Microbial populations in sweet sorghum juice during fermentation

Scope and method of study. The specific objective of this study was to enumerate the microbial populations in sweet sorghum juice immediately after pressing (fresh juice) and during fermentation under varied yeast inoculation conditions. The effect of different levels of yeast inoculation and different times of yeast inoculation on the microbial populations was investigated. Targeted microorganisms included total aerobic bacteria, lactobacilli, coliforms, E. coli and yeast. Sweet sorghum was pressed with a small scale roller press, and the resulting juice was fermented at room temperature in 0.5 L vessels. Yeast was inoculated at levels of 0.13 and 0.26 g/l, and inoculation time was varied from 3 to 48 hr after pressing. Samples were plated at intervals of 12, 24 and 48 hr after inoculation onto petrifilms containing selective media for the organism of interest.Findings and conclusions. Results indicated that growth characteristics of the microorganisms were not affected by yeast inoculation time. Yeasts, and especially lactobacilli, increased irrespective of the time of inoculation. Coliforms generally decreased during the fermentation process, regardless of inoculation factors. Growth characteristics of the microorganisms were unaffected by the level of yeast inoculation. It is hypothesized that varying environmental conditions (temperature, humidity and soil conditions) during harvest and the way the harvest is conducted contributes to the significant change in microflora of the harvested juice samples, in particular the bacterial populations. Juice inoculated soon after pressing yielded more ethanol than the juice inoculated later in time. But statistical analysis showed no significant difference in the ethanol yield between the two levels of yeast inoculum tested.