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Ultrasonic cleaning of salad leaves could reduce food poisoning instances

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Salad leaves could be more comprehensively cleaned, compared to current washing methods, using gentle streams of water that carry sound and microscopic air bubbles, researchers have said.

A team from the University of Southampton said that as well as reducing food poisoning, the method could reduce food waste and have implications for the growing threat of anti-microbial resistance.

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.

Washing with soap, detergent bleach or other disinfectants is not recommended and the crevices in the leaf surface means washing with plain water may leave an infectious dose on the leaf.

The researchers instead used acoustic water streams to clean spinach leaves directly sourced from the field crop, then compared the results with leaves rinsed in plain water at the same velocity.

Professor Timothy Leighton, who led the research, said: “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 researchers believe that improvements in how food providers clean fresh produce could have a major role to play in combating the threat of anti-microbial resistance.

Earlier this year, researchers developed a laser-based method for printing holograms on films of dried corn syrup that could one day be used to label products.

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