Charged Water Droplets Reduce Foodborne Microbes

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).

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.