Growth rate and growth–no-growth interface of Penicillium brevicompactum as functions of pH and preservative acids Original Research Article

The effects of pH and preservative agents such as sorbic acid (E 200), propionic acid (E 280) and sodium benzoate (E 211), on Penicillium brevicompactum growth or growth–no-growth interfaces were studied. Experiments were carried out on solid media maintained at a water activity of 0·9, and incubated at 20°C. Fungal growth was established by diameter measurements for up to 75 days and kinetics were fitted to Baranyiʹs primary predictive model. The no-growth phenomena was determined by no visible mycelium development after 75 days. A set of 198 experimental data was generated using two experimental designs. Firstly, pH effect on growth rate was analysed by a secondary predictive model including the inhibitory pH value, pHmin, as a parameter. This pH limit value, which depended on the nature of the preservative agent, was estimated to be 5 in the presence of sorbic acid at maximal authorized values in the European Community (2000 mg l−1). Secondly, sorbic acid and sodium benzoate inhibitory concentrations were evaluated at pH 5, by developing a predictive model taking both growth and growth–no-growth data into account. The sorbic acid had a greater inhibitory effect than sodium benzoate, however the limit value of growth of Penicillium brevicompactum depended on both sorbic and sodium benzoate. Finally, a model built with the laboratory medium data was tested on bakery products made at pH 5, a water activity of 0·9 with various concentrations of sorbic and sodium benzoate, and incubated at 20°C. Results were fairly satisfactory even if wide variability was observed for food products.