Dynamics of Active Fluorescent Units (AFU) and Water Activity (a w ) Changes in Probiotic Products-Pilot Study.

The flow cytometry method (FCM) is a widely renowned practice increasingly used to assess the microbial viability of probiotic products. Additionally, the measurement of water activity (a w ) can be used to confirm the presence of viable cells in probiotic products throughout their shelf lives. The aim of this study was to investigate the correlation between changes in a w and variations in active fluorescent units (AFU), a unit commonly used in flow cytometry method, during the aging of probiotic products containing freeze-dried bacteria. We controlled the stability of probiotic products for bacterial counts (using ISO 19344 method) and a w 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 ± 2 °C and 60% relative humidity) over a period of 24 months. During this time, the bacterial contents decreased by 0.12 Log 10 in the single-strain product, by 0.16 Log 10 in the two-strain product and by 0.26 Log 10 in the multi-strain product. With the increase in a w , the number of bacteria decreased but the a w at the end point of the stability study did not exceed 0.15 in each of the three tested products. FCM combined with a w is a prospective analysis that can be used to assess the stability of probiotic products, both for its ability to detect bacterial viability and for practical (analysis time) and economic reasons.