Ascorbic Acid Enhances Destruction of Escherichia coli O157:H7 during Home-Type Drying of Apple Slices

Abstract

Destruction of Escherichia coli O157:H7 was evaluated on inoculated apple slices dehydrated at two temperatures with and without application of predrying treatments. Half-ring slices (0.6 cm thick) of peeled and cored Gala apples were inoculated by immersion for 30 min in a four-strain composite inoculum of E. coli O157:H7. The inoculated slices (8.7 to 9.4 log CFU/g) either received no predrying treatment (control), were soaked for 15 min in a 3.4% ascorbic acid solution, or were steam blanched for 3 min at 88°C immediately prior to drying at 57.2 or 62.8°C for up to 6 h. Samples were plated on tryptic soy (TSA) and sorbitol MacConkey (SMAC) agar media for direct enumeration of surviving bacterial populations. Steam blanching changed initial inoculation levels by +0.3 to −0.7 log CFU/g, while immersion in the ascorbic acid solution reduced the inoculation levels by 1.4 to 1.6 log CFU/g. Dehydration of control samples for 6 h reduced mean bacterial populations by 2.9 log CFU/g (TSA or SMAC) at 57.2°C and by 3.3 (SMAC) and 3.5 (TSA) log CFU/g at 62.8°C. Mean decreases from initial inoculum levels for steam-blanched slices after 6 h of drying were 2.1 (SMAC) and 2.0 (TSA) log CFU/g at 57.2°C, and 3.6 (TSA or SMAC) log CFU/g at 62.8°C. In contrast, initial bacterial populations on ascorbic acid–pretreated apple slices declined by 5.0 (SMAC) and 5.1 (TSA) log CFU/g after 3 h of dehydration at 57.2°C, and by 7.3 (SMAC) and 6.9 (TSA) log CFU/g after 3 h at 62.8°C. Reductions on slices treated with ascorbic acid were in the range of 8.0 to 8.3 log CFU/g after 6 h of drying, irrespective of drying temperature or agar medium used. The results of immersing apple slices in a 3.4% ascorbic acid solution for 15 min prior to drying indicate that a predrying treatment enhances the destruction of E. coli O157:H7 on home-dried apple products.