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

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 (μm) 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 μm. Decreasing aw and pH, and increasing TEO concentration decreased μm and increased λ. At low aw 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 aw and pH reduction effectively inhibited P. expansum growth.