Effect of inoculum size and water activity on the time to visible growth of Penicillium chrysogenum colony

•Food products are often contaminated with a single fungal spore.•The time to visible growth for a single spore contamination is a good criterion to determine the shelf-life of products.•This time can be estimated accurately by the model of Gougouli et al. (2011) that takes into account the inoculum size.•The model parameters and the ratio time to visible growth over germination depended on water activity.•This time cannot be extrapolated satisfactorily from high to reduced water activities.

In order to assess the effect of the inoculum size on the time to visible growth for Penicillium chrysogenum, the correlation described by González et al. (González, H.H.L., Resnik, S.L., Vaamonde, G., 1987. Influence of inoculum size on growth rate and lag phase of fungi isolate from Argentine corn. International Journal of Food Microbiology 4, 111–117) was compared to the model introduced by Gougouli et al. (Gougouli, M., Kalantzi, K., Beletsiotis, E., Koutsoumanis, K.P., 2011. Development and application of predictive models for fungal growth as tools to improve quality control in yogurt production. Food Microbiology 28, 1453–1462). Based on the regression coefficient, the latter model performed better than the former one to fit the data obtained for P. chrysogenum grown on Potato Dextrose Agar at 25 °C. Inoculum sizes in the range 101–105 spores were tested at 0.930, 0.950, 0.970, and 0.995 aw. By extrapolation of the straight line, the model of Gougouli et al. (2011) provided accurate estimations of the time to visible growth for a single spore inoculum, tvg (N = 1). In order to avoid experiments at reduced water activities, the influence of water activity on the model parameters, and on the ratio tvg (N = 1) over the germination time was assessed.