Ultrasonic Cleaning of Leafy Greens Could Reduce Instances of Foodborne Illness

According to a new study, streams of water carrying sound and microscopic air bubbles can clean microbial contaminants from spinach leaves more effectively than current washing methods.

Salad and leafy green vegetables may be contaminated with harmful bacteria during growing, harvesting, preparation, and retail, potentially leading to outbreaks of foodborne illness. Because there is no cooking process to reduce the microbial load in fresh salads, washing by the supplier and the consumer is critical. 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. Even if chemicals are used, they may not penetrate the crevices.

Researchers used acoustic water streams to clean spinach leaves directly sourced from the field crop and compared the results with leaves rinsed in plain water at the same velocity. 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.

Timothy Leighton, a professor of ultrasonics and underwater acoustics at the University of Southampton in the U.K., invented the technology and led the research. “Our streams of water carry microscopic bubbles and acoustic waves down to the leaf,” he says. “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 report was published in the journal Ultrasound in Medicine and Biology. As well as reducing outbreaks of foodborne illness, the findings could reduce food waste and have implications for the growing threat of anti-microbial resistance.