• CORRECTION: The subhead of this story was changed on March 17, 2015, to correct the process used to create the water droplets.
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Web Date: March 13, 2015

Charged Water Droplets Reduce Foodborne Microbes

Nanotechnology: Highly charged nanosized droplets formed by electrosprayed water can reduce microbial contamination on food surfaces
Department: Science & Technology | Collection: Life Sciences
News Channels: Environmental SCENE, Biological SCENE
Keywords: food safety, food-borne pathogens, disinfection, electrospray, nanoparticles, reactive oxygen species
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Safer Salad
Charged nanosized droplets made of water and reactive oxygen species reduced bacterial concentrations on the surface of grape tomatoes.
Credit: Shutterstock
Photo of baskets full of grape tomatoes.
 
Safer Salad
Charged nanosized droplets made of water and reactive oxygen species reduced bacterial concentrations on the surface of grape tomatoes.
Credit: Shutterstock

Using only electricity and water, researchers have created highly charged nanosized droplets filled with free radicals. When exposed to food surfaces, these so-called engineered water nanostructures can inactivate food-borne microbes and could provide a new “green” method to disinfect food (Environ. Sci. Technol. 2015, DOI: 10.1021/es505868a).

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Nanobombs
Electrospraying water forms charged droplets that consist of reactive oxygen species surrounded by an electron-rich water layer. These nanosized droplets can act as a disinfectant.
Credit: Environ. Sci. Technol.
Schematic of engineered water droplet.
 
Nanobombs
Electrospraying water forms charged droplets that consist of reactive oxygen species surrounded by an electron-rich water layer. These nanosized droplets can act as a disinfectant.
Credit: Environ. Sci. Technol.

Fruits and vegetables run the risk of contamination from bacteria acquired on fields or during packaging and processing, so food processors currently disinfect produce using a mild bleach solution, peroxides, or other chlorine-based compounds. Charged water droplets could offer a sustainable alternative that uses fewer chemicals and could be applied at many different points during food processing, the researchers say.

Philip Demokritou of Harvard University and his colleagues create the droplets by passing an aerosolized stream of water through a strong electric field, a method known as electrospray. Some of the water molecules split into hydroxyl and superoxide radicals, reactive oxygen species that are usually unstable. But in the electrospray, a thin, electron-rich layer of water encapsulates the radicals to form highly stable, charged particles approximately 25 nm in diameter (Environ. Sci.: Nano 2014, DOI: 10.1039/c3en00007a). The strong electrical charge increases these particles’ surface tension and prevents evaporation, Demokritou says.

Previously, the researchers had used these nanosized droplets to kill microbes in the air and on surfaces, so they decided to test whether the particles could effectively disinfect food surfaces and stainless steel, commonly used in food processing equipment.

The researchers sprayed the surfaces of prewashed organic grape tomatoes and stainless steel chips with three lab strains of bacteria related to common food-borne pathogens: Escherichia coli, Salmonella enterica, and Listeria innocua. The researchers then exposed tomatoes to the water droplets at concentrations of 24,000 to 50,000 particles per cm3 in an enclosed chamber, for time periods ranging from 30 to 90 minutes. Steel surfaces were exposed for 15 to 45 minutes.

Depending on the bacterial species and type of surface, the exposure decreased microbial concentrations by 80 to 98%. Sensory tests showed no difference in the taste or color of the tomatoes.

Typical food safety regulations require a 99.999% reduction in bacterial loads, which is usually achieved within a few minutes, according to microbiologist Michael P. Doyle of the University of Georgia. The method would have to work much faster to be commercially applicable, he says.

Chemical engineer Michael A. Matthews of the University of South Carolina says the most promising aspect of the technique is that it can be used at normal temperature and pressure. However, he adds that the method may only be applicable to a few kinds of surfaces. “Tomatoes have a smooth surface that’s easily contacted by droplets, but for a vegetable like a cauliflower, it’s probably more difficult to uniformly contact the surface,” he says.

Demokritou says their study serves as a proof of concept that the nanosized water droplets can disinfect foods such as tomatoes. In ongoing studies, his team is increasing the concentration of droplets up to 400,000 particles per cm3 and their potency by packing more reactive oxygen species into each tiny sphere. Both tactics reduce bacterial loads significantly, and do so more quickly, Demokritou says.

 
Chemical & Engineering News
ISSN 0009-2347
Copyright © American Chemical Society
Comments
Bill Floyd (Thu Mar 19 12:23:23 EDT 2015)
Any thoughts as to how EPA and/or FDA would want to regulate this technology as it claims biological activity and is applied to food?
Robert Buntrock (Thu Mar 19 12:50:41 EDT 2015)
Since this spray is oxidative I wonder how the shelf life of the sanitized vegetables compared to non-sprayed due to enhanced oxidation of the produce.
Sher (Thu Mar 19 14:11:37 EDT 2015)
Is ozone doing the disinfecting?
Zubair Syed (Thu Mar 19 15:15:26 EDT 2015)
What will happens if the vegetables were dipped inside the water containing charged nanosized particles instead of spraying on them?
John Segreto (Thu Mar 19 17:10:13 EDT 2015)
Would this work well at killing bacteria if it were just sprayed onto food as opposed to keeping the food in an enclosed chamber.If so would this not be likely to increase shelf life of fresh meats such as fish. Which could make this easily applicable to commercial markets.
Jenna Wright (Sat Mar 21 17:02:54 EDT 2015)
Could this be developed into a treatment for human consumption as an alternative antibiotic against pathogenic bugs within the GI Tract?
Bruce Locke (Mon Mar 23 09:31:26 EDT 2015)
The energy cost seems quite high. If I calculate correctly for 5 Watts for 0.0726 log/min removal of E. coli with 10 microL per surface gives 4x10^5 J/ml/log reduction.

5 Wattsx60 / 0.0726/ 0.01 = 4x10^5 J/ml/log.

There are many plasma based process that are orders of magnitute much more efficient than this (at least for liquid suspensions) and also rely on water with a flowing gas. I may be missing something in their experimental set up - are they treating only one surface containing equivalent of 10 microL sample of bacterial solution?
steven lee (Fri Mar 27 12:35:15 EDT 2015)
What a novel way to disinfect. Now, to carry the discussion forward with length of exposure, could this process be used to sterilize?
Suchismita acharya  (Fri Mar 27 15:06:07 EDT 2015)
I wonder if ROS spray would destroy the natural proteins and vitamins present in the food or any study have been done to support this?
Jim Cwynar (Fri Mar 27 17:02:45 EDT 2015)
Whatever happened to using UV light for disinfection?
Dr. Joop Koster (Sat Mar 28 09:54:56 EDT 2015)
What about using a wetting agent, so the small droplets will better spread over the surfaces of the to be treated food articles ? Tiny amount of siliocne based wetting agents are needed ( 0.1 % ).
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