Perpetually updated with additional terms and definitions surrounding the fermentation, distillation and general production of America’s Native Spirit: Bourbon. 

learn about bourbon

Bourbon Definition

Learn everything about bourbon. First, a proper definition: The term "bourbon" is very specific. In fact, for a whiskey to be referred to as a bourbon, certain legal requirements have to be met. Bourbon must be made in the United States, comprising at least 51% corn, and have no other additives except water. Also, it must be distilled at not more than 160 proof and barreled at no more than 125 proof strictly in a brand-new charred oak barrel.

Finally, it has to be bottled at no less than 80 proof. Only whiskey that meets these criteria can be called bourbon. "Proof" is a term used to describe the potency of a spirit and is simply the alcohol by volume multiplied by two. If bourbon is 40% ABV, then it will be 80 proof.

As a rule, all bourbons can be legally called “whiskey” however, not all whiskey can be legally called “bourbon.”


Distillation is the process of separating liquids from other liquids by using heat (usually to a point of boiling), evaporation and condensation. As the liquid is heated (raw distillate), alcohol vapors evaporate and are then cooled rapidly – causing condensation (returning the vapor to a liquid state). In the world of bourbon distillation, the resulting distillate – a clear, high-alcohol liquid - is the precursor to a finished bourbon product.

This distillate is also sometimes referred to as “white dog” or “moonshine” and is basically unaged bourbon. Only after it enters a new, charred oak barrel will it then go from clear and colorless to a more viscous golden amber color due to the char on the inside of the barrel, sugars in the wood and natural congeners.

Distillation is the best way to go from high-alcohol cereal mash (cooked and fermented grains from the bourbon mash bill) to a finished distillate ready for barreling.

Aged vs Matured Bourbon

In the world of bourbon terminology, there are a lot of terms to get acquainted with. Sometimes, two words can seem like they are conveying the same message or describing the same process – however they can be radically different.

Aged/aging: This means the bourbon is getting physically older – as in the length of time is spends in a barrel. If a bourbon enters the barrel January 1, 2022 and is removed from the barrel in January 1, 2007, it has aged five years. Some would call it a “Five-year-old bourbon.”

Maturing: Much as the same as adult humans, age doesn’t necessarily indicate maturity. Just because a bourbon has spent a certain number of years in a barrel, does not by default mean it has matured appropriately. Maturation can be affected by climate changes, temperature variations, rackhouse placement (warehouse), etc.

So many bourbon enthusiasts immediately judge a bourbon based on it’s age statement (a declaration of the amount of time the bourbon spent in the respective barrel), however, just because a bourbon is “x” number of years old, does not mean it is matured. Some two year bourbons might very well taste much more “mature” than a bourbon aged six or more years. There are many factors which attribute to the final taste of a bourbon.


A cooper makes and repairs bourbon barrels used in the bourbon aging process. A cooper may also make similar vessels such as casks, vats, buckets, tubs and troughs. A cooper starts off as a journeyman, and may learn the craft of bourbon barrel making via the construction and craftsmanship of other wooden implements. The word “cooper” is derived from the German word “kuper.” A place where coopers manufacture bourbon barrels or other storage vessels for spirits aging is called a “cooperage.”

With the litany of regulations and requirements that bourbon-making must follow, many of which have to do with the barrel specifically, coopers are highly skilled tradesmen who are very sought after in the industry. Some notable cooperages produce thousands of bourbon barrels per day. Coopers may also undergo training in ironworking to make the rings and rivets used on a bourbon barrel to affix the staves to one another.

New Spirit/New Make/White Dog

These terms all refer to the clear liquid generated by the bourbon distillery that will eventually be aged into legal bourbon. All bourbon starts out as a “new spirit” or white dog / moonshine. However, not all raw distillate or new spirits are the same. Each has a unique flavor profile resulting from the bourbon mash bill – which has a definitive influence on the taste and character of the finished bourbon after aging.

Bourbon Barrel Char / Charring

Charring means the bourbon barrel was partially burned inside. This method is practiced as a way to infuse aromas and flavors into the bourbon that are created from the caramelization of the natural sugars in the wood and their interaction with the unique alcohol resulting from the distillation of a particular mash bill. Typically, a cooper working at a barrel cooperage will be the designated person to carry out barrel char once the bourbon barrel has been constructed.

Charring can be accomplished with different levels of intensity based on how long the barrel was burned or exposed to flame. A level 1 charring lasts for about 15 seconds, Level 2 is 30 seconds, Level 3 is 35 seconds, and Level 4 is 55 seconds. Some distillers (although not commonplace) have charred bourbon barrels to Level 7 char, which is 210 seconds or 3.5 minutes.

The main reason for charring bourbon barrels is to break down the natural hemicellulose in White American oak into simpler sugars which can be infused into the aging bourbon. Hemicellulose is the component in wood barrels responsible for the common notes of brown sugar, caramel and toffee found in bourbon.

Another reason for barrel char is to expose greater levels of lignin – which is a naturally occurring wood glue – which binds together microscopic wood fibers. Lignin is responsible for adding the vanilla taste to bourbon. It is commonly referred to as “vanillin.”

Tannins – a commonly observed component of finer wines – is also expressed through the barrel charring process. Too many harsh tannins and you have headaches and hangovers. Too few, and bourbon maturation slows to a crawl. As a rule of thumb, the higher the char level – the more tolerable the tannins are.

Finally, there are oak-specific lactones which are created as a result of barrel char. These lactones give bourbon it’s wood-centric, toasted coconut essence. If a bourbon barrel was not charred – these would be missing from the palate.

Alligator char is the visual anomaly created most commonly by a Number #4 char on a bourbon barrel. The deeper burn leaves the wood looking like a dry desert, or similar to alligator skin. Many cooperages and bourbon distillers aim for either right before or right at this alligator char level for their barrels.  

Bottled in Bond Bourbon

The Bottled in Bond Act was initially created to prevent people from flavoring and coloring other spirits and selling them as bourbon. Not only was this unethical, but people were getting sick and even dying from this practice. Today, the government standard for bottled in bond requires the bourbon to be at least four years old and bottled at 100 proof. Also, it must be bottled by a single distiller. The next requirement is that it must be distilled all in the same year, not a mix of several years.

The phrase "bottled-in-bond" on the label is a way to indicate that you have high-quality bourbon. Bottled in bond isn't limited to bourbons; any liquor can be bottled in bond as long as it meets these requirements.

Small Batch Bourbon

A bourbon batch is many barrels combined prior to bottling, which can also be referred to by the less technical and less common term "mingling."

"Small batch bourbon" is a term that has no required legal definition. It means basically whatever the distillery wants it to mean. However, some general standards have been widely accepted: A small batch is no more than 150 barrels of bourbon from married together and may also be as few as 15 as in the case of Rabbit Hole Bourbon.

When you see "small batch," you can assume it's a more premium blend that has been carefully crafted. Major bourbon brands may blend hundreds or even thousands of barrels together to ensure product consistency across tens of thousands of bottles. Only the most confident of distillers would use small or micro batches – as they are sure of the product quality and consistency in these small batches.

A small batch bourbon of about 15 barrels may be blended, however – there may be ten thousand barrels that were part of that original “run” or distillation. In this case, the distiller carefully selects barrels in groups of 15 or more and blends those. It is rare to find such precision and consistency when using so few barrels – as buying the same expression on the west coast will taste the same as buying it on the east coast. Fewer barrels in a small batch means greater control over the final bottled product, but with the caveat of exponential possibility for inconsistency in the finished bourbon unless great care is taken.  

Single Barrel Bourbon

A single barrel bourbon is a much more precise version of a small batch bourbon. As the name implies, this batch came from only one barrel. Essentially, tasters will go into the warehouse where the bourbon barrels are kept and pick out a couple that they think stand out from the rest and are truly exceptional. Various influences on a bourbon barrel can affect the taste after aging; from average temperature, relative humidity and barometric pressure to unique artifacts in the particular white oak tree used to manufacture the bourbon barrel – there are many reasons why one barrel might stand out among the rest.

Next, a bourbon distiller will bottle that one barrel at a time, and most often offer it as a founder’s collection or other labeled “limited release.” Sometimes, a bourbon distillery will allow a celebrity or industry influencer to pick the barrel in what is known as a “barrel pick” and offer it to select retailers on a promotional basis. Bottles originating from a small batch bourbon (several barrels all mixed together) will be virtually indistinguishable from one other, whereas a single barrel bourbon will always have a unique flavor compared to the usual offering – even if it is distilled from the same bourbon mash bill.

Bourbon Mash Bill

A bourbon mash bill is a recipe of sorts. It is either a written or verbal list outlining the ratio of various grains used to make bourbon or whiskey. With bourbon, the mash bill comprises at least 51% corn. The rest of the mash bill can be any grain the distiller prefers. Typically, the remainder is often some mixture of rye and barley. That isn't a requirement, and as such, there's another category of bourbon called wheated bourbon that uses wheat instead of the more traditional rye. These are known as “wheated bourbons.” But again, any grain can be used, as some distillers have even used grains such as brown rice.

Mash bills can number in the tens of thousands. For example, let’s say we are using a standard three-grain mash bill, and after the required 51% corn, that leaves 49% of various grain percentages left to experiment with – like more corn, or adding rye or barley. So – even at the most basic level, if we take 49 individual percentage points and ideate three various grain combinations (corn, rye barley), that number would be 503 which equals 125,000 possible mash bills. If we add in additional grains like malted barley, malted wheat, etc. that number goes even higher. Some would say the difference in one or two percent addition or subtraction of a particular grain won’t affect the finished bourbon all that much.

However, there are many who would argue that, and point out the fact all bourbon must be aged in new oak barrels (99.9999% use white American Oak) and aged at least two years. So – that constant says the magic is in the mash bill, yeast, fermentation process, distillation process, etc. In essence, every percentage, every point of differentiation – counts when crafting premium and one-of-a-kind bourbon.

Sweet Mash and Sour Mash Bourbon

The difference between sweet and sour mash bourbon refers to whether you reuse previously fermented mash or not. Sweet mash starts from scratch, only using fresh yeast, water, and grains in each fermentation process. Crafting a sweet mash bourbon requires precision to maintain quality and ensure success and can take more time to begin the fermentation process. Lots of variables can affect a sweet mash – from relative humidity to ambient temperature to barometric pressure. Just as with any other fermented foods and drinks – they usually use a “starter” culture which is essentially a little of the active fermentation left over from a previous batch.

This is how most breads like sourdough are made. The live cultures in the starter provide a jump start to the new fermentation batch – and allow for a predictable start to the new culture because the active yeasts are “vetted” for efficacy.  

Sour mash bourbon is one which is made from a fermentation that has already been successful and then included in a subsequent fermentation. Reusing the mash allows distillers to fine-tune it and maintain a consistent flavor profile more easily. The liquid saved from a previous distillation that will be used to sour a future mash is referred to as either backset or setback.

Bourbon Fermentation

Fermentation is how yeast converts sugar into various molecules, including carbon dioxide and alcohol. The fermentation process generally takes one to two weeks. Distillers add sour mash and yeast during fermentation to reduce the mash PH level, deter bacteria growth, and create an overall better environment for yeast activation.

Without adding yeast, there is no fermentation. A cooked mash bill is basically like a large bowl of multigrain oatmeal. Here’s how it all works:

When the grains of a mash bill are “cooked” it is basically the same as cooking a huge vat of oatmeal. In raw form, oats are dry and flaky. After cooking, they are sticky and “slimy.” That is because of the starches released. This is exactly the same for the grains of a mash bill.

Now those starches need to be converted to sugar. Why? Because yeasts do not “eat” starch, rather they feed on sugar. This is where barley comes in. barley contains a powerful enzyme “amylase” and when barley is released, amylase is released. Amylase naturally converts starch to sugar.

Next, single-celled fungi known collectively as “yeasts” are added to the cooked mash. Now they can feed on the sugars resulting from the amylase in the barley processing the starch form the corn, rye or wheat.

The byproduct of the process by which yeasts eat sugar are alcohol and carbon dioxide. This can be observed when taking a distillery tour – as you will see lots of gassy bubbles and a strong earthy smell being released from the mash vats/vessels.

Besides creating alcohol, the yeasts also create flavors via releasing various esters. Additional esters will be pulled from the wood by the alcohol during the bourbon aging process.

The yeast most commonly used during bourbon fermentation and eventual distillation is Saccharomyces cerevisiae (S. cerevisiae).

So, if all distillers use this same yeast, why do bourbons from various distilleries (or even the same distillery but different expressions) have distinct flavors? Just because the yeast is the same species, there can exist hundreds or thousands of different strains.

Yeasts are living organisms, and just like any other animal – there can be vast genetic variations between individual species of the same genus. For instance, all wolves are “wolves” however, one may have black fur with blue eyes, and another silver fur with brown eyes.

Esters are what give bourbon it’s flavor notes – some are derived from the yeast, and others are pulled from the wood barrels they are aged in. Here are just a few esters which may exist in yeast strains and the white oak barrels the alcohol is aged in:

  • Vanillin (Vanilla)
  • Cis-whiskey Lactone (Woody, earthy)
  • Trans-whiskey Lactone (Celery)
  • Furfural (Toasty)
  • 5-methyl furfural (Toasty, sweet, caramel)
  • 5-hydroxymethyl furfural (creamy fudge)
  • Cyclotene (maple, caramel, licorice)
  • Ethyl acetate (ethereal, solvent-like)
  • Dimethyl Sulfoxide (Flavorless, umami)
  • Guaiacol (smokey, leathery)
  • Ester pentyl acetate (banannas)
  • Octyl acetate (Oranges)
  • Methyl butyrate (apples)

As you can see, more or less of any of these esters or various combinations of just a few of them can give quite a pronounced flavor profile, nose and finish to the bourbon distillation.

Beer/Distiller's Beer

When grains are fermented, the resulting product is beer – a combination of cooked grains from the mash bill, esters and alcohol. When the sole purpose of the beer is to be distilled into bourbon or other whiskeys, it's more specifically referred to as distiller's beer.

Bourbon Bottling Proof

The most common proof that bourbon is bottled at is 80 proof, which is 40% alcohol by volume. Coincidentally, 80 proof is the minimum proof required to be bourbon — any less and it can't legally be called bourbon. Diluting to 80 proof serves a couple of different purposes. First, it helps reduce costs, as distillers get more volume for what has been distilled. Second, it takes a bit off the bite, making it more palatable to a larger audience. While most bourbons are 80 proof, there are several exceptions, and higher proof bourbons are relatively common.

Bourbon Barrel Proof

"Barrel proof" is another relatively self-explanatory term. Barrel proof means bottling the bourbon at the proof it was barreled. It's distilled at around 160 proof, barreled at about 125 proof, and then typically further diluted with water down to 80 proof. Barrel proof removes that last step and bottles straight from the barrel.

Barrel proofing can vary significantly from distillery to distillery and even barrel to barrel. The warehouse environment and weather conditions will substantially impact the barrel proof. Depending on conditions, more alcohol can evaporate or more water can evaporate. You may also hear this referred to as "cask strength."

Bourbon Age Statement

The age statement of a bourbon refers to the age of the youngest bourbon used in the blend. So if you buy a bourbon with an age statement of 10 years, then all bourbons used have been aged at least 10 years. For bourbons up to four years, the age statement is required. Older than that and it's optional, although most will choose to add the age statement, as the more the bourbon has been aged, the more expensive it is.

Straight Bourbon

To be called straight bourbon, additional legal requirements must be met. Straight bourbon has a minimum age of at least two years, and no added flavors or colors are allowed. A bourbon can still be labeled “straight” even if it is finished in another type of barrel (i.e. P.X. Sherry barrel/cask) if the prior legal requirements have been met in terms of being a legally defined bourbon. However, if actual P.X. Sherry is added, it can no longer be labeled as “straight.”

If a bourbon is not labeled as “straight” there is a chance it could have added colors or flavors like peanut butter, cinnamon or other additives.


Wash/wort is the liquid portion of the mash bill. Generally, bourbons are fermented and distilled using both the liquid and solid portions of the mash. However, some bourbons filter out the solids and simply distill the wash/wort.

Angel's Share

When bourbon, or any whiskey, is being aged, a small portion is lost due to evaporation and exacerbated by the porous nature of wood barrels. Since this has evaporated into the air and, thereby, the heavens, the portion that evaporates is referred to as the Angel's Share.

It is said that as much as 3-7% of total volume can be lost annually to the angels. The average, however, is closer to 4%. In total bourbon production across all brands and distilleries in the USA< over 20 million bottles’ worth of bourbon is lost each year as “Angel’s Share.”

Devil's Cut

Since the angels get their share, the devil likewise demands his. The expression “the devil’s cut” refers to the loss of bourbon distillate during the ageing process after being absorbed by the oak barrel. Since it is not evaporation (as in up and into the heavens for the angels), the devil’s cut is bourbon forever locked away in the barrel This is one of the main reasons why distillers use white American oak grown in colder temperatures like the forests of northern Minnesota, as it has a tighter grain and density – making it less porous and less likely to give the devil his “full” share.

The more porous the wood, the greater amount locked away as the devil’s cut. For a standard 53 gallon barrel, the loss due to devil’s cut can be as much as 2-4 gallons. Some brands market a bourbon expression called “devil’s cut” which is essentially a process of adding water to a used barrel and swishing it around for a few days to extract the tannin-heavy distillate form the wood and then diluting it with the usual bourbon distillate. While mostly a marketing ploy, some like the taste of these trapped congeners and tannins.

Finished Bourbon

Once a batch of bourbon has been distilled, the bourbon is stored in another finishing barrel to be aged again instead of bottling it at that point. The barrel(s) used for finishing aren't new barrels but rather barrels that have been used to age other spirits like rum, port, sherry (P.X. Sherry casks), or wine.

Unfiltered Bourbon

Bourbon is filtered before bottling by either chilling (chill filtering bourbon) or using activated charcoal (charcoal filtered bourbon). Filtering removes any fatty acids that would become cloudy upon chilling, such as served over ice.

Pot Still

Very similar to a pot you would use to boil water, a pot still is a pot that you add the mash to and heat to evaporate the alcohol. A condensing coil attached to the pot still is used to cool the vapor into a new spirit. The pot still is cleaned, and a new mash is added between batches.

Patent/Column/Continuous/Coffey Still

These terms reference a still comprising a large column, typically many stories tall, into which wort or mash is poured at the top while steam gets pumped into the bottom. Pipes along the side of the still are used to collect the alcohol vapor. The benefit of this type of still is it is continuous. The maker can keep going as long as they keep pumping in steam and adding mash.

The invention of the column still had many advantages over using a pot still. Most notably, the volume of bourbon production became exponentially greater. A column still also increase consistency between bourbon batches. Both pot stills and column still either are completely made of copper and/or contain many internal copper fixtures. Copper helps to remove sulfur which can give the bourbon a sour flavor. Copper also helps improve or amplify existing esters (flavor molecules) that give bourbon it’s vanilla, cinnamon and coffee flavors. Column stills provide a cleaner and more sanitary distillation process as well as result in a much higher ABV percentage.

In some cases, a pot still is preferred either because of tradition (Scotch whiskey), or as a means to hone in on specific flavors desired from certain distillates like rum, cognac and mezcal.

Hybrid Still

A hybrid still can be run as either a column or pot still. Using a hybrid still provides the flexibility to make various spirits and helps control the distillation specifics during the distillation process.

A hybrid still might be used in distilleries where there are multiple spirits distilled. A distiller might use the column option in one run to produce bourbon distillate, and on another run might utilize the pot still functionality to produce rum.


A doubler is used to refine the bourbon after it has been processed in a column still. Once the vapor is condensed off the column still, doublers distill it directly using thumpers (so-called due to the thumping noise made).

Low and High Wine

Low wine refers to the liquid produced from the first distillation of the mash, whereas high wine is the liquid from the second distillation. The term “high wine” is typically used in triple distillation processes.

Beer Still

A beer still is a still is another name for the column part of a column still. It may also be used to describe the column still functionality of a hybrid column/pot still.

Spirit Still

This is the counterpart to the beer still, as it's used only for the final run in which the wine is further distilled into the new spirit.

A spirit still, also called the “low wines still,” is the second and usually the final stage in the bourbon distilling process. Some bourbon distilleries distill more or less than two times, but double distillation is far more common than single distillation or triple distillation.

A spirit still captures the contents of the low wines and feints receiver. This is a type of tank which contains the low wines which result from the “wash still,” combined or made up of - the unwanted products from the last distillation run of the spirit still. The final result is a mixture of around 25% alcohol by volume ABV.

Single, Double, or Triple Distillation

As the names suggest, this refers to the number of times a liquid is distilled before barreling. The more times a spirit (like bourbon distillate) is distilled, the more of the flavor molecules or esters are removed. The spirit is “cleaner” from a congener and natural sediment perspective; however, the flavor diminishes as more distillations are performed.   

The majority of all bourbons are twice distilled. They are distilled once in a column still (also known as a beer still) which takes the total alcohol by volume (ABV) up to approximately 50-60%. Next, instead of steam, the distillate is direct heated in copper pot stills known as 'doublers' (or 'thumpers' since it creates a thumping sound due to steam expansion). This second distillation removes more fusil oils and impurities and also further increases the strength to around 63-80% ABV.

Heads, Hearts, Tails – Bourbon Distillation

Heads, hearts and tails are monikers used to describe the three phases of the bourbon distillation process. When a bourbon mash is heated – either in a pot still or a column still – alcohol vapor begins to evaporate.

The first phase of this vaporizing process is known as the “heads” portion. With the heads are all of the unwanted (and potentially lethal) vapors like methanol (not to be confused with the good ethanol), acetaldehyde, and lighter (more pungent and solvent/astringent-like) esters (flavor compounds).

The heads are not something a bourbon distiller would want to include in their final distillate. Early moonshiners – either due to ignorance or desire to increase yield – would sometimes include the distillate form the heads portion, and when their consumers drank it, they ended up going blind, getting very ill or even died.

Now, a little portion of the heads is fine and, in most cases, adds a bit of complexity to the distillate. It is important that a knowledgeable distiller is involved in this process to discern when enough is enough. The heads is also commonly referred to as “the first cut.”

After the first cut, the heads are typically thrown away, or used in a clean-burning fossil fuel converter to power other operations at a distillery. Sometimes the heads are re-distilled again until they are more suitable for consumption.

Next, the hearts are collected. The hearts are the “keepers” meaning it is what consumers end up drinking after the bourbon is aged. Some distillers refer to this phase as the “heart cut.” The hearts contain the majority of the ethanol (not to be confused with the toxic methanol) and also the desirable flavors, aromas, congeners and esters.

As the hearts collection continues on, eventually there comes a point where the hearts will start to smell and taste bitter or overall “distasteful.” In most modern distilleries – as this process is closely monitored, the distiller will then divert the rest of the vapor coming from the mash (via a diverter valve) to a third container. The tail end of the distillation is appropriately known as the “tails.”

The tails portion contains less alcohol by volume (parts per liter) and may be re-distilled much the same way as the heads to extract the cleaner and more desirable “second heads.”

A good way to understand this easily is imagine a water hose and three buckets. When you first turn the hose on, you are streaming the first part of the water into the first bucket – it is dirty. Once the water is running clean, you move the hose direction to the second bucket – you’ll be keeping this, it’s the hearts. When you shut off the water, you let the tail end of the water drain into the third bucket.

So, how does double or triple distillation factor in? This means the bourbon distiller will take the hearts portion and start the whole process over again. The same with a triple distillation. With each successive distillation, the distillate becomes purer and purer ethanol, but also will have little or no flavor as the esters and congeners are being removed with each distillation.  

Barrel Bung

The stopper used to seal the hole in the bourbon barrel, usually made of wood or sometimes plastic, is called a bung. A typical bung is two inches in diameter and fits the standard two-inch hole (bung hole) in a standard 53 gallon bourbon barrel.


When you add wood to the barrel to add flavors to the aging liquid, it's called chipping and can be any form of wood, such as chips, staves, sawdust, or planks. However, any chipping wood must be new to maintain the alcohol's right to be called bourbon. Additionally, chipping is not legally defined as the same as aging.

Dram [Of Bourbon]

A dram is a unit of measurement that equals exactly 1/8 of a fluid ounce. However, the term "dram" is more commonly used to refer to a small drink of any type of spirit. It is most commonly used in reference to bourbon, whiskey and scotch.


Ethanol is the type of alcohol produced by the fermentation of sugars resulting from enzymatic processes involving naturally occurring starches in cereal grains and via petrochemical processes such as ethylene hydration.

Ethanol is the base alcohol that can be consumed “as is” or placed into barrels and casks for aging as the case with bourbon.

Ethanol (also called ethyl alcohol, grain alcohol) is an organic chemical compound. Of the many types of alcohols, It is a simpler alcohol with a chemical formula C2H6O. Its formula can be also written as CH3−CH2−OH or C2H5OH (basically an ethyl group linked to a hydroxyl group.

Ethanol is volatile, and highly flammable. Unless polluted with various sedimentation, ethanol is a colorless liquid.

Bourbon Expression

Expressions are just variations of a particular bourbon or whiskey recipe or may originate from completely different mash bills. Distillers and consumers alike use the term “expression” to differentiate between featured bourbons that are distinct and unique.

A new expression can be the result of changing the mash bill, altering the distillation process, different aging techniques, the barrel char level, or any other alteration that does not change the core spirit too much from the original “recipe” and process. Some distilleries may only change the label or release a special edition of a known expression and label it as such.

There is no legal definition or guideline which dictates whether or not a distillery can claim diversity between expressions. In reality, a new expression of a particular bourbon brand should have a clear and distinct difference in terms of mash bill, barrel finishing technique, or age.

Finger [Of Bourbon]

A finger is a rough measurement that indicates the amount of liquor that would fill up a rocks glass to the width of a finger. Similarly, someone may order “two fingers” or three fingers” of bourbon. Not all fingers are the same in terms of exact volume. The diameter of the glass will play a role; for instance, there is a big difference between a finger of bourbon in a Glencairn glass and a double rocks glass.

Bourbon Foreshots/Feints

The first vapors released from heated mash during the bourbon distillation process are called foreshots and usually contain dangerous and volatile alcohols like methanol. Distillers refer to these foreshots as “the heads.” Meanwhile, feints are the last of the ethanol vapors left over after a distillation run is complete and will usually be reused in a later batch. Distillers refer to these as “tails.” The “hearts” are the “money” bourbon vapors and will go on to be barreled and aged.

Bourbon Nose/Aroma

Smell and taste are very much connected, which is part of the reason the nose of bourbon is so essential. "Nose" simply refers to the aromas given off by the bourbon. Nosing is the act of smelling the whiskey and identifying the individual aromas present.

The aromas are due to esters; tiny molecules created during the fermentation process as well as those extracted from the wood from the barrel during the aging process.

Bourbon Palate

As it pertains to bourbon, or any whiskey or spirit, the palate is the individual flavors in the bourbon and may also refer to the taster’s breadth and scope of capability for detecting certain flavor notes in the bourbon.

Common flavors whish even novice bourbon drinkers may notice include vanilla, caramel, oak, tobacco, citrus, and baking spices, among others. Someone with a distinguished palate has an either natural or trained aptitude for picking out (and describing) flavors in an expression with great skill and ability.  

It is said that anyone can improve their palate by observing a few fundamental practices. When preparing for a tasting, don’t smoke cigars, cigarettes, or cannabis. Try not to eat spicy foods, or drink coffee. These can all distort taste, or even inhibit the ability to pick up on subtle flavors in a bourbon.

Additionally, try being a more adventurous eater. It would be difficult to describe a flavor you’ve never experienced. If you’ve never tasted leather, try chewing on the end of an old belt. Sounds crazy maybe, but it will give you a straightforward idea as to what leather actually tastes like. Maybe try actual real vanilla bean, salted caramel, 100% cacao chocolate, etc. The more experiences you give your taste buds, the more diverse their ability to taste unique and subtle flavors in a bourbon.

Bourbon Finish

The finish is like the palate because it involves flavors and textures, but "finish" refers to the flavors that come out after you swallow the whiskey. This is not to be confused with “finished bourbon”  - a term used to describe a bourbon that has met legal requirements, but is then aged in specialty barrels, or with oaking/chipping to alter and enhance flavors of a particular expression.

Bourbon Notes

Notes refer to the nose, palate, and finish of bourbon. Most commonly, when someone uses the term “notes” they are referring to comparable foods, spices and scents to describe similar qualities in a particular bourbon. Notes “on-the-nose” may include vanilla, cinnamon, caramel, etc. On the palet (taste), a bourbon may possess the essence or hint of many different foods, spices, edibles and even non-edibles (i.e. leather, wood, grass, cardboard, etc.)

During the aging process, several ester-based compounds are created that gives bourbon it’s aroma and flavor notes (collectively called “notes”). The alcohol pulls these esters from the wood of the American white oak barrel as well as being a result of the proprietary yeast strain used in the fermentation process.  

These include:

  • Vanillin (Vanilla)
  • Cis-whiskey Lactone (Woody, earthy)
  • Trans-whiskey Lactone (Celery)
  • Furfural (Toasty)
  • 5-methyl furfural (Toasty, sweet, caramel)
  • 5-hydroxymethyl furfural (creamy fudge)
  • Cyclotene (maple, caramel, licorice)
  • Ethyl acetate (ethereal, solvent-like)
  • Dimethyl Sulfoxide (Flavorless, umami)
  • Guaiacol (smokey, leathery)
  • Ester pentyl acetate (banannas)
  • Octyl acetate (Oranges)
  • Methyl butyrate (apples)
  • Ethyl cinnamate (cinnamon)
  • And many others…

Bourbon Oxidation

A critical component of the whiskey tasting process, "oxidation" refers to what happens when bourbon is exposed to oxygen in the air. For the nose and palate to fully develop, exposure to oxygen is required. However, too much oxygen exposure can have a harmful effect on the quality, and oxidation begins upon opening the bottle and continues even if you reseal the bottle, so enjoy a bottle of whiskey within a short time of opening.

The typical shelf life of a bottle of bourbon after opening is about 1-2 years. This, among other reasons, is why some collectors and bourbon enthusiasts may have many unopened bottles in their collection.


A tun is a huge vessel, usually made from stainless steel or occasionally copper, that will contain all the ingredients for a whiskey recipe, used primarily for the mashing process. Copper is used as a means to increase ester production. Esters from the yeast give bourbon it’s flavors like vanilla, coffee, cinnamon, etc. Tun volume can be anywhere from about six feet in diameter to over 20 feet in diameter and 3-4 feet deep.

Unicorn Bourbon / Barrel

This term isn't official but has gained popularity among whiskey enthusiasts to refer to those bourbons that are extremely hard to find. Finding one of these is like finding a unicorn in the forest. Not to be confused with a honey barrel, which usually comes from a very well-known, predictable and easily accessible place in a rackhouse. A unicorn barrel is a once-in-a-lifetime, never before experienced anomaly.  

Glencairn Bourbon Glass

A Glencairn Whisky/Bourbon Glass is a type of whisky glass which is designed and engineered to amplify the taste and subtle complexities of whisky and bourbon. They are made with a tapered mouth opening, which helps to funnel and direct the aroma of the spirit straight to the nose.

The Glencairn whisky glass is a specific style of tasting glass. It was designed by Glencairn Crystal Ltd, in Scotland for drinking scotch whisky. The first and original design for the glass was accomplished by Raymond Davidson, who was at the time the  managing director of Glencairn Crystal. The purposeful intent of the glass design was inspired by the more traditional “nosing copitas” which was widely used in whisky labs throughout Scotland

The vast majority of Glencairn glasses in production are made of lead-free crystal. The capacity of a typical Glencairn whisky glass is approximately 175 ml (6 US fl oz), and it is intended to hold approximately 50 ml (1.7 US fl oz) of liquid for tasting.

Bourbon Neat

A way of drinking bourbon or whiskey; without any mixer, ice or other additive. Drinking your bourbon “neat” means straight from bottle to glass (to mouth). Taking a bourbon neat is widely accepted by bourbon purists as being the most accurate and formidable way to experience the full nose and palate of the bourbon expression.

Drinking a bourbon neat is also closely associated with tradition and history and prestige of bourbon culture. However, in more recent times – drinking a bourbon cocktail, adding ice and/or water is just as acceptable as drinking a bourbon neat.

Chill Filtering Bourbon

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 mouthfeel and flavor provided by the fatty acids, so some premium bottlings proudly declare a lack of chill-filtration on their label.

Some say that shelf stability, visual appeal, and consistency are the major benefits of chill-filtered bourbon. However, many who appreciate fine bourbon and all of it’s natural complexities would disagree.

Non-Chill-Filtered Bourbon

It simply means that the whiskey hasn't been subjected to the flocculation process and is not chill-filtered. The extra sediments and esters give a rich, creamy texture and peculiar earthy flavor to the bourbon. In non-chill-filtered bourbon, these natural congeners are not sieved away.

Double Malt Bourbon

Some bourbon distillers feature malted grains in their mash bill. In addition to the 51% corn requirement for the legal usage of the word “bourbon” other grains such as rye, barley and wheat can be used. Distillers may opt to use malted barley, malted wheat or even “honey malt” or “chocolate malt” grains in their mash bills. While there is no actual honey or chocolate, the custom malting process produces subtle nuances of these flavors – which carries over to the aged bourbon. In this case, a “double malt” bourbon would contain at least two additional grains besides corn, and these grains would be “malted.”

Triple Malt Bourbon

Similar to a double malt bourbon, a triple malt bourbon contains three malted cereal grains in addition to the legally required 51% corn grain. The triple malt grains might be standard malts or may be specialty malts such as chocolate malts or honey malts. Additionally, in the case of a triple malt bourbon, essentially, it would be appropriately named “Four grain triple malt bourbon” – whereas there are four grains including the core corn grain, and three additional grains – all malted. Typically, however, this would be labeled simply as a “triple malt bourbon.”

Chocolate Malted Barley & Chocolate Malted Wheat

Chocolate malted barley is typically a special type of barley labeled “two-rowed barley.” Two-rowed barley was named for the fact it develops two rows of kernels on it’s seed head. The barley kernels are plump, straight, symmetrical, and less tapered than the usual six-rowed kernels. Also, two-rowed barley has less protein, more starch, lower enzyme content, and a slightly textured (and thinner) husk than six-rowed barley.

The chocolate malting is not a flavor addition, rather a process. At some point in the history of making beer and dark stouts, it was discovered that after the usual process of malting – aka “germination” – the grains could then be roasted similar to the way coffee beans and/or cacao nibs were roasted to make coffee and chocolate.

In all actuality, the malted barley and wheat have a similar chemical profile to that of coffee and chocolate, possessing similar flavor precursors like peptides, amino acids, and sugars. In fact, many chocolate cereal malting producers also repurpose their equipment to roast coffee and raw cacao nibs for chocolate making.

Honey malted grains

Honey malted barley and wheat are produced using a special method designed to allow the grain to develop strong, distinctive “honey-like” flavors. The malting process creates a sweeter malt which has a flavor and taste similar to that of honey, bread crust and toasted bread.

Two-Rows and Six-Row Malt

In bourbon distillation and more specifically, fermentation, the most widely used form of grains is from 2-row. 2-row and/or 6-row is an indicator of how many heads the grain grows. A 2-row grain has distinct advantages over 6-row in terms of consistency and overall volume. A 2-row grain is typically used for extracts and syrups.

Barrel toasting / toasted barrels

Toasted bourbon barrels are exposed to less extreme temperatures than barrels charred in a fast burn. Toasted barrels are most commonly used for wine. Bourbon aged in a toasted barrel is a special treat that's more difficult to find. The toasting process exposes the interior of the barrel to a smaller flame for a longer period of time. This allows the heat to penetrate deeper into the wood. It also allows the distillate to pull more sugars, congeners, and notes naturally occurring deep within the wood.

Honey Barrel (Bourbon)

Long-standing bourbon mythology – a honey barrel is a particularly exquisite barrel which stands out in both taste and complexity. Although honey barrels typically share the same entry liquid as thousands of other barrels in the rackhouse, a honey barrel will have taken on unique qualities which are not present in other barrels. Some suggest a honey barrel was simply at the “right place and right time” in the rackhouse. Further, many lifelong master distillers say that within every rackhouse, there is a special place – about the size of one whiskey barrel – that maintains it’s own microclimate. It is at this “sweet spot” within the vastness of a rickhouse, that allows one barrel to age in a very magical way. Once discovered, these honey barrels rarely leave the distillery/distiller’s private vault.

Kentucky Limestone water

While bourbon can legally be produced in any state within the USA, Kentucky is most notably associated with bourbon. Long ago, distillers realized something unique about the water in Kentucky lakes, streams, and underground wells. It is known collectively as “limestone water.” Basically, this is naturally occurring water that has been filtered through layer upon layer of limestone bedrock which is abundant in Kentucky.

Limestone water has a high pH level, promoting faster and more thorough mash grain fermentation. Limestone also naturally adds trace minerals, like calcium. Limestone rock filters out many of the impurities found in natural water supplies, most importantly iron, which can give distilled spirits a foul taste and may even affect the color of aged spirits.

Many master distillers will say that the three main components of making consistently good bourbon are the water used, the grains used and the yeast used. While many distilleries can borrow similar sources for the grains and yeast used by Kentucky distillers, it is far more difficult to mimic natural Kentucky Limestone water.

Amylase/Alpha Amylase

Alpha-amylase (α-amylase) is an enzyme EC that hydrolyses the alpha bonds of large, alpha-linked polysaccharides, such as starch and glycogen, yielding shorter chains like, dextrin and maltose.

The technical process of turning starches into fermentable sugars is made possible with enzymes; more specifically: alpha amylase. Before alpha-amylase can be effective, the long chains of starch must first be exposed. Therefore the “mash” is cooked – basically to begin the breakdown process. When grains are mixed and cooked in hot water, starch absorbs the liquid and expands.

Ultimately, the starch expands so much, it explodes or “pops” – thereby spilling out all hidden sugars. It is at this point the mash becomes very thick and highly viscous – almost like slimy cooked oatmeal. Gelatinization is the term used because the mixture resembles a gelatin-like consistency. Reaching this state is key to achieving baseline ethanol (alcohol) yields.

Alpha amylase is then added to the mixture. Alpha amylase enzyme easily breaks down almost any grain starch it encounters. As Alpha amylase continues to break down starches, the mash mixture begins to be less gelatinous and more liquified. On a technical level, the long-chain starches are now short-chain starches.

Gelatinization and liquefaction is a process that happens rather quickly; spanning seconds, minutes or a couple of hours depending on the mash bill, temperature, and other distiller-specific qualities.

American White Oak

The American white oak (Latin: Quercus alba), is one of the most common hardwood trees in northern and central North America. It is common for these trees to live a couple hundred years as they are resistant to disease and rot. Some have been found to be well over 400 years old. They are, by majority vote, ideal for making bourbon barrels.

Although they are called “white oak,” it is uncommon to find one with white bark; the most common bark color is light gray or dark gray. The “white oak” name derives from the color of the wood once processed and eventually finished. New bourbon barrels have a whiteish or grayish look to them.

Most premium bourbons are aged in American white oak wood barrels. In fact, to legally be labeled as “bourbon” rather than simply “whiskey” the barrels must be made of new oak; meaning it has never been used to age bourbon before.

Unique to North America, American white oak is one of the most popular species of hardwood trees, growing abundantly from far regions to the north and south. It is a popular wood export used for endless purposes around the world – especially bourbon barrels and wine casks.

American white oak grown in forest regions in climates geographically more common with longer, harsher winters like Minnesota, around the great Lakes regions or even Canada are ideal because the climate encourages a tighter and less porous grain configuration than areas with warmer climates. Although any oak can be theoretically (and legally) used in bourbon barrel making, American White oak is known to release more natural flavors and sugars during the aging process.

Bourbon barrel volume and dimensions

A typical bourbon barrel or “cask” is a large wooden vessel usually constructed from American white oak. Since they are handmade, there are slight variations in finished size/dimensions. However, the most common measurements are:

  • Barrel Volume: 200-250 liters (53-66 gallons) the average is 53 gallons.
  • Barrel Height: 86cm (34-36 inches)
  • Barrel heads (top/bottom): 53cm (21 inches) diameter.
  • Barrel Weight: (empty) about 45kg (100 lbs)
  • Barrel Weight full: 225 kg (500 lbs or more)
  • Number of staves: 31-33
  • Barrel Diameter: (non-bilge) 21 inches
  • Barrel Bilge circumference (widest part): 26 inches
  • Steel hoops: six
  • Barrel Fasteners: steel rivets (usually branded by cooper/maker)
  • Barrel Stave width: 3-5 inches each
  • Barrel Stave height: 35 inches length
  • Barrel Bung Hole diameter: 2 inches

Parts of a bourbon barrel:

  • Barrel Stave: A narrow slat of wood (usually American white oak) with a beveled edge which fit together to create a leak-resistant vessel for aging bourbon.
  • Barrel Bilge: The center bulge, or widest diameter of a bourbon barrel.
  • Barrel Bilge hoop: Iron band located in the center of the barrel; holding staves together at the widest part.
  • Barrel Head hoop: Metal/iron band locates closest to top and bottom of bourbon barrel.
  • Barrel Quarter hoop: Metal/iron band located between the head hoop and bilge hoop.
  • Rivet: Steel fasteners that hold the hoops together with force by attachment from hoop beginning to end.
  • Barrel Cant: Machined/tooled slope carved into the barrel head.
  • Barrel Head: barrel lid which fits snugly into the top and bottom of a bourbon barrel; thereby sealing to for aging.
  • Barrel Chime: The beveled edge for connecting bourbon barrel staves, thereby holding the barrel heads in place.
  • Barrel Croze: A small groove carved at both ends of a stave of a cask into which the head fits is the 
  • Barrel Stave joint: A tongue-and-groove type joint which interconnects barrel staves along their longest sides. Forming a water-tight connection, these joints remain flexible yet self-seal.
  • Barrel Bunghole: Two-inch hole by which a bourbon barrel is filled and emptied. It is sealed with a bung which is a wooden puck that fits tightly into the bunghole.

Grain cooker

A large vat or vessel used to “cook” the grains of a bourbon mash bill. As the grains are cooked, their fibers and structural walls are softened and broken, allowing the release of hidden sugars and enzymes. As an example, when making bourbon, corn is added first, then the rye and then barley (or malted barley) is added last. Barley, or better yet, malted barley is critical to the fermentation process.  Malting barley is essentially allowing it to germinate, or “sprout.”  

This sprouting process allows for the natural release of enzymes designed to feed that plant. When the malted barley is added to the grain cooker, those enzymes immediately turn all starches in the mash bill to sugar.  After the grains are cooked in the grain cooker, and the enzymes turn the starches into all available sugars, the yeast added will feed on these sugars. The resulting byproduct is undistilled ethanol. 

Bourbon Yeast

After the grains of a mash bill have been cooked, and they have released their enzymes which in turn allow for sugars to be released in a bioavailable form, yeast is next added to start the bourbon fermentation process. Without yeast, there is no ethanol alcohol. Without ethanol, there is no bourbon. Without bourbon, you basically have a giant vat of cooked porridge (like oatmeal) in the grain cooker.

Yeast is described and categorized by “strain.” Yeasts are eukaryotic microorganisms. In nature, they can be found in a wide variety of ecological environments; primarily in water, soil, air and on plant and fruit surfaces. There are many different strains of yeast; each with different methods of breaking down sugars and turning them into alcohol.

The science? This is the chemical equation by which yeast breaks down glucose derived from starches (cereal grains) and converts it into undistilled ethanol:

C6H12O6(glucose)⟶2C2H5OH(ethanol)+CO2(carbon dioxide)

What’s going on here? Well, in oxygen-limited conditions, undistilled (raw) ethanol is a byproduct of acetaldehyde; additionally, two moles of ATP (adenosine triphosphate) are generated. Since this is not a comprehensive (total) reaction for yeast cells, they must consume high amounts of glucose (grain sugars derived from cereal starch) to deliver enough ATP to the specific ecosystem. Consequentially, undistilled ethanol is accumulates and in a natural “positive-feedback” looped system, fermentative activity is naturally stopped.

One reason bourbons can vary in aroma (the nose) and flavor notes (the palette) is that even though all are fermented by the same species of yeast, Saccharomyces cerevisiae, each distillery uses its own proprietary yeast strain or blend of strains from the same genus. The slight genetic variations within S. cerevisiae mean that different strains can produce different aromatic compounds called “esters.” In total, there are over 9000 different strains of S. cerevisiae in existence.

Some bourbon distilleries are using strains that have been passed down for hundreds of years. Others are experimenting with newer modified strains which are genetically altered for peak bourbon aroma and flavor. 

Congeners (in Bourbon)

The word “congener” is Latin in origin, and literally translates to “born with” (Latin “con” means “with” and Latin “genera” means “born”). So, essentially the word congener collectively describes all other compounds that are born in addition to ethanol (drinking alcohol) during the fermentation process.

When yeast breaks down sugars from cooked grains, ethanol alcohol is but one of many byproducts of the chemical process. The congener group of compounds includes:

  • Esters: Esters are desirable flavor compounds produced by the yeast and any contaminating bacteria present. Different esters can create different aromas and different flavors; ethyl octanoate (creates apple notes), ethyl dodecanoate (leans towards floral), and isoamyl acetate (mimics banana)
  • Acids: These can create strong, aged cheese aromas
  • Sulphur compounds: May create undesirable aromas, but adds more savory or “umami” flavors to the bourbon
  • Aldehydes: Adds sharp and intense aromas and flavors to the bourbon
  • Diacetyl: A very intense aroma; almost identical to buttery popcorn
  • High alcohols and fusel oils,: May be pleasant and fruity, or strong and solvent-like

In essence, congeners are what give bourbon its flavor. These are good and (in some cases) not-so-good. It all depends on the specific levels of specific congeners, and how they will be perceived by the majority. All bourbon drinkers experience different flavors in different ways; what might taste like banana to one might taste like walnuts to another. It is important to find balance when constructing a mash bill, yeast selection, fermentation process and distillation.

pH of Bourbon

During the fermentation process, the pH of the cooked grains and yeast should be between 5.4 and 5.8 – and ideal pH level for yeast reproduction. Depending on the yeast strain(s) used, these pH levels may be slightly lower or higher.

During aging, ethanol alcohol in the distilled spirits are extracting carboxylic acids which are also produced by the yeast during the fermentation cycle. The longer a bourbon ages in the barrel, the greater the acid or pH level. Of course, there is a point where this process slows and essentially stops.

Before being fully aged, bourbon typically has pH levels ranging from 4.4 to 5.1. After about 48 months/2 years, the pH levels ranged from 4.0 to 4.45 (lower pH number means higher acidity). The older the bourbon, the more acidic the bourbon is. At this level, the bourbon would have a marginally higher pH acidity level than black coffee but is still lower than citric acid/lemon juices.          

Carbon Dioxide (C02) (Bourbon Fermentation)

During fermentation of the cooked grains in a bourbon mash bill, yeast converts the sugars into (among other things) ethanol alcohol, carbon dioxide (C02), and heat. The bubbles and froth seen in a fermentation tank are caused by the release of C02.


When fermentation is complete, the bourbon mash/wort is a mixture of low ABV (alcohol by volume) ethanol, congeners, and cooked grains. The ethanol must be extracted and concentrated before entering the bourbon barrel in a process called distillation. This process involves heating the post-fermentation mash so the alcohol will evaporate. When it does, the ethanol is in vapor form, and very hot. It is directed through coiled metal tubing at the top of a column still. Surrounded by cold water, these coils convert the ethanol alcohol vapors back into liquid ethanol alcohol through the process of condensation. This is similar to having an ice-cold beverage sitting outside on a hot day. The water droplets which form on the outside of the glass are actually vaporized water which were then pulled from the air.

Spirit Safe

A spirit safe is an antiquated piece of distillery equipment with roots dating back to the early 1800’s. It resembles a large fish aquarium, and besides looking intriguing – performs an important function. At one time, only tax collectors had a key to this safe. The safe was initially required in Scotland to prevent unlawful selling of distillate before it could be barreled and taxed. 

The technical purpose of the spirit safe was to ensure proper cutting of the heads and tails sections of the distillation process. If a distiller needed to adjust, it was easy to do so within the spirit safe. This fine-tuning ensured a consistent and high-quality final distillate before being barreled. Many modern distilleries still use a spirit safe, but instead of it performing a necessary function, it is merely for an artistic visual; one which incorporates tradition and beauty.

Animal feed

Once a bourbon distiller had cooked the grains of the bourbon mash bill, and the fermentation process of the raw bourbon is complete, distillers can sell or donate their leftover mash to farmers as a source of high-quality animal/livestock feed. This livestock food supplement is often referred to as “distiller’s feed” and is commonly used in combination with other enriching feed types to provide livestock with a well-rounded dietary regime.

In summary, there are pro’s and con’s for using distiller’s grains as animal feed. Since the starch is removed via the fermentation process, there is a high concentration of protein and other trace minerals by volume (as much as three time as much protein and natural fats by weight compared to the same amount of un-fermented corn).

Special precautions should also be made regarding the higher levels of naturally occurring phosphorus levels, and Sulphur – which can lead to decreased trace mineral absorption issues. Another naturally existing, yet high level of molybdenum can bind to copper (a necessary mineral for livestock) and prevent absorption.

All in all, using distiller’s feed as animal feed is a great supplementation for livestock feed, as well as a great way to create a sustainable recycling of what would otherwise be waste.

Barrel Staves

A slightly curved, milled, and machined slat of wood with dimensions of 1” in thickness, 2-5” in width, having a curve and bevel called a “stave joint” which allows for interlocking with other staves to form a bourbon barrel circumference. The composition of a bourbon barrel stave is typically (and almost always) American white oak which has been kiln-dried and allowed to cure for a period of time determined by the barrel maker (cooper).

Charcoal Filtering Bourbon

Charcoal is used to filter bourbon whiskey straight from the barrel as a means to filter out impurities, lightly adjust the flavor, adjust the pH, reduce the grain-heavy esters, etc. Not to be confused with the making of Tennessee whiskey, the “Lincoln County Process” is a very specific technique and has a precise methodology that is similar to the nuances as bourbon making.

General charcoal filtering is a standard process for ensuring any potential foreign contaminants are removed from raw bourbon before bottling. General single-pass charcoal filtering has little or no effect on the overall flavor and finish of straight bourbon whiskey, whereas the Lincoln County process intentionally affects the flavor of the finished Tennessee whiskey.

Many distilleries utilize decommissioned bourbon barrels as a source of charcoal – after burning the bourbon barrel wood in a controlled temperature fire – it retains some of the characteristics lent by the aging bourbon and then as a filter adds unique character and finishing to the bourbon without violating the “straight bourbon whiskey” legal definition.

Grain silo

These are large storage containers for grains used in distillery mash bills. Typically, most distillers use a separate grain silo for each grain type, i.e. one for wheat, one for corn, one for barley, etc. Most grain silos have automated actuators and release chambers which precisely measure grain by type and percentage by weight to fill the grain cooker before the fermentation process starts. This automation prevents mistakes and ensures consistency in recipe-to-barrel.

Grain starts from delivery trucks and is sucked/pumped into the grain silos. From there, it is measured and sent to milling machines where it is broken down before being cooked for fermentation.


Reflux is the process where ethanol alcohol and water are vaporized and condensed. In terms of using a column still, the column is like a large bamboo stalk. It is a hollow design with multiple segments called “plates” through which the vapor passes through. The reason why column height is so important is the more plates and chambers the alcohol vapor passes through, the higher quality to final distillate (raw alcohol/white dog). Reflux is this process and can be similar to when a human has acid reflux whereas contents of the stomach (liquid and solids) reflux or move up through the esophagus. In bourbon making, this is a desired action; as it separates the ethanol, water and cooked mash.

At the top of the column, the distillate exits and is then condensed – everything else falls to the bottom. In essence, reflux simply means turning the alcohol created via fermentation into a vapor in a process collectively called “distillation.”


Acronym for “Direct To Consumer” which is a legal association where a distillery is permitted to sell spirits direct to the consumer or distillery-to-doorstep. Traditionally, all distilled spirits are a part of the three-tiered-system; a distillery ships bottled product to a distributor partner who then fulfills/supplies retail stores with the product. Currently, there are several DTC-reciprocal states in the USA that have opened laws. These include Alaska, Arizona, Connecticut, Washington D.C., Hawaii, Kentucky, Nebraska, Nevada, New Hampshire, North Dakota, Rhode Island (except  Bourbon).

More and more states are changing laws – especially after supply system issues caused by the COVID pandemic.


A tradesman or tradeswoman who is trained in the art and craft of making wooden bourbon casks and barrels using white American oak staved which are pressure, heat and steam treated. The cooper trade is also the source of the surname “Cooper.” Any final product a cooper creates through his/her craft is collectively known as “cooperage” which is also the name of a place where barrels and casks are manufactured by coopers.

A bourbon/whiskey barrel is a type of cask. A cask is the general definition for any wooden container that has a bouge, bilge, or simply “bulge” in the middle of it’s circumference. The art of barrel making by an experienced barrel maker is only one facet of the art and craft of a cooper or, more holistically, a “cask maker.”

A spinoff craftsman/woman is a “hooper” – and descriptively, is one that would fit the finished bourbon barrel or cash with the metal hoops or bands to make the barrel fit for travel.


Place where barrels and casks are manufactured by coopers (see above).



A cask is the general definition for any wooden container that has a bouge, bilge, or simply “bulge” in the middle of it’s circumference. Early evidence of cask making can be found as far back as ancient Egypt; where casks were created to store food, wine and grains. Traditionally, casks were also used as a means of fermenting beer in ancient Roman times, and fermenting food for longer shelf life. The cask, in general, played a vital role in storage throughout human history.

NDP Bourbon

NDP stands for “Non-Distiller Producer.” This describes any company that buys it’s bourbon or whiskey form another distiller and then bottles and markets it as their own.

The practice started long ago in Ireland where some Irish Whiskey houses would buy scotch form several different distillers and then blend them into a “blended scotch whiskey” and thereby creating a unique and highly desired blend. Another purpose was to offset the taste of a poorly aged group of barrels with those that were considered premium to balance out the less-premium ones and increase overall production volume with little waste.

In modern times, and with the advent of the internet and social media as viable marketing channels – celebrities and persons of influence may source raw distillate (vodka, bourbon, gin, rum, whiskey, etc.) from mega distilleries and market it using their own fame and likeness. Many of these celebrities have little or no experience or knowledge of the fermentation or distilling process, and simply rely on their fame to “push product.”

Not all non-distiller producers are rich and famous. Many are common label brands who buy their bourbon or whiskey from huge producers and market it as their own. In some cases, the bourbon or whiskey is distilled by someone else, but the distiller will follow a strict mash bill created by the receiving company. Most of the time, however, a non-distiller producer will simply buy thousands and thousands of gallons of bourbon and then bottle and market it as their own.

For more information, see full information on NDP Bourbon and whiskey.


Just as metal that contains iron can rust when exposed to water and oxygen, bourbon can oxidize as well. A sealed bottle of bourbon has a very long shelf life before age negatively affects taste. However, once opened and exposed to air, the taste can quickly deteriorate. Adding bourbon to a decanter may increase the rate of oxidation if the decanter is not well-sealed or it is opened often. The other effect oxidation has on bourbon is evaporation – which reduces the ABV (Alcohol percentage by volume) and thereby heavily affecting the flavor. In theory, an unopened bottle of bourbon has a near infinite shelf life. However, when exposed to oxygen or if opened, it can go bad or reach an unpalatable state in as little as 12 months. A simple example is to leave a glass of bourbon on the counter uncovered overnight, and then take a sip in the morning. It will become immediately apparent that something (negative) has changed.

Even the slightest of air gaps in the original seal will slowly allow oxygen to enter and ethanol (alcohol) to escape and may have dramatically adverse effects on taste over time.  


Bourbon (when adhering to the legal definition), requires a barrel proof (ABV of barrel entry) 62.5 percent ABV or lower, or more commonly, 120 proof. Bourbon increases overall proof as it ages, so some distilleries selectively choose a lower barrel entry proof. Legally, it cannot be above 125 proof, or it will lose the “bourbon” label and definition. Most bourbon brands bottle at 80 proof, 40 percent ABV or higher., and it is important to note this is a minimum.

The ”proof” of a bourbon is calculated by multiplying it’s overall ABV by a factor of two. For example, if a bourbon has a ABV of 50%, it is a “100 proof” bourbon.

ABV is an common abbreviation for “Alcohol By Volume” and is a measurement of the amount of ethanol alcohol in contained in a given solution. This measurement of alcohol may also be abbreviated as alc/vol and is typically followed by a percent (%) symbol. ABV can be used when describing the ethanol as a pure product from distilled grain, or as a way to declare the total ethanol in a produced beverage which contains grain alcohol, vodka, etc. in a mixed solution.

ABV is expressed or measured in milliliters (mL), and representative of the total amount of pure-form ethanol (drinking alcohol) present in a total measurement of 100mL or 3.5 imperial fluid ounces (3.4 USA fluid ounces) of any solution at a temperature of 20 degrees Celsius, or 68 degrees Fahrenheit. In calculable terms, the total mL of pure-form ethanol alcohol is equivalent to the mass of the total ethanol divided by it’s density at 20 degrees Celsius/68 degrees F (equating to 0.78924 grams per mL, or 0.45621 ounces per cubic inch)

The ABV as a measurement protocol is used worldwide, however, some countries such as France interchangeably use “degrees Gay-Lussac” named after the French chemist Joseph Louis Gay-Lussac. It is important to not there is a deviation in the precise measurement between ABV and Gay-Lussac, as the Gay-Lussac uses the International Standard Atmosphere value for temperature (15 degrees Celsius / 59 degrees Fahrenheit).


This is a term commonly used in America, and even more commonly as a descriptive term for higher ABV spirits. For instance, most consumers do not utilize a measurement of ABV or “proof” as a buying factor for beer. However, the proof and or ABV is a determining factor for bourbon, whiskey, vodka, etc. In the world of bourbon, there is cask strength (i.e. barrel strength), which is the typical 55% ABV (55% multiplied by two = 110 proof), and then there is the standard bottle proof which is a minimum legal 40 proof, or 80% ABV. Proof is simply an easy identifier used in bourbon and spirits marketing to say it is the ABV multiplied by a factor of two.

Historically, the proof measurement goes back to early 16th-century England, Kings and tax collectors would add an extra tax bill on “proof spirits”— which is distilled spirits which have a certain (rather unclear) level of ethanol. It was used as a way to show the measurement was accurate or correct.


Barley (genus Hordeum vulgare), is actually a member of the grasses family of plants that grows natively in the grasslands and woodlands of western Asia. It is a widely cultivated “cereal” grain and was one of the earliest cultivated grains for human consumption – dating as early as 10,000 years ago. About 70% of total barley production is for livestock feed, while about 30% is used as fermentable grains especially in the process of distilling bourbon. These grains, post-fermentation, usually end up being animal feed as well in a nutritionally dense animal feed additive called “distiller’s grains.”

Sometimes, bourbon distillers will allow the barley to first undergo a malting process, an ancient method of preparation where the barley is first germinated/sprouted and then quickly dried under controlled environmental conditions; thereby releasing embedded nutrients, enzymes and sugars. Of these enzymes, perhaps amylase is the most critical as it is what breaks down the starches in rye, corn and wheat into a simpler sugar for yeast to consume during the bourbon fermentation process.


Rye (Secale cereale) is a grass grown as a grain, a cover crop and a forage crop.  Rye shares a genetic profile with wheat and was among the first cereal crops to be systematically cultivated, around 13,000 years ago. It is a common ingredient in mash bills for whiskey, bourbon and “high-rye” variations of each. Sometimes referred to as “rye whiskey” or “high-rye bourbon” this cereal grain adds a noticeable spiciness to the finished distillate.


Wheat is a cereal grain and is grown for it’s seed. Although there are many variations, they all fall under the genus Triticum. Wheat was cultivated as early as 9600 BCE. Wheat is a variant of fruit; called caryopsis. Grown more than any other food crop, wheat trade globally is far more in volume than all other grains combined.


Corn is the primary ingredient in bourbon mash bills (recipes). In fact, by law, bourbon must contain at least 51% corn to be legally defined/labeled “bourbon.” Corn was readily available for whiskey and bourbon making – the fertile soil of what is now Virginia and Kentucky was the commonly agreed upon birthplace for bourbon. Previous to carrying the name “bourbon” the moonshine crafted by early settlers was mostly corn whiskey.

This early variety of corn was identified as “maize” and was actually a species of edible grass. While fermentation of this early corn is relatively new (within the last few hundred years or so), the cultivation of corn as a food staple has existed for thousands of years. Corn has a high sugar content and was a very versatile grain to use for both predictable and high-alcohol content fermentation – but also as a food staple to early settlers.


A blend, or as it is commonly called “A blend of straight bourbon whiskey” simply means that two or more legally defined bourbons (minimum of 51% corn, aged in new oak barrels, distilled no higher than 160 proof, barreled at 125 proof or less) are mixed or blended. Reasons may include blending older aged bourbons with younger ones to round out flavor and stretch volume, or simply to create unique flavor profiles resulting from similarly aged bourbons that have different mash bills. Bourbon blends are becoming more popular as a deviation from single-mash bill and aged bourbons.


Master distiller or other qualified personnel at a bourbon distillery who oversees and directs the blending of bourbons into large production batches for bottling.


A distillate is the end product formed from a distillation process. In the realm of producing bourbon, the distillate is known as “white dog” or “moonshine” and is the clear spirit before it enters a new charred oak barrel where it will take on a golden amber color and extract flavor notes, sugars, congeners form the charred wood.

Master distiller

A master distiller is a honorary title typically reserved for a person who has a significant amount of experience in the craft and distillation of a particular distilled spirit. A master distiller at a bourbon distillery may be the owner, or simply an employee who has a vast holistic knowledge of the bourbon distillation process. In some cases, a master distiller may oversee the mash bill creation, fermentation process, distillation process, aging process, bottling process, etc. Other times, the master distiller is more of a consultant to a large bourbon distillery – and will lend technical and experiential knowledge in order to produce a new bourbon expression, or simply oversee the ongoing production of a legacy bourbon expression.

Wheated bourbon

Traditional bourbon mash bills are pretty straight forward; although there may be slight deviations, primarily they include 51% corn (a legal requirement), rye and barley. If there is more than average rye used, the bourbon may be classified as a “high rye” bourbon. If the rye is replaced with wheat, it is known as a “wheated” bourbon. Many argue that a wheated bourbon is “softer” and sweeter than a standard or high rye bourbon. A simple way to see the difference is to take a bite of rye bread and then take a bite of wheat bread. You will immediately taste and “feel” the difference between the two grains and resulting profiles.

High rye bourbon

A high rye bourbon mash bill is – in essence – an amplified version of a standard or traditional bourbon mash bill. Most bourbon mash bills are three grains – the legally required 51% corn, rye and barley. In a high-rye bourbon, a distiller may opt to follow a mash bill where there is a much higher percentage of rye than what is typically used. For instance, a standard or traditional bourbon mash bill might contain 51% corn, 20% rye and 29% barley – whereas a high rye mash bill might be 51% corn, 40% rye and 9% barley. Of course, this is just an example. The term “high rye” holds no legal definition, and it is at the sole discretion of the distiller or distillery to label their bourbon as a high rye bourbon.

Low entry proof

Bourbon can be distilled at no higher than 160 proof and go into the barrel at no higher than 125 proof. It can be bottled at no lower than 80 proof. With this considered, a low barrel entry proof can technically be anything below 125 proof however, most would only consider a “low barrel entry proof” to be somewhere between an extreme low of 100 proof and on the higher end of the low entry proof at 115.

There are many arguments for low barrel entry proof. In fact, in the 1960’s, a couple of chemists did a study on barrel proof and found that many of the favorable associations with bourbon i.e. caramel, color, vanilla, baking spices, etc. are all more pronounced when the raw distillate (white dog) enters the barrel at a lower proof. Of course, there is a lot more to that study, and in reality – it would be impossible to definitively conclude that something as simple as lowering the barrel entry proof would ensure or guarantee a better bourbon – after all there are literally thousands and thousands of possible mash bills and percentages of various grains – and all react differently to higher distillation proofs and would taste differently upon higher or lower barrel entry proofs. In the end, it is up to the discretion of the distiller, the preferred finish of the bourbon expression and the consumer to determine what is “best.”

Cask strength

Cask strength (also known as barrel proof/barrel strength) is a term used by bourbon and whiskey producers and distillers to describe an expression (version) of either that has not been considerably diluted after legally required aging in new charred oak barrels. The level of alcohol by volume (ABV) strength for a cask strength whiskey is typically in the range of 52% ABV (104 proof) to 66% ABV (132 proof).

Most bottled bourbon is diluted with water to reduce its strength (i.e., ABV level) to a level that makes it both less expensive to produce (more profitability to increase volume) and palatable – usually to about 40% ABV (80 proof), which is the statutory minimum in United States. The degree of dilution significantly affects the flavor and general drinking experience of the bourbon. Further dilution with ice (“on the rocks”) or added to cocktails will even more greatly affect the true essence and flavors of the originally aged, cask strength spirit. Therefore, cask strength bourbon has become more popular as the level of knowledge has increased among consumers as well as the need to differentiate in an already crowded space. Bourbon is more popular than ever before, and consumers have higher expectations for distinguishable characteristics between brands.

Allocated Bourbons

The term “allocated” when referring to bourbon can be better understood by a simple reference. A distiller may choose to “allocate” a specific number of bottles to a specific number of retailers. This is a way some bourbon distillers artificially inflate the rarity and exclusivity of a particular bottle. In reality, the distiller may have tens of thousands of bottles in storage or in barrels – however, may only release a hundred or more to select retail locations to trigger a synthetic “frenzy.” While the term “allocated” is most often used by retailers, distillers and distributors – collectors and connoisseurs will more often use the term “Limited Release” or a shortened version “LE bottle” (Limited Edition).

Whiskey thief
A whiskey thief is a type of hollow tool that is usually made from copper and resembles a cylinder. Bourbon distillers use a whiskey thief to collect a small bourbon sample from a bourbon barrel for general tasting and/or quality control. A bourbon distiller first removes the barrel bung, which is a small circle of wood that tightly seals the barrel during the bourbon aging process. A bourbon distiller inserts a whiskey thief in the open bunghole (hole in the barrel). A distiller places his thumb over a small vent hole in the end of the whiskey thief, much like plugging one end of a drinking straw to form a vacuum, thereby trapping a small amount of liquid in the thief. Next, the bourbon taster/distiller will release his or her thumb from the hole, allowing it to drain into a proper tasting glass.

Bourbon Rickhouse / Rackhouse

A rickhouse (also called a rackhouse) is a warehouse type structure which holds aging bourbon barrels or other aged spirits. The bourbon barrels are sometimes stored many stories tall in shelving that contains “ricks” that allow the barrel to be securely stored for several years. In the past, bourbon distillers would simply store barrels stacked on top of one another, however, the compounding weight would cause the barrels at the bottom to crack or leak.

Ricks are designed in such a way that they do not place any compromising strain on the barrel or staves (wooden slats that come together to create a bourbon barrel). Unlike stacking barrels as they were in the distant past, ricks and shelving allow for better air circulation around the entire exterior surface of the barrel – which promotes even and thorough extraction of sugars and congeners from the barrel during various climates.

Another benefit is capacity; far more barrels can be stored in a single location when using racks lined with ricks and then storing barrels several deep in those shelves.

Esters – Bourbon Distillation


Esters are compounds responsible for producing some of the most common aromas and flavors closely associated with bourbon. They are formed throughout every stage of the bourbon mash fermentation process. For example, acetic acid esters (N-acetyl) form the glue-like smell or fruit-centric aroma of pears which is common among all bourbons. On the other hand, lactic acid esters are responsible for collective “fruity” smells (i.e., soft, mild, creamy) and are common to bourbons which utilize malted (sprouted) grains. Finally, Butyric acid esters also possess an artificially manufactured fruity smell (i.e. tropical fruit candy, bubblegum) to create the essential aged rum or younger bourbon flavors.

Ester formation during fermentation

During bourbon fermentation (and specifically during the yeast growth cycle) the yeast produces what are known as “fatty acid esters.” The yeasts do this in order to build up and strengthen their cell walls. Enzymes are responsible for forming these fatty acid esters.

The enzyme ester formation reaction can reverse chemical reaction directions. This means that the enzymes may also return the formed esters to an alcohol and an acid. Esters will become entrapped within the cell wall of the yeast and cannot be utilized as long as the cell wall is intact.

During a process known as autolysis (programmed cell death / yeast dies) the cell walls are ruptured by the protease enzymes. After the cell walls of the yeast have been ruptured, the enzymes will both create and destroy esters. Esters from yeast can also be released by including some of the yeast in with the wash for distilling. The heat of the still will break open the cell walls release the esters. For ferments the level of ester production increases until day 10-18 depending on wash make up, at that point ester levels equalize. Esters can also form within the still (during distillation), during the barrel aging process, and during a process known as transesterification.

Doping / Precursors: Bourbon Distillation

During the bourbon fermentation process, some distillers utilize a method known as “doping” the mash ferment with precursors such as Muck Pits, Dunder Pits and traditional Sour Mashing. These doping methods are used to boost or enhance/amplify ferments with more Carboxylic acids creating a surplus for expanded ester formation. Some bourbon distillers will target specific flavor/aroma esters through “doping” (artificially enhancing) the distillate with certain specific carboxylic acids. Doping may also be done with using the with tails (see heads/hearts/tails in this reference).  The tails contain fatty acids and fusel alcohols, when doping a bourbon mash ferment with tails it is similar to using muck. This allows for more ester formation during maturation.

Protein Rests: Bourbon Distillation

When working with partially modified malts (un-malted grains), bourbon distillers may do a “protein rest” (Proteolysis) which can help to break down proteins into smaller fatty acids (amino acids). As a result, the fatty acids are more bioavailable for esterification.

Copper Stills: Bourbon and Esters

Copper is often utilized in the bourbon mash fermentation process due to it’s inherent properties which can act as a catalyst for amplifying ester formation.

Fermenting Bourbon: Under Oxygenating Wash

During the bourbon fermentation process and specifically during yeast reproduction, the yeasts utilize oxygen in order to produce unsaturated (incomplete hydrogen bonds) fatty acids thereby depleting aCoA, which is a precursor to the production of esters.

When a bourbon distiller limits or restricts the level of available oxygen accessible to the yeast, it effectively increases the available ester precursor acetyl coenzyme A (aCoA), thereby increasing the production of “fruity” esters. Bourbon distilling best practices suggest refraining from oxygenating or aerating the wort. If oxygen reduction is desired for specific ester targeting, it is advised to increase the duration of the mash boiling process which will reduce the level of dissolved gasses.

Open Bourbon Fermenting

The opposite of the process of under oxygenating the wash, with open fermenting, there is a natural overall increase in ester production. Ester-heavy spirits such as bourbon, rum and scotch adhere to the open fermentation model.

Hydrostatic / Top Pressure During Fermentation

When C02 builds up in the fermentation vessel, the overall hydrostatic pressure increases to a limit where it inhibits ester formation and fusel alcohol formation. Controlling top/hydrostatic pressure is vital when forming a consistent distillate for bourbon production.

Pitching Rate

Most esters detected in bourbon are produced during the phase where yeasts are growing. If the pitching rate is low, ester formation is amplified. If the pitching rate is high, however, the ester production is decreased.

Degree Lintner

The term “lintner” or rather, “degrees lintner” is a measurement unit to determine the ability of a malt (or malted grain) to convert or reduce it’s starch volume to sugar. This is also known as the diastatic power of the malt. This measurement is universal to any amylase (enzyme which converts starch to sugar), however, the term is most used in reference to the combined α-amylase and β-amylase used in the bourbon fermentation process.

Diastase Enzymes

Diastase enzymes (from Greek, meaning to “separate”) are a class or group of enzymes that are used in bourbon fermentation in a process to catalyze the breakdown of grain-based starches into sugars – specifically maltose. For instance, Alpha Amylase can be synthetically added to a mash, or it may be naturally occurring form the cooking of barley. Alpha amylose converts starch to disaccharide maltose, the trisaccharide maltotriose, (which contains three α (1-4)-linked glucose residues), and oligosaccharides (also known as dextrins) that contain the α (1-6)-linked glucose branches.

Diastatic enzymes, in general, are used in the mashing process to convert starches from a state of unfermentable long-chain starches to short-chain sugars which can be fermented. The yeast in the mash convert the sugar to ethyl alcohol and C02 (carbon dioxide). A term called “diastatic power” or “DP” is a measurement of the volume of starch-converting enzymes are present in the bourbon mash bill.


Sacrrification is the process by which starches are converted to sugars and dextrins during the bourbon mash bill “mashing” process. The enzyme alpha anylase is the enzyme most often utilized for hydrolysis of long-chain starches into dextrins – next beta amylase breaks down dextrins into highly-fermentable sugars.

There exists a great balance in the saccharification process; the bourbon mash must be heated slowly and consistently in order to fully gelatinize, thereby making it bioavailable to enzymatic breakdown. However, as temperatures increase, the enzymes themselves become threatened with denaturation. A compromise temperature is often sought out to maintain a homeostatic environment which facilitates gelatinization of starches from the mash grains as well as protects the fragile nature of the enzymes. This process or balance is referred to as the “saccharification rest.”

Bourbon Wash

The “wash” refers to the liquid solution used for fermentation. The wash is typically made in a large vat called a lauter tun. Preparing the wash is the first step in producing the distillate that will eventually become bourbon. Other terms used for the wash are “mash” and “beer” (or distiller’s beer).

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