Control of Listeria monocytogenes in Caramel Apples by Use of Sticks Pretreated with Potassium Sorbate

Abstract

A multistate listeriosis outbreak associated with caramel apples from 2014 to 2015 prompted research on the survival of Listeria monocytogenes in fresh apples and caramel apples. Research indicated that stem end-inoculated caramel apples with stick insertion allowed for the survival and growth of L. monocytogenes at both refrigeration and ambient temperatures. This study aimed to assess the effectiveness of chemical preservatives as pretreatments for the wooden stick component to reduce L. monocytogenes loads in stem end-inoculated caramel apples during storage. Wooden sticks were pretreated with 1, 3, or 5% ascorbic acid (vitamin C), Nisaplin (2.5% nisin), potassium sorbate, and sodium benzoate and then inoculated with L. monocytogenes at 7 log CFU per stick. After storage at 25°C, the pathogen was reduced most effectively by the ascorbic acid pretreatments. At all three ascorbic acid concentrations tested, L. monocytogenes levels were reduced below the level of enumeration (2.5 log CFU per apple) at 24 h and were no longer detectable by enrichment after 72 h. Ascorbic acid (5, 10, and 20%) and potassium sorbate (10, 20, 30, and 40%) were further tested as wooden stick pretreatments for pathogen reduction on stem end-inoculated caramel apples stored at 5 and 25°C. The 40% potassium sorbate solution at 25°C was the most effective pretreatment condition in caramel apples and demonstrated a 3.1-log CFU per apple overall decrease in L. monocytogenes population levels after 216 h. Pretreatment of the wooden stick component of a caramel apple with potassium sorbate may be a viable preventive measure to reduce postprocess L. monocytogenes population levels and hence reduce consumer risk associated with caramel apple consumption.