Salad producers haven't succeeded in banishing E. coli and other dangerous microbes from fresh greens, though they've tried hard. As we've reported before, it's a major challenge to both growers and the environment. But one scientist thinks he's making progress – with a spinach spa that zaps bad bugs with ultrasound.
Ultrasound is nothing more than sound moving at a frequency too high for humans to hear. It's commonly used for medical tests, including those adorably fuzzy photos of babies in the womb. Turn up the intensity, though, and ultrasound can pack enough power to destroy bacteria. Ultrasound is increasingly used in food production, and has been used successfully to decontaminate other fresh foods, including apple juice. But using ultrasound on greens has had only mixed success.
"Leafy greens are difficult," says Hao Feng, an associate professor of food engineering at the University of Illinois who built the spinach spa. Zap a spinach leaf too hard, and it develops watery spots and rots. Zap it too little, and the germs live. "We need to be very careful. We don't want to damage this produce."
Yet a spinach leaf, delicate though it is, can block ultrasound waves from reaching bacteria behind it.
Feng tried to overcome these problems by submerging the spinach in a big trough of water, much like the tanks used to wash fresh greens for commercial production. He added Jacuzzi-like jets to move the water, so all the spinach gets exposed to about the same amount of sound waves. ) And he used transducers that were as deep and long as the tank to generate sound waves throughout.
As the sound waves move through water, they make areas with high and low pressure. That creates tiny cavities that pop like bubbles. That cavitation process can dislodge and destroy bacteria – or it can destroy the spinach. So Feng had to tweak his machine to cause just enough cavitation, but not too much.
Then he combined ultrasound and a time-tested industry technique – washing fresh greens in a solution of chlorine and water. That resulted in a tenfold reduction in E. coli, compared to a chlorine wash alone. The results were reported in the journal Innovative Food Science and Emerging Technologies.
Feng is confident he could invent a machine that would work on a commercial scale, but it would cost more than the chlorine-only treatment many processors use now, and so far no one's expressed interested in funding commercial development of his spinach spa.
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