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Mold, Mildew on Pallets, Lumber Can Be Prevented, Eliminated
Mold, Mildew Can Appear on Pallets, Lumber in Spring and Summer

By By Peter Hamner and Marshall White
Date Posted: 9/2/2003

Each spring and summer throughout most of the U. S., mold and mildew begin to appear on the exposed surfaces of pallets and containers as well as the lumber raw material used to manufacture them.

There are thousands of different types of molds, and they all fall within the realm of the highly prolific fungi kingdom. Mildew is a common term used to describe non-decaying mold fungi and the growth and discoloration they cause on raw wood surfaces.

Mold fungi are neither plants nor animals. They are microscopic organisms that contain enzymes for digesting and decomposing organic matter, and spores used for reproduction. In the spring and summer in temperate climates, mold spores are everywhere in the air and float onto wood surfaces. The spores will germinate and grow at amazing speed—between 24 to 48 hours. Optimal conditions include raw wood surfaces above 20% moisture content, air temperatures from 66 to 90 degrees F (see Table 1), dark or dim light, and little to no air movement.

Molds and wood rotting fungi are an integral part of the Earth’s ecosystem because they serve to rid the world of its used up organic debris (for reuse at a later time). Unfortunately, molds and other types of fungi do not distinguish between a dead tree in the forest and a stack of green pallets waiting for shipment.


Problems from Mold

Although closely related, molds are different from wood rotting fungi in that they do not affect the structural integrity of wood. They become a ‘problem’ when their airborne spores settle and grow on wood surfaces, producing a dark discoloration (see Figure 1) that can persist even after the mold is gone.

A characteristic musty or mildew odor is also prevalent where mold grows. Furthermore, the presence of mold fungi may be an indication that decay fungi are present as well.

Perhaps the greatest problems associated with mold occurring on pallets have to do with sanitation. Spores released from mold can contaminate a manufacturing environment. They can infiltrate equipment, contaminate production processes, threaten product quality, and pose a respiratory health risks to workers. Mold is therefore a potential problem for food and pharmaceutical manufacturers.

While there is no regulation specifically targeting mold itself, mold contamination is regulated under the Food and Drug Administration (FDA) Code of Federal Regulations (CFR); Title 21 – Food and Drugs, Part 820 – Quality System Regulation, Section 150 – Storage. This regulation is designed to control microbial contamination such that “no obsolete, rejected, or deteriorated product is used or distributed.”


Preventing Mold


1. Good housekeeping practices

Reducing spore density in the air and on surfaces around manufacturing and storage areas will help to minimize the potential for mold growth.

• Periodically wash production and storage facilities with soapy water and water diluted bleach (one part bleach to three parts water) to prevent mold build-up.

• General site clean up -- Remove messy unused woodpiles and other waste from the area where pallet and container materials are produced and stored. This will significantly reduce the ability of mold spores to locate hosts. Minimizing the threat of mold contamination is a good excuse to clean up production and storage site facilities!

2. Drying

Dry pallets to achieve less than 20% surface moisture content. Maintain this level throughout the shipping and storage process to dramatically reduce likelihood of mold formation. Because there is a moisture gradient across the thickness of wood pallet components, achieving an average moisture content of around 30-35% is usually sufficient to accommodate mold free surfaces.

• Air dry -- Green pallets should be stacked (typically 15 or more high) and protected from rain and sun. This is possible with portable covers or a pole structure with roof and open sides as shown in Figure 2.

Pallets should be stacked with stringers oriented parallel to the prevailing wind direction, no more than four stacks deep (front to back), and side-by-side extending as far along the open face of the shed as the structure will permit. Allow at least 2 feet of roof overhang. Pallet stacks should be raised at least 12 inches off the ground to allow cool, moist air to move down and away from the pallets. Pallets should not be stored near standing water; good drainage away from the storage area is imperative.

Air drying times vary considerably and depend on climate, pallet design and initial pallet part moisture content. Recent studies indicate that the 5/8 inch thick deck boards of mixed hardwood GMA pallets in the late spring in the Midwest air dried to an average of 30% in about 17 to 18 days. All wood surfaces were well below 20%. Similar drying in the winter of cold climates would take about twice as long. To prevent mold growth during air drying consider the application of a dilute bleach solution beforehand.

• Kiln dry -- Recent studies of kiln drying of green mixed hardwood GMA pallets with 5/8 inch thick decks (average initial moisture content of 75-80 percent) indicate deck board drying times at 108 degrees F of five days to reach an average moisture content of 30%. The surfaces were well below 20% in moisture on all pallet parts.

3. Disinfectants, mildewcides

Dipping or spraying with chlorine-based disinfectants or chemical mildewcides provides a thin chemical layer that acts as a barrier against fungal attack. Dip treatment with certain mildewcides can be effective for three to six months, depending on chemical concentration.

• Disinfectant -- Chlorine bleach solution (one part bleach to three parts water). Chlorine is effective for only 5 to 7 days.

• Mildewcide – Copper-8-quinolinolate is a recommended active ingredient.

Warning: mildewcides may not be acceptable to particular customers. Check with your customer before applying these chemicals.


Removing Mold

To kill and-or eliminate mold already present on the surface of pallets, containers, components, or raw materials, the following two strategies have been proven effective:

• Power wash moldy pallets or components with a scaled recipe of 1 gallon chlorine bleach to 3 gallons water and 1/3 cup of detergent.

• Heat sterilization of at least 130 degrees F at high humidity (chamber vents closed) for one hour after reaching treatment temperature.

Following these procedures, dry pallets or components to achieve wood surface moisture contents of below 20%.

(Editor’s Note: Marshall (‘Mark’) White is director of the Virginia Tech Center for Unit Load Design; Peter Hamner is research associate of the center.)



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