Laboratory and commercial examination of post-harvest chlorine dioxide antimicrobial applications for whole, fresh apples

Molly Dian Mayer

Chlorine and chlorine dioxide were evaluated as antimicrobial interventions for use in post-harvest treatment of whole, fresh apples in laboratory and commercial settings. In laboratory studies, Gala apples inoculated with generic Escherichia coli (E.coli) or E.coli O157:H7 were randomly assigned to treatments: inoculated untreated, chlorine or chlorine dioxide (oxidation-reduction potential {ORP} levels of 665mV, 750Mv and 850mV), phosphoric acid (pH 3.5) or water, for 2, 3.5 and 5 minutes. The chlorine dioxide study involved 4 replications (560 total apples) and the chlorinephosphoric acid study involved 5 replications (850 total apples). In the commercial study, Gala apples inoculated with generic E.coli (3 replications, 975 total apples) were randomly assigned treatments: inoculated untreated or treated in a commercial dump tank containing water, phosphoric acid (pH 3.5), chlorine (low ORP or high ORP), or chlorine dioxide (low ORP or high ORP). Samples were enumerated on violet red bile agar for generic E.coli and Cefixime-Tellurite Sorbitol-MacConkey Agar for E.coli O157:H7. Apple background microflora were evaluated: total coliforms, generic E.coli and aerobic bacteria. Chemical solutions were assessed for ORP and pH. For both chlorine-phosphoric acid and chlorine dioxide laboratory studies, the bacteria levels on the inoculated control were significantly higher (p<0.01) than all treatments examined, including water. Microbial levels after treatment with water, phosphoric acid or chlorine (665mV, 750mV, or 850mV) were statistically similar, and similar findings were observed in the chlorine dioxide study. Bacterial reductions compared to the inoculated control for chlorine ranged from 0.5-0.8 log10cfu/mL and for chlorine dioxide, 0.5-0.7 log10cfu/mL. In the commercial study, a significant reduction (p<0.01) was observed between the inoculated control and all antimicrobial treatments. Chlorine and chlorine dioxide treatments (low and high ORP) achieved significantly greater reduction of generic E. coli levels (0.9-1.1 log10cfu/mL) compared to phosphoric acid and water (0.5 log10cfu/mL and 0.6 log10cfu/mL, respectively). Phosphoric acid (pH levels 3.1-3.5) did not appear to be an effective antimicrobial treatment. Although laboratory studies did not indicate significant microbial reductions, the commercial study indicated that chlorine and chlorine dioxide (ORP from 581-852mV and 691-816mV, respectively) reduced generic E. coli levels.

Laboratory and commercial examination of post-harvest chlorine dioxide antimicrobial applications for whole, fresh apples

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