Use of gradient plates to study combined effects of temperature, pH, and NaCl concentration on growth of Monascus ruber van Tieghem, an Ascomycetes fungus isolated from green table olives.

The effect of temperature, pH, and sodium chloride concentration on the growth of the Ascomycetes fungus Monascus ruber van Tieghem, the main spoilage microorganism during storage of table olives, was studied by using the gradient plate technique. Gradients of NaCl (3 to 9%, wt/vol) at right angles to gradients of pH (2 to 6.8) were prepared for the plates, which were incubated at 25, 30, and 35 degrees C. Visible fungal growth, expressed in optical density units, was recorded by image analysis and graphically presented in the form of three-dimensional grids. Results obtained from the plates indicated that the fungus was salt and acid tolerant, being able to grow at NaCl concentrations of up to 9% (wt/vol) and pH values of as low as 2.2, depending on the incubation temperature. The inhibitory effect of NaCl increased as the pH decreased progressively at 25 and 30 degrees C but not at 35 degrees C. Growth was better at 30 and 25 degrees C as judged by the larger extent of the plates covered by mycelium compared with that at 35 degrees C, where no growth was observed at pHs below 3.7. Differentiation between vegetative (imperfect-stage) and reproductive (perfect-stage) growth was evident on all plates, providing useful information about the effect of environmental conditions on the form of fungal growth. When the growth/no-growth surface model was obtained by applying linear logistic regression, it was found that all factors (pH, NaCl, and temperature) and their interactions were significant. Plots of growth/no-growth interfaces for P values of 0.1, 0.5, and 0.9 described the results satisfactorily at 25 and 35 degrees C, whereas at 35 degrees C the model predicted lower minimum pH values for growth in the range of 7 to 10% NaCl than those observed on the plates. Overall, it is suggested that the fungus cannot be inhibited by any combination of pH and NaCl within the limits of the brine environment, so further processing is required to ensure product stability in the market.