Using a recipe that involved soil from a Yakima, Washington, apple orchard and exudates from fresh nicked apples and decayed apples, Zhu said they tried to mimic conditions in packinghouse dump tanks. The higher organic rate also was designed to simulate a worse-case scenario.
In lab testing, chlorine at 25 ppm had limited efficacy against Listeria in high-organic matter dump water. At 100 ppm, chlorine was not affected by low levels of organic matter, but its efficacy was reduced by high organic loads.
The efficacy of PAA, on the other hand, increased with concentration and contact time and was only minimally influenced by organic loads. PAA at 40 ppm had higher efficacy than 100 ppm chlorine.
“PAA and chlorine both can reduce Listeria when compared to no sanitizer,” Zhu said. “With those sanitizers, it definitely helps even though we couldn’t totally eliminate the pathogen, and it depends on the concentration.”
“We started using a high inoculation level because we tried to see the log reduction. In a real industry setting, contamination will be low and the (sanitizer) concentration will fluctuate.”
The researchers also are looking at chlorine and PAA combined with a handful of other chemicals considered GRAS, or generally recognized as safe. They include octanoic acid and lauric arginine, and were chosen based on literature reviews, Zhu said.
“We’ve done some bench-scale testing, and efficacy increased but we still weren’t able to totally prevent cross-contamination,” she said.
Their next step involves testing disinfectant efficacy in a WSU pilot dump tank that will also include apples. The researchers will use the same recipe to simulate dump tank water.
“This testing will be essential before we move into commercial packinghouses,” Zhu said. “This [tank system] will be much smaller than a commercial operation but much bigger than our bench-scale testing. If the system is only water, it will be an easy assessment. Once you put fruit in, it becomes complicated.”
The researchers plan to validate their bench-scale and pilot-plant findings in three to four cooperating packinghouses before the end of 2022. Zhu plans to work with the Washington State Tree Fruit Research Commission to recruit participants.