Microbial composition of turbid rice wine (Makgeolli) at different stages of production in a real processing line

Abstract

The aim of this study was to examine for changes in microbial populations during the production of turbid rice wine (also known as Makgeolli) at different production plants and to identify critical points for the control of microbial quality. Samples from raw ingredients (water, rice and wheat flour), materials from different production stages (steamed ingredients, the base of fermentation, primary and secondary fermentation stages at different time points), and the final rice wine products (non sterilized and sterilized) were analyzed. The microbiological content of samples was assessed by quantitative (aerobic plate counts, lactic acid bacteria, fungi, acetic acid bacteria, coliforms and Bacillus cereus) and qualitative (Escherichia coli and eight foodborne pathogens) analyses. Aerobic plate counts for rice and wheat flour were relatively low (3.1 and 1.9 log CFU/g, respectively), as were those for lactic acid bacteria (1.6 and 2.1 log CFU/g), and fungi counts (2.2 and 0.7 log CFU/g). All counts increased to 7.8-7.9 log CFU/ml in the base of fermentation after the addition of Nuruk and Koji, and these counts were maintained at 8-9 log CFU/ml during fermentation. Acetic acid bacteria were not detected in the ingredients, but were isolated from the base of fermentation (1.2-2.8 log CFU/ml). Heat sterilization reduced aerobic pate counts significantly from 8.4 to 2.1 log CFF/ml, and lactic acid bacteria, fungi and acetic acid bacteria were reduced to non-detectable or negligible levels. Isolated microorganisms were considered as autochthonous to the environment or were artificially introduced with the starter culture. B. cereus was widely distributed throughout the manufacturing process, and may have been introduced from the raw material (present in 100% rice and 41.7% wheat flour samples). B. cereus counts were not significantly affected by sterilization, suggesting that it may exist at low levels as spores in the final products. No coliforms and other pathogens were detected in any samples. The raw materials, the base of fermentation, and sterilization stage were identified as important points for the control of microbial quality during the fermentation process. (C) 2015 Elsevier Ltd. All rights reserved.