Health experts have warned against eating pre-washed spinach.
They say commercial washing techniques fails to remove 90% of the bacteria.
Small peaks and valleys in baby spinach leaves could be a key reason why there have been numerous bacterial outbreaks involving leafy green vegetables, they say.
But these methods, which can include water and bleach rinses or irradiation, are not completely effective, says Nichola Kinsinger of the University of California.
She says scientists have estimated that 99 percent of food-borne illnesses from leafy greens can be traced back to disinfection issues.
'In a sense the leaf is protecting the bacteria and allowing it to spread,' said Nichola Kinsinger, a post-doctoral researcher working with Sharon Walker, a professor of chemical and environmental engineering.
'It was surprising to discover how the leaf surface formed micro-environments that reduce the bleach concentration and in this case the very disinfection processes intended to clean, remove, and prevent contamination was found to be the potential pathway to amplifying foodborne outbreaks.'
As a result, as the leaves move through the processing facility after being rinsed the bacteria may continue to live, grow, spread, and contaminate other leaves and surfaces within the facility.
The researchers in the Bourns College of Engineering found that because of the varied topography of the spinach leaf nearly 15 percent of the leaf surface may reach concentrations as low as 1000 times that of the bleach disinfectant being used to rinse it.
'Despite current disinfection rinsing, bacteria are surviving on the leaf and causing cross contamination, resulting in the numerous outbreaks we hear about in the media,' Kinsinger said.
'If these bacteria are not all killed in the disinfection process, they can continue to live, grow, spread and contaminate other surfaces within the facility and other leaves.'
Using a parallel-plate flow chamber system that Walker developed, the researchers tested the real-time attachment and detachment of bacteria to the outer layer of spinach leaves.
At low bleach concentrations, the bacteria fell off the leaves, but remained alive. At the higher concentrations used commercially, however, all of the bacteria were killed.
'This result was perplexing,' Walker says.
'Our experiments were telling us that commercial bleach rinses should be much more effective than they are. But then we studied the leaf itself in more detail.'
Currently, the industry standard is to add 50 to 200 parts per million of bleach to the water used to rinse leafy green vegetables.
But that is just a recommendation, not a requirement or regulation, Kinsinger said.
For the research, Kinsinger and Walker designed a parallel plate flow chamber system to evaluate in real time the attachment and detachment of pathogens to the spinach in realistic water chemistries and flow conditions.
Their work focused exclusively on baby spinach, however the issue of reduced bleach concentration across the leaf surface and other surfaces within the processing facility translates beyond the specific scenarios tested and demonstrates the limitation of bleach disinfection causing significant concern over public health.
Future research will focus on a broader range of foods, surfaces in processing facilities and pathogen types, Kinsinger said.
Despite their findings, Kinsinger notes that the United States has one of safest food supply system. Still, she says, 'I recommend rinsing those leaves.'
Source: http://ucrtoday.ucr.edu/30972