Purpose: Laboratory inoculation studies with pathogenic and generic Escherichia coli on whole, fresh Gala apples examined antimicrobial activity of lactic acid (LA) and peroxyacetic acid (PAA).
Methods: Cocktails of generic Escherichia coli (E. coli) (ATCC 25922, 11755, 23716) or E. coli O157:H7 (43890, 43895, SEA13B88) were surface inoculated by massaging apples for 10 minutes. For each replication (n = 3), 220 fresh gala apples (660 total) were used. Lactic acid (1% and 2%) and water treatments were examined at application times of 5, 15 and 30 seconds to mimic commercial spray bar applications. PAA (60 and 80 ppm) and water treatments were examined at application times of 2, 3.5 and 5 minutes to mimic commercial flume applications. Samples were enumerated on violet red bile agar for generic E. coli and Cefixime-Tellurite sorbitol-MacConkey agar for E. coli O157:H7.
Results: Bacterial levels on apples treated with water and LA (1% and 2%) were significantly (P < 0.05) lower than the inoculated control (1 log CFU/ml reduction. Application time and the concentration of LA did not influence microbial reductions, and levels of pathogenic and generic E. coli were similar between apples treated with water and LA. For 60 ppm PAA at 3.5 and 5 minutes and for all application times at 80 ppm significant bacterial reductions (P < 0.05) were achieved compared to the water treatments and the inoculated control treatment (average 1.5 log CFU/ml reduction).
Significance: PAA treatments of 80 ppm for 2-5 minutes and 60 ppm for 3.5-5 minutes reduced microbial levels sufficiently for consideration as a dump tank or flume application for fresh apple packing.