Pre-washed, double-washed, triple-washed—nearly every package of salad and lettuce mix that you find in the grocery store will note how it was washed by the manufacturer. However, even pre-washing can still lead to the spread of food poisoning and bacteria, such as E.coli and listeria. Now, scientists are researching ways to get an even deeper clean using ultrasonic technology.
A team of scientists at the University of Southampton in England used acoustic water streams to clean spinach leaves that were directly sourced from the field crop and compared the results to leaves that were rinsed in plain water at the same velocity. The results showed that the microbial load on spinach 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 water streams did not shorten the life or cause damage to the spinach leaves.
"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," says Professor Timothy Leighton of the University of Southampton, who invented the technology and led this research.
This isn't the first time scientists have tested ultrasonic cleaning methods. In 2012, Hao Feng, an associate professor of food engineering at the University of Illinois, created a "spinach spa" using jacuzzi-like jets to wash spinach and other fresh greens. He combined ultrasounds with a common solution of chlorine and water to disinfect greens and reduce the spread of E. coli.
Fiding an effective cleaning solution for salad and leafy greens is important because it's common for them to be contaminated with harmful bacteria during the growing, harvesting, preparation, and retail process, which leads to outbreaks of potentially fatal food poisoning. For the last few years, there have been major outbreaks of different strains of E. coli on romaine lettuce in the United States and Canada, many of which were fatal.