Effect of Preharvest Application of Chitosan and Tea Tree Essential Oil on Postharvest Evolution of Lettuce Native Microflora and Exogenous Escherichia coli O157:H7

Abstract

The main objective was to assess the effectiveness of preharvest application of chitosan (CH) and tea tree essential oil (TT) in butterhead lettuce in controlling native microflora growth and counteract exogenous Escherichia coli O157:H7 contamination. TT and CH were applied to lettuce plants at 14, 10, 7, 3 and 0 days before harvest. CH showed significant reductions at harvest on mesophilic (−2.0 log cfu/g), psychrotrophic (−1.0 log cfu/g) and yeast and molds (−1.8 log cfu/g), compared with control samples. CH also reduced total coliform bacteria (−2.0 log cfu/g) compared with inoculated plants. Furthermore, CH exerted a bacteriostatic effect on E. coli. TT was only able to reduce total coliforms compared with inoculated plants. Preharvest application of CH exerted an inhibitory effect on lettuce native microflora and a bacteriostatic effect on exogenous E. coli during postharvest storage. Therefore, CH appears to be a promising method for enhancing the safety of lettuce, exposed to an inadequate postharvest handling. Practical Applications Consumers demand products with less use of chemicals; therefore, there has been an increasing research for natural antimicrobials. Chitosan (CH) and essential oils, such as tea tree (TT), have become promising alternatives to chemical decontamination because of its natural character, antimicrobial activity and elicitation of defense responses in plant tissue. The objectives of this work were to examine the residual effectiveness of CH and TT solutions, applied during preharvest, to control E. coli O157:H7 inoculated in lettuce heads, simulating an inadequate manipulation of the vegetable at postharvest. CH solution was a good alternative for controlling not only the native microflora in lettuce during storage, but also reducing the survival of E. coli inoculated in the vegetable at harvest. Preharvest application of CH in the late stages of lettuce head development could easily be introduce in the good agricultural practice routine, without consequences on the environment because CH is a generally recognized as safe substance.