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Ultrasonic washing of leafy greens

by Andrew Thomson on March 9, 2021

Salad and leafy green vegetables may be contaminated with harmful bacteria during growing, harvesting, preparation and retail. Therefore, improving how food providers clean fresh produce could have a major role to play in reducing food poisoning and food waste as well as possibly helping to combat the growing threat of antimicrobial resistance.

Scientists from the University of Southampton have shown in a recent study, published in the journal Ultrasound in Medicine and Biology, that gentle streams of water carrying sound and microscopic air bubbles can clean bacteria from salad leaves more effectively than current washing methods used by suppliers and consumers.

In the study, acoustic water streams were used to clean spinach leaves directly sourced from the field crop; the results were then compared with leaves rinsed in plain water at the same velocity.

Professor Timothy Leighton of the University of Southampton, who invented the technology and led the research, explained: “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 each bubble into a microscopic ‘scrubbing’ machine. 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.”

The results showed that the microbial load on samples cleaned with the acoustic streams for two minutes was significantly lower six days after cleaning than on those treated without the added sound and bubbles. The acoustic cleaning also caused no further damage to the leaves and demonstrated the potential to extend food shelf life, which has important economic and sustainability implications.

The work was sponsored by Vitacress, with Group Technical Director Helen Brierley saying: “Ensuring food safety for our products is an essential requirement. At Vitacress, we wash our produce in natural spring water, and this type of groundbreaking new technology helps to enhance our process whilst ensuring our commitment to protect the environment is maintained. We are always interested in new developments and are excited to see the results of this research.”

The research project was a collaboration between Sloan Water Technology, Vitacress and the University of Southampton, a collaboration formed and supported by Global-NAMRIP (the Global Network for Antimicrobial Resistance and Infection Prevention).