A spray washing process with slightly acidic electrolyzed water and UV-ozonated water for inactivating Escherichia coli O157:H7 on romaine and iceberg lettuces

Summary

Spray washing is a common sanitizing method for the fresh produce industry. The purpose of this research was to investigate the antimicrobial effect of spraying slightly acidic electrolyzed water (SAEW) and a combination of ozonated water with ultraviolet light (UV) in reducing Escherichia coli O157:H7 on romaine and iceberg lettuces. Both romaine and iceberg lettuces were spot inoculated with 100 ?L of a 3 strain mixture of E. coli O157:H7 to achieve an inoculum of 6 log CFU/g on lettuce. A strong antimicrobial effect was observed for the UV-ozonated water combination, which reduced the population of E. coli by 5 log CFU/g on both lettuces. SAEW achieved about 5 log CFU/g reductions in the bacterial counts on romaine lettuce. However, less than 2.5 log CFU/g of E. coli O157:H7 was reduced on iceberg lettuce. The difference may be due to bacteria aggregation near and within stomata for iceberg lettuce but not for romaine lettuce. The UV light treatment may stimulate the opening of the stomata for the UV-ozonated water treatment and hence achieve better bacterial inactivation than the SAEW treatment for iceberg lettuce.

Situation

Lettuce is a popular and important leafy vegetable in the human diet because of its delicious and nutritious characteristics. According to a US Department of Agriculture (USDA) report, about two million tons of lettuces were produced in the United States in 2014, and the value of production was over one billion dollars. Fresh lettuce can be contaminated with foodborne pathogens at farm by manure and irrigation water. Lettuce, as a minimally processed and ready-to-eat (RTE) food product, can act as a vehicle for transmitting foodborne disease. The Centers for Disease Control and Prevention (CDC) reported a multistate Escherichia coli O157:H7 outbreak in 2011 which was directly linked to romaine lettuce. In addition, pathogenic microorganisms such as E. coli O157:H7, Listeria monocytogenes, Salmonella and Cyclospora may attach to fresh vegetables during farming and post-harvest storages and hence an effective washing treatment during post-harvest handling is necessary to enhance the safety of fresh produce.

Response

SAEW at pH 6.0 were freshly generated by electrolyzing a dilute NaCl solution (ca. 0.03%) using an EAU EO water generator. Appropriate dilutions were made to achieve 50 mg/L free chlorine concentration in SAEW. About 100 L SAEW was prepared and used for spray experiments using a custom built spray cabinet. The distance between the nozzle and lettuce samples was set at 20 cm. SAEW was sprayed at 1.5 L per min. A UV-chilled ozonated water washer (UV-ozone washer) was also used in this study. The refrigeration system, ozone gas generator, and 2 UV-C (254 nm) lamps were three major components of this washer. The concentration of O3 in the ozonated water (0.5 mg/L) was determined using the Indigo method. Three nalidixic acid-resistant strains Escherchia coli O157:H7 were used to inoculate romaine and iceberg lettuces. Half of the lettuce leaf samples was immediately treated with SAEW, while the remaining half of the samples was covered with plastic wrap and held at 4 ?C for additional 24 h before SAEW treatment. The efficacy of different washing treatments (e.g., UV, ozonated water, SAEW, and a combination of UV and ozonated water) was also studied.

Impact

The combination of UV and ozonated water treatment had significant higher reduction than either UV or ozonated water treatment alone. Although no significant difference was observed in the surviving population of E. coli O157:H7 on romaine lettuce among three treatments at 5 min, the combination of UV and ozonated water achieved an additional 1.8 to 2.5 log CFU/g reductions at 10 min treatment and 2.1 to 2.7 log CFU/g additional reductions at 15 min of treatment when compared to either UV or ozonated water treatment alone for the same treatment time. The synergistic effect of UV and ozonated water combination treatment may be due to ozonated water plus ozone gas released from ozonated water can get to the folded leaves where UV-C light was not able to reach. Therefore, the combination treatment of UV and ozonated water could deliver a close to 5 log reductions of E. coli O157:H7 on romaine lettuce compared to 2 to 3 log reductions using UV or ozonated water treatment alone. For romaine lettuce, the antimicrobial effect of UV-ozonated water combination and SAEW were able to deliver close to 5 log reductions on E. coli O157:H7 at 10 and 15 min treatments. However, for iceberg lettuce, a significant difference in microbial inactivation was observed between UV-ozonated water combination treatment and SAEW. UV-ozonated water combination could achieve similar antimicrobial effect on iceberg lettuce as compared to the romaine lettuce. Whereas SAEW can only achieve up to 2.5 log reductions on iceberg lettuce regardless of treatment time. This difference may be due to the different behavior of bacteria on romaine lettuce vs. iceberg lettuce. This is because bacteria tend to aggregate near and into stomata of iceberg lettuce leaves, while no such attraction of bacteria to stomata was found on romaine lettuce leaves. In the current study, lower antimicrobial efficacy on iceberg lettuce than romaine lettuce for SAEW may be due to SAEW cannot reach/penetrate into the stomata on iceberg lettuce where the bacterium was internalized. However, for UV-ozonated water combination, UV can stimulate the opening of the stomata and hence allowed ozonated water and/or ozone gas to inactivate bacteria that may be internalized in the stomata. Hence, a 5 log reductions of E. coli O157:H7 using UV-ozonated water combination was observed for both iceberg lettuce and romaine lettuce.

State Issue

Food Safety

Details

• Year: 2016
• Geographic Scope: National
• County: Spalding
• Program Areas:
  • Agriculture & Natural Resources