They looked at attachment and detachment of E. coli from whole baby spinach leaves and isolated outer layers of the spinach using a parallel plate flow chamber.
Companies could add the photocatalyst to the rinse water or to coat equipment surfaces that come into contact with the leaves as they are processed. When TiO2 absorbs light, it produces a strong oxidant that kills bacteria.
The work was presented at the 250th National Meeting and Exposition of the American Chemical Society (ACS).
Methods of washing greens
Greens are washed by commercial processes but these methods, which can include water and bleach rinses or irradiation, are not completely effective, said Nichola Kinsinger, postdoc in the lab of Sharon Walker, at the University of California, Riverside.
Kinsinger said scientists have estimated 99% of foodborne illnesses from leafy greens can be traced back to disinfection issues.
“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," she said.
“Pathogens can come from irrigation waters or from water used during processing, and they can adhere to spinach leaves. 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 the parallel-plate flow chamber system that Walker developed, researchers tested the real-time attachment and detachment of bacteria to the outer layer of spinach leaves.
Uneven spinach leaves
At low bleach concentrations, the bacteria fell off the leaves, but remained alive. At the higher concentrations used commercially bacteria were killed.
The team modeled how the bleach would move across the surface of a spinach leaf, taking its bumps and grooves into account.
This revealed that the concentrations of bleach on leaves may not be consistent.
“We found that because of the topology of the spinach leaf, nearly 15% of the surface may ‘see’ a bleach concentration that is 1,000-times less than that of the rinse solution,” said Kinsinger.
In some cases, that translated to a 90% bacterial survival in tests and a high risk for cross contamination.
Cross-contamination from 0.1% of produce accounts for 99% of illnesses due to ineffective disinfection rinses used post-harvest.
The scientists plan to conduct more studies on the photocatalyst and will look at a broader range of foods, engineered surfaces and pathogens.
Work was funded from the USDA National Institute for Food and Agriculture (NIFA) under the Agriculture and Food Research Initiative.