Suppression of Outgrowth

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The application of effective antibacterial agents to the surface of fresh-cut melons may suppress outgrowth of the native microflora and any human pathogens. Studies showing antilisterial activity of nisin in TSB or PBS62, and demonstrating its activity against native microflora on whole and minimally processed cantaloupe have been reported [17]. However, total elimination of salmonella on the surface of whole or fresh-cut melon could not be achieved, probably due to surface irregularities and internalization which reduced the ability of antimicrobial treatments to contact or remove bacterial cells. However, treatment with the combinations sodium lactate-potassium sorbate or nisin-sodium lactate may lead to an increased shelf life and a reduced risk of foodborne illness from salmonella or other human pathogens; such treatments also appeared acceptable from a quality standpoint [17,63]. The use of nisin for treating fresh-cut melon may reduce the risk of L. monocytogenes outgrowth [64].

Bacteriophage was used to control growth and reduce population of S. Enteritidis on fresh-cut melons [65]. In our most recent study, we found that the native microflora of cantaloupe and honeydew melon was inhibitory to L. monocytogenes [66]. Lactic acid bacteria were used to improve microbial safety of minimally processed fruits and vegetables [67]. Other researchers have used antagonistic microorganisms isolated from the field to control postharvest pathogens and colonization of apple surfaces [68].

10.7 methodology for microbiological evaluation of melons

Accurate assessment of the microbiological quality and safety of melons requires use of suitable sampling, recovery, and detection methods that take into account the mode of attachment of microorganisms to the melon surface. This is especially important with cantaloupes because of their complex surface morphology characterized by netting and the presence of fissures, both of which are absent on honeydew melons [14,69]. The cantaloupe surface morphology provides numerous microbial attachment sites and opportunities for inaccessibility not present on other non-netted melons. However, all melons will show variations in surface features that could affect microbial attachment and growth, especially in the stem scar and ground spot regions.

Beuchat and Scouten [70] conducted a detailed study of survival and recovery of S. Poona on spot- and dip-inoculated cantaloupes sampled at three sites: the intact rind, a wound, or the stem scar. Recovery was accomplished by stomaching excised rind in a wash solution containing 0.1% peptone, with or without added Tween 80, or by rubbing melons in the same wash solution within a plastic bag. They demonstrated the equivalence of a number of combinations of preenrichment broth, enrichment broth, and selective agar medium in detection of S. Poona recovered from the rind surface. They reported no difference in recovery of S. Poona from the three sites compared to when the inoculum was suspended in water or an organic matrix (horse serum); growth occurred in both spot- and dip-inoculated wounds over 24 hours at 21 and 37°C but not at 4°C. Addition of up to 1.0% Tween 80 to peptone may have enhanced detachment of S. Poona, recovered by the washing procedure. The stomaching and wash solution procedures appeared to give equivalent results.

Annous et al. [71] examined recovery and survival of E. coli NRRL B-766 on spot- and dip-inoculated cantaloupe rind. Less than 1% of the inoculum applied by spot inoculation to the rind surface could be recovered by excising plugs containing the inoculation sites and blending. E. coli survival on inoculated cantaloupe after treatment with 300 ppm chlorine or water at 60° C was greater if applied by dip inoculation of the melon surface compared to spot inoculation. The investigators compared two sampling methods for recovering bacteria from the melon rind surface: (1) excision and blending of 20 replicate plugs containing inoculation sites for spot inoculation or taken at random locations for dip inoculation, and (2) removal of the entire spot- or dip-inoculated rind with an electric peeler. With both methods, the rind samples were homogenized with peptone water, serially diluted, and plated. The methods were applied to melons inoculated with E. coli B-766 or S. Poona. A method was developed for calculating the melon surface area from measurements of the polar and equatorial diameters, based on an assumption that the cantaloupe was a sphere, oblate spheroid, or prolate spheroid. When expressed on an area basis, the population estimates for the two methods were the same with both test organisms (Table 10.5). Expression of the population estimate on a weight basis would be invalid, however, because of poor correlation between the rind weight and external surface area. The whole rind method is less time-consuming and requires less handling than the rind plug method.

Barak et al. [42] compared two elution methods with peeling and blending for recovery of S. Poona from inoculated cantaloupes. They reported better recovery with Butterfield's buffer containing Tween 80 as the eluant than with phosphate-buffered saline, similar recovery when agitation was provided by shaking or rolling, and better recovery by the elution methods than by peeling and blending. The last result was attributed to the release of inhibitory substances during blending.

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