Application of flow cytometry and measurement of water activity to evaluate the stability of probiotic products

The flow cytometry method (FCM) is a widely renown practice increasingly used to assess the microbial viability of the probiotic products. Also, water activity (aw) can be used to verify of viable cells in probiotic products throughout their shelf lives. The aim of this study was to analyze and demonstrate the effective, time-efficient and economical method of combining FCM and aw for assessing the shelf life of various probiotic products and guaranteeing their quality. We controlled the stability of probiotic products for bacterial counts (using flow cytometry, SYTO24 and PI dyes) and aw levels in commercially available capsules containing freeze-dried bacteria such as Lactobacillus sp. or combinations of Lactobacillus sp. and Bifidobacterium sp. in standard conditions (25°C ± 2°C and 60% relative humidity) over a period of 24 months. During this time, the bacterial contents decreased by 0.1 Log10 in the single-strain product, by 0.14 Log10 in the two-strain product and by 0.25 Log10 in the multi-strain product. The aw at the end point of the stability study did not exceed 0.15 in each of the three tested products. FCM in combination with aw is an innovative, valuable and prospective method for assessing the stability of probiotic products both for their ability to detect bacterial viability, but also for practical (analysis time) and economic reasons.