Mold. The word can cause panic among wood users and especially among pallet manufacturers whose customers often demand mold-free wood.
By Jeff Morrell, Ph.D.
Mold means different things to different wood users. There are literally thousands of fungi that can land on a wood surface and grow. Some of these fungi degrade the structural polymers that give wood its unique properties. Others only use sugars, proteins or other compounds stored within the wood, and others still may just be growing on material on the wood surface (such as dust or pollen). Some are not visible to the naked eye, while others markedly discolor the wood.
For the purposes of this short review, we will consider molds to be those fungi that produce pigmented spores on the wood surface. These spores are typically capable of becoming airborne and can land on any wet wood where they germinate to produce hyphae that grow into the wood to use stored sugars, proteins and other compounds. Once the fungus obtains enough energy, it produces spores on the wood surface to start the cycle again on another piece of wood. This process can take a few days but can also extend to months or years, depending on the fungus and the environmental conditions.
Wood species can also affect the risk of mold. Species with more sapwood or more stored sugars are at a higher risk of fungal attack. Mold spores can usually be brushed from the wood surface, leaving little evidence of their presence; however, the fungus is still very much alive in the wood and can continue to grow and produce new spores if the wood remains wet.
Mold fungi have very little effect on wood properties; their most important effects are increases in permeability caused by removal of the pits connecting the wood cells. The primary concerns with mold fungi on wood and other materials are related to their spores, although there have also been concerns over some of the secondary compounds they can produce. Mold spores, like many other airborne particles, can induce asthma. Obviously, more spores increase the risk of asthma attacks.
Some mold fungi are also capable of producing toxins and these fungi have attracted considerable media attention. While toxins produced by fungi such as Stachybotrys chartarum, Fusarium spp. and a number of other species can cause human health effects at very high levels, the levels produced by fungi on wood tend to be low (in fact, many of these fungi do not grow well on wood) and the compounds are not highly volatile. Some people, however, are extremely sensitive to mold spores and there is probably no safe level of exposure for this exceedingly small segment of the population. For the vast majority of people, mold spores are a nuisance. The primary risk associated with molds on pallets is the potential for the spores to either contaminate or grow on the materials being transported.
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.
Limiting moisture can be difficult with pallets for several reasons. First, many pallets are constructed using green or wet wood. The wood will eventually dry below 20 percent MC, but mold fungi can grow into and sporulate on the pallet while this process occurs.
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.