Acoustic water streams shown to reduce the risk of infection from salad

An ultrasonic cleaning method invented at the University of Southampton has been found to be more effective at cleaning bacteria from salad leaves than current washing methods.

In the new study, published in the journal Ultrasound in Medicine and Biology, scientists used gentle streams of water carrying sound and microscopic air bubbles to clean spinach leaves directly sourced from the field crop.

The results showed that the microbial load was significantly lower six days after cleaning than on those rinsed in plain water at the same velocity. The acoustic cleaning caused no further damage to the leaves, demonstrating valuable potential to extend food shelf life.

The acoustic water streams were invented by Professor Timothy Leighton in Southampton’s School of Engineering, who is the founding Chairman of Global-NAMRIP (the Global Network for Antimicrobial Resistance and Infection Prevention).

Professor Leighton explains: “Our streams of water carry microscopic bubbles and acoustic waves down to the leaf. There the sound field sets up echoes at the surface of the leaves, and within the leaf crevices, that attract the bubbles towards the leaf and into the crevices.

“The sound field also causes the walls of the bubbles to ripple very quickly, turning them into microscopic ‘scrubbing’ machines. The rippling bubble wall causes strong currents to move in the water around the bubble, and sweep the microbes off the leaf. The bacteria, biofilms, and the bubbles themselves, are then rinsed off the leaf, leaving it clean and free of residues.”

A diet containing uncooked salad, fruit and vegetables is key to reducing a range of conditions, including cardiovascular diseases, Type II diabetes and certain types of cancer.

However, salad and leafy green vegetables may be contaminated with harmful bacteria during growing, harvesting, preparation and retail leading to outbreaks of food poisoning which may be fatal in vulnerable groups. The latest findings underline the new technology’s ability to reduce food poisoning, whilst also minimising food waste and bypassing the growing threat of anti-microbial resistance.

The research project was a collaboration between Sloan Water Technology Limited, Vitacress and the University of Southampton.

The paper was co-authored by Dr Tom Secker, Dr Craig Dolder and Professor Bill Keevil, and included data from PhD student Weng Yee (Beverly) Chong, whose research followed a BSc Acoustics with Music with an Engineering Foundation Year.