The microbial ecology of a rum production process

Abstract

Rum is an alcoholic beverage made from the distillate of a microbial fermentation of sugar cane molasses. Although commercial rum production started in the 16 th century, the microbial ecology of the process has remained relatively unexplored. This thesis reports an investigation of the microorganisms associated with rum production at a distillery in Queensland, Australia. Samples of raw materials (molasses, dunder, water, additives), starter cultures and fermenting molasses were systematically examined for the populations and species of yeasts and bacteria. Molasses contained low populations (< 102 CFU/mL) of yeasts, Bacillus species and lactic acid bacteria. Saccharomyces cerevisiae, used as a starter culture, was the main yeast of molasses fermentation, growing to populations of about 107 CFU/mL. Lactic acid bacteria were consistently isolated from the molasses fermentation and reached populations of 107 CFU/mL. The main species isolated were Lactobacillus fermentum, Lactobacillus plantarum, Lactobacillus brevis and an unidentifiable Lactobacillus spp. These species were indigenous contaminants within the processing environment, colonising sites that escaped effective cleaning and sanitation operations. Species of Clostridium, Zymomonas and Propionibacterium were not detected in the production system. Dunder, which originated from the distillation operation, was considered to be sterile, but developed a population of lactic acid bacteria (the unidentifiable Lactobacillus spp.) on storage. Dunder had significant concentrations of organic acids and amino acids. At concentrations of 10% and above, it significantly inhibited the growth of S. cerevisiae in molasses medium and to a lesser extent lactic acid bacteria. Laboratory scale molasses fermentations and distillations were performed to investigate the effect of lactic acid bacteria on the growth of S. cerevisiae, process efficiency and production of flavour volatiles. Both single and mixed cultures using S. cerevisiae, L. fermentum, L. plantarum and Lactobacillus spp. were undertaken. The bacteria did not restrict the growth of S. cerevisiae but enhanced utilization of molasses sugars and ethanol production. The work presented in this thesis is the first comprehensive and systematic study, of its type, into the microbial ecology of a rum distillery.