Beer from the Wood
by Jason Dunson-Todd (Brewing Techniques)
From Ancient Past to Present, Oak Remains a Perennial Source of Special Character
The use of wood, particularly oak, was commonplace in brewing until the 20th century. Though wood is still widely used for the fermentation or storage of wine and spirits, its use for beer is one example of an increasingly uncommon tradition being kept alive by a handful of tenacious brewers.
Throughout history, wood prevailed as the material used in key brewery processes. Even after wood kettles were replaced with directly heatable metal, wood continued to be used for fermentation and aging vessels until (with a few notable exceptions) the middle of this century. Though difficult to maintain and keep clean, wood was durable and easy to handle, and its various flavor contributions also became an integral part of many classic beers’ flavor profiles.
Purists lament the end of tradition and the loss of wood flavor and character as brewers move toward modern materials and more efficient methods. The classic Pilsner Urquell is one example of a brewery that, under pressure from market forces, recently broke with a longstanding tradition of beer from the wood. The increasing rarity of wood in the brewery lends an air of romance to an era dominated by stainless steel.
Whether it’s nostalgia or flavor that calls, many brewers have been making an extra effort to return to brewing’s roots. This article reviews the history of the use of wooden casks in the fermentation and aging of beer and explores modern-day applications in home and microbreweries.
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A Design Ingrained with Function
The lineage of the cask stretches through time and culture to the wine and song of the Roman Empire, if not beyond. Stone bas-reliefs from Caesar Augustus’s time (63 B.C.–14 A.D.) depict casks with all the recognizable features of modern barrels. The French and German languages both contain words, douve and daube, meaning staves (the narrow strips of wood placed edge-to-edge to form the sides of a barrel); these words were likely derived from the medieval Latin word doga or dova, meaning “vessel”.
Casks and barrels in the United States are most commonly built from white oak because of the species’ strength, resistance to decay and leakage, and workability, although other woods have been used.* In fact, white oak has earned the nickname “stave oak” because of its extensive application in barrel production. White oak also has particularly good water-tightness, and its high tannin content makes it resistant to decay. Tannin is also an important extractable flavor component. (See the box, “The Heart of the Oak,” for more on the making of barrels.)
For thousands of years, Europe supplied most of the oak used for brewery applications. White oak also grows throughout much of the eastern half of the United States, though it is harvested for commercial cooperage applications only in the South, especially in Kentucky, primarily for the Bourbon industry. Quercus alba is the American species of white oak most used, Q. robur and Q. sessilis ire the European species.
Oak’s Flavor Contributions
Oak’s contribution to the flavors of wines and spirits is well-documented, and it comes as no surprise that the use of wood is still alive and well in these industries. The desirability of an “oaky” flavor in beer is disputed, however; in fact, most breweries that do use oak use various techniques to minimize its flavor contribution. Breweries often used casks lined with pitch, for example, both to protect the wood against the beer and to protect the beer from the strong oak flavors that might otherwise leach from the cask (see the box, “The Evolution of Pitching”). Many breweries tried different species of oak to achieve a mellower flavor, in the belief that some give stronger flavors than others. Other breweries used older casks that most likely had much of their strong flavors leached out. Yet others embraced the unique flavor contributions of oak and mastered the methods needed to provide the desired character in the beer. Held in check, oak can add nicely to the complexity of a beer’s flavor profile.
*Other woods used for casks or vessels include red and chestnut oak, red and sweet gum, sugar maple, yellow and sweet birch, white ash, Douglas fir, beech, black cherry, redwood, elm, and basswood. Many types of wood, for example, redwood, are unacceptable for brewing applications because of excessive flavors that could be extracted from them. Others, such as red oak, are too porous and must be lined before use.
Flavoring compounds: Oak contains many flavor components. The primary compounds that may influence beer are tannins, which are a type of phenol. Although brewers generally seek to minimize the amount of tannins in their wort and beer, tannins may offer benefits to cask-aged beer. It is generally accepted, for example, that tannins act as body builders, contributing to a beer’s mouthfeel. Ellagitannin and gallotannin are known to clarify wine by reacting with proteins and precipitating them out of solution; it is likely that they have a similar effect on beer. Ellagitannins also offer antiseptic properties and may therefore afford beer some protection against microbiological invasion. The woody odor of whiskey has been attributed to 9-carbon y-lactones, which are found in white oak. The hydrolysis and oxidation of lignin, also found in white oak, produce vanillin syringaldehyde and other compounds in alcoholic beverages during cask aging. Vanillin odor will increase with time and mellowing, but extended aging is not a luxury most beer brewers can afford.
| The Heart of the Oak |
| Making staves: Staves (the narrow strips of wood placed edge-to-edge to form the lining of a barrel) are made of the heartwood of the oak. This is the hardened center of the tree, made up of older cells that are no longer growing. The heartwood is surrounded by the sapwood, the cambium layer, and finally the bark. Water resistance. When a tree is cut, its conductive tubing is normally exposed, rendering the wood inappropriate for holding liquids. White oak, however, has the ability to effectively seal off this tubing, making the wood water-tight and thus particularly good for ships and barrels. Tyloses (roughly, “callouses”) in the wood arise from the ballooning of the cell walls of nearby living cells into the channel of the vessel. (Because French oak generally has less tendency to form tyloses, the trees must therefore be carefully hand-cut to seal off the vessels, adding to the cost of production; American oak is usually machine cut.) Drying: After the cutting, stave-length sections of the tree are then split or cut lengthwise into quarters (bolts) through the center. The staves are cut from the faces of each quartered bolt. Optimally, as is normally the practice with European coopers, the fresh-cut staves are exposed to the elements for at least two years to age and mellow the wood. Though more and more American coopers are now also treating their wood this way, many continue to disdain extended aging in favor of quick oven-drying because they primarily sell to the less-demanding distilleries rather than the very particular wineries. Barrel-making and charring: Once aged or dried, casks are “raised” by joining the staves in a circle and temporarily hooping them at one end. This shell is heated over an open fire to steam moisture from the pores; this process allows the staves to be bent without cracking. The first firing is finished by drawing the other ends together until the structure takes on the characteristic cask shape. The staves are bound together by a temporary hoop and shaped by sight. The second firing conditions the cask by drying the interior and softening and stretching the exterior. This is also the point at which casks will acquire varied degrees of toasting, depending on each cask’s future use. Bourbon casks are charred; wine and beer casks are usually given a light or medium toast. Heads are then fitted onto each end of the cask by inserting them into the grooved ring, or croze, cut out of the interior surfaces of the staves. The outside surface is smoothed, and hoops, usually of galvanized steel, are put into place. Wood hoops, also sometimes used, were considered doubly durable — most likely because they were so thick that they protected the cask itself, which was rolled from one place to the next. The cask’s egg-like shape and skillful production make it incredibly strong. Wine and whiskey casks have one hole bored through a stave at the highest point of the bilge (the point of largest circumference, usually in the middle of the barrel). Beer casks typically used for dispense have an additional tap hole bored into their lowest point in one of the head boards. |
Much of the research on these compounds is based on products with higher alcohol content than beer, and the precise effect of these compounds on beer is not fully known. Beer is generally lower in alcohol and therefore less protected from off-flavors. It is a comparatively delicate product; it cannot afford to age in a cask as long as wine and spirits can. History and experience have, however, taught us many things about successful applications of oak in the brewery.
| Glossary of Terms |
| Bilge: Point of largest circumference (usually the middle) of a cask or barrel. Cask: Wooden vessel used to ferment or store beer. Chime: The portion of the stave of a cask that extends from the croze to the rim. As a verb, chime means “to chamfer the end of the staves to form the rim.” Cooper: One who makes or repairs wooden casks. Croze: The groove near either end of a barrel stave in which the barrel head is inserted. Flagging: A cooper’s term for the reedy material (often cattail stem) used to caulk small leaks in casks. Head boards: The circular boards placed at either end of the barrel. Heartwood: The section of the tree no longer growing; used for staves of barrels. Hoops: Metal bands encircling the cask at each end. They squeeze the staves against the headboards to form the cask’s sturdy egg-like shape. Quercus alba: The name of the American species of oak; Quercus robur and Quercus sessilis are European species often used for barrels. Sodium meta-bisulfite: Sanitizer used in solution to keep unused kegs free of contamination. Staves: The narrow strips of wood placed edge-to-edge to form the sides of a barrel. Stillage: A small table or stand, sometimes equipped with wheels, that is used to support barrels or casks. Tannins: Phenolic compounds found in wood. Contribute to mouthfeel and astringency to beer. Tyloses: Roughly, “callouses” that seal vessels in the heart-wood of certain types of wood, making it water-tight. |
| Tips on Purchasing Wood Barrels |
| Sizing: Barrels come in sizes ranging from 3 to 50 gallons; you’ll probably encounter metric measurements, so remember that a liter is roughly the same as a quart, so a 5-gallon barrel would be roughly 20 L. A barrel full of beer is going to weigh 8.3 lb/gallon of liquid, plus the weight of the barrel, so if you choose to buy a 10-gallon or larger cask consider ahead of time the logistics of filling and storing it. Due to the weight, you may need a hoist or a few strong friends to lift it. Also, be sure you have the brewing capacity to fill the barrel. Partially filled barrels may lead to dry staves and leakage and excessive oxidation and spoilage. New versus used: Purchasing a new barrel can be a costly enterprise. Bear in mind that the prices do not increase linearly with size. For example, a 10-gallon barrel typically costs only about $20–30 more than a 5-gallon barrel. Typical prices for new barrels are $90 for 5 gallons, $100 for 10 gallons, $150 for 20 gallons, and $175 for 30 gallons. These prices include mark-up; they will vary among suppliers. Second-hand barrels are often the way to go, particularly if you live near a vineyard. Many wineries in the United States use 200-or 225-L European barrels, however, and finding smaller sizes may be difficult. At least one company I know reconditions large barrels down to 30-gallon sizes. Home winemakers might be a good source for 5–10 gallon casks. If you want a charred barrel, you might contact a distillery directly. By law, barrels used for Bourbon must be new, so some surplus used barrels can always be found. Used whiskey or brandy casks can also add character from leftover flavors. If possible, obtain the cask immediately after it was last used, so it does not dry out. Condition: Make sure that the cask does not leak, and find out what recourse you have if it does. Find out the material used to make the cask and its origin, if that is important to you. Perhaps most important, be sure to find out whether the cask might be contaminated. It is increasingly common to find wineries selling barrels after using them for only 3–4 years. This is related to an overall trend in the industry to curtail the use of sulfite preservatives; unfortunately, the trend has resulted in an increased risk of Brettanomyces infection in the barrels — a problem for anyone brewing beers other than lambics. For lambic brewers, barrels that have become contaminated or that are at risk are often available at marked-down prices of around $50. |
Differences between new and used: The use of older barrels that have been used previously for the fermentation of wine or spirits will reduce some of the sharpness typically found in a new cask but also presents its own flavor problems. Lining new barrels with pitch will keep the oak flavoring to a minimum, but cracking of the pitch might result in unintended consequences for the beer. It is also possible to pre-age new casks using any of several techniques. (See the boxes, “Tips on Purchasing Wood Barrels” and “The Preparation and Repair of New and Used Barrels”).
American vs. European oak: American oak is commonly thought to contribute harsher flavors than European oak, and many traditionalists find the brash American oak too overpowering for their styles. Terry Foster, for example, disdains American oak for use with traditional pale ales, suggesting that it is “unsuitable, since the beer will leach out various undesirable flavors from it”. Foster leans toward the traditional use of English oak, even though it is now quite rare and therefore expensive. (Many “new” English oak barrels are actually pieced together from old ones.) Similarly, most vintners typically prefer the subtle flavors of French oak, even though it is quite a bit more expensive than American.
| The Preparation and Repair of New and Used Barrels |
| Preparation: Although new casks are tested for water-tightness, the wood dries when not in use, which can lead to leaks; it is therefore necessary to prepare a new cask by soaking. The whole cask can be filled and left to soak until it stops weeping, topping it off each day to replace lost volume. Tannin control: Flavor contributions from oak vary depending on the age of the cask, the length of exposure, the aging process used to make the cask, and even the species of oak. An extensive soaking in hot water (no higher than 160 °F [71 °C]) and rinsing will leach out some tannins; repeated rinsings may be desirable to mellow the stronger flavors found in new casks. This process will also remove eugenol, a colorless aromatic oil (C10H12O2), that can be tasted in the water even after a brief soak. Longer soaks with a sodium metabisulfite and citric acid solution will also help remove tannins. A good starting place for a newly purchased cask is to soak it in hot water for two weeks first. (Add sodium metabisulfite at 200 ppm if the climate is humid.) Then add sodium carbonate at the rate of ½ oz/gallon and let soak. Drain, rinse, repeat 4–6 times. The water should be clear. Then add citric acid to drop the pH to 3 and let it sit. Rinse and fill with sodium metabisulfite to inhibit mold until brew day. Repair: Leaks are most likely to be found between the head boards and the chime, where the surface is flat. The head board can be lifted out by removing one or two of the hoops. The leak can then be caulked with flagging, a cooper’s term for a reed material, often cattail stem. I have found this flagging wrapped around entire headboards in some of the older wooden casks used by commercial breweries. This same method can be used to stop leaks between staves without removing the head. It is important when taking a cask apart to remember to mark the position of the head and replace it in the same position. One antique cask I took apart looked round, but the shape of the headboard was actually slightly oval. Storage: If a cask is going to be left empty, burning a sulfur candle in it will remove oxygen and prevent microbiological growth. It’s best, however, to keep the cask filled with a bacteria-resistant metabisulfite solution to keep it from drying out. It is a good idea to periodically refurbish the solution if the cask is going to be left for a while. Ongoing care: Much can be said about preventive maintenance when it comes to wood; it is a delicate material. Something to consider first is where the casks will be stored. Wood is like a sponge, so keep your casks in a clean area that is not too damp and that is raised off the floor on some sort of stillage. A stillage is a device designed to support the cask near the hoops to keep it off its bilge. Any length of time that the cask rests on its bilge, especially when full, puts it at risk of leaking. Little in the way of routine cleaning and sanitization need be done if the cask is stored properly as described (that is, filled with sanitizer). If you do need to clean and treat a cask, consider your materials carefully because of the unplaned, porous nature of the cask’s interior. Don’t use anything on your wood that you do not want to taste later. Therefore, standard chemicals used in the brewhouse should not be used for wood. If you put a chlorinated cleanser in the cask, your beer will taste like chlorine. Caustic soda and phosphoric acid are also out of the question. Sodium metabisulfite, a bacterial inhibitor that releases sulfur dioxide, is the strongest chemical you should use in a sound cask. High-pressure steam can also be used on a commercial level for sanitization. Sulfur is excellent for keeping oxygen out of the cask, but sulfured casks should be rinsed before use. Not only does sulfur tend to lower the concentration of acetovanillone, vanillan, and eugenol — the components of wood that add positive character to the beer — but the sulfur itself can leave undesired off-flavors when present in high concentrations. Mold and other contaminants: When dealing with wood, preventive maintenance is your best friend. A sodium metabisulfite solution can keep the outside as well as the inside of the keg clean. Casks that already show possible signs of mold or fungus might be saved, if you like taking risks, by being opened and brushed out (if they cannot first be rinsed). Casks in this state, however, are not acceptable for commercial use. Wood absorbs an incredible amount of liquid, and because the liquid in question is often beer or wine, the conditions are set for acetobacter, lactic bacteria, and Brettanomyces contamination. If you have any doubts about how deep the mold is, or for how long it has been in the wood, throw the cask out. Brettanomyces can be an acceptable inhabitant for the recreation of some British styles and Belgian lambics, but it is far safer to inoculate the beer rather than guess at what is making your cask smell funny. If that funny smell is vinegar, use the cask as decoration, or for making vinegar. Nothing can remove acetobacteria from the wood. Questionable casks and options for salvage: If you are hell-bent on using your cask no matter what, you must put it through a series of cleanings. Soda ash, Iye, or a combination of these and other chemicals such as Barolkleen may be used. A solution of these chemicals in hot water should be left to soak for up to three days. Then, drain and flush out the cask and add a solution of metabisulfite and citric acid. Allow the solution to stand for one day to neutralize the previously used carbonates and hydroxides. Again, there are no guarantees, and cleaning is no substitute for a sound cask. |
Though some of this debate can likely be chalked up to snobbery or differences in aging practices, distinct differences do exist between the various oak species. American oak may be higher in lactones than European oak, imparting a coconut-like taste, but it is lower in phenols, especially ellagintannins. One study reported that French barrels had twice as many phenols as American barrels, yet French oak still had “less obvious ‘oaky’ flavors and aromas”; for some reason, American oak seems to have a more aggressive flavor contribution. It also has more vanillins. These genetic differences can be affected by a number of factors, however, and brewers must choose a wood according to their needs and tastes.
Influences of drying and toasting: A barrel’s exposure to heat may be the most important factor affecting the flavor contribution from oak.
Differences between American and European oak may be accentuated by the typically shorter drying/aging periods for American-produced oak barrels, or by the drying process used. Whereas most European wood is dried by exposure to the elements over a period of one to two years, American oak is usually forced or kiln-dried in a week’s time. Much of this wood will then be charred for whiskey barrels, making aging less critical. Winemakers who cannot afford the pricier European wood, however, may be putting some pressure on American coopers to more closely emulate European aging practices.*
Toasting levels will also affect a casks’s flavor contribution. Heating the lignin in the wood develops aromatic compounds, with aromas such as toasted almonds, cocoa, toasted bread, coffee, chocolate, and caramel all possible. Toasting also affects lactone, an ester of a hydroxyl acid that occurs naturally in oak; a high toast will completely destroy the lactones, while a medium or light toast will preserve some of the lactones, adding subtle woody notes to the beer. Winemakers vary the toasting specifications they give to the cooper depending on the wine style; similarly, brewers should consider the level of toasting when purchasing their casks.
Contact time: The length of time the wood contacts the beer is extremely important to a cask’s flavor contribution; the longer the beer is left to age, the stronger its flavor will be. The length of time to allow will vary greatly depending on each of the other variables and the desired flavor profile of the beer you are producing. Even in a cask that has been well-leached of tannins, two weeks may be enough time to give a light beer a strong oak flavor.
Beer style: Full-bodied beers, such as American, British, and Belgian high-gravity ales, are the most likely candidates for oak aging. All pale ales, even medium-gravity bitters, take oak flavors well, though the maturation period will be short. The higher alcohol of strong beers protects them from attack, and their intricate flavors can often be enhanced with oak components. Barleywine is an excellent candidate for oak, as is any beer requiring a Brettanomyces fermentation or finish (for those bold brewers willing to experiment with wild yeast).
*American vintners seem to agree that American oak is “improving with age.” Many winemakers now use a combination of French and the less expensive American oak, depending on the wine.
Adding to the options, a species of American oak that does particularly well in the Pacific Northwest is now catching some attention. The species Quercus garryanna (often referred to as “Oregon oak”) has been said to have flavor characteristics similar to French oak. A representative of Francois Frere, which owns the Demptos Napa Cooperage in California, said he believes that the combination of lower costs, flavors rivalling French oak, and aging times that parallel European cooperages will begin to turn some heads. Contact Mel Knox, 415/751-6306 for more information.
Cask size: The size of the cask is also an important consideration. The surface-to-volume ratio in a smaller cask will allow for more contact with wood than will a larger one. According to The Chemistry of Winemaking, “Increasing the container volume 10-fold gives a little less than one-half as much surface/liter volume, and containers 1,000 times as large in volume have exactly one-tenth as much surface/liter volume”. A small cask will mature the beer much more quickly.
The Tradition of Wood Persists Abroad
Many beers certainly seem to have been influenced by oak aging. Roger Protz finds, for example, “rich, fruity, oaky aroma and palate” in England’s Green King Strong Suffolk Ale, which is a blend that includes one beer aged in oak vats for two years. Rodenbach in Belgium has a distinct oak flavor to it, mostly that of tannin and caramel. Belgian brewer Frank Boon’s lambic also has a delicate oak flavor.
In fact, lambic breweries have been the most consistent in their use of wood. The wood’s porous nature provides a perfect breeding ground for the all-important wild yeasts and bacteria at work in lambic fermentations, supporting the beer’s spontaneous, wild fermentations; the wood also lets the fermenting beer “breathe.” (See the article “Practical Strategies for Brewing Lambic at Home” on page 38 for more on the use of wood in lambic brewing.)
Other breweries also take advantage of wood’s tendency to harbor microorganisms. Rodenbach, in the province of West Flanders, Belgium, produces a sour red beer aged in huge uncoated oak tuns. Michael Jackson writes, “The wood, with its tannins and caramels, makes a direct contribution to the beer’s palate and colour. I once expressed surprise that the wood would continue to impart character after constant employment, and it was explained to me that the inside of each tun was scraped after every use”. The tuns may also contribute to its red color, while the acetic and lactic acids remaining in the wood from previous batches lend it its characteristic sourness.
In Britain, Guinness relies on wild yeast and bacteria from its oak vessels to give sourness to its Foreign Extra Stout, which is produced by blending the soured beer with a younger stout.
Marston’s in Burton upon Trent continues to ferment in wood with a traditional Burton union setup, which consists of a series of linked casks lined up beneath a wooden trough. As the wort ferments, wort and yeast get pushed up through pipes to the slightly sloped trough, where most of the pitched yeast gradually collects while the beer runs back to the cask.
Some British breweries still use wood to dispens