Modeling Penicillium Expansum Growth Response to Thyme Essential oil at Selected Water Activities and pH Values Using Surface Response Methodology

Abstract

Our objective was to evaluate, using a full factorial design, the effects of selected water activities (0.990, 0.945, or 0.900), pHs (5, 4, or 3), and thyme essential concentration (TEO, 0, 25, 50, or 100ppm) oil on Penicillium expansum lag time (λ) and radial growth rate (μ_m) obtained by modeling mold response using Gompertz equation, and corresponding polynomial quadratic models. Potato-dextrose agar formulated with every studied factor combination was inoculated with 10³ spores/ml, and incubated at 25°C up to 30 days. Mold colony diameter was periodically measured during incubation and adjusted with Gompertz equation to determine λ and μ_m. Decreasing a_w and pH, and increasing TEO concentration decreased μ_m and increased λ. At low a_w and pH, the increase in TEO concentration had a dramatic effect on P. expansum response since 25ppm of TEO inhibited its growth for 30 days at 25°C. Gompertz parameters exhibited that P. expansum was sensitive to the evaluated combined factors, allowing us to construct a secondary predictive growth model. TEO in combination with a_w and pH reduction effectively inhibited P. expansum growth.