The search for safer sanitisers

Trials have identified potential alternatives to calcium hypochlorite for sanitising pack-house flume water. By Anna Mouton.

Many pome pack houses use granular calcium hypochlorite to sanitise their flume water because it’s effective, inexpensive, convenient, and readily available. But consumer concerns about potential health risks associated with chlorine by-products are pushing pack houses to find alternatives.

The problem with hypochlorite

When dissolved in water, both sodium and calcium hypochlorite produce hypochlorous acid. Hypochlorous acid is considered the most potent disinfectant of the various chlorine compounds that form in water. The molecule readily and rapidly penetrates and kills microbes.

Hypochlorous acid is safe — our white blood cells produce it to protect us against germs. However, hypochlorites react with organic matter in water to generate disinfection by-products such as trihalomethanes and haloacetic acids. These compounds have been linked to cancer risks in people consuming chlorinated drinking water.

“This creates the fear that the European Union and the United Kingdom could reduce the use of hypochlorite in our pome pack houses,” says Dr Pieter Louw, Portfolio Manager: Crop Protection at ExperiCo Agri-Research Solutions.

Louw has conducted two Hortgro-funded projects to test other sanitisers. “There are quite a few alternatives currently available,” he says, “but we’re unsure how effective they are, hence our reason for conducting this research.”

Putting sanitisers to the test

Louw ran laboratory and pack-house trials on potential pack-house sanitisers. In the laboratory, he tested their efficacy at different concentrations against the spores of common fungal postharvest pathogens. Efficacy against spores matters because fungi generally spread as spores, which are relatively hard to kill.

For the first laboratory trials, Louw tested seven sanitisers, representing the active ingredients bioflavonoids, fulvic acid, glutaraldehyde, hydrogen peroxide, organic acids, peracetic acid, persulfate salts, and potassium sorbate. Calcium hypochlorite was the control.

Four products that showed promise in the laboratory were further assessed for their ability to sanitise flume water obtained from pack houses.

Louw found that only one product — a combination of hydrogen peroxide and peracetic acid — was as effective as calcium hypochlorite against Botrytis cinerea and Penicillium species. The other products were either ineffective or only worked for some fungi.

“The concern with this product is that it has a pungent smell, making it difficult to use in closed environments,” said Louw. “And if you know peracetic acid, you will know why.” The compound has a strong vinegar odour, and its fumes can irritate the eyes and nose.

The second round of trials

Louw subsequently tested nine products, representing the active ingredients 2-butoxyethanol, bioflavonoids and other biofungicides, hypochlorous acid, ionised water, inorganic and organic acids, ozone, and persulfate salts. Calcium hypochlorite was again the control.

Using different product and spore concentrations and contact times, he first evaluated their efficacy against Botrytis cinerea, Penicillium expansum, and Alternaria alternata spores in the laboratory. He also investigated whether they could be combined with calcium hypochlorite to reduce hypochlorite concentrations.

Sanitisers and sanitiser combinations that showed promise were further assessed on fruit. Louw inoculated Golden Delicious apples and Packham’s Triumph pears with fungal spores before treating them with the different sanitisers. Control fruit were untreated. The fruit were then stored at -0.5 °C for five weeks, followed by a five-day shelf life at 10 °C.

Only three products were effective against all three fungi: hypochlorous acid, ozone, and the high dose of ionised water. No phytotoxicity was observed for these products. Louw reports that he was very impressed with the hypochlorous acid product. “It was excellent — it was more effective than calcium hypochlorite at the recommended dose.”

In tests of contact periods, the hypochlorous acid product also worked faster than any of the other products, including calcium hypochlorite.

Hypochlorous acid was 100% effective against Botrytis cinerea and Alternaria alternata spores on Golden Delicious and Packham’s Triumph.

However, hypochlorous acid and ozone were less effective against Penicillium expansum than ionised water and some sanitiser combinations on Golden Delicious. None of the sanitisers or combinations were more than 60% effective against Penicillium expansum spores on Packham’s Triumph.

Two top performers

A good postharvest sanitiser must meet specific criteria, explains Louw. It must be effective against bacteria as well as postharvest fungal pathogens, and it must work fast. Then, it must be affordable, available, and easy to transport and use within the existing pack-house systems.

Based on his recent trials, he is excited about the performance of the hypochlorous acid product. “This molecule is unique in that it is not known to produce harmful by-products. It’s already used in wound treatments and cosmetics.”

Although the potency of hypochlorous acid has long been recognised, scientists have struggled to create a stable formulation. Even hypochlorous acid produced by dissolving granular calcium hypochlorite in water generally has a half-life of about four hours.

However, recent chemical advances have brought about a hypochlorous acid solution that manufacturers claim has a six-month shelf life. “We want to do more tests to see how long it lasts in the flume water,” says Louw. “How long will it continue working? And we need to do more tests on fruit, look at the commercial implementation, and do a cost analysis.”

In addition to hypochlorous acid, Louw thinks there’s a place for ozone. “It works. The issue is that it’s a corrosive gas with an extremely short half-life, so the question is, how will you apply it? And where, at what dose and contact time? There’s still a lot of work to be done.”

Besides acting as a sanitiser, ozone can potentially provide phytosanitary control as it can kill insects. Louw is planning to work with PHYLA, Hortgro’s bespoke phytosanitary research facility, to test biological control of postharvest pathogens and phytosanitary pests using ozone in controlled atmosphere storage of pome fruit and cherries.

To see more of Louw’s results, view his Research Showcase presentation on the Hortgro YouTube channel.