Postharvest contamination and subsequent spread of Escherichia coli O157:H7 can occur during shredding, conveying, fluming, and dewatering of fresh-cut leafy greens. This study quantified E. coli O157:H7 transfer from leafy greens to equipment surfaces during simulated small-scale commercial processing. Three to five batches (22.7 kg) of baby spinach, iceberg lettuce, and romaine lettuce were dip inoculated with a four-strain cocktail of avirulent, green fluorescent protein-labeled, ampicillinresistant E. coli O157:H7 to contain ∼10(6), 10(4), and 10(2) CFU/g, and then were processed after 1 h of draining at ∼23°C or 24 h of storage at 4°C. Lettuce was shredded using an Urschel TransSlicer at two different blade and belt speeds to obtain normal (5 by 5 cm) and more finely shredded (0.5 by 5 cm) lettuce. Thereafter, the lettuce was step conveyed to a flume tank and was washed and then dried using a shaker table and centrifugal dryer. Product (25-g) and water (40-ml) samples were collected at various points during processing. After processing, product contact surfaces (100 cm(2)) on the shredder (n = 14), conveyer (n = 8), flume tank (n = 11), shaker table (n = 9), and centrifugal dryer (n = 8) were sampled using one-ply composite tissues. Sample homogenates diluted in phosphate or neutralizing buffer were plated, with or without prior 0.45- m m membrane filtration, on Trypticase soy agar containing 0.6% yeast extract supplemented with 100 ppm of ampicillin to quantify green fluorescent protein-labeled E. coli O157:H7 under UV light. During leafy green processing, ∼90% of the E. coli O157:H7 inoculum transferred to the wash water. After processing, E. coli O157:H7 populations were highest on the conveyor and shredder (P<0.05), followed by the centrifugal dryer, flume tank, and shaker table, with ∼29% of the remaining product inoculum lost during centrifugal drying. Overall, less (P<0.05) of the inoculum remained on the product after centrifugally drying iceberg lettuce that was held for 1 h (8.13%) as opposed to 24 h (42.18%) before processing, with shred size not affecting the rate of E. coli O157:H7 transfer.

Escherichia coli O157:H7 contamination of fresh-cut leafy greens has become a public health concern as a result of several large outbreaks. The goal of this study was to generate baseline data for E. coli O157:H7 transfer from product-inoculated equipment surfaces to uninoculated lettuce during pilot-scale processing without a sanitizer. Uninoculated cored heads of iceberg and romaine lettuce (22.7 kg) were processed using a commercial shredder, step conveyor, 3.3-m flume tank with sanitizer-free tap water, shaker table, and centrifugal dryer, followed by 22.7 kg of product that had been dip inoculated to contain ∼10(6), 10(4), or 10(2) CFU/g of a four-strain avirulent, green fluorescent protein-labeled, ampicillin-resistant E. coli O157:H7 cocktail. After draining the flume tank and refilling the holding tank with tap water, 90.8 kg of uninoculated product was similarly processed and collected in ∼5-kg aliquots. After processing, 42 equipment surface samples and 46 iceberg or 36 romaine lettuce samples (25 g each) from the collection baskets were quantitatively examined for E. coli O157:H7 by direct plating or membrane filtration using tryptic soy agar containing 0.6% yeast extract and 100 ppm of ampicillin. Initially, the greatest E. coli O157:H7 transfer was seen from inoculated lettuce to the shredder and conveyor belt, with all equipment surface populations decreasing 90 to 99% after processing 90.8 kg of uncontaminated product. After processing lettuce containing 10(6) or 10(4) E. coli O157:H7 CFU/g followed by uninoculated lettuce, E. coli O157:H7 was quantifiable throughout the entire 90.8 kg of product. At an inoculation level of 10(2) CFU/g, E. coli O157:H7 was consistently detected in the first 21.2 kg of previously uninoculated lettuce at 2 to 3 log CFU/100 g and transferred to 78 kg of product. These baseline E. coli O157:H7 transfer results will help determine the degree of sanitizer efficacy required to better ensure the safety of fresh-cut leafy greens.

Chemical sanitizers are routinely used during commercial flume washing of fresh-cut leafy greens to minimize cross-contamination from the water. This study assessed the efficacy of five commercial sanitizer treatments against Escherichia coli O157:H7 on iceberg lettuce, in wash water, and on equipment during simulated commercial production in a pilot-scale processing line. Iceberg lettuce (5.4 kg) was inoculated to contain 10(6) CFU/g of a four-strain cocktail of nontoxigenic, green fluorescent protein-labeled, ampicillin-resistant E. coli O157:H7 and processed after 1 h of draining at ~22 °C. Lettuce was shredded using a commercial slicer, step-conveyed to a flume tank, washed for 90 s using six different treatments (water alone, 50 ppm of peroxyacetic acid, 50 ppm of mixed peracid, or 50 ppm of available chlorine either alone or acidified to pH 6.5 with citric acid [CA] or T-128), and then dried using a shaker table and centrifugal dryer. Various product (25-g) and water (50-ml) samples collected during processing along with equipment surface samples (100 cm(2)) from the flume tank, shaker table, and centrifugal dryer were homogenized in neutralizing buffer and plated on tryptic soy agar. During and after iceberg lettuce processing, none of the sanitizers were significantly more effective (P ≤ 0.05) than water alone at reducing E. coli O157:H7 populations on lettuce, with reductions ranging from 0.75 to 1.4 log CFU/g. Regardless of the sanitizer treatment used, the centrifugal dryer surfaces yielded E. coli O157:H7 populations of 3.49 to 4.98 log CFU/100 cm(2). Chlorine, chlorine plus CA, and chlorine plus T-128 were generally more effective (P ≤ 0.05) than the other treatments, with reductions of 3.79, 5.47, and 5.37 log CFU/ml after 90 s of processing, respectively. This indicates that chlorine-based sanitizers will likely prevent wash water containing low organic loads from becoming a vehicle for cross-contamination.

Cross-contamination of fresh-cut leafy greens with residual Escherichia coli O157:H7-contaminated product during commercial processing was likely a contributing factor in several recent multistate outbreaks. Consequently, radicchio was used as a visual marker to track the spread of the contaminated product to iceberg lettuce in a pilot-scale processing line that included a commercial shredder, step conveyor, flume tank, shaker table, and centrifugal dryer. Uninoculated iceberg lettuce (45 kg) was processed, followed by 9.1 kg of radicchio (dip inoculated to contain a four-strain, green fluorescent protein-labeled nontoxigenic E. coli O157:H7 cocktail at 10(6) CFU/g) and 907 kg (2,000 lb) of uninoculated iceberg lettuce. After collecting the lettuce and radicchio in about 40 bags (∼22.7 kg per bag) along with water and equipment surface samples, all visible shreds of radicchio were retrieved from the bags of shredded product, the equipment, and the floor. E. coli O157:H7 populations were quantified in the lettuce, water, and equipment samples by direct plating with or without prior membrane filtration on Trypticase soy agar containing 0.6% yeast extract and 100 ppm of ampicillin. Based on triplicate experiments, the weight of radicchio in the shredded lettuce averaged 614.9 g (93.6%), 6.9 g (1.3%), 5.0 g (0.8%), and 2.8 g (0.5%) for bags 1 to 10, 11 to 20, 21 to 30, and 31 to 40, respectively, with mean E. coli O157:H7 populations of 1.7, 1.2, 1.1, and 1.1 log CFU/g in radicchio-free lettuce. After processing, more radicchio remained on the conveyor (9.8 g; P < 0.05), compared with the shredder (8.3 g), flume tank (3.5 g), and shaker table (0.1 g), with similar E. coli O157:H7 populations (P > 0.05) recovered from all equipment surfaces after processing. These findings clearly demonstrate both the potential for the continuous spread of contaminated lettuce to multiple batches of product during processing and the need for improved equipment designs that minimize the buildup of residual product during processing.

Chemical sanitizers are routinely used during commercial flume washing of fresh-cut leafy greens to minimize cross-contamination from the water. This study assessed the efficacy of three chlorine treatments against Escherichia coli O157:H7 on iceberg lettuce, in wash water, and on surfaces of a pilot-scale processing line using flume water containing various organic loads. Iceberg lettuce (5.4 kg) was inoculated to contain 10(6) CFU/g of a 4-strain cocktail of nontoxigenic, green fluorescent protein-labeled, ampicillin-resistant E. coli O157:H7 and held for 24 h at 4°C before processing. Lettuce was shredded using a Urschel TransSlicer, step conveyed to a flume tank, washed for 90 s using water alone or one of three different sanitizing treatments (50 ppm of total chlorine either alone or acidified to pH 6.5 with citric acid or T-128) in water containing organic loads of 0, 2.5, 5, or 10% (wt/vol) blended iceberg lettuce, and then dried using a shaker table and centrifugal dryer. Next, three 5.4-kg batches of uninoculated iceberg lettuce were processed identically. Various product (25 g), water (50 ml), and equipment surface swab (100 cm(2)) samples were homogenized in neutralizing buffer, diluted appropriately, and plated on tryptic soy agar containing 0.6% (wt/vol) yeast extract and 100 ppm of ampicillin without prior 0.45- m m membrane filtration to quantify E. coli O157:H7. Organic load negatively impacted the efficacy of all three chlorine treatments (P < 0.05) at the end of processing, with typical E. coli O157:H7 reductions of >5 and 0.9 to 3.7 log CFU/ml for organic loads of 0 and 10%, respectively. Organic load rarely had a significant impact (P < 0.05) on the efficacy of chlorine, chlorine plus citric acid, or chlorine plus T-128 against E. coli O157:H7 on iceberg lettuce. Reduced sanitizer efficacy generally corresponded to changes in total solids, chemical oxygen demand, turbidity, and maximum filterable volume, indicating that these tests may be effective alternatives to the industry standard of oxygen/reduction potential.