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FIGURE 18.10 Kinetics of microbial reduction in ground black pepper after 6.7mg/l ozone gas treatment at a flow rate of 6l/min. (From Zhao, J. and Cranston, P.M., J. Sci. Food Agric., 68, 11, 1995. With permission.)

60 minute exposure and more than a 3 to 4 log reduction of Penicillium spp. and Aspergillus spp. after 10 minutes. Figure 18.10 shows the kinetics of microbial reduction in ground black pepper after the ozone gas treatment. Zhao and Cranston [68] also treated black peppercorns by bubbling gaseous ozone (6.7mg/l; 6l/min) in a peppercorn-water mixture (2:5, w/w) for 10 minutes. This treatment reduced the microbial population by 3 to 4 log numbers. Kim et al. [64] reported that bubbling gaseous ozone (4.9%, v/v; 0.5l/min) in a lettuce-water mixture under sonication and high-speed stirring was the most effective ozonation method that inactivated up to a 1.9 log of the natural microbial load in 5 minutes.

The effectiveness of ozone gas treatments is summarized in Table 18.5. Low concentrations (<100ppm) of ozone gas treatments with long exposure times (days) have been used for growth inhibition and inactivation of spoilage microorganisms on fresh fruits and vegetables. Barth et al. [86] found that fungal development was suppressed when blackberries were stored at 2°C for 12 days in the air with 0.3 ppm ozone, with 20% of control fruits showing decay. Growth rates of yeast surviving ozone treatment were markedly decreased under longer exposure times and higher concentrations, RH, and temperatures [83]. Constant exposure to ozone throughout storage has also been reported to be effective in inhibiting storage pathogens on lemons and oranges at 1 ppm (14°C, 85% RH) [88] and peaches at 0.25 ppm (4 to 15°C) [89]. However, ozone was reported to be ineffective in preventing fungal decay in strawberries after 4-day treatment with 0.35 ppm ozone at 20°C [90]. Other early reports also indicated that ozonated apples, cantaloupes, and cranberries demonstrated more decay or damage than those not ozonated [91-94].

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