Monday, 28 July: The few remaining weeks before apple harvest begins provide an ideal time for cleaning and sanitizing apple storage rooms. All storage rooms should be swept clean and washed out with water to remove any dirt and debris on the floors. However, cleaning with water will not eliminate the fungal spores from the Penicillium species that cause blue mold decay in stored apples. Blue mold accounts for most of the losses to postharvest decays during apple storage, and most apple storage rooms contain at least moderate populations of Penicillium spores even after rooms have been swept and washed. The spores survive washing by becoming airborne while the room is being washed, then settling back to the floor when there is no longer any air movement in the room. They remain on the storage room floors until the cooling fans are turned on in the fall, at which time they become airborne again and are available to infect wounds in any freshly harvested apples that are moved into those rooms.
In an article that appeared in today’s issue of Scaffolds Fruit Journal, I describe some options for sanitizing storage room floors and the rationale for doing so. As pointed out in that Scaffolds article, quaternary ammonium sanitizers (quats) have been considered the best option for sanitizing hard surfaces such as bins and storage rooms. Unlike chlorine solutions or peroxide-type sanitizers, the quats are more stable and provide some residual activity so that spores landing on treated surfaces after the treatment has dried may still be inactivated.
However, the European Union has recently enacted a maximum residue limit of 0.5 ppm for quats (see this for a list of EU countries and this for the EU document). No one has enough data to know if a quat sanitizer applied to a storage room floor could later be transferred on bin runners in sufficient quantity to generate excessive residues on apples as the bins are lowered into water flotation tanks on grader lines. Because of this uncertainty, storage operators who expect to export apples to the EU may wish to avoid using quats in their storage rooms or other fruit contact surfaces until more information becomes available.
The extremely low residue limit established by the EU should not be construed as an indication that residues from quats pose a health risk. Rather, it appears that the EU instituted a very conservative residue tolerance primarily because quats have been considered so safe that they have not undergone much testing. The EU uses the precautionary principle of establishing very low limits in situations where companies have not provided a full regimen of safety data.
Finally, it is worth noting that bacterial pathogens associated with food safety concerns are much more easily controlled with sanitizers than are spores of Penicillium species. Penicillium spores can survive for several years on dry surfaces, are easily dislodged and moved around by air currents, and require higher doses of sanitizers and/or longer exposure times for effective control. Thus, any treatment that eliminates Penicillium spores should be more than adequate for controlling any bacteria that might otherwise survive in storage rooms.
Penicillium spores do not in themselves pose any risks to human health, but high spore concentrations in storage rooms can result in higher incidences of fruit decay during storage. Fruit infected with Penicillium species often contain the mycotoxin known as patulin, and patulin levels in cider are regulated by law. Therefore fruit with blue mold decay should not be used in cider production.