak barrel industry produced slack cooperage from red oak for dry goods and tight cooperage from white oak for liquids.

Barrels were made from other trees, but white oak is so abundant in our forests, and the trees so large, that tight cooperage depended almost entirely on white oak . Today we use cardboard, plastic and other materials for storing our stuff,  but white oak has remained the only material permitted for making real bourbon.  The village cooperage is gone in most of the US, but Kentucky today has dozens of busy cooperages meeting the growing demand of bourbon distilleries.

Oak barrels are not easy to make. Wood has to be harvested, sawn, carefully dried to avoid decay and mold, resawn into staves and assembled into barrels, a process that can take up to 3 years. In a modern cooperage, staves are steamed to bend them into place while the hoops are added to hold the staves. No glue or nails are used.  Before the industrial revolution, steam was not easy to come by. Early coopers would assemble the staves, then heat the inside of the barrel with fire to make the wood flexible enough to bend into place.  The charring of a barrel was the result of the manufacturing process. Only later did it become clear that charring or toasting the inside of the barrel added to the flavor of the bourbon.

The charring or toasting process is very important to the flavor of bourbon. Toasting breaks chemical bond in the lignin, cellulose and hemicellulose that make up wood, creating smaller molecules that add flavor. For example, heating lignin creates vanillin, the characteristic vanilla fragrance of good bourbon.  Charring burn the inside of the barrel into charcoal, which absorbs some of the wood chemicals and mellows the flavor. Each distillery specifies the degree of toasting and charring for their product.

Bourbon barrels in a warehouse

Barrels are moved around in the rick house during aging

Bourbon is aged in rick houses or barrel houses. These are unheated buildings in which barrels are stacked in oak ricks. The master distiller periodically has barrels moved around the warehouse to even out the impact of temperature.  The rising and falling temperatures in the rick house are important to the flavor of the finished product.  As the whiskey heats and cools, it moves by diffusion and mass flow in and out of the barrel staves. This increases the extraction of flavor from the wood.  Use of a climate controlled rick house would probably produce a poor product. Several distilleries are now experimenting with the effect of various rick house designs on heat flow among the barrels.

A little bit of history

The history of bourbon is clouded in mystery, with at least three stories about who created bourbon, when and where.  Distillery tours are a great source of information about the history of bourbon, but there is a fair amount of marketing in claiming to be the “original” distillery. Even the name ‘bourbon’ is of uncertain origin. I won’t go into the controversies over the history, but only want to focus on the barrel.

1812 cabin

1812 cabin built by Elijah and Sarah Pepper. Elijah was one of the first bourbon distillers

A few general facts are known. Farmers produced abundant grain in Kentucky, but had no means of getting the product to market – it was impossible to ship grains over the mountains to the urban east coast, and flatboat transport down the Mississippi River made Kentucky grain too expensive by the time it reached New Orleans.  Instead, farmers created distilleries and shipped whiskey to New Orleans, where it went on to the east coast.  Barrels were filled at the distillery and shipped by flatboat, taking weeks or months to reach New Orleans or the east coast. By the time it arrived, the barrels had been heated, cooled and sloshed around, all the while extracting the oak ingredients that make bourbon.  It appears likely, then, that the first bourbon to reach market was an accidental result of the shipping process. Soon, distillers in Kentucky learned the benefits of aging in barrels, and the first rick houses were built, creating a  more uniform product that became a branded product instead of a commodity.

Where do the barrels go?

Bourbon barrel used for art

Barrels can only be used once for bourbon. Most are reused and some become art works

Barrels can only be used once in the bourbon distillery – the law requires new oak barrels.  Fortunately, bourbon barrels have a long career after the whiskey is poured out at the distillery.  There is a thriving industry in the re-use of bourbon barrels.  Many other products can be aged in used oak barrels, including wine, scotch, rum and beer. Brokers and cooperages ship used barrels all over the world.  Each barrel contains some residual bourbon, and I like to think that all those other beverages get a flavor boost from good Kentucky bourbon.

Some barrels remain in Kentucky and are cut in half for planters or made into art objects. There used to be a secondary market in the production of counterfeit bourbon – using grain alcohol to extract the last bit of color and flavor from the barrel and selling it as cheap bourbon. I think this market is largely gone because distilleries and cooperages maintain better control over their barrel inventory.

Is white oak sustainable?
With the rapid growth of the bourbon industry, it is reasonable to think about whether there is sufficient supply of white oak to support an increase in cooperage.  The short answer is that there is enough white oak, for now at least.

Map of white oak

Current range and importance value of white oak. Importance value is a relative measure of abundance and size. US Forest Service. Darker green indicates higher importance value. Click map for details.

White oak grows throughout the eastern United States and Southern Canada, especially in the midwestern oak hickory forests. These forests were largely cut down in the 19th and early 20th centuries, as the insatiable demand for wood and arable land consumed our forests.  In the early 20th century, Kentucky was only about 19% forested.  Today, farmers only use the best land and the abandonment of marginal land has allowed forests to come roaring back with a very large component of white oak. Kentucky is about 50% forested now. More importantly, the growth of oak trees in our forests exceeds the harvest.

The United States has an important program called Forest Inventory and Analysis (FIA). Each year, foresters and technicians working for state agencies and the US Forest Service measure and count trees in plots, revisiting each plot about every five years.  This remarkable program provides foresters with statistically valid measurements of forest health and productivity, and is a critical component of modern forest management.  FIA allows foresters to detect changes in resource availability. So far, the white oak resource is more than sufficient for current levels of harvest.

White oak forestry is highly sustainable with proper forestry methods.  As long as a harvest uses best management practices,  forests come roaring back.  There are some problems. Many harvests are done without the advice of a professional forester (even though such advice is often free).  Land ownership is a barrier to good forest management –  most forest land in the east is in very small parcels whose landowners do not live near the property. The truth is that although oak forests are growing well, there is not enough monitoring of the source of barrel staves to know whether the harvest is sustainable.

Certification is a solution to this problem.  There are several wood products certification programs available in the US, including the Forest Stewardship Council and the Sustainable Forest Initiative. Certification allows the buyer of a wood product to have confidence that the wood was obtained from a sustainably managed forest. The cost of barrel staves is a tiny component of the price of a bottle of bourbon, and the marginal cost of certification is also small. However, until consumers begin requesting certified wood, or distilleries take the initiative to distinguish their market, I don’t think we will see much demand for certified barrel staves.

In Kentucky, Maker’s Mark Distillery is setting the standard for sustainable whiskey production, buying grain near their plant in Bardstown, using waste materials for energy, and reducing their carbon footprint.  They have a sustainable forest management plan for their own property. I am not aware that Maker’s Mark or any other distillery or cooperage is currently planning to use certified wood in their operations.

Climate and the Future of Bourbon

Climate change is upon us. We need to do whatever we can to reduce our emissions of greenhouse gases, but we will have to live with the consequences of the climate change we have already set in motion.

White oak range in 2100

Potential habitat for white oak in 2100. Importance value declines and range moves north. Click map for details

When it comes to bourbon, there are at least four factors that will be influenced by climate change and policies to reduce emissions.

  • Effects on grain production.  It presently looks like Kentucky will experience warmer summers, more rainfall and less stable weather as we move through this century.  It does not appear that this will have a huge impact on grain production, but it may cause distilleries to consider local grain sources. Maker’s Mark is already doing this.
  • Effects on aging and flavor. Bourbon is aged in rick houses that are not climate controlled.  Changes in temperature within the rick house are a major factor in moving the aging liquor in and out of pores in the barrel wood.  It is difficult to predict what impact warmer summers will have on the final product, except that aging in a warmer climate will increase the “angel’s share” – the amount of product lost to diffusion through the wood and evaporation.
  • Effects on wood supply.  Research by the US Forest Service allows us to make some limited predictions on the future habitat for tree species. The Climate Change Tree Atlas is a first attempt to compare various climate models and see where tree species might thrive in the future.  The results of the Atlas should not be taken as a certain prediction, but a rough estimate.  It appears that while white oak will still be growing in our region in the next 100 years, the abundance of the species in the Central Hardwood Forest Region may decline.  However, habitat north of the current range of white oak may become more conducive to its growth, so the net supply may not change.
  • Effects on energy consumption.  Distilling is an energy-intensive process.  Most distilleries use gas boilers but many are supplementing their gas heat with wood burners.  This is likely to continue. It is unlikely that distilleries, especially urban ones, can become energy self-sufficient, but they can go a long way toward improving their carbon balance sheet.

For right now, it appears that bourbon is a sustainable product even in the face of climate change. Only the future will tell. In the meantime, enjoy your bourbon if it suits your taste, and have a Happy New Year.

Note: This essay is based on my own experience and research related to my second book. If you’d like to know more, drop me a line and I’ll point you to some references.   Photographs are mine, taken with permission at Woodford Reserve Distillery, Woodford County, KY.


Bourbon barrels

Barrels of white dog on their way into the rick house. They will emerge in a few years filled with bourbon


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8 Responses

  1. Tom,
    Great article…really helpful information and I appreciate the level of detail you provided. Our firm, SustainAbility, works for clients for which the climate change impacts you talk about are very important to understand now and into the future. Thanks for the piece.

    Lindsay Clinton

    • Tom Kimmerer says:

      Thank you, Lindsay. I hope that the bourbon industry will make stronger moves in sustainability this year. In general, the industry is lagging, but they sell a premium product to a sophisticated clientele, so they should have the capacity.

  2. Cliff Berry says:

    Superb article by a guy who’s the “real deal.” No B.S.! Just responsible, accurate, informative facts. With a well written style!

  3. Adam Skiles says:

    A great article, but the percentages listed for alcohol by volume are not right. I stumbled upon this article when looking for how many barrels can be made from 1 oak tree on average. Sorry to rain the article, but I don’t want to see people getting the wrong info.

  4. Tom Kimmerer says:

    The abv figures are correct

  5. Interesting article. Do you know if there is any info available on the general age of the oak trees being cut for barrels these days?

    How about on the relative ages of oak trees cut for older barrels versus newer? Clearly older barrels would have been made with older trees, but how much older?

    • Tom Kimmerer says:

      The rotation time of white oak (time between harvests in a given stand) is about 60-80 years, so the majority of barrels used for bourbon are in this range. I’m not sure what you meand about older barrels. By law, bourbon is aged in new oak, and a barrel cannot be reused for bourbon. So we have no older barrels in the industry, and the oldest barrels are no older than 27 years (the age of the longest-aged whisky).

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