In recent decades, the market for fresh produce – and, in particular, minimally processed, ready-to-eat products like fresh-cut lettuce – has grown substantially. At the same time, outbreaks related to foodborne pathogens, and consequently, food recalls, have also increased. Fresh-cut products are suspected of providing the vehicles for those pathogens.
To reduce pathogen loads, processors have been using chlorine. However, in recent years the practice has been called into question. The microbial safety of fresh-cut products is a top concern for producers, processors and retailers. As a result, food researchers are looking for alternatives to chlorine, more efficient decontamination treatments that not only preserve freshness, but also improve the quality, safety and shelf life of those products.
The collaborative SAFEFRESH project has taken on that task. The project is working to develop innovative methods for the deactivation of pathogenic microorganisms on the surface of fresh and minimally processed products, like salads, melons and sprouts. One of the methods involves the use of chlorine dioxide, which research shows efficiently deactivates microorganisms on the surface of plant-based foods without changing the product’s properties.
Kristina Naudorf of Jürgen Löhrke gave a presentation on the topic at this year’s Anuga FoodTec conference in Cologne, Germany. After being cleaned, cored, trimmed and cut, salads, she said, are washed in untreated water.
“All of the microbiologicals that were present on the surface go into the washing water and spread maybe to other salad leaves, which are not contaminated,” she said.
The goal of the project was to see if treated process water could provide a potential alternative. The researchers did this using chlorine dioxide (ClO2) and electromagnetically activated water (ECA or anolyte water). Chlorine dioxide is an oxidizing agent that is very reactive. It has a high reaction rate and offers fast and broadband disinfection. Depending on the concentration, chlorine dioxide is a yellow-green to orange-colored gas. It has good solubility in water.
One of the advantages of chlorine dioxide compared to other disinfectants, said Naudorf, is that it is oxidizing, not chlorinating.
“So this reaction mechanism of chlorine dioxide differs from that of chlorine, and that’s the reason why there’s no forming of THM, chloramin or chlorophenols,” she said. “It has a good effect in the range of pH 4 to 10. It can eliminate biofilms and also prevent their regeneration. And it shows factor ≈2.5 better disinfection capacity than chlorine.”
The other option they tested was electrochemically activated water, or an anolyte treatment, which also acts as a disinfectant.
Compliance with specified critical values set by legislation is key. Chlorine and chlorine dioxide are both allowed to be used in the treatment of water in some countries, and in processed food. In water, the addition of 1.2 ppm chlorine is permitted; however after treatment, only 0.1 to 0.3 ppm can remain. For chlorine dioxide, only 0.4 ppm may be added, and only 0.05 to 0.2 ppm may remain after treatment is complete.
In food, however, chlorine dioxide and chlorine can be used because nothing is actually added to the food. The treated wash water is a processing aid.
“But there are requirements,” said Naudorf. “The unavoidable residues of the substance in the final product must be safe in terms of health and must not have any technological effect on the final product. Furthermore, odor and flavor cannot be adversely affected.”
The researchers tested endive salad at their lab in Berlin using three treatments: water only, water plus chlorine dioxide and water plus ECA treatment. Results showed that cross contamination did occur where processing water was used without a disinfectant. However, where ClO2 or ECA-water was used, no cross contamination occurred.
Throughout the trials, concentration levels were also measured to see if losses occurred throughout the wash process including after pre-wash, after pre-rinse, after one wash tank, after two wash tanks and after the rinse. In both ECA-water and chlorine dioxide trials, losses occurred where only one dose was used. Researchers, therefore, concluded that continuous dosing is necessary for optimal results.
With regards to disinfection byproducts, produce that was washed in ClO2 water showed fewer byproducts than with water containing anolyte. ClO2 water showed little amounts of chlorate and no THM (trihalomethanes), whereas washing with anolyte water left higher concentrations of both chlorate and THM.
Some results are still pending, as the project finishes up in September. Conclusions to date, though, show that chlorine dioxide and anolyte-water do make good options in the decontamination of fresh produce. They both prevent microbial cross contamination from process water, although continuous dosing is necessary for best results. Chlorine dioxide, however, makes for a better option since it leaves significantly fewer disinfection byproducts on the produce. Finally, the integration of a pre-wash of uncut salad heads with ClO2-containing wash water show an effect of >1.3 log more reduction possible than without pre-wash, and results in less cross contamination during cutting and the following processing steps.
SAFEFRESH project partners include the Fraunhofer Institute for Process Engineering and Packaging IVV, the Leibniz Institute for Agricultural Engineering Potsdam-Bornim (ATB), the Leibniz Institute for Plasma Science and Technology (INP), Jürgen Löhrke, RIPAC-Labor, neoplas, inter 3 and GARTENFRISCH Jung. The Federal Ministry of Education and Research (BMBF) funded the project under the “Research for Civil Security” program of the German government.
