Source material and fermentation

The world’s rums span an enormous breadth of flavor profiles, more than any other type of spirit. But all rums have one fundamental common element: They’re made from sugarcane juice or something derived from sugarcane juice.

All distilled spirits are made by using yeast to convert sugars in a solution into alcohol, then distilling the result to concentrate the alcohol. Spirits like rum just require fewer steps to make, compared to say, whisky. In what follows, we’ll focus the early stages of rum making and prior to distillation.

Sugarcane

Sugarcane is a tall grass which grows best in hot climates near the equator. It takes about a year for the plant to reach maturity, achieving at least three meters in height and around 50 mm in width.

Shortly the sugarcane is cut, the stalk passes through a series of grooved rollers to press out the juice. The juice contains several types of dissolved sugars, but primarily sucrose. Some rum is made directly from sugarcane juice, but most rum is made from what remains after further processing of the juice to make sugar crystals.

To cause crystals to form, the sugarcane juice is heated, forcing much of the water to evaporate and the liquid to thicken. Eventually, sucrose crystals begin to form and are removed. The thick, dark liquid left behind is molasses. Most of the world’s rum comes from molasses, which still contains a substantial amount of dissolved sugar.

At this point in the rum making process, we have lots of dissolved sugar—in the cane juice or molasses—but no alcohol.

Fermentation

Fermentation is the organic process which transforms sugar into alcohol, along with various flavor compounds. For example, if you ferment grape juice the result is wine. The same concept applies when we use cane juice or molasses.

Yeast cells, which are living microscopic organisms, power the fermentation process. Into our sweet liquid (cane juice or diluted molasses) we introduce large quantities of yeast cells. The hungry yeast consume the sugar molecules and rapidly reproduce, growing in number. The waste from the yeast’s activity is alcohol (mostly but not entirely ethanol) along with carbon dioxide, heat, and small amounts of flavor compounds.

About those yeast cells: There are thousands of different yeast varieties and they don’t all emit the same flavor compounds. Thus, the type of yeast feeding on the sugary solutions influences the resulting flavors.

Fun fact: Rum was made for two hundred years before scientists fully understood yeast and fermentation. The fermentation of those early rums was triggered by airborne yeast cells that naturally found their way into the sugary liquid. Fermentation typically took a week or longer, as the initial number of yeast cells was fairly small. Also, the strains of airborne yeast in one location were often very different from the yeasts found just a few kilometers away. Thus, rum made in each locale had its own flavor profile, what we call terroir today.

By the late-1800s, as our understanding of yeast grew, rum makers began directly adding large quantities of yeast to fermenting liquid, enabling it to proceed much faster. In addition, by controlling which variety of yeast they used, they could better control the resulting flavor.  What was once a several week process could now be completed in a few days. It should be noted that some rum makers still use airborne yeasts today because they desire the particular flavor profile of their airborne yeast.

Fermentation stops when all the sugar is consumed. In its place are flavor compounds and alcohol - usually between six to ten percent by volume. Next time we’ll learn how this cane wine becomes rum.