Inactivation of listeria monocytogenes on and within apples destined for caramel apple production by using sequential forced air ozone gas followed by a continuous advanced oxidative process treatment

Abstract

This study evaluated the efficacy of using sequential forced air ozone followed by an advanced oxidative process (AOP) treatment to inactivate Listeria monocytogenes on and within Empire apples. The forced air ozone treatment consisted of a reactor that introduced ozone (6 g/h) into an airstream that flowed through an apple bed (ca. 30 cm in depth). Before treatment, the apples were conditioned at 48C to ensure that condensate had formed before the apples were transferred to the reactor. The condensate ensured sufficient relative humidity to enhance the antimicrobial action of ozone. Air was passed through the apple bed at 9.3 m/s, and the ozone was introduced after 10 min. The ozone concentration measured after exiting the apple bed reached a steady state of 23 ppm. A 20-min ozone treatment supported a 2.12- to 3.07-log CFU reduction of L. monocytogenes, with no significant effect of apple position within the bed. The AOP-based method was a continuous process whereby hydrogen peroxide was introduced as a vapor into a reactor illuminated by UV-C and ozone-emitting lamps that collectively generated hydroxyl radicals. Operating the AOP reactor with UV-C light (54-mJ cm2 dose), 6% (v/v) hydrogen peroxide, 2 g/h ozone, and a chamber temperature of 488C resulted in a 3-log CFU reduction of L. monocytogenes on the surface of the apples and internally within the scar tissue. Applying a caramel coating, from a molten solution (at 808C), resulted in a 0.5-log CFU reduction of L. monocytogenes on the apple surface. In apples treated with the sequential process, L. monocytogenes could only be recovered sporadically by enrichment and did not undergo outgrowth when the caramel apples were stored at 228C for 19 days. However, growth of L. monocytogenes within the core, but not the surface, was observed from caramel apples prepared from nontreated control fruit.