The rapid assessment of viable cells in freeze-dried formulations of probiotic Lactobacilli is an important issue in the probiotic industry. The probiotic Lactobacillus reuteri DSM 17938 is the active ingredient of BioGaia Protectis whose viable cell concentrations are currently determined using the classical and time-consuming viable count technique. In this paper we present a rapid method based on Isothermal microcalorimetry (IMC) for viability assessment of freeze-dried DSM 17938. Fresh and rehydrated freeze-dried cells were incubated in MRS medium at different cell concentration and the thermal power (mu W) at 37 degrees C was measured over time. A linear dependence between the time to reach a certain thermal power value and the decimal logarithm of viable cells was found. This allowed rapid viability assessment of rehydrated cells, reducing the detection time from 48 h (viable count) to 10 h (microcalorimetry) in samples with 3 x 10(3) cfu/mL, and to 4 h in samples with 4 x 10(6) cfu/mL. No significant differences were found between the two methods when freeze-dried rehydrated cells were analysed (p > 0.05). Therefore, IMC is a promising tool to be used in the quality control of freeze-dried DSM 17938 as well as other bacteria-based products. (C) 2017 Elsevier Ltd. All rights reserved.