Dynamics of Active Fluorescent Units (AFU) and Water Activity (a) 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) 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 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 levels in commercially available capsules containing freeze-dried bacteria such as sp. or combinations of sp. and 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 in the single-strain product, by 0.16 Log in the two-strain product and by 0.26 Log in the multi-strain product. With the increase in a, the number of bacteria decreased but the a at the end point of the stability study did not exceed 0.15 in each of the three tested products. FCM combined with a 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.