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Tree Selection

• Physical Properties
• Tree Species
• Oak Silviculture

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Properties of a Tree That Make for a Good Barrel: 

• Tall and can be milled into a long length of wood
• Contains straight-grained wood with vessels and fibers that run parallel to the length of the trunk
• Contains large rays and has significant presence of tyloses.1
• Rays are elongated cells that are positioned radially along the trunk axis and conduct water and nutrients between the bark and wood.  In a barrel, ray tissue gives oak the flexibility to easily bend into the curved sides (bilge) of the barrel without cracking.
• Tyloses are a gummy balloon-like swellings that fill the vessels so that the vessels can no longer conduct water. The presence of tyloses is what differs sapwood from heartwood and what allows wood to be generally impenetrable to the movement of liquids or gases. 
• Oak heartwood in particular, meets all these criteria, making it an ideal wood for barrel aging. 

Grain
Wood grain is directly correlated with the tree’s rings and is influenced by growth rate.  

Grain is composed of:

• “Earlywood”/”Spring wood”: Cells formed early in the growth season that become relatively long, hollow, and have thin cell walls.
• “Latewood”/”Summer wood”: Cells formed late in the growing season when hormone levels in the cambium fall, and have thick-walled cells.

Grain Size:
Tight grain (1-3 mm)  Wide grain (3-10 mm) as defined by Chêne & Cie Cooperage.9 

Grain Porosity:
Tight grain is more porous because it has more vessels therefore contains more void.

Grain Influence: 
Wood grain primarily influences oxygen permeability. See the section on Oxygen to learn more.  
Grain does not influence tannins and aroma compound extraction, as found in Mirabel et al. (2011) and Doussot et al. (2000) 10, 11. These findings, however, go against typical industry conversation about grain.

Imact of Growth Rate
Slow Growth 
Generally results in the development of less-dense (softer and more pliable) heartwood, relative to rapid-growing wood, due to the higher proportion of large-diameter vessels produced in the spring.  

Rapid Growth
Rapid growth results in the development of denser (harder and more rigid), heartwood relative to slow-growth wood because of a higher portion of small vessels caused by growth continuing into the summer months. 

 Impact of Oak Growing Climate
Variations in growing conditions, including sunlight and water, impact both physiological and chemical development. However, in North America, the specific growing region of the oak is given little attention, whereas in Europe the identification of oak by origin is traditional, and geographical designations which may even include the specific forest in which the tree grew are common.  This may be because European forests are on public land, which is specifically managed for timber production and therefore “branded,” whereas oak harvested in America typically comes from private property.  Additionally, as trees are so large, research has revealed as much variability between trees in a single stand as there is between trees from different forests.  For more on the cultivation of oak see the section on Oak Silviculture

​Oak Species
“White oak,” is the name of the primary “section” of genus Quercus used for barrel production2.  The White oak “section” is made up of different species including the oaks most commonly used for barrel production.  When it comes to barrel production, the main differentiation is by the common names American oak and European oak because the species within those general types of oak have similar properties.  This was specifically found by Marco et al. 1994 who compared American oak and French oaks grown in different regions by gas chromatography and high pressure liquid chromatography analysis. He found that while oak from the U.S. and France can be distinguished from each other, most of the samples from France can be unambiguously assigned to their actual classes of geographical origin even though they have been taken from neighboring forests.3  Additionally, oak sub-species hybridization is common with Quercus alba4 making delineation between species difficult in practice, whereas hybridization does not occur as often in Quercus robur and Quercus petraea.5   

American White Oak
Primary Species:
Quercus alba (provides about 45% of the white oak lumber produced in North America)

Secondary ​Species:
Q. bicolor (swamp white oak), Q. lyrata (overcup oak), Q. macrocarpa (bur oak), Q. muehlenbergii (Chinkapin oak), Q. prinus (chestnut oak), and Q. stellata (post oak), Q. garryanna (Oregon white oak)

Staves produced from different American white oak species are almost indistinguishable to the naked eye.

Primary American Oak Growing Regions: 
Missouri, Minnesota, Wisconsin

For more on Native oak species of Eastern North America: www.fs.fed.us/foresthealth/technology/pdfs/fieldguide.pdf

European (White) Oak
Primary Species:
Quercus robur, commonly known as pedunculate oak or common oak

Secondary Species:
Quercus petraea also known as Q. sessilis, Q. sessiliflora is commonly known as: sessile oak, cornish oak, Irish oak or durmast oak

Primary European Oak Growing Regions: 

• France: Allier (forest of the Troncais), Limousin, Cher (forest of S. Palais), Nievre (forest of Never and Bertange), Borgogna (forest of Citeaux), Vosges (forest of Darney) and Argonne6
• Slovenia (Q Robur)
• While Quercus robur and Quercus Though commonly found together, there may be some very general preferences for climate conditions. Ronald Jackson in Wine Science listed the following generalizations:
• Quercus robur generally prefers 
• Sunlight: High
• Soils: deep, rich, moist soils 
• Q. sessilis 
• Sunlight: Shade Tolerant
• Soils: drier, shallow, hillside soils

For more on the physical differences between European oak species:

The Difference Between American Oak and European Oak
The variations in oak’s influence on barrel aging as dictated by the genetic differences between American White Oak species and European Oak species (Q. sessilis and Q. robur) include:

Structural Differences:
American oak has a higher quantity of tyloses than European oak

Volatile Aromatic Compounds: 
Generally, American oak has higher aromatic potential.  (See Table 1)

Tannins: 
French oak generally has more tannin than American oak. 
​
For General Reference on the physical differences between oak species:
Diaz-Maroto IJ, Tahir S. 2018. Testing of wood physical properties in oak species (Quercus robur l., Q. Petraea (matts) liebl. And Q. Pyrenaica willd.) For cooperage. Part ii: wood grain. Wood Res 63: 959 969. www.woodresearch.sk/wr/201806/04.pdf

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​Other Species of Oak

• Section Mesobalanus:
• Quercus frainetto, commonly known as Hungarian oak or Italian oak
• Quercus cerris, commonly known as Turkey oak or Austrian oak
• Subgenus Cyclobalanopsis: 
• Qurcus crispula, commonly known as Mizunara
• Eastern European oak:
• Eastern European oaks appeared to be intermediate between French and American oak (Prida and Puech, 2006). 

Other Types of Wood Barrels and Other Barrel Materials
Barrels are made of other woods and man-made materials. This, along with aging implications, will be discussed in more detail in another article. 

Other wood barrels are used to age spirits 
Common alternatives include chestnut (Castanea sativa), acacia (Robinia pseudoacacia), hickory, maple, redwood, walnut and cherry.  

Other barrel materials
Stainless steel, cement, fiberglass, and plastic storage containers.  
 

Oak Silviculture: The Purposeful Growing and Cultivation of Oak Trees  

Silvicultural Techniques: 
Timber stand Improvement (TSI), also known as tree thinning: Undesirable trees may be removed when they are young, damaged or declining in order to maximize the health and overall vigor of the remaining trees by increasing sun exposure and space.  The exact tree density will vary upon the sivilcultralists.

Tree selection:
Oak, due to its slow growth and high value, is carefully chosen, cut down, and then sold by the individual tree.  This is unlike pine, which is cut and often sold in bulk.

Sustainability and forest management7

Europe
Programme for the Endorsement of 
Forest Certification (PEFC)
www.pefc.org The world’s largest forest certification system that promotes sustainable forest management.

L'Office National des Forêts (FSC)|  www.onf.fr L'Office National des Forêts, since the 17th century, has managed the majority of France's forests. Their standard method of management, "Futaie Reguliere" or Even-age Management, has a goal of producing tall, straight stands of trees with relatively little difference in age and size. This is done by making improvement cuts (thinning/TSI), early in the growing cycle, to select the highest quality trees and allow them to reach full maturity with little competition. Once matured, trees are slowly harvested to leave behind the healthiest trees, or “Mother Trees,” which continue with the regeneration of the forest through the spread of acorns. When the last of the mature trees are cut, the stand begins a new life cycle.

​North America
The majority of American oak forests are privately owned by individuals instead of the government. This results in a wide spectrum of management techniques that depends upon the objectives of the landowner.  There are some general timber management guidelines and organizations.

The White Oak Initiative:  
“The White Oak Initiative works to ensure the long-term sustainability of America’s white oak and the economic, social and conservation benefits derived from white oak dominated forests. While currently white oak growing stocks are sufficient to meet demand, forest monitoring, and long-term projections indicate problems in maintaining high-quality white oak regeneration.” This is done through providing technical assistance, on-the-ground implementation, communication strategies, and policy solutions to landowners (whiteoakinitiative.org/). 

The White Oak Initiative is Financially backed by:

• Governmental sector: U.S. Forest Service, the State Departments of Forestry of Alabama, Arkansas, Illinois, Indiana, Iowa, Kentucky, Maryland, Michigan, Minnesota, Missouri, North Carolina, Ohio, Pennsylvania, Tennessee, Virginia, West Virginia, and Wisconsin
• NGO’s: University of Kentucky, American Forest Foundation, Kentucky Distillers Association, DendriFund, W L Lyons Brown Foundation
• Distillers: Sazerac, Brown-Forman, Beam Suntory, Old Forester Bourbon
• The Cooperages: Independent Stave Co, Kelvin Cooperage, Robinson Stave, Speyside Bourbon Cooperage
• Others, including WestRock and The Forestland Group 
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General sustainability practices
According to ISC, loggers work with landowners to survey the stand of timber to estimate the value of the trees and the costs of harvesting. The logger and landowner then discuss the best sustainable harvesting methods to accomplish the landowner's objectives.

Tree Harvesting and Preliminary Milling

• Harvesting
• Milling

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Tree Harvesting 12
The logger surveys the stand of timber and estimates the value of the trees and the costs of harvesting, a process known as “cruising.” Successful loggers have expert knowledge of the different species, size, and quality requirements of the various timber mills in a region.  Many states have a voluntary Master Logger Certification to help ensure long-term forest sustainability.

Tree Selection and Log Purchasing

• Trees with diameters between 17.75 to 25 (~45 - 60 cm) are chosen.  Trees typically reach this size between 50 and 150 years old for Q. alba and approximately 200 years old for French oak.  White oak will reach 80 -100 feet tall at maturity.13
• The Doyle Scale Method may be used to determine the amount of board feet in a log.
• Larger trees are typically reserved for the production of head staves (headings).

Barrel Wood Milling
Barrels are made at a Cooperage, or Tonnellerie in French.  The first step in the process is to break down the oak log into a rough stave which will then be seasoned.  

Types of Wood within a Tree

Sapwood:
The living metabolically active cells of a tree.

Heartwood
Found in the center of the tree, is structural non-living wood which has had the cytoplasm replaced by phenolics (primarily ellagitannins).  

• The higher phenolic deposits, higher lignin content, and lower water content makes the heartwood highly resistant to decay and makes the lumber less liable to crack or bend when dried.14
• Heartwood is the primary wood used for barrel construction.  
• According to Gollihue et al. (2021), “A large percentage of sapwood in staves is generally thought to decrease liquid recovery per barrel and create undesirable flavours in the distillate (personal communication, Andrew Wiehebrink). However, it should be noted that there is no evidence of this hypothesis being either tested or published.”

Stave Milling 15
There are two ways to mill a barrel, and each is dependent on the type of oak being used. In a study by Chatonnet P, Dubourdieu D. (1998) found that:  

Sawing 

• Cuts across some irregular vessels, which increases surface roughness and potentially increases permeability. 
• Ideal for American oak (Q robur) with its thick and tightly packed tyloses in the heartwood, which are more tolerant to sawing. Sawing produces a higher yield than splitting.

Splitting 

• The separation of the wood along the planes of elongated vessels.
• Ideal for European oak (Quercus petraea and Q. robur) because it avoids rupturing the vessels, as the thylosis do not provide an adequate barrier against leaking.  However, with splitting, only about a quarter of a log (including both sap and heartwood) can be converted into stave wood because  the sections may be somewhat twisted

When cutting or splitting, the wood is partitioned into the following segments:
Bolts: Lengths of whole trunk, which are roughly the size of a stave

• Quarter Bolts: Each bolt is cut into quarters 
• Staves: Are split (or sawed) out of the quarters. 
• Heading pieces are cut out similarly, but are removed from shorter lengths of wood.
• Discarded scrap wood: 
• Portions too narrow for stave production. 
• Wood removed from the plank when they are being made into a uniform width. 
• Any sapwood associated with a stave piece.

​
Stave Size Impact on Aging 

• For standard 225-liter barrels, staves can range from .75 inches to 1 inch in thickness and are 4.25 inches wide at the bilge and 13-14 inches long.
• To accelerate maturation, thinner (22mm) Château-style staves may be used.

Sawing

Splitting

​Photos provided by Demptos Napa Cooperage

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Stave Seasoning

Seasoning of Staves
Once cut, the staves and heading pieces are dried to a specific moisture content and to degrade astringent ellagitannins, and increase aroma compounds through a process called “seasoning.” This can be done in the open air, exposed to the elements, or through kiln drying. 

• Open Air Drying
• Kiln Drying

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Natural Seasoning/Open Air Drying
Natural seasoning is when staves are left outside in the elements, using rain and natural agents like light, humidity, and fungal activity to help flavor development through the decomposition of the wood constituents lignin and hemicellulose.19 In addition, wood tannins begin to soften and the wood becomes less acidic.20

Duration: 
Seasoning is typically 18 months and can go up to five years (~1 yr per cm thickness) (Gollihue et al. 2021). 

Process: 

• To create more uniformity between barrels, staves from different logs may be mixed together.
• Air drying is typically done in conjunction with kiln drying because the latter can help to sterilize any microbial growth on the staves.21 Kiln drying is also more controlled allowing for the staves to be brought to a specific moisture content.

​Benefits of Open Air drying:

• Increased thermal conduction as the difference between kiln drying and 24-month open-air seasoning under the same toasting conditions was a temperature difference of nearly 20°C after toasting22.
• Open Air drying may create fungal development, some of which can reduce wood quality while other types of fungal growth can be beneficial.  Canton Cooperage studied the microflora of seasoned staves and found23:

Species and their functions:  

• E. gallinarum: Ligninase, xylanase and cellulase 
• E. casseliflavus: Ligninase, xylanase and cellulase 
• Bacillus cereus: Cellulase and hemicellulase 
• Aureobasidium pullulans: Cellulase 
• Penicillium glabrum: Secondary degradation of cellulose 
• Penicillium roqueforti: Secondary degradation of cellulose
• The researchers noted that these microbes are ”not pathogenic and they do not produce toxic compounds.”
• On average, 60-month staves have 10 times more microflora, while 18-24-month and 24-30- month staves have similar average levels.

​Kiln Drying
Kiln drying can be done in conjunction with air-drying or as a stand-alone process.

Process
Kiln drying typically occurs between 40 and 60 ºC and can reduce the moisture content of wood to approximately 12% (Jackson 2008). 

For more on the technical operations of Kiln Drying:
www2.ca.uky.edu/agcomm/pubs/FOR/FOR128/FOR128.pdf 

Benefits of Kiln Drying24

• Kilning of greenwood is faster than air drying and can have similar effectiveness regarding the desired reduction of off-flavors compounds in  American (Quercus alba) and French (Quercus petraea) oak.25
• Beyond providing precise control of moisture content, kilning can sterilize the wood so that the microbes that were developed during seasoning do not influence the contents of the barrel.  For this reason, kilning is often used in conjunction with air-drying.

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Barrel Formation

• Stave Cutting
• Stave Softening and Bending

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Barrel Construction
After drying, rough staves are checked for faults before undergoing “dressing,” which includes the following steps: 
Equalizer saws cut the staves to the exact barrel length. 

• Listing: Planers create the convex curvature on the exterior surface that matches the circumference of the finished barrel. The interior is finished with a concave surface. The amount of listing depends on the desired “height” of the barrel.
• Backing chisels a small amount of wood from the ends of the stave.
• Hollowing: A small amount of wood is chiseled from the center of the stave to facilitate bending.
• Jointing: A bevel is planed along the inner edge of the sides of each stave. This requires skill because the angle changes along the length of the stave and that precise jointing determines the tightness between the staves. 
• Raising: Barrel raisers arrange jointed staves in a set-up stand which ensures a consistent amount of wood is used in each barrel. The combination of wood amount and joint shape determines the final size and shape of the barrel. Temporary hoops including a trussing hoop are put on the barrel to help support the staves during the next set of processes, which are listed below.

Softening: 
A process using steam, hot water, or fire occurs after the barrels are raised.   This process may be accomplished in multiple ways.  According to R. Jackson in Wine Science (2008) they are: 

Fire Bending

• Usage: Spirit and wine barrels.
• Technique: Barrels are sprayed with water then inverted approximately 5 cm above the toasting pot (brazier). Additional water is sprayed or splashed during this process. 
• Aroma Influence:  Fire bending can cause charing which is fine for spirits aging, but not ideal for winemaking. 
• Additional Benefits: The toasting fire can be used to soften, bend and toast the same barrel which creates space efficiencies.

Water Bending

• Usage: Common in French Cooperages for wine barrels
• Process: Soaking the entire barrel for 10-30 minutes in hot, but not boiling, water before using fire to bend the staves.
• Aroma Influence: Water bending causes lignins to soften and releases some of the harsh, water-soluble oak tannins into the water, and combined fire temperature and high moisture content allows toasting to occur, without charring, at a higher temperature than in the fire-bent process.
• Additional Benefits: Good for French oak because of its high tannin levels.
• Water bending may modify the wood’s flavor potential compared to that of traditional fire bending and extract unwanted tannins.35
  
  

Steam Bending 

• Usage: Steam is also used to produce wine barrels.  This is an uncommon process.  
• Technique: In an advanced version of waterbending, 160 to 180°C steam is applied to barrels for 10-20 minutes.
• Aroma Influence: Steaming may also help to break down biopolymers, like tannins, by applying similar thermal pressure. However, we have not found research that specifies its impact.
  
  

Earth Bending and Air Bending
Earth bending, in particular metal bending, is used for creating hoops. And Ang was The Last Airbender.  

Setting
After the staves are steamed, temporary hoops are put on the barrel to hold the staves in place for an additional 10-15 minutes of heating.  In this process, called setting, the innermost wood fibers are shrunk, which releases the tension caused by bending and sets the staves into their curved shape (Jackson 2008). 

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Barrel Treatments

Chemistry of Toasting and Charring

At above 200°F (93.33 °C) toasting and charring breaks down the lignin hemicellulose components of the wood and releases the building blocks (precursors) for aroma compounds and involves two main changes:

Toasting and charring breaks down components of the wood torelease the building blocks (precursors) for aroma compounds andinvolves two main changes:

• Pyrolysis, the thermal decomposition of materials in a minimal to no- oxygen environment to prevent combustion.  The result is carbonation, also known as char/charcoal/biochar, and a combination of compounds also known as “pyrolysis oil” or “bio-oil” and synthetic natural gas (Syngas).  The pyrolysis of hickory produces a bio-oil sold as the food seasoning “liquid smoke”.  The production of each compound in bio-oil can be modified by temperature and duration of heat application.
  
  Interestingly, the pyrolysis of plant matter and in particular cellulose, hemicellulose, and lignins, is a major topic of research by the alternative fuel industry as a replacement for petroleum (bio-oil) and natural gas (syngas). 
  
  For more on this topic: Sadaka, S., & Boateng, A. A. (n.d.). Pyrolysis and Bio­Oil. Retrieved October 25, 2021, from  uaex.edu/publications/PDF/FSA-1052.pdf. 
  
  
• Thermolysis/Thermal hydrolysis: Chemical decomposition caused

​Impact on Volatile Phenols
Cadahía et al. (2003) found that toasting led to high increases in the concentration of volatile phenols, furanic aldehydes, phenyl ketones, and other related structures, but the effect on w-lactone levels depended on species and origin.  For example, the volatile composition in Spanish oak species evolved during toasting, similar to French and American oak, but quantitative differences were found, which were especially important in American species.

Pyrolysis and Thermolysis Break Down:

Pyrolysis of Cell-lumen (interior of heartwood cells)
Heartwood contains compounds that are deposited into its lumen during its maturation from sapwood. The lumen’s phenolic compounds are 7 - 20 times higher than in the corresponding sapwood (Jackson 2008).

• Cell lumen:  The central cavity of a dead cell (one without cytoplasm) formed by the cell wall. 
• The list of compounds can be found in the "Oak Aroma Compund" Section.

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Barrel Toasting

• Process Approach
• Methodology

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 Toasting
Toasting is a relatively recent addition to barrel production as there is no textual reference prior to the mid-nineteenth century.  It is done to both further dry the wood and then to thermally degrade the wood’s components into non-volatile precursors, which will turn into aroma compounds when reacting to beer, wine or spirits. [36]

Temperature and Duration

• Short hot toasts caramelize wood sugars on the initial layers and can leave the tannins in deeper layers intact. 
• Longer time at lower temperatures also allows heat to transfer from the surface of the barrel to deeper layers.  The longer the maturation, the deeper the penetration of the liquid into these layers, up to approximately 6mm.
• As the stave surface is heated, the deeper layers of the stave are heated through conduction. However, as wood’s cells do not grow uniformly which makes thermal transfer and toasting uneven (Gollihue et al. 2021)
• In combination, cooperages will use this combination of time and temperature to create different barrel profiles that have different flavor attributes.

​Toasting levels
There are no industry-wide definitions or standards for light, medium, or heavy toasting, and the colors created by toasting can be achieved with higher heat applied for a shorter time or lower heat applied for a longer time, both of which cause different flavor developments. R. Jackson (2008).

Methodology
Traditionally, the process of toasting was done using an open fire. This has changed with modern technology that has allowed for more precise control.  The toasting methodologies are: Direct fire and infrared. 

Pretreatment of wood  
Soaking the wood in water allows higher temperatures to be used for the toast level because the moisture promotes heat transfer deeper into the wood without the risk of charring.  ​

​Direct Fire Toasting (Traditional Method)
A “toasting pot” fueled by oak scraps is preheated to the desired temperature, then a barrel is placed over the pot.  As the toast occurs, some cooperages will use a computer system with sensors to track the toast’s temperature and will spray water or add more oak wood to the fire, depending on what is needed for the particular temperature curve trying to be achieved.  The barrel will be periodically flipped to achieve a consistent toast. After toasting, the barrel is visually inspected and moves to the next phase of production, for spirit barrels this is typically charring [37]. In research conducted by Matricardi and Waterhouse 1999 [38], they found that:

• Limiting air supply by covering the top of the barrel with a metal cover (called “closed-top toasting”) resulted in lighter oak color despite higher toasting temperatures and almost total loss of the original ellagitannins. 
• Water usage decreased toasting temperature and affected color inconsistently, but did not have a large effect on the content of the extractable phenolics. 
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Additional Direct Fire Toasting Notes

• Carbonization (charring) of the wood begins at about 250ºC. This process is difficult to control, and the barrel may be unevenly toasted. There is also an increased potential of blistering, which increases microbial contamination.
• Typically large cooperage is not toasted [39].

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​Infrared Toasting
Computer-controlled infrared machines use a range of the electromagnetic spectrum to heat barrels without the use of fire. According to World Cooperage, the wine barrel subsidiary of ISC, “Shorter wavelengths penetrate the surface of the wood and toast deeper within the wood layers, whereas longer wavelengths reflect more, and thus, toast the outer layers of wood. Utilization of this technology also allows us to create high impact barrels without smoke – something that can be quite challenging to do with fire toasting.” [40]

Image by Mariana Calderon Photography at Demptos Napa Cooperage

Barrel Charring

Charring, which begins at temperatures above 250°C (Jackson 2008), may be done on its own or in conjunction with toasting.  This process is used to create a layer of activated carbon that absorbs unwanted compounds from the barrel’s contents and is only used in spirit aging since it would absorb the coloration of red wine. 

• Process Approach
• Methodology

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​Physical Reactions
Charring a barrel creates two distinct layers on the stave: 

The Char Layer: 
After toasting, spirit barrels are still charred to benefit from the char layer’s filtration properties [43]. This is done by the application of Intense heat to a barrel in order to create a layer of charcoal on the outer surface of the stave.

• The char layer serves as an important filter to remove aromas and odors.  For this reason, charred barrels are not used in red wine production because the charcoal would absorb color.
• Little flavor comes from this layer because most of the wood has been broken down severely. ​

The Red Layer
Rich and varied concentrations of aromatics are created in this layer from a much less severe thermal breakdown of wood constituents.  If there is no toast, the red layer is akin to the toasted layer.  The red layer is also  what gives bourbon its color.

Char Level
Barrel char is typically measured on a scale from 1 to 4. Char 1 is “flash char” and is approximately 15 seconds. Char 4 is a deep char that takes approximately 55 seconds and is commonly referred to as an “alligator char.”

​Cooperages can use different techniques to char a barrel [41].

Charring by Toasting Fire
This traditional method may yield more combustion products (mainly charcoal) than charring by natural gas flame.  

Charring by Natural Gas Flame
Natural gas produces a hotter flame than charring by toasting fire.  This creates conditions closer to those found during fast pyrolysis, a process that creates less char and can increase the amount of “bio-oil” production relative to traditional pyrolysis [42].

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Additional Barrel Construction Steps

• Bung hole is bored and enlarged with a special auger to receive a tapered wooden, rubber, or plastic peg. A tap hole may also be bored near the end of the central head-stave.
• The barrel head, consisting of 12 - 16 untoasted head pieces and constitute approximately 25% of the barrel surface, is created with untoasted staves.  
• The barrel head is then installed by inserting the bottom of the head first. 
• Temporary hoops are replaced with permanent hoops before being hammered tight which in turn forces the staves together and closes most cracks. The barrel is then soaked in water for approximately 24 hours.  Hoops may come in different colors for aesthetic reasons.  
• Larger-volume oak vessels including vats are made using similar techniques to smaller cooperage but with staves that are longer, thicker and wider with less of a degree of curvature. Large cooperage is also typically not toasted.

For more information on the barrel construction process see:
 blog.heavenhilldistillery.com/crucial-partnership-distilleries-cooperages/ 

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Maturation in Oak

Cooperage Size

Up until the early nineteenth century move to smaller barrels, mid-size to large ( 1000L - 10 hl) oak cooperage was common in many European regions and was used as both fermenters and storage containers because the size minimized oxidation and was easy to clean.  However, as power tools made for easier barrel production, as barrels became easier to move, and because oak barrels allowed for earlier maturation and the imparting of oak flavors, widespread adoption of barrels with a capacity of between 200 and 250 liters occurred (Jackson 2008).  

Cooperage Size Influence
The major component of cooperage size is the surface area to liquid ratio.  The larger the surface area, the more oak interaction.  The specific influence of barrel size varies by what is being aged within the barrel, and will be discussed in a future article. It should be noted that some cooperages will also cut specific shapes into barrel staves to increase barrel surface area. 

Maturation Process

Beyond the compounds released by oak, other processes occur. These are:

• Sorbtion and Absorbtion by Oak
• Oxygen Exposure

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Compounds sorbed and absorbed by oak
Though sorption (the physical and chemical process by which one substance becomes attached to another) and absorption may differ by liquid being aged in the barrel. The following are some examples:
​
In whiskey:
Volatile sulfur compounds of Dimethyl sulfide, 3-(methylthio) propyl acetate, dihydro-2-methyl-3 (2H)-thiophenone and ethyl 3-(methylthio) propanoate, in a  study by Masuda and Nishimura 1982, were found to decrease rapidly during aging and disappear within three years [44].
In wine: 

• Aroma compounds were sorbed without any interactive effects of co-sorbents, except for 2-phenylethanol and benzaldehyde, which were sorbed in larger amounts from a mixture than when they were the sole volatile solute [45].
• A sherry cask may absorb as much as 15 liters into its staves [46].

Oxygen Exposure
Oxygen in the atmosphere interacts with the liquid in the barrel as oak is semi-permeable. 

Places where oxygen enters the barrel47
Demptos Research Centre Scientist Dr Nicholas Vivas found in a 1997 study:

• Bung:  21%
• Between Stave Joints:  63%
• Through the staves: 16%

What Influences oxygen rates
Oxygen’s entrance into the barrel by rate per location can be changed by a multitude of factors that include the permeability coefficient of the wood as influenced by grain, the machining profile/tightness of joints, pressure from steel strapping, and the wood’s moisture content. 48
For more information on oxygen diffusion into the barrel, the following study provides great insight and aggregates of a oxygen diffusion rates as found through a multitude of studies: 

Junqua, R., Zeng, L., & Pons, A. (2021). Oxygen gas transfer through oak barrels: a macroscopic approach. OENO One, 55(3), 53–65.  doi.org/10.20870/oeno-one.2021.55.3.4692

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Angel's Share [49]

The “Angel’s Share,” measured in percent, is the quantity of wine or spirit lost from the barrel due to evaporation. The remaining liquid in the barrel will then concentrate. The Angel’s Share is highly variable by producer and is often known and shared upon request.  As a general range, the Angel’s Share in Scotland is 2% and in the tropics can be as high as 10% per year.

• Factors that influence Angel’s Share
• Angels Share Calculations

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 [Factors that influence Angel’s Share

Humidity: 

• Low humidity environments: Water evaporates faster than ethanol, resulting in higher alcohol content in the wine.  
• High humidity environments decrease the quantity of water that is lost relative to ethanol evaporation.  High relative humidity suppresses water evaporation, but has no influence on alcohol loss.
• Ideal humidity for wine cellar: 65%

Temperature:  

• In warmer temperatures, evaporation is higher.  
• Temperature Fluctuations: Are colloquially said to increase aging as the pores of the wood expand.  However, we have not found scientific research that mentions this impact.
  ​

Barrel Refilling Practices
As the volume of liquid in the barrel decreases, the amount of liquid that is in contact with air increases.  The void left is called headspace, or “ullage.”  This causes:

• Increased evaporation rates as the amount of air in the barrel increases.  
• ​If a producer wants to minimize evaporation, barrels are usually “topped off.”  Also, too much air exposure to wine may cause acetic acid development (turning the wine into vinegar).  

Additional Components lost in Angel’s Share
Hasuo and Yoshizawa found that in whiskey aging “the evaporating ratios of various substances evaporated from the barrel for 1 year's storage were as follows: Acetaldehyde 32.0%, ethyl alcohol 12.7%, n-propyl alcohol 10.6%, ethyl acetate 10.5%, isobutyl alcohol 8.7%, isoamyl acetate 5.4%, isoamyl alcohol 5.2%, ethyl caproate 1.3%, acetic acid 1.0%.”[50]

Angels Share Calculations

For Spirits:
del Toro del Toro N, Fong Casas F, Ayan Rial J, Caridad Portuondo González M, Crespo Sariol H, Navarro Campa J, Yperman J, Vandamme D, Carleer R. Boltzmann-Based Empirical Model to Calculate Volume Loss during Spirit Ageing. Beverages. 2019; 5(4):60.  doi.org/10.3390/beverages5040060 

For Wine
Ruiz de Adana, Manuel & López-González, Luis M. & Sala, J.M.. (2005). A Fickian model for calculating wine losses from oak casks depending on conditions in ageing facilities. Applied Thermal Engineering. 25. 709-718. 10.1016/j.applthermaleng.2004.07.021. Retrieved from:  www.academia.edu/7527210/A_Fickian_model_for_calculating_wine_losses_from_oak_casks_depending_on_conditions_in_ageing_facilities 

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Barrel Conditioning and Care

• Conditioning 
• Care and Cleaning
• Lifespan and Renewal

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Barrel Conditioning 
Different conditioning techniques may create different oak flavors and the preferred procedure depends on the type and style of wine as well as the attributes of the barrel. Jackson 2008 suggested the following: 

Types of Barrel Conditioning

Minimal treatment: Rinsing and presoaking with cool to lukewarm water which is then dumped. The warmer the water the more wood flavor extracted and removed by the water. 

In-barrel fermentation can also be used as a barrel conditioning procedure since the constituents’ most easily extracted compounds, ellagitannins and phenols, when combined with wine constituents, precipitate out and are lost with the lees. Because the desirable flavors, including oak lactones and aromatics found deeper in the wood dissolve more slowly, they are not extracted by in-barrel fermentation. This, however, is labor-intensive because of not only the fermentation process but the additional cleansing required before being refilled with wine being matured in that barrel. 

Barrels not subjected to toasting (heated only to facilitate stave bending and setting) may be conditioned with a solution of 1% sodium or potassium carbonate. The alkaline solution accelerates both phenol oxidation and extraction. Subsequently, the barrels are given a thorough rinsing with a 5% solution of citric acid and, finally, a water wash.

Conditioning Process Influence on liquid

• Lighter treatments preserve the intensity and complexity of oak flavors.
• Heavier treatments are used to remove oak flavors and are ideal if an inert barrel is desired.

General Barrel Care and Cleaning

• Precipitate/Sediment may be removed by a hot water ( 80 ºC) rinse under high pressure.
• Tartrates build up and may require treatment with 0.1 to 1% sodium or potassium carbonate, followed by a thorough rinsing with hot water. After draining, burning a sulfur wick in the barrel usually provides the inner surfaces with sufficient disinfection.
• After cleansing and disinfection, if a barrel is left empty for more than a few days, the barrels should be thoroughly drained, sulfited, and tightly bunged.
• Barrels stored empty for more than 2 months should be filled with an acidified solution at 200 ppm SO2. Before barrel reuse, the residual sulfur dioxide can be removed by water.
• Oak should be kept at 30% moisture content to prevent shrinkage and cracking 

​
Barrel Sanitization

Inner Barrel Sanitization: 

• If the cooperage has become contaminated with spoilage microorganisms, various disinfection treatments may be used. 
• Acetic acid bacteria: Hot water treatment of 85–88 ºC for 20 minutes.51 
• Other treatments can use SO2 and ozone.

Outer Surface Treatments

• Exterior mold growth does not affect barrel strength or influence the sensory properties of its contents, therefore treatment is typically cosmetic.  
• Boring Insects: 1% rotenone in boiled linseed oil.

AWRI Recommendations
 www.awri.com.au/industry_support/winemaking_resources/storage-and-packaging/packaging-operations/barrel-cleaning-storage-and-maintenance/ .

Barrel Lifespan
New barrels impart significantly more flavor (aroma compounds) than used barrels.  However, the amount of remaining oak aroma compounds varies by what was initially aged within the barrel and for how long.  The research on these declines will be examined in a future article.  However, here are some generalizations:
​
Spirits

• American bourbon by law requires new barrels
• Many other spirits industries will use ex-American bourbon barrels because there is an abundance of them, making them economical, and because they impart less oak flavor, thereby allowing the base spirit be more prevalent than the oak notes.  Some spirits producers, for example, will use a blend of 2nd fill, 3rd fill and 4th fill spirits.
  
  

​Wine

• Some regions have laws that dictate the duration of time barrels may be used. 
• In regions with no barrel usage rules, wine may be aged in new or used barrels.  Wine may also be used in a combination of used and new barrels which is then blended. These choices vary by producer.
• If oak flavors are not desired and oxygen is the main reason for barrel aging, then barrels may be used until they leak and cannot be repaired.
• Large oak cooperage can be used for decades, whereas small cooperage is usually replaced after several uses (Jackson 2008).  

For Beer
Beer is primarily aged in used barrels.  The number of usages depends on the producer and the condition in which they remain after the first usage.  As beer production is particularly heavily focused on controlling microbial inoculants, the approach of barrel aging beer is highly variable even within a single producer.

Barrel “Renewal”
Wine extracts material from oak staves only to a depth of 4 - 6 mm in 25mm staves, while spirits may reach 9mm.  Additionally, barrels can develop leaks or other faults over the course of their lifetime. Instead of discarding the casks, if allowed by the production rules, barrels can be renewed after a thorough cleaning.  It should be noted that renewed barrels are not identical to used barrels as the extractive characteristics are not identical with new barrels, being lower in ellagitannins, furanilic aldehydes, eugenol and vanillin and there being an increase in the concentration of lignin breakdown products which produce a burnt flavor such as methyl phenols and dimethoxyphenols (Jackson 2008).

The barrel renewal process may use one or more of these processes:

• Replacing any defective staves
• Retoasting the barrel
• Shaving off approximately 4 mm from the inner layer then retoasting the barrel. Access to oak flavors may be renewed, but the barrel’s structural integrity will be reduced.  

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Oak Aroma Compound Chart

• Lignin Derived 
• Hemicellulose Derived
• Cell Lumen Derived

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4-allylsyringol
Species Impact

• The high concentration of 4-allylsyringol shown by American wood with respect to the Spanish and French woods was exceptional.

Impact of Seasoning: N/A

• The most abundant volatile phenols in seasoned wood were eugenol, 4-vinylguaiacol, and 4-allylsyringol, contending for the first place depending on wood species and origin.

Impact of Toasting/Charring

• The main components in all toasted woods were 4-allylsyringol, 4-methylsyringol, eugenol, isoeugenol, syringol,4-methylguaiacol, and 4-vinylguaiacol with a different abundance order according to species and origin (Cadahía et al. 2003).

Benzoic aldehydes
Species Impact: N/A
Impact of Seasoning: N/A
Impact of Toasting/Charring

• Majority of the increase due to toasting (Cadahia et al. 2003)

Aroma Notes
Almond (goodscentscompany)

Cinnamic acids and aldehydes 
Species Impact: N/A
Impact of Seasoning

• Increase during seasoning depending on the duration of this process (Cadahia et al. 2003)

Impact of Toasting/Charring

• Majority of the increase due to toasting (Cadahia et al. 2003)

Aroma Notes
Cinnamon notes

​​Coniferyl alcohol (2-methoxyphenol)
Species Impact: N/A
Impact of Seasoning: N/A
Impact of Toasting/Charring
Primary degradation by toasting especially at 200 ºC

Aroma Notes

• Precursor of vanillin (worldcooperage.com)
• Under acidic conditions, coniferyl and sinapyl alcohols are oxidized into: 
• Aromatic phenolic aldehydes, such as syringaldehyde and vanillin
• Phenolic alcohols, such as eugenol, quaiacol and syringol
• Phenolic ketones, such as acetovanillone and acetosyringone. 
• Aldahydes: coniferaldehyde and sinapaldehyde (Puech, 1987).

Coniferylic aldehydes
Species Impact: N/A
Impact of Seasoning: N/A
Impact of Toasting/Charring

• Increase from 150-200ºC, then decreases when Charred to approximately the same level as at 150ºC (Nishimura).  

Aroma Notes
Bready and graham cracker notes

4-ethylguaiacol 
(4-ethyl-2-methoxyphenol)
Species Impact: N/A
Impact of Seasoning: N/A
Impact of Toasting/Charring: N/A
Aroma Notes
Spicy and clove-like (thegoodscentscompany.com)

​Guaicol
Species Impact: N/A
In a different abundance order according to species and origin (Cadahía et al. 2003).

Impact of Toasting/Charring
Increased by toasting and a main component of toasted woods (Cadahía et al. 2003).
Aroma Characteristics
Smoke  (worldcooperage.com)

4-methylsyringol
Species Impact

• In a different abundance order according to species and origin (Cadahía et al. 2003).

Toasting Impact

• Increased by toasting and a main component of toasted woods (Cadahía et al. 2003).

Aroma
Smoke

4-methylguaiacol
Species Impact

• In a different abundance order according to species and origin (Cadahía et al. 2003).

Impact of Toasting/Charring
Toasting breaks down syringaldehyde into 4-methylguaicol (worldcooperage.com) 

• Increased by toasting and a main component of toasted woods (Cadahía et al. 2003).

Aroma Characteristic: 
Spice/clove (worldcooperage.com)

Phenol
Species Impact: N/A
Impact of Seasoning: N/A
Toasting Impact

• Produced by the breakdown of guaiacol (world cooperage).

Aroma Impact
Smoke

​Sinapyl alcohol (2,6-dimethoxyphenol)
Impact of Seasoning

• Increase by seasoning (Cadahía et al. 2003)

Impact of Toasting/Charring

• Primary degradation by toasting especially at 200ºC

Aroma Notes
Cinnamon
​
Syringaldehyde
Species Impact: N/A
Impact of Seasoning: N/A
Impact of Toasting/Charring

• Increases slightly from 100- 150 ºC with significant increase at 200 ºC.  Falloff if charred to approximately same level as at 150 ºC (Nishimura et al)

Aroma Notes
Plasticy, tonka chocolate

Syringol
Species Impact

• In a different abundance order according to species and origin (Cadahía et al. 2003).

Impact of Toasting/Charring

• Increased by toasting and a main component of toasted woods (Cadahía et al. 2003).

4-vinylguaiacol
Species Impact

• Varies by species and origin (Cadahía et al. 2003).

Impact of Toasting/Charring

• Increased by toasting and a main component of toasted woods (Cadahía et al. 2003).

Vanillin, vanillic acid
Impact of Seasoning: N/A
Impact of Toasting/Charring

• Majority of the increase is due to toasting. (Cadahía et al. 2003)
• Toasting increases level significantly between 150- 200 ºC before decreasing when charred to levels below that of 150 ºC.  (Nishimura et al)  

Aroma Notes
Vanilla

Furanic aldehydes: Furfural 
Species Impact: N/A
Impact of Seasoning

• Kiln-drying at 65°C produces an increase in the levels of furfural and hydroxymethylfurfural which may be explained by a slight degradation of xylane and glucomannan fractions of  

Impact of Toasting/Charring

• Toasting is the primary source of Hemicelluloses degradation
• Product of heavier toasting (worldcooperage.com)
• The highest increments during toasting and that the quantities were higher in French Q. robur and American woods than in the other studied woods under light toasting (115 - 125 ° C). (Cadahía et al. 2003)

Aroma Notes
Dark toasty (worldcooperage.com)

5-methylfurfural
Species Impact: N/A
Impact of Seasoning: N/A
Impact of Toasting/Charring

• Product of Medium Toasting (worldcooperage.com)

Aroma Notes

• Medium Toasty, sweet caramel flavors (worldcooperage.com)

5-hydroxymethylfurfural
Species Impact: N/A
Impact of Seasoning: N/A
Impact of Toasting/Charring

• Toasting is the primary source of Hemicelluloses degradation
• Product of Light Toasting (worldcooperage.com) 

Aroma Notes
Light toasty, creamy-fudge flavors (worldcooperage.com)

Maltol and Cyclotene
Species Impact: N/A
Impact of Seasoning: N/A
Impact of Toasting/Charring

• Toasting is the primary source of Hemicelluloses degradation

Aroma Notes

• (sweet and toasty)
• caramel and toffee

​

Eugenol, isoeugenol 
Species Impact

• Showed significant differences among species and provenances (growing locations), and its concentration was higher in American woods with respect to Spanish and French woods. Considering European species, Spanish Q. pyrenaica and French Q. petraea showed eugenol concentrations higher than the others, even though no significant differences were

​were observed because of the high standard deviations (SD) shown by these variables (Cadahía et al. 2003). 

​Impact of Seasoning

• Sefton et al. observed a regular and constant decrease of eugenol content during natural seasoning and Masson et al. observed the same during kiln drying, Chatonnet found the opposite effect: a little increase in relation to the seasoning duration (Cadahía et al. 2003).
• The most abundant volatile phenols in seasoned wood were eugenol, 4-vinylguaiacol, and 4-allylsyringol, contending for the first place depending on wood species and origin (Cadahía et al. 2003).

Impact of Toasting/Charring

• The effect of toasting on eugenol was different according to species and origin of wood, but in general, no drastic changes were observed (Cadahía et al. 2003).
• The levels obtained from Spanish, French, and American toasted woods to those obtained from the same seasoned woods, the average contents of most of the compounds, especially isoeugenol, syringol, 4-methylsyringol, and 4-allyl-syringol, increased during toasting process (Cadahía et al. 2003).

Aroma Notes
Clove


​β-ionone
Source: N/A
Impact of Seasoning

• β-ionone (floral aroma) increases through the seasoning process, especially in American oak (Ticket-Lavandier/Caton Cooperage) 

Impact of Toasting/Charring: NU/A
Aroma Notes
Floral aroma (goodscentscompany.com)

​Oak Lactones: Cis-methyl octalactone and trans
Species Impact

• Spanish Q. petraea was especially poor in cis-methyl octalactone, but the other Spanish species Q. robur, Q. pyrenaica, and Q. faginea showed concentrations halfway between French Q. petraea and American Q. alba.  The only significant difference between species was by American Q. alba compared to Spanish Q. petraea and French Q. robur.  These results reinforce those obtained by Masson et al., These also revealed important differences in the proportion of cis oak lactone for American with respect to Pedunculate and Sessile oaks (French Q. robur and Q. petraea) and higher proportions of trans isomer in some French woods from different forests.

 American oak was characterized by higher levels of methyl octalactones, particularly of cis isomers, than French oaks in other published studies.(Cadahía et al. 2003) which showed high variability of levels and ratios of cis/trans-methyl octalactone in French and American oaks, among trees, forests, and species. 

​Impact of Seasoning

• Whisky lactone (coconut flavour) in American oak decreases during the seasoning process down to a level comparative with French oak staves seasoned for 36 months. (Ticket-Lavandier/Caton Cooperage)

Impact of Toasting/Charring

• The effect of toasting on methyl octalactone levels depended on species and origin of studied wood. (Cadahía et al. 2003)
• French Q. petraea and American Q. alba woods showed an increase of both c is - and trans -lactones.
• Spanish species and French Q. robur showed a decrease of cis -methyl octalac-tone, (the isomer more abundant in seasoned woods). This was especially evident in the cis -methyl octalactone concentration of Spanish Q. robur. The  contradictory effects may be attributed to variations in toasting intensity and method as the first toasting in laboratory or cooperage conditions shows an increase of methyloctalactone contents in the superficial wood layer, but under prolonged toasting, a total destruction of these compounds took place.
• Sometimes, methyloctalactone concentration varied more among trees, species, and origins than among different heat treatments in cooperage, and the results from Cadahia corroborated that the chemical and structural characteristics of wood and toasting conditions may influence the reaction to heating, and this process is complicated by the possible loss of notable compounds, like w-lactones.

Aroma Notes

• In lower concentrations these oak lactones give a woody aroma which improves the quality of red wine, but at higher concentrations they can give resinous and coconut-like aroma which may be less desirable Chatonnet et al (1990).
• Trans lactone: Celery Like (worldcooperage.com)
• Cis Lactone: Coconut (worldcooperage.com)

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Trade Associations and a few Cooperages

Associated Cooperage Industries of America: acia.net

Fédération des Tonneliers de France: tonneliersdefrance.fr/en

Various American Cooperages and Used Barrel Brokers
Adirondack Barrel Cooperage:  adirondackbarrelcooperage.com

Allary Tonnellerie: tonnellerie-allary.com

Barrel Associates Int’l: barrelassociates.com

Barrel Builders Napa Valley: barrelbuilders.com

Barrels Direct: barrelsdirect.com

Barrel's Unlimited Inc:  barrelsunlimited.com 

Bouchard Copoperage:  bouchardcooperages.com

Brown-Forman Cooperages: brown-forman.com

Canton Wood Products: cantonwood.com

Charlois Cooperage USA: charloiscooperageusa.com

Demptos Cooperage: demptos.fr/en/
Demptos Napa Cooperage, Inc.: demptosusa.com

Gainesville Cooperage: gainesvillecooperage.com

Gibbs Brothers Cooperage: .gibbsbrotherscooperage.net

Independent Stave CO: independentstavecompany.com
ISC Spirits Barrels: www.iscbarrels.com 
ISC Kentucky Bourbon Barrel: kentuckybourbonbarrel.com
ISC Oak Solutions Group: oaksolutionsgroup.com

Kelvin Cooperage: kelvincooperage.com

Kentucky Bourbon Barrel: kentuckybourbonbarrel.com

McGinnis Wood Products: mcginnisbourbonbarrels.com
​
Nadalie USA: nadalie.com

Robinson Stave: robinsonstave.com

Rocky Mountain Barrel Co:  rockymountainbarrelcompany.com

Seguin Moreau Napa Cooperage: seguinmoreaunapa.com

Speyside Cooperage (Scotland): speysidecooperage.co.uk
Speyside Bourbon Cooperage, Inc.: speysidebci.com
Speyside Cooperage Kentucky: speysidecooperageky.com

The Barrel Mill: thebarrelmill.com

The Oak Cooperage: theoak.com

Tonnellerie O: tonnellerieo.com

Tonnellerie Radoux - USA:  tonnellerieradoux.com

Tracy Cooperage, LLC: tracyloggingandsawmill.com

Vicard Cooperage:  groupe-vicard.com/en 

WV Great Barrel Company, LLC: www.wvgbc.com

 ZAK Cooperage: ZAKcooperage.com

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Resources and Suggested Reading 

​1. “Tree Biology.” Tree Biology - Pruning - Landscape Plants - Edward F. Gilman - UF/IFAS, University of Florida, 2025. Retrieved from  hort.ifas.ufl.edu/woody/compartments-tyloses.shtml.

2. List of Quercus species. (2021, September 05). Retrieved October 23, 2021. Retrieved from  en.wikipedia.org/wiki/List_of_Quercus_species

3. Marco J, Artajona J, Larrechi MS, Rius FX. 1994. Relationship Between geographical origin and chemical composition of wood for oak barrels. Am J Enol Vitic 45: 192- 200. Retrieved from  www.ajevonline.org/content/45/2/192

4. Hardin, J. W. (1975). HYBRIDIZATION AND INTROGRESSION IN QUERCUS ALBA. Journal of the Arnold Arboretum, 56(3), 336–363. Retrieved from http://www.jstor.org/stable/43781979

5. Morphological variability of oaks (Quercus robur L, Quercus petraea (Matt) Liebl, Quercus pubescens Willd) in northeastern France: preliminary results.  JL Dupouey and V Badeau.  Ann. For. Sci., 50 Supplement (1993) 35s-40s.
Retrieved from  doi.org/10.1051/forest:19930702 

6. Frangipane, Maria Teresa & Santis, Diana & Ceccarelli, Antonella. (2007). Influence of oak woods of different geographical origins on quality of wines aged in barriques and using oak chips. Food Chemistry - FOOD CHEM. 103. 46-54. 10.1016/j.foodchem.2006.07.070. Retrieved from  www.researchgate.net/publication/238378715_
Influence_of_oak_woods_of_different_geographical_origins_on_quality_of_wines_aged_in_barriques_and_using_oak_chips

7. Forest management: ISC Barrels. (2020, March 12). Retrieved from  www.iscbarrels.com/2020/03/12/forest-management/

8. Punches, J. (2004, September). Tree Growth, Forest Management and Their Implications for Wood Quality. Retrieved from http://owic.oregonstate.edu/sites/default/files/pubs/pnw576.pdf

9. De Pracomtal, G., Marie Mirabel, M., & Du Cros, R. T. (2014, July). Types of oak grain, wine élevage in barrel. Retrieved from  www.cantoncooperage.com/pdf/WV_July2014_Types-of-grain-elevage.pdf

10. Mirabel, Marie & Cros, Rémi & Beauregard, Dominique & Balu, François & Giraud, William & Comtat, Maurice. (2011). Aging wine in barrel: is there a link between oak wood grain size and porosity? Retrieved from  www.researchgate.net/publication/267266839_Aging_wine_in_barrel_is_there_a_link_between_oak_wood_grain_size_and_porosity

11.  Doussot, F. & Pardon, P. & Dedier, J. & Jéso, Bernard. (2000). Individual, species an geographic origin influence on cooperage oak extractible content ( Quercus robur L. and Quercus petraea Liebl.). Analusis. 28. 960-965. doi.org/10.1051/analusis:2000162. Retrieved from analusis.edpsciences.org/articles/analusis/pdf/2000/10/doussot.pdf

12. Mirabel M, Monteau A-C, Cros RTd. (2013). American oak: the benefits of extended natural ageing. Retrieved from  www.cantoncooperage.com/pdf/WV_July2014_Types-of-grain-elevage.pdf

13. From forest to barrel: Harvesting: ISC barrels. (2017, July 19). Retrieved from  www.iscbarrels.com/2017/07/20/from-forest-to-barrel-harvesting/

14. Quercus alba. (2021, August 16). Retrieved from  en.wikipedia.org/wiki/Quercus_alba

​15. Todaro, L., Dichicco, P., Moretti, N., & D'Auria, M. (2013). Effect of combined steam and heat treatments on extractives and lignin in sapwood and heartwood of turkey oak (Quercus Cerris L.) wood. Retrieved from http://ojs.cnr.ncsu.edu/index.php/BioRes/article/view/3558

16. Chatonnet P, Dubourdieu D. 1998. Comparative study of the characteristics 
of American white oak (Quercus alba and European Oak (Quercus petraea and Q. robur) for production of barrels used in barrel aging of wines. Am J Enol Vitic 49: 79- 85. Retrieved from  www.ajevonline.org/content/49/1/79.short  

17. Helton, B., & Jackson, N. (2017, October 31). From forest to barrel: Stave Mill: ISC Barrels. Retrieved from  www.iscbarrels.com/2017/11/07/from-forest-to-barrel-stave-mill/

18. United States Department of Agriculture. (1956, September). DURABILITY OF WATER-RESISTANT WOODWORKING GLUES. USDA. Retrieved from https://www.fpl.fs.fed.us/documnts/fplr/fplr1530.pdf.

19. Stave thickness has an important enological impact. Kadar Hungary. (n.d.). Retrieved from http://www.kadarhungary.com/products-thickness.

20. Barrel profiling – part I: ISC barrels. (2019, September 17). Retrieved from  www.iscbarrels.com/2017/04/12/barrel-profiling-part-i/

21. Barrel profiling - part II: ISC barrels. (2017, June 09). Retrieved from  www.iscbarrels.com/2017/05/25/barrel-profiling-part-ii/

22. Ward, A., Hale, M.D., & Cardias-Williams, F. (1998). The Isolation of Fungi from Air and Kiln Drying Oak Wood Used for the Maturation of Alcoholic Beverages.   doi.org/10.1515/hfsg.1998.52.4.359
 citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.852.5277&rep=rep1&type=pdf 

23. Hale MD, McCafferty K, Larmie E, Newton J, Swan JS. 1999. The influence of oak seasoning and toasting parameters on the composition and quality of wine. Am J Enol Vitic 50: 495- 502.

24. Stave yard and microorganisms: A living universe. (2011, March). Retrieved October 23, 2021, from  www.cantoncooperage.com/pdf/Stave-Yard-and-Microorganisms_A-Living-Universe.pdf

25. Masson E, Baumes R, Moutounet M, Puech J-L. 2000. The effect of kiln-drying on the levels of ellagitannins and volatile compounds of European oak (Quercus petraea Liebl.) stave wood. Am J Enol Vitic 51: 201- 214.

26. Martínez J, Cadahía E, Fernández de Simón B, Ojeda S, Rubio P. 2008. Effect of the seasoning method on the chemical composition of oak heartwood to cooperage. J Agric Food Chem 56: 3089- 3096.  doi.org/10.1021/jf0728698

27. Masson G, Moutounet M, Puech JL. 1995. Ellagitannin content of oak wood as a function of species and of sampling position in the tree. Am J Enol Vitic 46: 262- 268.

28. Chatonnet, P., & Dubourdieu, D. (1998). Comparative study of the characteristics of American white oak (Quercus alba) and European oak (Quercus petraea and Q. robur) for production of barrels used in barrel aging of wines. American Journal of Enology and Viticulture, 49, 79-85.

​29. Prida, Andrei & Boulet, Jean-Claude & Ducousso, Alexis & Nepveu, Gérard & Puech, Jean-Louis. (2006). Effect of species and ecological conditions on ellagitannin content in oak wood from an even-aged and mixed stand of Quercus robur L. and Quercus petraea Liebl.. 
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