Print Page | Contact Us | Sign In | Join NWPCA
Mold: Tips of the Trade
Blog Home All Blogs
There are pro-active approaches to prevent mold, or reactive ones to treat mold once it is found. Experts share best practices.


Search all posts for:   


Top tags: mold  challenges  chemicals  environment  industry  kiln drying  pallets  preservative  prevention 

Mold Prevention

Posted By Administration, Tuesday, June 30, 2015

By Jeff Morrell, Oregon State University’s College of Forestry – Wood Science & Engineering Department

As with all activities, prevention is always preferred and there are a variety of methods for limiting mold growth on wood used for pallets. Virtually all fungi that attack wood require oxygen, an adequate temperature, a food source, and free or liquid water to grow. It is fairly difficult to limit oxygen or control temperature on pallets except where the wood is kept in a cooler and some fungi can even grow in cooler conditions. As a result, making the wood unusable to the fungus or controlling moisture remain the best ways to limit mold.


In addition, pallets can be rewetted when stored outdoors, allowing other mold fungi to colonize the wood. Free water may only be present on the wood surface for a short period of time, but it may only take a few days for a fungus to germinate, grow into the wood, and produce spores on the surface. The wood may already be dry by that time, but the damage will already be done.

Kiln drying is one method for limiting mold colonization on pallets, but this adds costs to a product that is often designed for only a single use. In addition, even kiln-dried wood can experience mold attack when it is rewetted and, as noted earlier, the wetting period need not be long.

Many producers also have a misconception that heat treatment will provide protection against mold attack. While heat treatment will kill fungi on or near the wood surface, it generally does not dry the wood. As a result, any spores falling on the wet wood surface find a substrate free of competitors. The result is a potentially higher risk of mold attack. Some producers have also tried to use their heat treatment chambers as modified kilns. This will only work if the operators understand the drying process and have good air flow so that they do not produce pockets where airflow and therefore drying are limited. Failure to produce uniformly dry material can result in moldy pallets.

Molds can also be limited by application of prophylactic fungicides. These chemicals are applied by dipping or spraying shortly after sawing and can provide protection for three to six months. There are at least 20 chemicals currently registered for limiting colonization of freshly sawn wood by molds and other fungi. Many of these chemicals are active against a few groups of fungi and are used in combinations designed to provide broad protection to wet wood. These chemicals generally work when used at the proper levels, but their use is often restricted because of concerns about possible chemical contamination of the materials being transported. While the amounts of chemical applied to pallets are generally small (5-10 ppm on the wood surface), many pallet users do not allow any chemical treatments to be applied to the wood they use for pallets and this creates a challenge when the customer demands mold-free pallets without chemical protection.

There have been a number of attempts to protect wood without conventional pesticides, but this is a major challenge because of the wide variety of fungi that can land on and grow into the wood. The most promising treatments use natural extracts of various plant materials. For example, cinnamon leaf oil, which is a food-grade material, has been shown to limit fungal attack on freshly-sawn lumber. However, this material only works well when the oil is dispersed in ethanol, which would not be suitable for most mill operations.

There are a variety of other essential oils with biological activity that might be useful for mold control, but most function best when they are dispersed in an organic solvent instead of water. This creates fire risks; however, work is underway to formulate these systems in water while retaining their effectiveness.

Another approach to wood protection is the use of chitosans, which are derived from chitin in shrimp and other seafood farming operations. Modified chitosans have some activity against fungi, but they have also not been widely used for protecting wood.

Finally, a number of so-called “non-biocidal” wood protectants have emerged on the market. Some of these products coat the wood surface to prevent spore germination. While these products may work, there are few third-party reports on their effectiveness.

So how does a manufacturer decide if a treatment will protect their pallets without taking the risk of treating thousands of pallets with a potentially ineffective system? The simplest way to decide is to test on the same type of material used at that facility. Oregon State University typically uses a severe mold test where we dip or spray the surface on half the length of a board in the proposed treatment. The other half of the board remains as an untreated control. We treat 15-20 boards with that system. If appropriate we use different levels of a system and then include a known treatment to serve as a positive control (i.e. it will protect the wood). The boards are stacked by treatment and then covered with plastic to retard drying. The bundles of wood are stored in a shady area for one to three months. The boards are visually examined at monthly intervals for degree of discoloration, which serves as the measure of effectiveness. These trials can be run any time of the year and provide a very severe measure of treatment effectiveness on actual production wood.

So, given the difficulties in limiting mold using chemical treatments, what steps can be taken to reduce the risk? It is important to remember that time is the biggest enemy for wood. The longer wood remains wet under the proper temperature conditions, the higher the risk of mold development. Logs or lumber can be colonized during storage and these fungi can quickly grow on to the surface to produce their spores. Fungi established within the wood are also more difficult to control using chemicals or barriers, placing added importance on preventing colonization.

There are a number of steps in the process that can help reduce the risk of mold.

From a producer perspective:

Do not store logs for long periods after cutting. Older logs will already have fungi growing in them and these fungi can quickly grow to the surface.

Do not store wet lumber in solid piles for long periods (i.e. more than three days), especially under warm conditions. Create good airflow so that natural breezes accelerate surface drying that will reduce the risk of mold growth.

Avoid having collections of older moldy wood; these can increase the risk of fungi colonizing your freshly cut wood

From the pallet manufacturer perspective:

Know your supplier and make sure the wood you receive is freshly sawn.

Do not store green materials for long periods; the longer you do so, the higher the risk of mold.

Do not store heat-treated wood under non-drying conditions; create good airflow to allow the wood surfaces to dry. This will help reduce the risk of mold.

From a pallet user perspective:

Store pallets where they can dry and will not be rewetted.

If you can accept mild fungicides, do so. This will reduce the risk of mold.
Jeff Morrell is a university distinguished professor at Oregon State University’s College of Forestry – Wood Science & Engineering Department

Tags:  kiln drying  mold  prevention 

Share |
PermalinkComments (1)

Mold: Ongoing Discussion, Best Practices Shared by Industry Experts

Posted By Administration, Monday, November 17, 2014

By Brad Gething, NWPCA Technical and PDS Manager

An increasing demand for mold-free wood packaging in the food and pharmaceutical sectors has led to a greater need to understand the finer points of the mold issue. In response to this need, NWPCA brought together two members of the wood packaging industry and one member of the wood preservation industry to discuss what can be done to combat mold, and how the industry can better educate customers to ensure that the wood packaging remains mold-free throughout its lifecycle. The discussion panel occurred at the Annual Leadership Convention at Ft. Lauderdale in March, 2014 and was moderated by NWPCA Technical and PDS™ manager Brad Gething.

The panel consisted of: John Dye, Product Development Manager, Scott Pallets Michael Pompeo, PhD, Director for Technical Development of the Osmose Inc. Ralph Rupert, Manager Unit Load Technology, Millwood Incorporated

Wood packaging manufacturers need to recognize that wood material is potentially susceptible to mold growth the moment the tree is felled, and that susceptibility may increase as the tree is cut into cants and cut stock. A firm understanding of what mold is and what causes it will provide manufacturers with the foundation necessary to prevent mold growth. Mr. Rupert gave a strong presentation on this topic. He explained: “Mold spores are present everywhere in the environment and require several things to grow: oxygen, a food source and moisture. Realistically, moisture reduction is one of the only available methods to prevent mold growth. Overall, reduction to 19 percent moisture content (MC) has been shown to significantly reduce the likelihood of mold growth.”

The amount of moisture in wood can vary greatly depending on the tree species and how green the material is, and drying wood, particularly stacked as pallets, is inherently a variable process. John Dye explained that research performed in conjunction with Edinburgh Napier University showed that a target moisture content must be reported as a range, and not an exact number “because it cannot be precisely predicted or guaranteed.” This range does not mean that a reduction in moisture cannot provide increased confidence in mold prevention, but it does require improved communication between you and your customer to set the proper expectations. Consequently, Dye added, “TIMCON is focusing on efforts to educate end-users about environmental conditions that promote mold growth, instruct them on proper pallet storage and handling, and provide best practice guidance on kiln drying pallets manufactured from green timber.”

Mr. Rupert also described some popular misconceptions that exist about mold. “The most widely held misconception is that heat treatment prevents mold growth. While heat treatment may kill existing mold spores on the surface of the wood, its intended purpose is to kill insects and does not prevent future spores from growing, so post heat-treated material is not immune. In fact, the heat-treatment process tends to pull out internal moisture to the surface of the wood, which could actually lead to more attractive conditions for mold growth.” Secondly, in spite of the evidence that drying below 19 percent MC greatly limits mold growth, it does not guarantee that mold will not grow. In the right conditions, some of the remaining moisture in the wood will eventually come to the surface, and if it can accumulate sufficiently, mold can grow.

When moisture reduction is not suitable for your business, or the customer desires greater assurance against mold growth, chemical treatments can be an option. Dr. Pompeo outlined the major types of chemical treatments and how they are applied to wood packaging. He said, “There are several major moldicide suppliers in the U.S. which produce a variety of formulations. These formulations tend to be a mixture of several active ingredients and they are all EPA registered pesticides. A few substances are approved or exempted by the FDA as safe to use for food contact application, and they are known as Copper-8 and potassium sorbate. Potassium sorbate is in fact a food additive that functions as a preservative/moldicide. All of these products can either be applied through dipping or spraying processes.”

Dr. Pompeo was asked about the efficacy of different chemical treatments and was quick to point out that “effectiveness varies based on the type of formulation and environmental conditions. It is best to discuss these variables with a preservation expert and conduct field trials on specific applications to determine a probable timeframe that the wood will be protected from mold. Effective protection time, based on these variables, can range from as little as a few weeks to a few months or longer.” In general, if mold is already present on received cut stock and cants, or mold is found on existing wood packaging, the panelists recommended that a simple solution of water with bleach (10 to 1 ratio) or 3 percent concentration of hydrogen peroxide can be used to scrub or pressure wash the wood clean. Proper drying is needed to ensure that the mold does not return.

All three panelists emphasized that storage conditions are a critical aspect to mold prevention. In moist environments with little to no air flow, mold will grow on most anything, not only wood. Rupert said, “Trailers should be unloaded as soon as possible. They shouldn’t be called trailers, but incubators, since mold grows so easily inside them. Wood should be stored in ventilated areas to promote drying and inhibit mold growth. In fact, air velocity as low as a few feet per second can deter mold growth.”

Also, Mr. Dye pointed out that “keeping wood packaging elevated off the floor or ground to prevent any uptake of moisture is a simple and effective way of keeping the wood dry. This includes when pallets are placed in a kiln to dry.” Dr. Pompeo warned against outside exposure: “Outside storage in the rain or in warm/humid conditions is the worst case scenario for storage of wood articles.” If outdoor storage is the only option, covering should be provided to prevent such exposure to the rain, but should also provide enough air circulation as pointed out by Mr. Rupert.

From this discussion an overriding theme resulted: while the increasing desire for mold-free wood packaging is an ever-present matter for the industry, there are methods available to effectively control mold growth. The battle against mold must be fought on two fronts. First, diligence within each step of the supply chain is necessary to ensure that wood does not grow mold. Second, it is imperative for the wood packaging manufacturer to communicate clearly to the customer what environmental conditions are favorable for mold growth and what actions he or she must take to ensure prevention. Such a comprehensive approach can be a challenge, but it will provide an opportunity for companies to differentiate themselves in this competitive industry. 

Brad Gething, PhD, is NWPCA's technical and PDS manager. He can be reached at 703-519-6104. Follow him on Twitter.

Tags:  challenges  chemicals  environment  industry  mold  pallets  preservative 

Share |
PermalinkComments (0)