Apple peel morphology and attachment of Listeria innocua through aqueous environment as shown by scanning electron microscopy

The industrial apple packing process begins with submerging the harvested or stored apples into the water dump tank and carrying them by water flumes through various washing steps before drying and packing. Constant introduction of organic matter and water recirculation introduces the risk of apples being cross-contaminated with pathogenic bacteria such as Listeria monocytogenes. This study investigated the relationship between the apple peel microstructure and attachment of bacteria in an aqueous environment. Whole fresh apples of three varieties (viz., Gala, Golden Delicious, and Granny Smith) were inoculated with L. innocua, a surrogate strain of L. monocytogenes, by submerging the apples into the inoculum solution containing ∼106 CFU/mL level. Apples were carefully turned around in the inoculum for 10 min simulating industrial dump tank and fumes conditions. Attachment of L. innocua, to different apple peel sections, was demonstrated using scanning electron microscopy (SEM). Textural characteristics varied considerably among apple varieties. Generally, the apple peel was covered with an amorphous layer of wax, disrupted by microcracks, lenticels, and occasionally overlaid with various types of epicuticular wax platelets. Listeria innocua attached primarily to the apple peel in the stem bowl and calyx sections, embedding in the microcracks, lenticels and on the surface of the trichomes. Bacteria were difficult to locate on the smooth peel surface in the equatorial section of the apples. Results provide valuable information on Listeria attachment on apples which can be used in developing new intervention methods of microbial decontamination.