Bourbon vs Rum: Technical Head to Head

To my great delight, interest in rum from bourbon enthusiasts has gathered steam recently. As a writer I’m keenly aware that many topics in the bourbon world are well-tread ground, while rum has far more undeveloped land, so to speak.

Many of the recent stories here on Cocktail Wonk have focused on rum, but it shouldn’t be surprising that I’m also a fan of bourbon, Scotch whisky, and whiskey in general. Take a look at my bourbon stories here if you don’t believe me! Need further evidence? I’m also the Vintage columnist for Bourbon Plus magazine.

With that in mind, it’s time to bring together three of my loves: Bourbon, rum, and wonky comparisons. More specifically, how are bourbon and rum different? And what attributes do they share?

Terms of Engagement

Talking in general terms about distilled spirits is fraught with peril. Nearly any declarative statement like “bourbon is column distilled” will elicit an “Actually…” rebuttal that cites a counterexample.

Nonetheless, if we are to get through this in time for happy hour, we must deal in generalities. My focus here is the ninety-five percent of the time case, and not the minutiae of a one-person distilling operation on Vanuatu. I will cover important subtleties but make no claim that what follows is exhaustively comprehensive.

Before we begin, it’s important to note that rum is a subcategory of sugar cane spirits. Some people use rum as a catch-all term for cane spirits, while others take great pains to differentiate the different subcategories, e.g. cachaça from Brazil or clairin from Haiti. I shall not attempt to tease apart all of the associated subtleties here. I will use rum here in the general case, while citing non-rum cane spirits as appropriate.

Where Made

By U.S. law that’s also recognized by major countries around the world, bourbon must be made in the U.S. Whiskey can be made anywhere in the world, and bourbon is a subcategory of whiskey – made only in the U.S.

Like whiskey, rum is also an uber-category that transcends national boundaries. That isn’t to say that there are no rules in rum. Rather, any rules regarding rum (as with whiskey) are specific to certain countries.

Rather than comparing the entire uber-category of rum to bourbon, a fairer comparison uses national identities, e.g. Jamaican Rum to Bourbon, or Cuban rum to Irish Whiskey. But I’m not going to be fair here, so will compare rum production across many countries to good old USA Bourbon.

It should be noted that cane spirits are primarily made in countries where sugar cane is grown or was once grown. That is, the tropics of Capricorn and Cancer: The Caribbean, Central and South America, India, southeast Asia, and Australia.

Fun fact: Brazil makes the most cane spirit (Cachaça) of any country in the world.

Source Material

Bourbon must be made from a fermented mash of grain, e.g. corn, barley, wheat, and rye. Furthermore, bourbon’s mash bill must be at least 51 percent corn. In practice, bourbons are made from several grains, i.e., corn and one or more other grains. Often the other grains include malted barley which supplies the necessary enzymes that turn the grain’s starch into fermentable sugars.

An essential part of bourbon making (and any whiskey making) is the mashing process. The dry grains are milled into a coarse powder, then added to a giant tank, along with a significant amount of water. Heat is applied to cook the thick soup, causing the starches to turn into sugar.

After the cooking has completed, the soup, or wort has a substantial amount of fermentable sugars.

Each grain brings its own flavors to the party, and the ratios used affect resulting flavor. In high enough proportion, non-corn grains in a mash bill lead to informal names like high rye bourbon or wheated bourbon.

As grains are to whiskey, sugar cane is to rum. The “cane” here is vitally important. Rum is not made from sugar. It’s made from sugar cane – no beet sugar need apply. Begone beets!

When it comes to sugar cane, there are multiple ways to process it – or not process it. The three primary forms of processed sugar cane products (for rum production) are:

• Cane juice – Must be used immediately after crushing
• Cane syrup – Cane juice with substantial amounts of water removed
• Molasses – Cane juice from which a substantial amount of water and sucrose (table sugar) has been removed.

The majority of rum made is from molasses, which is less expensive than cane syrup, as it has less sugar per unit volume.  A key advantage of molasses is that it’s stable, so will not spontaneously begin fermenting on its own and can easily be stored and transported.

Rum for cane juice comprises the smallest amount of rum made globally but expresses local terroir far more than a molasses rum. Since sugar cane must be crushed very quickly after cutting to prevent spoilage, you’ll only find cane juice rum (sometimes referred to as agricole) made where sugar cane fields are close by. The French islands of Martinique, Guadeloupe and Reunion Island are particularly known for their cane juice rums, or rhum, in French.

In the same way that barley, corn, wheat, and rye are varieties of grains, there are hundreds of sugar cane varietals used to make rum. So, if you think about a bourbon whiskey versus a rye whiskey, you might also differentiate a blue cane rum from a red cane rum. However, the flavor difference between cane varietals is typically more subtle than grains, e.g. corn versus rye.

The essential point about both grain and sugar cane as raw materials is that the end result is fermentable sugars in liquid form. The mashing process at a bourbon distillery converts the starches into sugar. In rum making, the molasses is already loaded with sugars, so no transformation is needed before fermentation.

Yeast

Yeast is a living organism that converts sugar into alcohol and congeners, i.e. flavor compounds! It’s where the magic starts for any type of distilled spirit.

There are countless different yeast strains, and each makes its own unique set of flavors. The vast majority of bourbon and rum is made from carefully cultured yeast strains. Cultured yeast strains are intentionally kept the same from fermentation batch to batch, so each batch tastes like the prior batch – in theory. This consistency has high commercial value. Once a distillery finds a yeast strain that works for them, they stick with it, taking great effort to make sure the strain doesn’t mutate and can be recovered from offsite if a disaster occurs.

All of the major bourbon producers use cultured yeast. They can’t take the risk of one batch of bourbon tasting substantially different than another. That said, many distilleries use their own proprietary strain rather than an off-the-shelf variety.

While cultured yeast is the norm in rum, some producers use natural, airborne yeast to elicit a much more wild, funky, or unusual flavor profile. Among the better-known producers using wild yeast are certain Jamaican distilleries like Hampden Estate, Clairin from Haiti, Guyana’s Demerara Distillers Ltd., and St. Lucia Distillers.

Fermentation

The length of time spent fermenting affects a spirit’s flavor. In general, longer fermentations creates a more flavorful, often “funky” profile.

The typical bourbon fermentation period is between two and four days. As for rum, the vast majority is made with a relatively short duration of a day or two. The Spanish heritage rums of Cuba and Puerto Rico are great examples of these short fermentations.

However, some rum fermentations stretch out for weeks. This supercharges the resulting wash with esters, one of the primary types of flavor compounds. Jamaican rum is particularly associated with long fermentation, although it’s used in many locales.

It should be noted that during a longer fermentation the vast majority of the yeast’s work completes in the first few days, with further activity largely bacterial in nature. It makes for flavors particularly associated with rum, especially high ester rum.

As a final note of comparison, both bourbon and rum making have a long tradition of using waste from a prior distillation (vinasse in French) as a component of a subsequent fermentation. That is, liquid remaining in a still after distillation is part of the next fermentation. The acidic liquid waste creates a favorable environment for the yeast to work and enhances the creation of esters, making for a more flavorful end result.

Bourbon makers call this use of acidic waste sour mash. In rum making, it’s known as dunder by the Jamaican distilleries, who are the primary modern users of the process. Use of dunder by non-Jamaican distilleries is rare but not unheard of. For instance, Clairin Le Rocher from Haiti uses dunder.

Distillation

The big bourbon manufactures almost exclusively distill continuously, rather than in batches. The typical bourbon still at a distillery like Buffalo Trace, Jim Beam or Wild Turkey is a column still, with a single column. (There are column stills with more columns – More on this in a moment.)

What many people don’t realize is that there’s more to big bourbon distillation than a single column still. The large producers almost always send the distillate through another specialized still to further increase the alcohol content. This second still has some physical similarities to a pot still, but it operates differently, continuously receiving column still distillate while continuously emitting stronger distillate. There are no heads, hearts, and tails as in batch distillation.

These special stills come in two varieties – thumpers and doublers. Prior to distillation the thumper is partially filled with liquid, typically with alcoholic waste from prior distillations. An input pipe goes into the thumper, terminating below the liquid’s surface. Hot distilled vapor from the column flows through the pipe and emerges below the surface of the thumper’s liquid. (Picturing blowing bubbles in a cup through a straw.)

The latent heat in the vapor causes the liquid in the thumper to slowly vaporize and flow out via another pipe to be condensed and collected.

Simply put, a thumper is just an optional part of a continuous distillation pipeline; it constantly takes in distilled vapor at one alcoholic strength and emits re-distilled vapor at a higher strength. Think of it like a car engine’s turbocharger, although the analogy is not particularly exact.

As for doublers, they’re fairly similar to thumpers. However, rather than taking in distilled vapor from the column, it receives somewhat cooled liquid distillate from the column. That is, the vapor from the column first condenses back to liquid before entering the doubler. As with a thumper, a doubler is also part of a continuous distillation process.

Of course, there are pot stilled bourbons. Both Woodford Reserve and Willett are well-known examples of bourbon distilleries who make some of their bourbon using pot stills. They also both use column stills in addition to their pots.

Smaller bourbon distilleries are more likely to use batch distillation for some or all of their production. A typical setup is a batch kettle feeding into a short column for further rectification.

Around the world, rum distillers use a far broader variety of distillation setups. For continuous distillation, you’ll find single columns, double (Coffey) columns, four columns (Savalle) and multi-columns (typically five columns).

The vast majority of rum sold worldwide, by producers like Bacardi, Tanduay, and Havana Club are made using a multi-column setup that creates a very light rum. In contrast, the French rhums of Martinique and Guadeloupe are almost exclusively distilled on single column stills and are extremely flavorful.

Heavier rums are usually associated with batch (pot) distillation. Interestingly, the largest pot distilled rum manufacturers aren’t using simple batch stills like used in Scotland. Rather, they use a setup known as a double retort pot still.

A double retort still has three main components – the pot (similar to what you’d find in Scotland), and two retorts. A retort works like a bourbon still thumper, albeit in a batch process. Distillate vapor flows from the pot into the liquid of the first retort. This boils the liquid within, and the resulting vapor flows into the second retort, where the same process repeats. The advantage of the double retort pot still is that a single pass through the still yields alcoholic strengths up to around 85 percent ABV.

Every Jamaican rum distillery has at least one double retort pot still. You’ll also find them in Barbados, St. Lucia, and Guyana.

Alcoholic Strength

Legally bourbon cannot be distilled above 80 percent ABV.

As for rum, there are no worldwide standards for maximum alcoholic strength. However, certain countries have regulations. For Martinique AOC compliant rhum, distillation must be between 65 and 75 percent ABV, and come from a single column.

Brazilian Cachaça must be bottled at between 38 and 48 percent ABV. In fact, artisanal pot distilled Cachaça uses a single pass through a simple pot, resulting in ABV in the 50 percent range. This is on the very low end of the range of distilled spirit ABVs from the still.

Of course, a country may have a minimum ABV for distilled spirits. In the U.S., it’s 40 percent ABV.

Aging Containers

When it comes to aging spirits, American oak casks are the dominant type of vessel used.

Bourbon legally must be aged in “charred new oak containers”.  The vast majority of such containers are 53-gallon bourbon barrels. Note that a bourbon barrel doesn’t have to be American oak, but in practice it overwhelmingly is.

The new means that nothing else can have been previously aged in the cask. Once the cask is used it can’t be re-used to make more bourbon. Thus, the bourbon industry goes through a phenomenal number of casks at great expense. In fact, there are more bourbon casks aging in Kentucky than people.

This requirement that bourbon makers use new casks is a boon for many other spirits producers who’ve come to rely on ex-bourbon casks to age their spirits. Scotch, Irish, and Canadian whisk(e)y makers, along with rum producers are heavy users of ex-bourbon casks.

Some rum makers also use other wood types, with French oak being a particularly popular choice. Ex-sherry casks, or more accurately, sherry-treated casks, are also a popular rum aging vessel.

Of course, there’s any number of rums, bourbons and other whiskies that are finished in other types of casks, including port, madeira, red wine, Islay whisky, and just about anything else you can imagine. We shall not speak here of the recent Tabasco Barrel finished whiskey.

Aging strength and Angel’s Share

Cask entry strength matters because it impacts how much wood-extract flavors like vanillin are pulled from the wood.

Bourbon must enter the barrel at no more than 62.5 percent ABV.

Rum cask entry ABVs fall within a very wide range, with very little if any country-specific regulations. The majority of rums fall in a range between 50 and 80 percent ABV. Spanish heritage producers like Cuba and Puerto Rico are known for aging at the lower end of the range.

The evaporation rate from a cask, aka angel’s share, is highly dependent on where aging occurs. The closer to the equator, and thus higher average temperature, the higher the angel’s share.

In Kentucky, where the vast amount of bourbon is made, the angel’s share averages around five percent per year. In contrast, Caribbean angels (or duppies, if you prefer) may take over ten percent in the first year of aging before dropping to around eight percent.

Aging Duration

The general category of bourbon has no required minimum aging duration. However, to be labeled as “straight bourbon whiskey” it must age for a minimum of two years. Also, bourbon aged less than four years must have a minimum age statement on the label.

Unaged cane spirits are very common in many countries including Brazil, Jamaica, and Barbados. (Of course, these countries also have aged spirits.)

Required/minimum aging durations for rum is an immensely challenging area to keep track of. Many countries have required minimum amounts of aging, and/or define what terms like XO and VSOP can be applied to different ages. Cuba, Puerto Rico, and Australia are among the counties that have a minimum requirement for aging before it can be labeled as rum.

Unfortunately, rum is awash is terms like solera or slow aged that have no legal meaning.

By and large, bourbon aging is relatively static. The bourbon goes into the casks and remains there until it’s bottled. In contrast, some producers, especially in Spanish heritage countries, have a tradition of aging rum for a bit, blending multiple casks together, and then casking it again to further develop.

Blending and additives

When it comes to bourbon, the rules are very simple. No additives of any kind, including caramel. Only water to proof down to bottling strength is allowed.

On the other hand, rum is a minefield when it comes to additives. Caramel coloring is allowed in many countries, as are other blending materials, including sweet wines sugar, and glycerol.

There has been a substantial amount of discussion recently about geographic indications for rum, and what they have to say (or not say) about additives. It’s a contentious and far larger topic than can be addressed in this context. It’s been written about extensively elsewhere, so I shall not attempt to summarize it here.

In the absence of national regulations, people wanting to know if there are additives (usually sweeteners) in their rums are forced to look at each expression individually. Sites like Drekon have lists of estimated sugar levels in many different rum expressions, but the values cited are rough estimates and can’t account for certain additives like glycerol.

Wrap Up

Having looked at many aspects of bourbon and rum production, its clear that rum is far more diverse than bourbon. This isn’t a pejorative statement – Bourbon making is constrained to one country. If you limited your scope to a single rum making country like Jamaica or Cuba, you’d see a much smaller range of differences across distilleries.

The key takeaway here for bourbon lovers is that rum offers a huge spectrum of different styles to explore. You might not enjoy them all, but odds are you’ll find something that tickles your spirited fancy.

Finally, a huge thank you to Nicholas King of the WSET for reviewing the above and providing his always insightful commentary.