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Thursday, December 20, 2012

Dishwashing doesn’t kill norovirus

The leading cause of food borne illness is not Escherichia coli or Salmonella. No, that honor goes to the norovirus, a group of viruses named for the Norwalk virus. Transmission of noroviruses can be via direct contact or contaminated food or water. In general though, if you got sick with a norovirus, you probably ate someone’s poo.

Most restaurants do their best to make sure their food is safe. They use techniques known to kill bacteria such as E. coli. But how well do these food safety practices work against a virus like the norovirus? To test this, researchers led by Lizanel Feliciano from the Ohio State University allowed cream cheese contaminated with either E. coli or norovirus stock to harden onto ceramic plates and stainless steel forks. Contaminated milk was left to dry in drinking glasses. The tableware was then washed with detergent and sanitized with either chlorine-bleach or quaternary ammonium compounds. Titers of bacteria or virus were taken both before and after air-drying and washing.

Not surprisingly, the bacteria and viruses did fine on the air-dried surfaces prior to cleaning. If a surface has been contaminated, the bugs on it can survive for an extended period of time, up to a month in the case of norovirus. Bacteria are more sensitive but can still last quite some time.

Washing the tableware dropped the bacterial load significantly. Unfortunately, that wasn’t the case for norovirus. There was a reduction in the viral titer after washing, but not nearly enough to prevent disease transmission. This was true for both mechanical and manual dishwashing methods, though mechanical washing was slightly better at killing bacteria. Interestingly, sanitization did not improve the outcomes for either bacteria or virus. The greatest drop in microbe levels was achieved solely by washing, subsequent sanitization was no better than spraying with tap water.

I should point out that it only takes a handful of norovirus particles to cause an infection. In other words, you have to eliminate all trace of the virus from tableware to ensure food safety. Standard washing methods simply can’t achieve this goal, though vigilance on the part of restaurant management to ensure that sick people stay out of the kitchen may do so. Bon appétit.

Lizanel Feliciano, Jianrong Li, Jaesung Lee, & Melvin A. Pascall (2012). Efficacies of Sodium Hypochlorite and Quaternary Ammonium Sanitizers for Reduction of Norovirus and Selected Bacteria during Ware-Washing Operations PloS ONE : doi:10.1371/journal.pone.0050273.

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