Yeast Type
Some rum distillers use wild yeasts, whereas others use specific cultivated strains. Using natural yeast will impart certain characteristics of the local environment into the rum, characteristics that can actually change from batch to batch. To control such variables, some companies use specific cultivated strains that retain the same characteristics from generation to generation. The Los Angeles Times recently wrote about the yeast used by Bacardi, detailing how the Bacardi distillery smuggled its yeast out to Puerto Rico when its original Cuban facilities were nationalized by the Castro government in 1960.
Length of Fermentation
The other variable that yeast introduces is fermentation speed. Some yeast strains convert sugar to alcohol more quickly than others. Bacardi, for example, has bred its yeasts to ferment molasses so quickly that it forms fewer esters and congeners—the chemical substances that create flavor compounds. So one reason Bacardi tastes so light and mild is thanks to its yeast and the speed at which it works.
