Annual consumption of muskmelon (Cucumis melo reticulatus) is 4.1 kg/capita in the United States (USDA-ERS, 2012). Muskmelon is a good dietary source of provitamin A, vitamin C, and folate (Hodges and Lester, 2011), but its consumption in the United States has been linked to a number of severe outbreaks of food poisoning due to contamination with Escherichia coli O157:H7, Listeria monocytogenes, Salmonella enterica, and other human pathogens (Bowen et al., 2006; Walsh et al., 2014). Pathogens may be trapped in the exterior netting (reticulation) of the muskmelon rind (Ukuku and Fett, 2006) and transferred to the edible tissues when melons are cut for consumption (Ukuku and Sapers, 2001). Addition of disinfectants to wash water is routine during postharvest cleaning to prevent cross-contamination, but few of these are effective for direct microbial control because they fail to achieve the desired reduction of 2–3 log colony forming units per gram (cfu/g) (Hammer and Carson, 2010; F. Critzer, personal communication). Washing with water or chlorinated water [(CL) 200 µL·L−1 free chlorine] (Parnell et al., 2005), hydrogen peroxide (Fan et al., 2009), acidified calcium sulfate, acidified sodium chlorite, and peroxyacetic acid (Ukuku et al., 2001) all failed to effectively reduce microbial loads on produce surface. Chlorine dioxide gas (5 µL·L−1) has achieved 3 log cfu/g reduction of E. coli O157:H7, L. monocytogenes, and S. enterica Poona (Mahmoud et al., 2008), but the requirement for on-site permitting, handling, and usage of this gas limits its application in small-scale operations (Gómez-López et al., 2009).
Many EOs, such as oregano, rosemary, cinnamon, clove, and thyme, have strong antimicrobial properties (Burt, 2004). Studies with lettuce (Ponce et al., 2011), cherry tomatoes (Yun et al., 2013), grape (Tripathi et al., 2008), and banana and papaya (Maqbool et al., 2011) have been conducted to determine the efficacy of EOs for postharvest microbe control and indicated promise for the use of EOs as antimicrobial agents on fresh produce.
EOs can influence microbial concentrations in postharvest muskmelons and on the quality of the fruits. A >5 log cfu/g reduction of L. monocytogenes was achieved when fruits were washed with 2% thymol at 65 °C for 5 min (Upadhyay et al., 2014), but the quality of fruit after this treatment was not reported. Zhang et al. (2015) showed that applying an alginate coating with 2% cinnamon bark oil to whole fruits achieved a >4 log cfu/g reduction of S. enterica and E. coli O157:H7 and a >5 log cfu/g reduction of L. monocytogenes. The ripening rate was slower, as indicated by firmer fruit and lower color indices (a and b) in whole muskmelon 15 d after treating with a coating containing 2% cinnamon bark oil compared with the use of a coating that did not contain the oil. In contrast, Raybaudi-Massilia et al. (2008) reported that flesh firmness decreased when cut melons were treated with an edible coating containing 0.7% lemongrass oil in comparison with melons without EOs. No phytotoxicity to muskmelons in response to these EOs was reported.
One challenge in using EOs as postharvest washing disinfectants is the incorporation of the hydrophobic oils into aqueous solutions. A whey protein–based emulsion has been used to successfully disperse 0.5% clove bud oil (CBO) into water. This emulsion has been shown to reduce contaminants in wash water by 2–3 log cfu/g and prevent cross-contamination of S. enterica Enteritidis, E. coli O157:H7, and L. monocytogenes (Luo et al., 2014), offering a potential for use in postharvest wash water. The objectives of the following experiments were to determine postharvest effects of EO emulsions on muskmelon external and internal attributes to determine their suitability for larger packhouse wash system.
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