Distilling Craft: Ep 006


Barrels of Fun with Henric Molin

In this episode, we are talking about barrel maturation and warehousing. Later in the podcast, I bring in Henric Molin of Spirit of Hven in Backafallsbyn, Sweden and we talk about the chemical analysis of spirits and how it benefits your distillery.

This is our introductory episode on barrels since it’s such a large topic we are mainly giving a broad overview. As part of that overview let me talk briefly about the fire code issues of barrel warehousing. This is something that Dalkita specializes in and if you’d like to know more check out one of the presentations we give around the country or give us a call. That being said barrels are exempt from the IFC requirements for hazardous storage but not exempt from the IBC requirements. That means that once you have more than the Maximum Allowable Quantity (MAQ) — 240 gallons of liquor in barrels in a room/facility with sprinklers — you automatically have a hazardous occupancy according to the building code. The Fire Code offers some relief for barrels but does not alter the building code requirements. Generally, this means that you are required to provide proper ventilation, have a certain percentage of the perimeter on an exterior wall – if the room is large enough, emergency alarms, minim fire separations, and sprinklers. The minimum required ventilation helps but the classified electrical area — everything within 3’ of the floor and 25’ radius of the barrels PLUS everything within 5’ of a barrel — needs to be class/div rated per the National Electric Code. Typically, this includes things like forklifts and lighting. It is possible to declassify the space with ventilation to decrease the Angel’s Share to a fraction of the Lower Flammable Limit.

Since it is generally cheaper to declassify then buy an explosion rated forklift most distilleries opt for air flow through their barrel warehouse. The downside of the required air flow is that it increases the rate of evaporation from the barrels. This happens because having a concentration of water and ethanol in the air create some “pressure” against more water and ethanol entering the air by removing the ethanol concentration then it is easier for the next bit to evaporate. That being said just letting the ethanol concentration build is a terrible idea and very unsafe.

With the safety talk behind us let’s start talking about what is happening in the barrel. There are hundreds of papers written about the chemical processes that occur in barrels from the spacing of the wood grains to the level of toast and char and the effect of temperature and humidity. Unfortunately, I’m not going to be able to summarize even a small part of them here. There are some people who think they’ve got it figured out and we’re trying to get them on the show so if you know someone please pass along their contact info. That being said the reactions in barrels basically break down into three parts (technically 4 but close enough) There is the absorption of chemicals out of the wood, the oxidation of the spirit, and the evaporation of chemicals out of the spirit (the fourth would be all of the chemicals combining and breaking down in the spirit). The chemicals that are extracted from the oak break down into four primary categories, Furfurals (toasty, creamy), Aromatic Aldehydes (vanilla), Volatile Phenols (smokey, spicy), and Lactones (buttery, coconut flavor). The oxygen in the head space of the barrel will then react with various components in the spirit to change their flavor mainly through the process or esterification and transesterification. Finally, parts of the spirits are lost to evaporation which allows the most volatile compounds to be lost but also causes a concentration of the more complex flavor chains. Over time these reactions will occur over and over and that is what will result in the flavor of the aged versus new make spirits.

Well, that and the interactions of the chemical with each other. Also, the char formed creates a surface that allows for filtering the spirit as it moves into and out of the wood the adsorption onto its surface. Generally, this best remove nonpolar molecules with high molecular weight. This is a good thing since our flavor esters are more polar than most of our undesirable compounds but less so than alcohol. Also, concentration matters so the highest concentration chemicals will be absorbed in the greatest quantities. The amount of char is also important and is affected by your choice of char. The second impact of charing is that it toasts the wood behind the char that doesn’t get hot enough to char. This toasting is similar to what is found in wine barrels except further into the wood. This level of heat help to break down the lignins to make available the flavor compounds that we desire and to help remove any woody flavors after the drying process of the wood.

All of these processes are then affected by the ambient conditions around the barrel. The temperature that the barrel is stored at effects, the speed at which these reactions occur, the ability for chemicals to be absorbed and the volume & density of the spirits. Vanillin (what makes the flavor of vanilla if you couldn’t have guessed from its name) concentration can increase as temperature increases and peaks around 76-77°F. As heat expands the spirit it is able to penetrate further into the wood because it is less dense and there is additional pressure. Different studies have looked at where toasting and charring create flavor and a recent study I reviewed showed that vanillin concentration was the highest 6mm into the stave by forcing the spirit deeper into the wood these concentration layers are able to be reached depending on the drying, toasting and charing process used.

The ratio of square feet of wood to the volume of the spirit is another thing that impacts the absorption of the oak flavors into the spirit. This is because the more wood (cubic feet) there is per volume the more flavor compounds are available to be absorbed and the larger the exposure surface the faster the spirit has access to those compounds. In a barrel, the volume of wood is proportional to the exposure surface since barrels from 10 gallons to 53 gallons have about the same stave thickness since it’s easier that way so people mostly talk about the exposure surface but if you were able to get a sphere of toasted oak with the same volume as a 53-gallon barrel you would eventually be able to get the same flavors out of it as the barrel. I say eventually because the sphere would have the smallest possible exposure surface and access to those chemicals would be much slower than in the barrel.  Since this chemical absorption is only part of the overall flavor creation of a spirit, smaller barrel matured whiskeys will typically have more oak flavor and less complexity but they will be able to extract those flavors much more quickly than their larger counter parts. Moving from smaller barrels to larger barrels will change the flavor of a spirit even if the days of exposure per square foot per gallon remain constant. There are other techniques that try to make up for this like the sonic vibrations that help with chemical breakdown and oxidation or sloshing/agitation methods. The agitation method is common in the wine industry where barrels are stored on rollers both to ensure that the barrel is wet all around to minimize the separation of the staves to minimize evaporation and to provide oxygenation. The barrels are rolled about 1-1/4 turn once a week to help mix the wine and the air cap.

The proof of a spirit will make it absorbs certain chemicals preferentially if the proof declines with Angel’s share then you will pick up different flavors out of the oak over time and you can see spikes in relative concentration as vanillins are picked up at higher proof and then decline in relative concentration as proof declines and other chemicals are added to the mix. For shorter aging cycles or places where proof doesn’t decline over time, it is possible to dilute your spirit over time to simulate this. Also, the lower the proof the more complex the total absorption of flavors will be. Some of this is due to the water in the spirit absorbing flavors rather than just the ethanol and then being blended with unflavored water prior to bottling. In order to minimize this effect, some distilleries are aging their blending water in oak prior to dilution the water is treated with a low concentration of ethanol to prevent any bacterial fouling and then allowed to age. If these barrels are not to be stored in the barrel warehouse it is important to keep their alcohol concentration below 20% although this may require more frequent monitoring to prevent fouling. One 20% barrel could be used to dilute almost 3 barrels from 60% to bottling proof.This idea was talked about in the last episode with Jason Barrett. The other reason that lower proof creates more complex spirits is the preferential absorption of flavors at 120 proof vanilla will be the major flavor not but at 110 you’ll still be getting vanilla but not you’ll be picking up caramel and clove from the barrel in greater proportion.

The humidity in the barrel warehouse makes a large difference because it influences the evaporation of water as well as the evaporation of ethanol. On the highest end, the humidity can reach a saturation point (dew point) that the air is unable to accept any more liquid while at more reasonable concentrations elevated humidity will create osmotic pressure on the barrel that will prevent the evaporation of water and cause the ethanol to evaporate preferentially. Low humidity will cause both water and ethanol to evaporate more quickly and could be a solution to attempting to speed mature products.

Once the barrel profile has been determined, barrel entry proof (possibly dilution schedule), room temperature, humidity, barrel size it is time to start looking at how the barrel is stored. The inclination of the barrel while aging will affect the oxygenation surface since primarily the head space in the barrel is where oxygen gets into the spirit. When barrels are stored on their side the head space will have a larger surface area to interact than a vertically stored barrel. This is only true after a minimum amount of air is in the barrel typically 1” down on a 53-gallon barrel has the same surface area as 1/3” down on a head before that the head end has a greater area and after that the side. Traditionally spirit barrels are not topped off as they age in order for this aeration surface to grow over time and allow for increased interactions. For large distilleries, this is also not done because the barrels are stored on end, palletized, and stacked so there is no practical way to top off or agitate the barrels during their aging. Some will permeate through the wood as well so this is hardly a deal breaker and I know of no studies comparing the flavor differences in the storage position. If one of you have done this let me know, I’d love to know the results.

Designing your warehouse configuration is a function of how you want to interact with your spirits during their aging process and how much room you have. Barrels stored on their side in barrel racks will take up more space than palatized barrels but depend on how you want to access your barrels may be the only option. For people with a fixed barrel warehouse footprint being able to cram 24 53-gallon barrels into a space that is 4’ x 4’ x 20’ can allow for a lot more barrel in storage before needing to move to a new facility. The downside of this dense packing is that is can become a chore to access the barrel when it’s time to work with them. Either leaving 8’ rows for the forklift costs you two pallets all of the ways down the length of the row or you have to move all of the pallets you put in the row and then dive into the stack when it’s time to access the barrels. Either time or space is the typical barrel warehouse trade off. That being said you have those aisles available to you until you warehouse is half way filled either way so this only impacts the second half of the warehouse life. Palatized barrels should only be stack six pallets high so that is a limitation to this storage method particularly if you are using smaller barrels since six 10-gallon barrel pallets are much shorter than six 53-gallon barrel pallets.

Once in the warehouse, the barrels need to be tracked so that they are recovered at the right time both from a flavor perspective (over oaking a small barrel is a real possibility) and to capture the revenue associated with that barrel on the cash flow plan. Losing a barrel makes for a great marketing story but it really means that either that barrel didn’t need to be made when it was or that revenue that could have been captured when that barrel was ready to be sold was delayed. The easiest way to do this is to walk the warehouse at least monthly and inspect the barrels for leaks or other problems. This is obviously much more difficult with the tight staked warehouses.

After the barrels are emptied they need to be cared for in order to maximize their usefulness either to whoever you sell the barrel to or so that you can reuse them. The worst condition for a barrel is when it is just a little wet this will cause the wood to stay wet but without the alcohol that can be used to prevent the growth of mold. If the barrel is at least completely full as with the aging water then oxygen is prevented from access to the mold. Sulfur is the most common way to prevent the barrels from molding if there is going to be a delay between emptying and filling a barrel. Typically, sulfur candles are burned to create a sulfur rich atmosphere that is toxic to the mold. Another method is to dry the barrel as completely as possible store the barrel without the bung in upside down so that any liquid can drain once it is drained completely leaving the bung out over a drain so that air has access and the humidity level in the barrel is as low as possible. Doing this will cause the staves to separate and the barrel will need to be resoaked prior to the next filling. Of course, if you have the capacity immediate filling of the barrel is best since it requires the least amount of work.

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  1. Sam Harbison says:

    Great episode. Looking forward to the next one on barrels as well.

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