How Rum is Made
Rum production begins with sugar cane. To this day, mature sugar cane is harvested by hand - grueling machete work - in many parts of the world.
The cut cane is promptly transported to the mill where it is crushed in a machine. The crushing extracts the sugar cane juice from the fibrous pulp. The crushed pulp waste is burned to generate power to crush the cane and sometimes to heat a distillation column. Depending where you are, the pulp waste can later be burned to generate power or to heat the still.
Now that you have the sugar cane juice, three different things can happen:
One, you can proceed directly to fermenting and distilling the sugar cane juice. This is what they do in the French West Indies (Martinique and Guadalupe). Understandably, this most direct approach yields rum that most closely preserves the vegetal characteristics of the cane.
Two, you can cook down and concentrate the sugar cane juice into a syrup. This syrup is itself a stable sweetening product, but the syrup can also be fermented and distilled. A few distilleries work with fermented syrup, because it gives them most of the characteristics of sugar cane juice that they are looking for, while enabling them to distill all year, not just at the harvest.
Three, you can process the juice into molasses and crystallized sugar. The crystallized sugar is sold as a sweetening product, and the molasses is sold to a distillery to be fermented and distilled into rum. Most rum is distilled from fermented molasses.
The fermentation process varies by distillery, and there's a great deal of variation involved. At one extreme is "natural fermentation," where yeasts inherent in the environment are relied upon to ferment the sugars in open vats. At the other extreme, fermentation is tightly controlled under laboratory-like conditions. For efficiency and predictability, most commercial rum fermentation processes fall between these two extremes; distilleries purchase and add the specific yeast cultures they want and take basic precautions appropriate to their environment. Time is also factor for fermentation; some ferments last only several hours while others can take up to two weeks.
The next step is distilling. The concept and basic mechanics of distillation are simple. A fermented liquid is heated in a sealed vessel to approximately 175 degrees Fahrenheit, evaporating the alcohols from the liquid. The alcohols are then re-condensed and collected, yielding the raw spirit. However, the reality of distillation is extremely complex. There is virtually no seemingly trivial detail that lacks the potential to affect the end product. Distillation is a science, and success depends on a great deal of expertise, but craftiness, habit, mother nature, superstition, and luck all play a role.
The designs of stills (the primary apparatus of distillation) also vary wildly. They are broadly organized into two groups: pot stills and continuous stills. Many stills combine elements of both pot and continuous stills. Nearly every still is a unique design (if only by virtue of how it is installed), and each still is hand-built. Every still has its innumerable quirks, and those quirks have their mysterious consequences. Some stills are profoundly simple devices, where others have multiple stages and features that enable different components of the alcohol to be separated. (Some distillers want to remove particular chemical components from their rum. Sometimes these chemicals that are undesirable for a rum are useful for other purposes. Large distilleries may collect byproducts to sell to other industries, such as the artificial flavor industry. Regardless, what comes out the end of the still is raw, hot rum that absolutely, positively couldn't have been made anywhere else.
Note: some liquor products are marketed - proud and loud - as pot-still products, as if that distinction alone were definitive. Another common marketing conceit is touting the number of distillations a spirit has undergone. Don't ascribe too much importance to these statements.
What happens next greatly varies depending on the product the distillery is aiming for. The raw spirit from the still will be between 70% and 95% alcohol by volume (although cachaca can distilled as low as 38% and no higher than 48%abv). Some rums, particularly for local Caribbean markets, are simply bottled and sold, fresh from the still. Most rum is aged, which radically alters the character of the spirit. Almost all rum is blended with other batches from the same distillery, often marrying rums of different ages. Some rum gets infused with herbs, fruits, and spices or deliberately blended with juices or extracts to produce a flavored rum product. Most rum is diluted with water - at some point prior to bottling - to 40%-50% alcohol by volume.
Although there aren't that many major steps, per se, to producing a given rum, the possible variables, beginning with the cane itself and ending with the post-distillation handling, are innumerable. These variables make rum the most varied of all the distilled spirits.