Fresh and fresh-cut produce are often washed with chemical sanitizers such as chlorine to minimize cross contamination and improve microbial safety. However, soft stone fruits, such as tree-ripened peaches, cannot tolerate washes due to their advanced ripeness and tenderness. Therefore, tree-ripened fruits are often packed under dry conditions (Crisosto and Valero, 2008; Crisosto et al., 1995). Although the chances of tree fruits being contaminated with human pathogens is low, there have been reports of foodborne disease outbreaks and recalls associated with the fruit (Bennett et al., 2018; Jackson et al., 2015). Therefore, producers and packers of tree-ripe stone fruits are in need of nonaqueous measures to sanitize fruit.
Ultraviolet-C radiation at short wavelengths (100–280 nm) has germicidal effects, resulting in damage to DNA (Sinha and Häder, 2002). The U.S. Food and Drug Administration (FDA) has approved the use of ultraviolet light for disinfection of liquid (water and juices), and decontamination of food preparation surfaces and food containers (Koutchma et al., 2009; U.S. FDA, 2000). Earlier studies have demonstrated that ultraviolet-C is able to reduce pathogens on the surface of various fruits (Bialka and Demirci, 2007; Yaun et al., 2004). However, there are several challenges for the commercial application of ultraviolet technology (Fan et al., 2017; Yan et al., 2014). First, some fruit may develop discoloration after ultraviolet-C treatment, particularly during post-ultraviolet storage. Second, bacterial pathogens injured by exposure to ultraviolet light may later be repaired by dark and/or by enzymatic mechanisms, leading to potential cell survival and re-growth (Fernández Zenoff et al., 2006). In addition, high doses of ultraviolet-C may potentially damage and weaken fruit tissues, which could increase the growth of surviving pathogens. For these reasons, possible re-growth of human pathogens and impact on fruit quality must be evaluated following ultraviolet treatments. In our previous study (Yun et al., 2013), we found that, even though ultraviolet-C at 442 mJ/cm2 produced limited reductions (1.2–1.9 log CFU/fruit) of E. coli and Salmonella on inoculated apricot (Prunus armeniaca L.) fruit, the differences in bacterial population between treated and control fruits increased during storage. It is unclear whether the rapid decline in inoculated bacterial populations would occur on other stone fruits, such as peaches.
In addition, many types of fruit, such as peaches, have a layer of nonsecreting trichomes (fuzz). Commercially, peaches are often wet-brushed to remove the trichomes (fuzz) for consumer appeal (Taylor and Rushing, 2012). The fuzz, which is single-cell extensions of epidermal cells, may contribute to protective ultraviolet-shielding of bacteria during treatment (Crisosto and Valero, 2008; Kays and Paull, 2004).
The objective of the present study was to study the changes in E. coli O157:H7 populations and fruit quality following ultraviolet-C exposure, as affected by the removal of fuzz during post-ultraviolet-C storage of peaches.
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