A new study has shown 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.
As well as reducing food poisoning, the findings could reduce food waste and have implications for the growing threat of anti-microbial resistance.
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.
Because there is no cooking process to reduce the microbial load in fresh salads, washing is vital by the supplier and the consumer.
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.
In this new study, published in the journal Ultrasound in Medicine and Biology, scientists 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 of the University of Southampton, who invented the technology and led this research, 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.”
The results showed that the microbial load on samples cleaned with the acoustic streams for 2 min was significantly lower six days after cleaning than on those treated without adding sound and the micro-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.
Improving how food providers clean fresh produce could have a major role to play in combating the threat of anti-microbial resistance. In 2018 and 2019, there were fatal outbreaks of different strains of E. coli on romaine lettuce in the USA and Canada and samples from humans infected showed strains that are resistant to antibiotics.