Ascorbic acid enhances destruction of Escherichia coli O157 : H7 during home-type drying of apple slices

Destruction of Escherichia coli O157:H7 was evaluated on inoculated apple s lices dehydrated at two temperatures with and without application of predry ing treatments. Half-ring slices (0.6 cm. thick) of peeled and cored Gala a pples were inoculated by immersion for 30 min in a four-strain composite in oculum of E. coli O157:H7. The inoculated slices (8.7 to 9.4 log CFU/ g) ei ther 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 degree sC immediately prior to drying at 57.2 or 62.8 degreesC for up to 6 h. Samp les were plated on tryptic soy (TSA) and sorbitol MacConkey (SMAC) agar med ia for direct enumeration of surviving bacterial populations. Steam blanchi ng changed initial inoculation levels by +0.3 to -0.7 log CFU/g, while imme rsion in the ascorbic acid solution reduced the inoculation levels by 1.4 t o 1.6 log CFU/g. Dehydration of control samples for 6 h reduced mean bacter ial populations by 2.9 log CFU/g (TSA or SMAC) at 57.2 degreesC and by 3.3 (SMAC) and 3.5 (TSA) log CFU/g at 62.8 degreesC. Mean decreases from initia l inoculum levels for steam-blanched slices after 6 h of drying were 2.1 (S MAC) and 2.0 (TSA) log CFU/g at 57.2 degreesC, and 3.6 (TSA or SMAC) log CF U/g at 62.8 degreesC. In contrast, initial bacterial populations on ascorbi c acid-pretreated apple slices declined by 5.0 (SMAC) and 5.1 (TSA) log CFU /g after 3 h of dehydration at 57.2 degreesC, and by 7.3 (SMAC) and 6.9 (TS A) log CFU/g after 3 h at 62.8 degreesC. 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 im mersing apple slices in a 3.4% ascorbic acid solution for 15 min prior to d rying indicate that a predrying treatment enhances the destruction of E. co li O157:H7 on home-dried apple products.