This study aimed to follow the viability of a probiotic culture in a soy beverage and in a fruit juice blend using quantitative PCR with propidium monoazide (PMA-qPCR). Free and microencapsulated (alginate and poly-L-lysine system) cells of Lactobacillus reuteri NCIMB 30242 were added at 108 CFU/mL in each food matrix and stored for 8 weeks at 4 or 8°C. In both matrices, viability losses during the 8 week storage period were less than 1 log CFU/mL. The pH of the fruit juices did not change during storage, but acidification occurred in the soy beverage, particularly when storage was carried out at 8°C. As a result, at a pH below 6.3, coagulation of the soy beverage occurred. It was found that qPCR could ascertain the total dead and viable population of L. reuteri in both food matrices. At day 1, the PMA-qPCR data in fruit juice were approximately 0.5 log cells/mL lower than in soy, which points to an effect of matrix itself on the qPCR analysis; the methodology was nevertheless successful in following the changes in L. reuteri viability during storage. Microencapsulation did not enhance the stability of the cultures.
Lactobacillus reuteri, microencapsulation, food matrix, storage temperature PMA-qPCR