Studying and modelling the combined effect of temperature and water activity on the growth rate of P. expansum

The effect of solutes, water activity (aw, 0.890–0.980) and temperature (5–25 °C) on the mycelial growth rate of Penicillium expansum was evaluated. The growth rate dropped as the temperature and aw of the medium decreased. NaCl was the solute causing the greatest growth rate reduction, followed by glucose, glycerol and sorbitol. Statistical analysis of the results showed a significant effect of solute, aw, temperature and combinations of two or three of these factors (P < 0.0001). Whatever the solutes and aw values, the initiation of colony growth required an additional day at 15 °C and 5 °C as compared to initiation at 25 °C. Growth models based on the results obtained with sorbitol and glycerol differed only slightly, with R2 values of 97.00% and 97.95%, respectively. The response surfaces of both quadratic polynomial models showed that P. expansum should be able to grow at low aw (0.890) and that growth at 25 °C should be fastest at aw values ranging from 0.960 to 0.980. Both models presented a good fit between predicted and observed values.