How is rum made?
How is rum made?
A usual starting point when discussing rum making is the colonial context and heritage of the country where it is made from.
Why? Because each one of these countries have their own culture, tradition and economic situation which translate in different ways to process rum.
However, the main steps of rum production are common wherever rum is created. The common processes involve the sugar cane harvest, its grinding in order to extract the sweet sugar cane juice, then this sweet juice is either sent to a distillery right away (“agricole” rum) or to a sugar refinery in order to transform the juice into molasses before sending it to the distillery (“traditional” rum). At the distillery, both molasses and virgin sugar cane juice (“vésou”) are fermented. Then, the fermented product is distilled in different kinds of stills (pot stills, column stills, coffey stills or combination like multiple-column stills). The distilled alcohol is either stored in stainless steel tanks (white rum) or in wooden barrels (mainly oak) to produce fine aged rums. When it comes to prestigious aged rums, the aging, grading and blending processes represent a whole universe where methods and classifications are highly diversified. As a consequence, comparing aged rum from different brands can be delicate.
At the end, this whole diversity makes the rum experience of each one of us such a beautiful and enjoyable journey!
Harvest and press
There are many species of sugar cane, which are more or less resistant to diseases and have various sugar (sucrose) levels. The main natural ones are : “saccharum robustum”, “saccharum officinarum”, “saccharum spontaneum”, “saccharum sinense”. In order to increase productivity and resistance against diseases, hybrid species have been created by crossing natural ones. Another trick used by a couple of cane farmers is to plant a selection of varieties to avoid having the whole field wiped out by disease.
At eleven months, sugar cane is harvested (manually or mechanically) before blossoming. When done manually, canes must be chopped as low to the ground as possible since the highest levels of sucrose are in the base. Leaves as well as the very top of canes are cut and left on site. Only the base of the cane is hastily transferred to the sugar factory to limit any waste. Once the lower end of the canes has been shredded, warm water is added to extract a sweet liquid or syrup called “vésou”. When this liquid is directly sent to a distillery to be used for rum, we are looking at an agricole rum. However, this syrup can also be processed further through centrifugation to extract all the crystalized sugar; the viscous, thick and sticky black-brown remaining byproducts constitute the so-called molasses. When rum is made from molasses we talk about traditional rum (also named industrial rum). These molasses are more stable than vésou, which is one of the reasons why most distilleries buy them from sugar industries. As a consequence, most of the rum around the world comes from molasses.
Fermentation process
The principle of fermentation is based on the consumption of the sugars by the yeasts converting them into alcohol, carbon dioxide (CO2) and heat. This is a key step in the production of rum, which will shape the profile of the future spirit. First, the vésou or the molasses are watered down and filtered to remove any unwanted compounds before being transferred into stainless steel vats where specific types of yeast are added. The most common types of yeast added are Schizosaccharomyces and Saccharomyces. This fermenting process can last between 24 hours to three weeks. The type of yeast and fermentation time depends on the distillery. Jamaican distilleries, for example are known for their longer fermentation time and their secret, unique yeast cultures.
There are three main types of fermentation processes:
Spontaneous fermentation, which relies on yeasts and other microorganisms naturally present in the air and in the vésou or molasses. This spontaneous fermentation is done in open vats and can last up to two weeks.
Controlled fermentation using yeasts that have been developed and refined in cultures, which can last up to three days, and helps with consistency in flavours and ABV.
Continuous fermentation, which consists in keeping the vat in constant activity by constantly adding vésou or molasses, allowing yeasts to remain continuously active by extracting must at different levels.
Importantly on the flavour side, yeast do not only produce alcohol, CO2 and heat, but they also provide through secondary reactions these non-alcoholic substances (congeners) as acids, aldehydes, esters and other compounds. These and especially esters constitute the flavours within the spirit. Once the process is finalised, a sugar cane wine at 4% up to 10% ABV is generated.
Distillation
A second key step playing a major role in the properties of the rum produced. The distillation process consists in concentrating the alcohol and its associated aromas through evaporation in a still. In other words, it relies on boiling the liquid obtained after fermentation before condensing it by cooling in another section of the still. This principle of distillation is based on the fact that water boils at 100°C (121°F) while alcohol at 78.3°C (165°F), so heating up the wine (either from molasses or sugarcane juice) in between those will allow water to separate from alcohol. This process can be repeated multiple times to amplify and purify the alcohol content of the liquid. Typically, after a first distillation, the wine which is somewhere between 4 to 10% ABV usually progresses to 25 and 30% ABV. Then, a second distillation will bring it all the way up to 70%. However, as the distillation process concentrates the alcohol through evaporation, some of the non-alcoholic substances rich in aromas are lost each time the process is repeated.
As usual in rum production, there are variations in the way distillation happens, with a variety of stills offering different properties. The two main types of stills:
The first one is the traditional pot still, which has been in use since the 16th century. The process associated with this type of stills is more time-consuming as each cycle requires the still(s) to be cleaned and cooled before repeating a new distillation.
During the 19th century, distillation was modernised and streamlined with the introduction of the column still, which allowed continuous distillation by combining several levels across the columns. Given the practicalities of this type of stills, and the associated cost-efficiencies, column stills have become widely used for rum production. One of the other advantages of column stills is that they allow to better control the aroma profile of the rum thanks to the multi-level structure. The lighter fumes, which have less aromas, reach the higher levels, whereas the heavier fumes, with stronger aromas, stay in the lower levels of the column.
Column stills have evolved over the years with: the Creole columns (used in the French Caribbean) that have fewer levels; the Savalle columns that are using a convoluted path to slow down fumes; or the Barbet columns with bell-shaped levels. But one of the biggest innovations is the addition of columns. When a typical column still process uses two columns (one for heating called the analyser, and one for cooling called the rectifier), innovative producers have started to use stills with three or more columns. Multiple columns have the ability to eliminate a specific property of the rum that needs to be rectified in each column. The specific shape and technique used has an influence on the profile of the final distillate, which at that point is typically between 70 and 95% ABV.
Aging
Once again, the beautiful diversity of rum expresses itself fully when talking about aging.
Before talking about “premium“ aged rums, we have to mention that even white rums are in theory “aged” from a couple of weeks up to three years. However, this short aging or resting will most of the time takes place in stainless steel, but can in some cases take place in wooden tanks. The rum resting in wooden tanks will by default gain some golden shade. Moreover, rum coming straight from its distillation process has a sharp and vivid taste due to its youth. So resting it in wooden tanks will soften the outline of this young rum by giving it subtle wooden notes. Moreover, color is to be taken with a grain of salt since rums aged for several years which usually look from light brown to darker shades, can also look white after charcoal-filtering.
Barrel vs cask
As we said earlier, the aging step itself carries so much diversity! Indeed, aging relies on a myriad of features on which different economic contexts, cultures and traditions will express their singularities ending up on defining a wide range of processes. But before getting into these details, back to the basics. A very simplified explanation of aging consists in storing freshly distilled rum in wooden casks from few to several years.
First, let’s consider the time factor. Longer is this storing, the more subtle notes the rum will develop. These notes are the outcome of two things, aromas being earned from the wooden barrel, and the chemical reactions happening inside the rum. Through its aging, the rum will gain a specific shade/color, and due to oxygen, oxidation will take place developing the personality of the rum. Interaction between alcohol and fatty acid from the wood will allow esterification to happen, which provides a lot of aromas. Nevertheless, it does not mean that aging rum for a maximum amount of years is a must reach in order to obtain the best quality and balanced rums. Actually, highly aged rums can be too stringent, sharp, specific and exclusive depending on the aging process. Therefore, there is a difference between age and maturity!
Now we know some hints about the involvement of wood in the aging process, let’s explore another parameter which is the quality of the wood. Indeed, new and used barrels are used to age rum. Virgin barrels being very active will transfer high levels of color, flavor and tannin. At the opposite, used barrels being less active will provide a mellower and less stringent aged rum. These types of barrels offer a lot of options for rum blending but we will develop this aspect later. Moreover, talking about the quality of the wood means different species as well. Up to date, white american oak barrels are still mostly used since american Whiskey must be aged in virgin white american oak to be called Bourbon. By consequence, used white american oak barrels are massively available on the market. However, other essences can obviously be used especially when you are aware that the essence of the wood defines the aromatic profile of an aged rum. Different essences contain variable levels of vanilla, lactones, tannins, hemicellulose, etc. Plus, their roughness, size and number of pores are different, which will impact the personality of the aged rum (oxidation, color, aromas, etc.).
Since we are talking about the wood, let’s now mention the treatment. Barrels can be roasted or charred. This action carries various functions. First, when it comes to used barrels, the charring process will rejuvenate and re-activate them and thus allow some chemical reactions to happen again. Second, by affecting wood’s physical and chemical composition, charring will promote some aromas (spices, vanilla, smoke, caramel) while inhibiting some others. For instance, light charring will provide spices notes while strong charring will caramelize sugars of the wood. Third, burning the inside layer of the wood will generate a carbonized layer which can charcoal filter substances the fresh rum to age (like sulfur), and trap oxygen thereby increasing oxidation reactions. Finally, charring will also participate in rum’s dark color.
The previous features (time, wood quality and treatment) can be combined for various and greater outcomes, however additional layers of optimization are still possible. For instance, the location will tremendously affect the whole aging process whatever the previous features are. Humidity from tropical weather is known to increase aging. Under high humidity, the rum enters further into the oak’s pores meaning higher exchange between the rum and the wood, but meaning higher evaporation losses too. In other words, humidity and heat increase the so-called “angel’s share” (percentage of loss of rum per year). Mastering tropical aging is a delicate expertise. Plus, due to their geographical location, some rums are aged at different altitudes affecting the pressure in the barrel therefore impacting the aging process (chemical reactions, pores penetration, etc.). Few other brands age their rums on a boat, at 42 feet under the Caribbean sea (unmatchable pressure and humidity), or in a chemical reactor (patent), while others slowly spin barrels on themselves. All these processes aim to impact and improve the interaction between the wood and the rum.
Now, taking into account all the previous possibilities, imagine all the aged rum you can create by blending rums with different aromatic profiles and core properties by playing with their proportions. That it is the craft and knowhow of Master blenders. Knowing how to handle aged rum with specific quality in order to get a unique overall profile. For instance you can blend rum from different ages, different distillation methods (pot stills and column stills), different raw material (molasses, virgin sugar cane), different grades (new barrel and used barrel) and so on. Once you found a nice signature the challenge is being able to reproduce this aromatic pattern. From time to time, Master blenders fall onto such an exceptional batch that they bottle it straight from one barrel, the so-called “single cask.”
We have to mention the solera system, which is a very peculiar blending technique and unique way to age rum faster (cf article about the solera system).
Finally, rum can have a double maturation in an additional cask in a same or different location. For example, a rum can be aged for several years in a Bourbon used barrel before being transferred in a Rye used barrel. This practice is called “cask finish” and consists in using a quite “marked” cask to sleek the gustative profile of the aged rum with subtle hints of aromas coming from different liquors. In this regard, casks that previously contained port, Madeira, whiskey, cherry, xeres, cognac, wines (sauternes, bourgognes), calvados and so on can be used for this practice. The rum will only age from a couple of months up to two years in this second cask otherwise new flavors will end up dominating the initial aging process.
Rum making is a fascinating endeavor, and as we saw here, it offers a lot of parameters to play with. This can explain the great variety of styles, profiles and aromas that one can find when exploring the category.
While this is a fairly simplified description of the process, we felt it was important to capture the overall method so we can all better understand what are these parameters that shape the rums we like. In future articles, we plan to cover each process in greater detail.