As part of our rum consulting practice, we are frequently asked by our clients about the different types of rum available in the world. Most of them are only aware of differences based on color and are very surprised to learn about all the variables that can affect how a rum tastes. Following is a brief summary of the different types of rum, along with explanations that we hope will help you understand the complexity and diversity in the rum industry. These categories are not mutually exclusive (i.e., you can have a Pot Stilled, Naval Style, Aged rum).
The previous blog focused on classifications of rum based on raw material, it is now time to talk about how rum is classified based on the fermentation method employed in the conversion of sugar to alcohol.
Rum types based on fermentation method
Natural Fermentation: Natural fermentation is similar to the process used in the beer industry to make Lambic-style beer. Distilleries rely on wild, naturally occurring yeast, present in the air and in the cane juice to convert the sugars (sucrose) in the mash into alcohol. Natural fermentation takes place in open containers to maximize the exposure of the mash to the air. This type of fermentation, depending on the size of the vat, can take from one to two weeks, and the results are not always 100% duplicable.
Controlled Fermentation (batch): In this method, a particular strain of yeast, which is usually guarded as one of the distillery’s most valuable assets, is introduced into the mash and allowed to perform its job. To reduce the risk of natural fermentation from occurring, the controlled yeast is first mixed with a small batch of the mash, in some cases just a couple of ounces. Next, the yeast is allowed to multiply and reach a predetermined concentration. This starter is mixed with a larger amount of liquid, around a gallon, from the mash. This process is repeated two or three times until a large amount of highly concentrated starter is achieved, which is then added into the large fermentation tanks. Controlled fermentation done in this way usually takes only two to three days and the results are very predictable and reproducible.
Controlled Fermentation (continuous): One of the latest trends in the world of fermentation is that of fermenting in a continuous process rather than in batches. As the name implies, this method consists of a main fermentation tank that continuously receives a stream of diluted molasses. While the influx of molasses keeps the yeast thriving in the medium, an equal amount of liquid is extracted from a different place in the fermentation tank, already “digested” and ready to be distilled. While the concept of continuous fermentation is relatively new to the rum industry, it is not so in other fields, such as the medical industry. An early continuous process was a vinegar generator in which acetobacter attached to wood shavings inside a container with one opening on top and another one at the bottom. Trickling a sugar solution down through the container packed with the wood shavings produced vinegar. The acetic acid discourages contamination at conditions where the acetobacter thrive.