Temperature is a critically important factor at every step of a distiller’s craft. Its affect on grains begins with the sun’s energy shining on the growing crops, and continues through the mash temperature of the malt. In fermentation, temperature is crucial for gelatinization and the enzymatic rest, and for yeast to properly do its work. In distillation, it provides the propelling forces of evaporation and condensation. Temperature even has its place in heat-shrinking and labeling during bottling. But one of the most important yet least understood functions of temperature is in the barrel. Temperature is critical during spirit maturation… but in what ways? What is going on in the damp darkness of an oak barrel and, what does temperature actually do for the maturing distillate?
Let’s start with the physical aspects of temperature in the barrel. Increasing the temperature of the barrel increases the physical penetration of the spirit into the wood. But why does it do this? A theoretical explanation can be found within a concept known as the Ideal Gas Law. Technically, evaporated distillate is not an ideal gas—a true ideal gas does not chemically react. But if we assume that the reactions are negligible, this principle serves as a useful model for what happens inside the barrel.
The formula for the Ideal Gas Law is: PV = nRT
P and V are pressure and volume, T is temperature, n is the amount of gas (in moles—a special unit only necessary for computations). R is a constant (having different values for different gases), meaning that it is a finite number. If you rearrange the equation, it can be stated like this:
PV/nT = R
Expressed this way this equation is extremely powerful. Theoretical math computations aren’t everyone’s idea of a good time but the above equation can explain many of the physical changes happening in the barrel. It states that pressure, volume, temperature and quantity of substance (the gaseous state of the distillate) form a relation equal to a finite number.
For the sake of argument, let’s assume the value of R equals one.
PV/nT = 1
This means that when any one of those variables change, the others have to change accordingly. For example, if the volume (V) of a gas increases by a factor of 10, but there is no change in the temperature of the gas or the amount of the gas, this formula can be written as:
(1/10)P(10V)/nT = 1
Since the temperature and amount of gas aren’t changing, we can ignore them to focus on what is changing. In this case, since the volume of the gas is increases by 10 times, pressure must decrease tenfold in order to balance the equation:
(1/10)(10)PV/nT = 1
(1/10)(10)PV/nT = 1
1(PV)/nT = 1
PV/nT = 1
In the case of the barrel, we know two things: Temperature is changing (seasonally and diurnally), and liquid distillate is constantly evaporating, therefore n and T are constantly changing. For the traditional heat cycling, when temperature increases, more spirit changes from liquid to gas form, therefore n also increases. This means that the pressure and/or volume inside the barrel must increase to maintain the constant ratio during the barrel-heating phase. Note that it is pressure and/or volume, as either one (or both) could change to maintain the ratio. In reality, it’s probably both. But no matter which changes, pressure or volume, it means that spirit is being pushed into the wood. When the barrel cools, n and T decrease (decreasing temperature causes gas to condense). This means that the pressure and/or volume inside the barrel must decrease to maintain the ratio of 1 during the barrel-cooling phase.
Viewing it this way gives insight into what is going on and what you can do to change things. To maximize your spirit push into the barrel, fill barrels up to a higher level. This minimizes the volume (V) of gas that can be in the barrel (gas head space) so the only variable that can change is pressure. Want more room for the vapor to react? Put the barrel somewhere with a lower pressure (P), and the only thing that can change is the volume. Granted, there are physical and chemical limits to this because the vapor is not an ideal gas (it will react with itself) and the barrel has structural limitations. That said, changes in volume, pressure, temperature and amount of distillate in the barrel can be altered to change flavor profiles by fine-tuning spirit to wood interaction.
Within the barrel, three major chemical changes occur. The first is the solubility change when the temperature increases. Much like making simple syrup, as the spirit in the barrel heats up, it becomes easier for chemicals to dissolve into it. When spirit is forced into the wood (either by pressure changes or volumetric changes) it will begin to dissolve the wood sugars. Any sugars (such as xylose, mannose or glucose) and phenolic aldehydes (like syringaldehyde or coniferaldehyde) created from the toasting/charring of the wood will be dissolved into the distillate quicker when the temperature is higher. This leads to a higher concentration of base flavor compounds in the distillate.
Second, and perhaps more importantly, is the breakdown of cellulose, hemicellulose and lignin that make up the structure of the wood. This depends on the amount of water in the distillate, the amount of ethanol and the temperature. The breakdown of the cellulose and hemicellulose into their constituent sugar molecules is done through a process known as “hydrolysis.” Hydrolysis uses the acids in the distillate to break the bonds linking the sugar molecules of which cellulose and hemicellulose are composed. However, the acids within a distillate are very weak due to dilution, which normally would be a problem. But when the barrel is heated or the pressure is raised, the acidic breakdown of the cellulose and hemicellulose increases. For lignin, the ethanol/water mixture itself breaks down the structure into its constituent parts. There is some debate as to what actually breaks it apart: the ethanol alone or the ethanol and water combined. But no matter what is breaking down the lignin, this process too is rendered more effective by higher temperatures.
And third, both in breaking down the wood itself and dissolving its products, temperature plays the role of “catalyst.” Heat technically isn’t a catalyst; a catalyst can only be a chemical or biological substance added to the reaction. In general, increasing temperature speeds up chemical reactions. Adding heat speeds up the process by providing some, or all, of the activation energy required for the reaction. Catalysts lower the required energy (called “activation energy”) to start chemical reactions. Imagine a chemical reaction as a roller coaster. If it’s sitting in the station, it’s not much of a ride. To get the cars moving through the loops and turns, it first has to ascend a large hill before it can gain momentum. Now, imagine how the same roller coaster can function with a hill that is three-quarters smaller—much easier to get going. Reducing the height of that hill is the purpose of a catalyst. Some normal reactions require a lot of energy just to get going, but add a catalyst, and less energy is needed for the reaction to start. In a barrel, temperature can act like a catalyst. By raising the temperature, any reactions within the barrel require less activation energy. However, these are only an explanation of the known reactions. Barrel science is still somewhat a mystery—there’s not much information published in scientific journals about the chemistry of barrel maturation. Raising the temperature can make it easier for the favorable reactions to run—but may also speed unknown and unwanted reactions.
Looking at the maturation process through the lens of physics and chemistry gives us a rough idea of what goes on when spirit, oak and heat convene, although in only the most basic way. It demonstrates that variable heating can have an almost cascading effect on the maturing spirit. Heating (and cooling) barrels will have a large effect on your new make, no matter the size of the barrel. And by maintaining a maturation program and carefully monitoring the results, heat can speed up basic maturation/extraction and lay a foundation for long-term, complex maturation.