Microbial Cross-contamination of Tomatoes during Washing with a Peroxyacetic Acid-based Sanitizer in a Commercial Packinghouse

Introduction: Post-harvest packing of tomatoes typically involves the use of a sanitizer in dump tank water during washing. However, sanitizer efficacy is known to decrease with increasing organic load.

Purpose: The goal of this study was to 1) assess the efficacy of a peroxyacetic acid-based sanitizer (Tsunami 100, Ecolab, St. Paul, MN) at 13, 52 and 72 ppm for reducing the microbial load on tomatoes and in dump tank water during commercial packing and 2) compare the efficacy of Tsunami 100 at 50 ppm in a pilot-scale packing line.

Methods: During three visits to one Michigan tomato packer, a series of tomato (~900 g), water (50 ml), equipment surface (100 cm²) and brush samples (one bunch of bristles) were collected during 4 h of processing. All samples were appropriately neutralized, hand-rubbed/stomached, diluted and surface-plated on Standard Method Agar and acidified Potato Dextrose Agar with or without membrane filtration to enumerate mesophilic aerobic bacteria (MAB) and yeast/mold (YM), respectively. Water samples from the dump tank were also assessed for sanitizer concentration, oxidation/reduction potential, pH, temperature, chemical oxygen demand, and total solids. Additional unwashed tomatoes obtained from the same packer were submerged for 2 min in 50 ppm Tsunami 100 and then brush-washed using a pilot plant-scale roller conveyor. Tomato, water and brush samples were similarly collected and quantitatively analyzed for MAB and YM using standard plating methods.         

Results: Initially and after 3 h of commercial processing, MAB populations decreased 0.29 and 0.17, 1.54 and -0.21, and 1.13 and 0.52 log on tomatoes using 13, 52 and 72 ppm Tsunami 100, respectively. Similarly, YM populations decreased 1.24 and 0.66, 1.71 and -0.11, and 1.65 and -0.5 log on tomatoes using 13, 52 and 72 ppm Tsunami 100, respectively. Microbial counts in the dump tank water tended to increase with organic load during processing. Both sets of brush rollers sampled before and after waxing were heavily contaminated. After pilot-scale processing, MAB and YM populations on tomatoes decreased 1.21 and 0.89 log CFU/g, respectively. The brush rollers were cross-contaminated by tomatoes with MAB and YM populations of 2.39 and 2.17 log CFU/bunch, respectively.

Significance: Based on inconsistencies in sanitizer concentration observed commercially, build-up of organic load in the dump tank water and the heavily contaminated brush rollers, more effective microbial intervention strategies are needed to minimize cross-contamination during tomato packing.