Streams of water carrying sound and microscopic air bubbles can remove microbial contaminants from spinach leaves, according to a study.

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

Salad and leafy green vegetables may be contaminated with bacteria during growing, harvesting, preparation and retail, leading to foodborne outbreaks that may be fatal. There is no cooking process to reduce the microbial load in fresh salads and other fresh produce dishes so washing by the supplier and consumer is vital.

Washing with soap, detergent, bleach or other disinfectants is not recommended and the crevices in the leaf surfaces mean washing with water may leave behind an infectious dose. Chemicals may also not penetrate the crevices.

Microscopic scrubbing machines
Professor Timothy Leighton of the University of Southampton in England invented the technology a few years ago and led the research. He said 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,” said Leighton.

“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.”

Microbial loads on samples cleaned with and without ultrasonically activated stream (UAS) were enumerated and compared against unwashed samples on day zero and six after cleaning. The effects of UAS cleaning on leaf quality were also examined.

Lower contamination findings
Results, published in the journal Ultrasound in Medicine and Biology, showed the microbial load on samples cleaned with UAS for two minutes was significantly lower on day six after cleaning than on those treated without ultrasound.

Cleaning for two minutes also did not produce significant surface damage, which can affect overall leaf quality, according to the research.

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

The work was sponsored by Vitacress, a fresh produce supplier. Helen Brierley, group technical director, said the firm was always interested in new developments.

“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,” she said.