Strong Oxidizers Present Alternatives to Chlorine Washes

Chlorine is the most common sanitizer used in fresh-cut fruit and vegetable production. It is an effective sanitizer that has its advantages, but fresh-cut processors are finding new ways to use older chemicals to ensure a safe food supply.

When Les Lipschutz started in the produce industry more than 25 years ago, he said, it was common to hear recommendations that the industry move away from chlorine. He now has a food-safety consulting company, Albuquerque, N.M.-based Food Safety, Inc., and the industry is still using chlorine, but as a whole, processors are a lot more sophisticated, he said.

“The larger companies get, the more they know they have to invest in their food-safety procedures,” Lipschutz said.

By now, chlorine is a well-understood sanitizer that is easy to regulate, effectively kills microorganisms and breaks down in less than 24 hours. But if not kept between pH 6.0 and 7.5 it can become ineffective or too corrosive, and it can create trihalomethanes, which are carcinogenic.

Research into other chemical applications is yielding new uses from known compounds, such as chlorine dioxide. Studies as far back as 1967 found chlorine dioxide to be 2.5 times as effective as chlorine at killing microorganisms, but there was no good delivery system. Chlorine dioxide is difficult to generate and must be generated on site because it has the risk of explosion if transported in gaseous form. At high concentrations, it can cause respiratory discomfort in workers, and the generating process can create chlorites and chlorates.

However, chlorine dioxide is considered an effective antimicrobial for a number of reasons. It is a strong oxidizer that kills microorganisms through molecular diffusion across cell walls that disrupts metabolic function. It can be used at a lower concentration than chlorine and be just as effective, Lipschutz said, but at high concentrations it can reduce flavors or bleach produce.

“The best and safest application is on whole fruit,” he said.

The delivery system for chlorine dioxide is becoming more efficient as well. Fresh-cut processors and researchers are now able to use sachet-type chlorine dioxide generators that are easy to use and safer to transport. The dry sachet creates a chlorine dioxide gas or a gas stabilized in water by mixing a package of sodium chlorite with an acidic activator. Another type of sachet generator works like a teabag in water. The bag “steeps” in tap water for six to 12 hours to create the predetermined concentration of chlorine dioxide in water.

Bassam Annous, a microbiologist with the USDA Eastern Regional Research Center, has been studying the non-thermal cleaning properties of chlorine dioxide. He’s studying how the gaseous form of chlorine dioxide sanitizes cantaloupe skin before it’s processed for fresh-cut uses.

“We can knock out 99.9 or 99.99 percent of pathogens on the skin of cantaloupe,” Annous said.

His laboratory research involves applying human pathogens to melon skins, and then subjecting them to chlorine dioxide washes.

The chemical washes are as effective as hot water, but Annous claims chlorine dioxide has advantages. For one, it is a strong oxidizer that replaces the air on the skin of the cantaloupe, killing the pathogens more effectively.

“It is better from my point of view because the cantaloupe has netting and cracks, in the skin that the pathogens get into,” Annous said. “They become inaccessible to wash, and they also form a biofilm that makes them harder to kill.”

Annous’ next study will be using chlorine dioxide on melons that were freshly harvested to see how they’re affected by the gas. His previous studies used stored melons.

“I need fresh produce, fresh coming out of the field,” he said. “I want to start with a good quality melon.”

He plans to look at different application techniques to avoid bleaching the melon skins, which is more important for the fresh market than for fresh-cut processors. He also will measure the byproducts of the gas to see if there are any chlorites created in the production.

Chlorine dioxide is just one compound that is gaining in popularity. Peroxyacetic acid, also called peracetic acid or PAA, is another strong oxidizer that is similar to hydrogen peroxide. It also is effective at a lower volume than chlorine and is effective against biofilms, Lipschutz said, but can be corrosive and some microorganisms are resistant. Ozone is used as well, but it is expensive and requires air scrubbers to prevent workers from getting sick.

While not a chemical application, ultraviolet light has shown success in reducing biological load in water, Lipschutz said. UV light is usually used in a closed flume system, and adding additional steps to kill microorganisms could help reduce the potential for contamination.