Purpose: Surrogate and pathogenic bacteria were evaluated for survival at thermal exposure shown to be non-injurious to whole cantaloupe.
Methods: Seven cantaloupes were washed in 4 l of tap water and 200 ml was heated to 95 or 55°C alone or with peroxyacetic acid (PAA) (20, 50, and 80 ppm) or lactic acid (LA) (0.5, 1, and 2%). After 0.5 to 30 min, samples were quantified for total heterotrophs and total coliforms using Plate Count Agar and Quanti-Tray, respectively. 9 strains of Salmonella, 2 Listeria spp., 6 strains of Escherichia coli, or 1 strain of Pseudomonas fluorescens were inoculated onto detached cantaloupe periderm and, after air-drying, contained ~4-8 log CFU/g. 0.1-g were added to 9 ml of 70°C Trypticase Soy Broth (TSB) and held for 0.25 to 30 min before rapid cooling. Samples were quantified or enriched to determine presence.
Results: 95°C exposure for 2 minutes (no oxidizer) resulted in 3.6 and > 6.0 log reductions of indigenous heterotrophic bacteria and total coliforms. 95°C and 55°C water with 80 or 50 ppm PAA were significantly more effective (P < 0.05) at reducing populations of total heterotrophs after 2 min (avg. 3.60 log CFU/ml) than other treatments. After 30 min, 95°C and 55°C with 80 ppm PAA were more effective (P < 0.05) at reducing total heterotrophs than 55°C + 2% LA; all four treatments achieved a 6-log reduction of total coliforms. Four strains survived 30 min at 70°C – L. monocytogenes, L. innocua, Salmonella Cubana, and Salmonella Senftenberg.
Significance: Heated oxidizers during cantaloupe washing enhance reduction of indigenous bacteria used as an index for process verification. Additionally, these results delineate the importance of proper challenge strain selection when assessing thermal inactivation treatments.