Aspergillus niger time to growth in dried tomatoes.

Individual and combined effects of aw and incorporation of selected concentrations of Mexican oregano essential oil on the time to growth (TTG) of Aspergillus niger intentionally inoculated into dried tomatoes were studied during storage at 25°C for 100 days. For aw 0.96, 1,000 ppm of Mexican oregano essential oil inhibited A. niger growth during 100 days, whereas 500 ppm were sufficient at aw 0.91 and 250 ppm for tomatoes with aw 0.78. A. niger growth was evident at different incubation times depending on tested tomato aw and concentration of essential oil; these data were utilized to model TTG. Regression analysis revealed good agreement between experimental and predicted data with a correlation coefficient higher than 0.98. Analysis of mold growth data through TTG models makes possible to include observations detected as no growth and can be utilized to predict mold time to growth for specific preservation factor combinations or to select preservation factor levels for an expected shelf-life based on A. niger growth.

Thermal inactivation of Botrytis cinerea conidia in synthetic medium and strawberry puree.

Botrytis cinerea is one of the most important post-harvest molds that cause quality deterioration of strawberries and other fruits even during refrigeration storage. This research studied the effects of thermal inactivation of B. cinerea in synthetic medium and strawberry puree using hot water baths at different temperatures. These media were studied in order to determine if results obtained in a solution with the major components of the fruit (synthetic media), are comparable to the ones obtained in fruit purees. The results demonstrated that B. cinerea spores can be inactivated by heat treatments using relatively low temperatures (42-46 °C). Inactivation curves were well described by first order kinetics (R² 0.91-0.99). B. cinerea conidia inoculated in synthetic medium required less time to achieve one log reduction in population than those inoculated in the fruit puree. D values were 22, 8.5, 4 and 1.4 min at 42, 44, 46 and 48 °C, respectively, in synthetic medium; while D values in strawberry puree were 44.9, 13.8, 4.7 and 1.4 min at 42, 44, 46 and 48 °C, respectively. The z values obtained were 4.15 and 5.08 °C for the strawberry puree and synthetic medium respectively, showing higher sensitivity of B. cinerea in fruit purees than in the synthetic medium. Thus, a change in the medium composition had a marked difference in the heat inactivation of B. cinerea conidia, and the results obtained in synthetic medium are not accurate to describe the behavior of the microorganism in the fruit.

Colletotrichum gloeosporioides growth-no-growth interface after selected microwave treatments.

To study microwave heating for potential postharvest treatments against anthracnose disease, Colletotrichum gloeosporioides growth-no-growth response after selected microwave treatments (2,450 MHz) was fitted by using a logistic regression model. Evaluated variables were power level, exposure time, presence or absence of water in the medium during treatment, and incubation-observation time. Depending on the setting, the applied power ranged from 77.2 to 435.6 W. For the experiments on dry medium (mold spores over filter paper), exposure times were 1, 2, 3, or 4 min, whereas spores dispersed in potato dextrose agar, a wet medium, had exposure times of 3, 6, or 9 s. Growth (response = 1) or no growth (response = 0) was observed after two different incubation-observation times (4 or 10 days). As expected, high power levels and long exposure times resulted in complete inhibition of C. gloeosporioides spore germination. In a number of cases (such as low power levels and short treatment times), only a delay in mold growth was observed. Scanning electron micrographs showed signs of mycelia dehydration and structural collapse in the spores of the studied mold. Cell damage was attributed to heating during microwave exposure. Reduced logistic models included variables and interactions that significantly (P < 0.05) affected mold growth, and were able to predict the growth-no-growth response in at least 83% of the experimental conditions. Microwave treatments (4 min at any of the studied power levels in dry medium, and 9 s at power levels of 30% or more for wet medium) proved effective in the inhibition of C. gloeosporioides in model systems. These no-growth conditions will be tested further on fresh fruits in order to develop feasible postharvest microwave treatments.