Modelling the effect of ethanol on growth rate of food spoilage moulds

The effect of ethanol (E) on the radial growth rate (μ) of food spoilage moulds (Aspergillus candidus, Aspergillus flavus, Aspergillus niger, Cladosporium cladosporioides, Eurotium herbariorum, Mucor circinelloides, Mucor racemosus, Paecilomyces variotii, Penicillium chrysogenum, Penicillium digitatum, Rhizopus oryzae and Trichoderma harzianum) was assessed in Potato Dextrose Agar (PDA) medium at aw 0.99, 25 °C. In order to model this effect, the Monod type equation described previously by Houtsma et al. (Houtsma, P.C., Kusters, B.J.M., de Wit, J.C., Rombouts, F.M., Zwietering, M.H., 1994. Modelling growth rates of Listeria monocytogenes as a function of lactate concentration. Int. J. Food. Microbiol. 24, 113-123.) was re-parameterised: μ = μ opt K ( E max − E ) K E max − 2 ⁢ K ⁢ E + E max E ; Emax (%, wt/wt): ethanol concentration at which no growth occurs, K (%, wt/wt): ethanol concentration at which μ=μopt/2, μopt (mm day−1): growth rate at 0% ethanol. The model was capable of describing curves, μ vs. E, with either a concave shape (K<Emax/2) or a convex one (K>Emax/2) with a good accuracy (root mean square error (RMSE)≤0.136) with the notable exception of R. oryzae and T. harzianum. After growth rate data were square-root transformed to stabilise the variance, Emax was estimated in the range 3% to 5% for all moulds with the exception of T. harzianum (Emax 2.14%) and P. variotii (Emax 6.43%). Ethanol would appear an effective additional barrier to inhibit fungal growth in food products and would represent an interesting alternative to the use of preservatives.