Center for Low-Moisture Food Safety

Apple fruit is covered with an extracellular cuticle layer and an epicuticular wax layer responsible for the glossiness of an apple (Veraverbeke et al., 2001). The extensive commercial packinghouse processing such as dump tanks and spray sanitizer processing compromise the natural waxy cuticle layer of fresh apple (Kitinoja and Kader, 2015). Thus, wax coating is a common practice for conventional apples packed in tree fruit packinghouses (Saftner, 1999). Besides improving the glossiness of apples, the wax coating also acts as an oxygen and carbon dioxide gas diffusion barrier on the apple surface to slow down apple respiration, reduce moisture loss of apples during storage, and extend the shelf-life of apple (Bai et al., 2003, Baldwin, 1994, Hagenmaier and Shaw, 1992, Saftner, 1999). Shellac and carnauba are natural waxes secreted by Kerria lacca and carnauba palm trees (Copernica cerifera) (Srivastava et al., 2016, Steinle, 1936), respectively, and are generally recognized as safe (FDA, 2020a, FDA, 2020b). PrimaFresh 360, a carnauba-derived wax, and Shield-Brite AP-450, a shellac-based wax, are the most commonly used wax coatings in the Pacific Northwest tree fruit industry (NHC, 2018), which contain morpholine as a solubilization agent (Baldwin et al., 2011). PrimaFresh 606 is designed for the European market, which only allows morpholine-free waxes, as an alternative to PrimaFresh 360.

Listeria monocytogenes have caused multiple outbreaks (Angelo et al., 2017, Marus et al., 2019) and recalls (FDA, 2015, FDA, 2017) due to the consumption of contaminated fresh apples, highlighting the importance of controlling L. monocytogenes contamination on fresh apples. Fresh apples are commonly subjected to postharvest sanitizers such as peroxyacetic acid and chlorine interventions to reduce the potential risk of foodborne pathogens and minimize cross-contamination before wax application. However, spray-bar sanitizer treatments in the packinghouse reduce but do not eliminate L. monocytogenes on fresh apples (Shen et al., 2020). Apples can be further re-contaminated with L. monocytogenes during waxing and other steps after spray-bar antimicrobial interventions. L. monocytogenes-contaminated food contact surfaces, including waxing/polishing brushes and drying brush conveyors, are recognized as important contamination sources linked to the 2014–2015 caramel apple L. monocytogenes outbreak (Angelo et al., 2017). Listeria spp. was most frequently isolated from the food-contact surfaces of wax packing lines in five eastern Washington apple-packing facilities (Ruiz-Llacsahuanga et al., 2021). Apples coated with wax solutions are typically dried simultaneously when passing through a hot air tunnel with a typical drying temperature of 42–45 °C, ranging from 38° to 60°C (per apple industry communication). Different wax drying temperatures were reported to affect the survival of Escherichia coli O157: H7 and Salmonella Muenchen on apple surfaces (Kenney and Beuchat, 2002). However, little is known about the fates of Listeria on wax-coated apples processed using different drying temperatures.

The packed apples after waxing are commonly stored at commercial refrigerated air (RA) conditions of 0–1 °C and 90–95 % relative humidity (RH) for 4–6 weeks before shipping to retail. Our previous study showed twelve weeks of lab storage at 1 °C led to 0.8 log₁₀CFU/apple reduction of L. monocytogenes on unwaxed Fuji apples (Sheng et al., 2017). L. monocytogenes can even proliferate in the microenvironment created between the apple surface and the caramel coating layer (Glass et al., 2015). The immediate application of shellac wax, Shield-Brite AP-40 on Listeria-inoculated apples led to 1.2 log₁₀ CFU/apple reduction of L. monocytogenes on Fuji apples; however, a protective effect of Shield-Brite AP-40 on the survival of L. monocytogenes on wax-coated Fuji apples was observed during the prolonged simulated cold storage at 3 °C (Macarisin et al., 2019). In contrast, the population of L. monocytogenes was not impacted by the shellac (Shield-Brite AP-450) or carnauba (PrimaFresh 360) coated Fuji apples immediately after waxing or during twelve weeks of simulated cold storage operated at 1 °C and 90 % RH (Shen et al., 2023). During twelve weeks of cold storage, a higher reduction of S. Muenchen was found on shellac-coated apples compared to that on unwaxed apples (Kenney and Beuchat, 2002). On the other hand, decay can potentially develop when the packed fruit was stored temporarily in a commercial RA room before sending to the supermarket. Even a very low presence of decay in the packed fruit is associated with substantial market risks. Currently, no studies have evaluated the fate of Listeria on waxed apples during post-packing commercial RA storage, and neither were studies on behaviors of decay organisms on the waxed apples.

This study aimed to assess 1) the fate of L. innocua, a validated surrogate microorganism of L. monocytogenes (Sheng et al., 2020), on Fuji apples coated with different commercial apple waxes during eighteen weeks of commercial RA storage operated at ∼ 0.6 °C and 90 % RH; 2) impacts of wax coating application on the persistence of endogenous yeasts and molds on apples; 3) influences of wax drying temperatures on the survival of L. innocuaand endogenous fungi on waxed apples; 4) quality attributes of apple fruit coated with different wax coatings after storage.