Bourbon - How is bourbon made?

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A whisky distiller must first determine the 'mash bill', essentially a recipe of different grains from which the whiskey is to be made, literally meaning 'the bill of goods to cook'. In the case of bourbon this must consist of a minimum of 51% corn (but is usually around the 70% mark) with the balance made up by 'small grains', usually malted barley and either rye or wheat.


Rye is the grain that packs the punch: it's peppery, spicy in character, and I have heard its flavour in a whiskey described as being like a tortilla chip back of your throat. Use more rye in the mash bill and you will produce a heavier whiskey such as Old Grand-Dad; opt for more wheat (which incidentally smells like a pampered baby when freshly ground) and you impart a sweetness that results in a lighter bourbon such as Maker's Mark. Malted barley adds bolder characteristics and aids the fermentation process.

The process begins with the grains (corn, rye, wheat, malted barley) being coarsely ground and mixed with water. (Producers usually using a hammer mill, though Maker's Mark uses a roller mill). This mixture is then heated so enabling Alpha Amylase enzymes naturally present in the grains to hydrolyse (break down) starch in the grain into fermentable (simple) sugars.

Amylase is present in seeds containing starch and enable plants to convert its food reserve - starch. Malted barley is a particularly good source of Amylase, hence its use is mash bills. Malted barley is produced by encouraging the barley grain to germinate (sprout) under controlled conditions, before drying it to obtain higher levels of the enzyme. Some distillers also add commercially prepared enzymes to ensure an efficient conversion to fermetable sugars

Corn is mixed with water and cooked first, the temperature is dropped and either rye or wheat added. Lastly barley is added to the mix and cooked at the lowest temperature. Cooking is sometimes under pressure and importantly paddles agitate the mix throughout. Americans are the only whiskey makers to cook different grains together. Other whiskey makers around the world making multi grain whiskey, such as blended Scotch whisky, blend the different spirits after distillation.

Around 95 per cent of all bourbon is produced in Kentucky and the state's nine bourbon distilleries lie within about 40 miles of each other, partly due to the limestone-rich waters in the area around Louisville. The limestone aquifer that underlies Kentucky's Bluegrass region acts as a natural water filter, removing iron salts and adding minerals, particularly magnesium and calcium. This unique, sweet-tasting hard water is perfect for distilling, as the lack of iron and the presence of calcium benefits the yeast during fermentation.

After cooking, whiskey producers in other parts of the world separate the sugary liquid, called wort, from the solids in the mash and send only the wort to the fermentation tanks. In Bourbon, and indeed all American whiskey production, the entire mash, solids included is pumped to a fermentation vat where yeast is added, along with modern American whiskey's essential ingredient - 'sour mash'. This refers to what distillers call 'set-back' (or backset) being added to the mash. 'Set-back' is the residue or spend mash that remains from the previous distillation. The term 'sour mash' is a reference to the sour taste of this acidic ingredient.

One of the main challenges of fermentation is the need to control bacteria in the fermenter. Set-back lowers the pH of the mash so preventing the growth of bacteria and giving the yeast a competitive advantage. (Yeast also suffers from the low pH but not nearly as much as the bacteria).

Some American whiskey brands use the term 'sour mash' on their labels, perhaps suggesting that this is a unique selling point. To the contrary, sour mash is now standard practice in pretty much all bourbon production, to the extent that its absence has become a USP in certain top-shelf bottlings, such as in Woodford Reserve's 1838 Sweet Mash Bourbon.

Unless a bourbon's label specially says 'sweet mash' you should assume it is made using sour mash. Historically, sweet mash would have been used to make all bourbons but the practice of sour mashing quickly took over when the benefits of set-back were discovered in around 1823 by Dr James Christopher Crow, a Scottish distiller (who established what is now the Woodford Reserve Distillery) in Kentucky. Prior to the use of sour mash, hops were often added to help sterilise the mash.

The mash, a beer-like fermented slurry, is then distilled. Most bourbons are distilled twice, usually once in a column still (beer still) which takes the strength up to approximately 50-60% abv and then again in direct heated copper pot stills known as 'doublers' (or 'thumpers' if the still is steam heated so emitting a thumping sound), which continue to remove fusil oils and impurities and also further increases the strength to around 63-70% abv. (Few bourbons are triple distilled). Unusually for pot stills, these doublers are often continually fed so operate semi-continuously.

After the second distillation the spirit produced is sometimes called 'white dog', and in recent years some distillers have started to bottle and sell it as it is. A few refer to white dog as being 'unaged whisky', which is both a misleading term and not permitted in Europe. Whisky is a term for an aged spirit so it is not possible to have an unaged whisky. It's like calling dough 'unbaked bread'.

Perhaps the most important part of bourbon making is the barrel aging process. White dog must be aged for at least two years in a new charred oak barrel before it can be called straight bourbon. A general misconception is that bourbon must be aged in American white oak barrels. While bourbon distillers almost always do use this excellent home-grown variety, no particular type of oak is specified in the law and several distillers are experimenting with small numbers of French oak casks.

American white oak grown in forests in colder regions such as northern Minnesota are preferred for barrel making as climatic conditions cause the trees to grow more slowly so consequently have a tighter grain.

The oak used to make the barrels is split and sawn into blanks that are stacked and left exposed to the elements for at least six months. The weathering bleaches and washes out bitter tannins in the oak and allows the development of vanillin. Steam and toasting over gentle flames helps shape the staves and also converts some of the starches in the wood into sugars. Finally, regulations dictate that the barrels must be charred and burning of the inside of the barrel caramelises these sugars. This layer of charred oak adds vanillin and caramel flavours to the whiskey and also gives bourbon its distinctive brown-amber colour. Distillers can choose the degree of charring from one (light char) to four (deep char) depending on how they want the barrel to affect their bourbon.

Due to being hand-made, the size of each barrel varies slightly, but the standard size is 263 litres (or 53 gallons) capacity, standing 86cm (or 34 inches) high, with the 'heads' (top and bottom) measuring 53cm (21 inches) in diameter. An empty whiskey barrel weighs about 45kg (100 lbs); when it's full, a weighty 225 kg (500 lbs).

White dog must be diluted to 62.5% alc./vol. or below before barrel filling, using demineralised water. It is commonly accepted that lower maturation strengths elicit better extraction of flavours from the cask, but this results in more spirit to mature, a need for more barrels, more manpower to move those barrels and more facilities in which to age them.

The sheds in which barrels are matured are known as rickhouses and these large buildings, which can be built of masonry, wood or tin sheet-clad wood, are a distinctive feature of the Kentucky landscape. Different distillers paint their warehouses various colours so as to affect the convection of the sun's heat - black, brown, cream and red are popular hues. During the summer the sun's heat will be retained in the barrels so that in winter the temperature will be warmer inside than out.

America's whiskey belt happens to sit in the part of the country known as Tornado Alley so dispersing rickhouses over a large area also spreads the risk of tornado or fire damage. In addition, barrels from the same year are dispersed among several different rickhouses: a fire or some other sort of accident that wiped out a whole year's batch of whiskey would be an economic disaster and this dispersal lessens the risk.

Temperature variations vastly affect maturation. When the wood expands with heat the whiskey seeps into the wood; conversely when the temperature drops and the wood contracts the whiskey is forced out of the wood. This action is referred to as a cycle. Mother Nature provides around four cycles per year due to seasonal changes. Some distilleries artificially heat their rickhouses to increase the number of cycles - some obtain around 11 cycles per year, giving the whiskey more flavour and colour.

Rickhouses can be anything from a single storey to ten high, with barrels typically stacked three high on each floor, resting on wooden racks, or 'ricks'. The temperatures within multi-storey rickhouses can differ by as much as 35°C (95°F) between the top and the bottom floors, and the differences in barrel position and the degree of charring of that barrel will greatly affect the flavour of the finished bourbon. The top floors of the rickhouse are dry and hot and here the barrels lose the most water, so gain a couple of per cent in alcohol strength. Conversely the bottom floors are cooler and moist: here the barrels evaporate more alcohol than water so lose strength.

Sometimes you see bourbons bottled at 'barrel proof' with a strength exceeding the 62.5% alc./vol. legal limit for barrelling bourbon. The reason is that although the whiskey may have gone into the barrel at 62.5% alc./vol. it has gained strength due to water evaporation during the aging process. Although the minimum bottling strength is 40% alc./vol. there is no maximum strength stipulated.

An analogy to help remember the effects of the different rickhouse floors references the spirit lost to evaporation during aging, known as the 'angels share'. "The angels at the top of the rickhouse are hot and dry so drink water from the barrels while those in the moist cool conditions at the bottom drink alcohol to alleviate their rheumatism. The lucky angels in the middle drink a cocktail of water and alcohol from the barrels."

Some distillers, most notably Maker's Mark, move barrels between different floors of their rickhouses to help balance the effects of temperatures on different levels. Others blend from barrel to barrel. While most rickhouses tend to be six to nine floors high to better make use of land, Four Roses has low, single storey rickhouses for more even maturation profiles across its aging barrels.

Bourbon will typically be blended from around 150 casks. These will be drawn from various floors of several different rickhouses, according to tastings of those casks and the master distiller's experience. Each 150 barrel blend will fill around 3,000 cases.

Prior to bottling it is common to chill filter bourbon to remove long-chain protein molecules which can precipitate out of the spirit. In other words they change from being dissolved in the liquid into solids again. If these are not removed the bourbon could become hazy when stored at low temperatures and/or diluted with water. The bourbon is chilled down to temperatures between -2°C and as low as -12˚C, causing the protein to precipitate and so allowing it to be filtered out using particle filters. However, some claim that chill filtration strips out mouth-feel and flavour provided by the fatty acids, so some premium bottlings proudly declare a lack of chill-filtration on their label.

Whiskies termed 'small batch' are currently in vogue. These tend to be bottled from barrels stored in one particular section of the warehouse, or even from an individual barrel. As mentioned above, the position of a barrel in the warehouse can greatly affect its maturity, as will the level of charring. Some small batch bourbons will also be characterised by differing proportions of grain from the standard mash bill. Whilst small producers may produce small-batch bottlings from less than a dozen barrels, larger volume small-batch bourbons are usually made by taking selected barrels from specific areas of the warehouse which are then blended together to achieve a taste that is consistent.

The 'small batch' category is still to be recognised in law and is a concept rather than a specific term where legislation can be enforced. This is not helped by the fact that the many different bourbon producers have not agreed, and probably never will, on a specific number of litres and barrels to determine what constitutes 'small batch'.

'Single barrel' whiskies drawn from one barrel tend to have no age statement as the distiller will have to keep sampling the contents to find when the spirit has reached the appropriate maturity.

Rickhouses are actually bonded - i.e. government-supervised - warehouses and the barrels must be government stamped, and revenue must be paid twice yearly on each barrel. Regulations dictate that only new oak barrels can be used to make bourbon. However, after use the barrels are not discarded: instead they are sold on for aging other spirits, mainly Scotch whisky and rum.

More on bourbon


What is bourbon
Bourbon origins and history

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