Influence of fermentation on the acid tolerance and freeze drying survival of Lactobacillus rhamnosus GG

The aim of this work was to investigate the effect of fermentation pH and time on the acid tolerance, glycolytic activity, and survival during freeze drying of Lactobacillus rhamnosus GG. Two fermentations were performed, one was pH controlled (pH 6.8), and the other was pH uncontrolled. Cells were collected after 10 h (late exponential phase), 25 h (mid stationary phase) and 37 h (late stationary phase) of fermentation. In the case of the pH controlled fermentation, the acid tolerance of the cells collected at 10 h was remarkably high, as the viability decreased only by 0.2 logs; in the case of the 25 h and 37 h, it decreased 1.5 and 6.0 logs, respectively. In the case of the pH uncontrolled fermentation, for all three time points the cell reduction was high, ranging between 3 and 4 logs. The cells from the pH controlled fermentation, and in particular the cells from the 10 h sample, demonstrated considerably higher glycolytic activity compared to the cells from the uncontrolled fermentation. The type of fermentation had a lesser effect on the ability of the cells to survive during freeze drying, as in all cases the cell reduction was small (<1 log). The cells from the 10 h sample from the pH controlled fermentation survived significantly (P < 0.05) better than the cells from the 25 and 37 h sample; the opposite was the case for the cells from the pH uncontrolled fermentation. However, in all cases, the differences between the samples were less than 0.5 log. In conclusion, both the fermentation pH and time were found to have a significant impact on the glycolytic activity of L. rhamnosus GG, and on its ability to survive under acidic conditions and during freeze drying.