Salad Wars

-How a team in Harvard is sterilising food with pure water.-


Typically, we don’t eat raw meat and eggs because we are aware of the infection risks, yet in Europe, around 10% of food poisoning outbreaks are traced to contaminated vegetables. These infections are especially deadly: accounting for almost half of food-poisoning-related deaths. The continued resilience of bacteria on food, on kitchen surfaces, and in the air, alongside increasing resistance to antibiotics is a big problem: irradiation with UV light can be unreliable, whilst treatment with chloride-based compounds disqualifies food from an organic label, and can only be foolproof doses which may cause harm in the long-term, hence the demand for a continuous, environmentally friendly, and reliable sterilisation method. The only way to destroy, or “inactivate” bacteria involves breaking its cell wall; but the plethora of harmful bacteria have walls that are too chemically varied, and despite their small scale, too tough, for a blunt chemical or radiation dose to damage them without also damaging the food or animal tissues.

One thing that sets bacteria apart is their small size. Whilst mammalian tissues are enveloped in skin, or fluid, and plants cells have cellulose cell walls which are seldom thinner than 0.3 micrometres, bacteria are far smaller lifeforms, which have walls in the 10 to 80 nanometre range. A nanoscale structure might selectively target the bacteria. Attempts to disinfect using conventional nanoparticles have backfired, not just because they tend to secrete harmful chemicals as much as conventional disinfection, but they also permanently transform the food surface, unpredictably altering taste and texture. Improvements in the electrospraying technique, where fluids such as water are shredded into ultra-fine aerosols by an electrical field, have brought a new solution to the forefront.

An EWNS, or Engineered Water Nano Structure, is an innovation under development in Harvard University which puts the unusual physical properties of nanoscale water to good use. In this conformation of the electrospray technique, the EWNS is especially small, at just 25 nanometres, and especially strongly charged, with an average of 10 free electrons on its surface. The high charge brings the EWNS a high surface tension, effectively hardening it, preventing vaporisation, and allowing it to be directed by electrical fields. Yet an EWNS is more than a cannon ball: owing to its negative charge, its core is packed with ROSs (reactive oxygen species) such as peroxide and superoxide, which degrade and destroy cell structures and proteins. The EWNS has been shown to maintain itself, and its payload, for a considerable length of time in spite of its small scale, with a half-life of approximately 1 hour in the closed system the team in Harvard developed.

EWNS vs Bacterial Wall.png

In this environment, EWNSs are proving effective against bacteria in the air, on stainless steel, and on the skin of tomatoes. Even at far higher air concentrations, EWNSs prove harmless against mouse epithelial cells and create no apparent damage to food. It appears that vegetable cell-walls, cuticles, or mucus coatings easily neutralise an EWNS’s payload into water, whereas thin walled, low-volume bacteria are much more susceptible.

As a cheap, reliable, disinfection technique that leaves no residues, this technology has immense potential, not just in the food supply chain, but in hospitals and housing. Proving effective against, waxy-walled mycobacteria, such as M. Parafortuitum, it may be employed against M. Tuberculosis. Although peroxide itself is known to be a powerful inactivator of H1N1 influenza EWNS’s actions on the virus remains to be tested.

It is also important to test whether EWNS technology can really extend shelf-life. This will require the development of constant, consistent, nanoscale spraying over an extended period of time: engineering challenges which are being ably met by Spraybase®.