Ultrasound waves and a chlorine wash cut the survival rate of E. coli O157:H7 on spinach about threefold more than chlorine alone in tests done at the University of Illinois at Urbana-Champaign.

The resulting paper, “Continuous-flow ultrasonic washing system for fresh produce surface decontamination,” has been published in Innovative Food Science & Emerging Technologies.

Food Technology Noord-Oost Nederland B.V. funded the project. The researchers — Bin Zhou, Hao Feng and Arne Pearlstein — added ultrasound capability to a small commercial washer.

“My understanding is that (Food Technology) is working on larger-scale systems and targeting some U.S. companies,” said Feng, an associate professor in the university’s Department of Food Science and Human Nutrition.

The ultrasound killed more cells by a factor of just over three when spinach was processed in batches. Added to the number killed by chlorine alone, overall reduction was by a factor more than 2,200 — or 3.35 log.

It’s not the holy grail — an absolute kill step — but it’s progress, said Feng, who envisions more improvements.

“We can reduce the risk of foodborne illness outbreaks but it’s still not the target,” he said. “The (U.S. Department of Agriculture) is looking for technologies that can achieve 4 to 6 log reduction.”

“This is just chlorine. I’m working with a few other sanitizers,” Feng said. “I think we can get more log reduction but we need to look carefully at the quality of produce. We are very careful to control the level of ultrasound and not damage it. Under current conditions it’s fine.”

“The combination of ultrasound with a chemical sanitizer can significantly enhance safety,” he said. “But every new technology will be more expensive compared to traditional chlorine wash.”