The Effect of Distillation Conditions and Molasses Concentration on the Volatile Compounds of Unaged Rum
Caribbean rums are made from blackstrap molasses and during the production process, four critical stages can affect the flavor of rum. These four stages include fermentation, distillation, aging, and blending. During the distillation process, a copper alembic pot is used. The rum distillate is collected in three fractions or ’cuts‘; heads, hearts, and tails. Major compounds in the head fraction contain ethanol, ethyl acetate, and acetaldehyde, the latter being undesirable at high concentration. The heart fraction is generally of commercial value because it contains ethanol, esters and fusel alcohols that gives the rum the unique aroma. The tail fraction usually contains a high concentration of fatty acids that give off a rancid and unpleasant odor. In industry, an experienced distiller will use a sensorial approach (taste and smell) to determine these cuts. To minimize such a subjective approach, this study proposed to use gas chromatography fitted with a flame ionization detector to monitor specific compounds (congeners) in the rum distillate. By understanding the congener profiles, it is possible to determine when to make appropriate cuts objectively. The head fraction was determined to be the first 400ml of the total rum distillate of 3000ml. At this point, the concentration of acetaldehyde starts to drop below the flavor threshold of 200 mg/L. Propionic acid, owing to its high boiling point, are collected at the later stage of distillation as the ‘tails’ fraction. According to the findings from this study, the tail cut is made at 2600 ml of distillate collected as the concentration of propionic acid starts rising rapidly above the odor threshold of 60 mg/L. High concentration of propionic acid in the rum distillate would give a sweaty-like and rancid odor. In addition, it was found that rum that has been distilled twice had higher purity in ethanol and lower concentration of undesirable congeners like acetaldehyde and propionic acid. However, the desirable ethyl ester congeners were also removed during after the double distillation process.
During the fermentation stage, a commercial rum yeast from a strain of Saccharomyces cerevisiae was used to ferment a mixture of varying molasses and dextrose concentration in the rum must (solution before fermentation). It was found that the raw material also affected the congeners of the distilled rum. Using the solid phase microextraction (SPME) with GC-MS technique, it was found that 1-butanol 3 methyl-acetate, octanoic acid ethyl-ester, acetic acid 2-phenylethyl ester, decanoic acid ethyl ester, octanoic acid ethyl ester, 3-methylbutyl ester, dodecanoic acid ethyl ester were the most abundant esters in rum distillate. However, there was no obvious trend to correlate molasses concentration in the must to the ester profiles of rum. The results from this study also showed that the amino acids in the blackstrap molasses did not have any impact on the concentration of esters in the final rum distillate.