The growth of Lactobacillus plantarum and Streptococcus thermophilus was modelled by fitting isothermal calorimetric traces to arbitrary mathematical functions. There was a correlation between certain features on the thermal trace and parameters derived from a Baranyi fit to viable count data. In the presence of 75 g L−1 lactose, S. thermophilus showed a slower rate of growth than in the presence of 5 g L−1 lactose when measured by both viable count and heat flow. A simple equation described the variation in glass transition temperature with water activity for all preparations. A master curve fit for all mortality data for S. thermophilus showed the importance of a transition temperature for the survival of freeze-dried preparations of bacteria. The temperature at which survival began to be impaired was 15–20 °C below the glass transition temperature determined by differential scanning calorimetry. Copyright © 2007 Society of Chemical Industry