Modelling the effect of temperature and water activity on growth of Aspergillus niger strains and applications for food spoilage moulds

Aims To develop a model for the combined effect of water activity (a(w)) and temperature on growth of strains of Aspergillus niger, and comparison with data on food spoilage moulds in the literature. Methods and results An extended combined model describing the growth of two strains of A. niger, as a function of temperature (25-30 degrees C) and a(w) (0.90-0.99) was developed. The growth rate (micro) was expressed as the increase in colony radial growth per unit of time. This extends the previous square root model showing the relationship between temperature and bacterial growth rate developed by Ratkowsky et al. (1983) and the parabolic relationship between the logarithm of the growth rate and a(w) developed by Gibson et al. (1994). A good correlation between the experimental data and the model predictions was obtained, with regression coefficients (r(2)) > 0.99. In addition, the use of this model allowed predictions of the cardinal a(w) levels: a(w(min)), and a(w(opt)). The estimation of the minimum a(w) levels (a(w(min))) was in accordance with data in the literature for similar and a range of other Aspergillus and related species, regardless of the solutes used for a(w) modification. The estimation of the optimal a(w) (a(w(opt))) and the optimal growth rate (micro(opt)) were in good agreement with the experimental results and data from the literature. Conclusions This approach enables accurate prediction of the combined effects of environmental factors on growth of spoilage fungi for rapid prediction of cardinal limits using surface response curves. Significance and impact of the study This approach is a rapid method for predicting optimal and marginal conditions for growth of a wide range of spoilage micro-organisms in relation to interacting environmental conditions and will have applications for improving shelf-life of intermediate moisture foods.