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

The effect of solutes, water activity (a(w), 0.890--0.980) and temperature (5--25 degrees C) on the mycelial growth rate of Penicillium expansum was evaluated. The growth rate dropped as the temperature and a(w) 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, a(w), temperature and combinations of two or three of these factors (P<0.0001). Whatever the solutes and a(w) values, the initiation of colony growth required an additional day at 15 degrees C and 5 degrees C as compared to initiation at 25 degrees C. Growth models based on the results obtained with sorbitol and glycerol differed only slightly, with R(2) 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 a(w) (0.890) and that growth at 25 degrees C should be fastest at a(w) values ranging from 0.960 to 0.980. Both models presented a good fit between predicted and observed values.