(Note: The Virginia Tech Center for Unit Load Design and the U.S. Forest Service Wood Education & Resource Center (WERC) cosponsored a conference in June on preventing and eliminating mold on wood pallets. The conference offered a consolidated and comprehensive educational curriculum. It was designed to teach primarily Eastern hardwood lumber and pallet manufacturers and their customers about mold-related problems, the causes of mold growth on raw wood surfaces, and how to prevent and eliminate it. The conference included topics relating to the science of mold, how mold affects food processors, various chemical treatments for mold, pallet drying, and a case study. Exhibitors from chemical treatment and drying and heat-treating sectors were represented. The following article summarizes the scope and content of the information that was presented at the conference.)
Mold growing on newly manufactured wood pallets is becoming less tolerated by many customers. Increasingly they are sending back trailer-loads of pallets that have mold growing on them and demanding mold-free pallets.
Unlike wood rotting fungi, molds occur only on the surface of wood and do not damage its structural integrity. So, why all the fuss?
The reasons are simple:
1) Mold growing on pallets can contaminate or discolor the packaging and, ultimately, the product placed on them.
2) Mold on pallets can contaminate the workplace, directly or indirectly contaminate product, and pose a health risk to people working in the same environment.
3) Moldy pallets present an ugly, unprofessional appearance that contaminates the packaged appeal of the finished unit load that is ready for distribution.
Whatever the reasons, it is clear that mold on pallets is a problem that is not going away.
Unfortunately, there is no ‘silver bullet’ to remedy the situation. However, with a little understanding and insight into the nature of this beast and a thorough commitment from management to develop and implement a comprehensive mold prevention strategy, mold can be substantially minimized.
The Science of Mold
According to Dr. Jon Eisenback, plant pathologist at Virginia Tech, microscopic mold spores are everywhere in the air all the time — as many as 10,000 per cubic yard. There are thousands of different species of molds that produce a variety of mold colorations: blue, black, green, yellow, orange, red — you name it.
Mold spores are constantly floating around and settling onto wood and other surfaces. Under optimal conditions, these spores will germinate within 24 to 48 hours. Growth requirements for fungal molds include oxygen, a food source, water, and warmth.
For wood, this means wood surfaces above 20% moisture content, air temperatures between 60-90 degrees, and little to no air movement. Mold spores can germinate and grow at lower temperatures that are still above freezing — for example, in the winter when pallets are stored indoors with poor ventilation and at temperatures somewhere above freezing.
Once the spores germinate, mold begins to grow by forming an extensive network of mycelium or hyphae that spread out across the surface of the wood, and all the while producing more spores for reproduction.
As we all know, mold will cause food stuffs to spoil. Mold is also the cause of many plant diseases, such as the infamous Irish potato famine of the mid 1800s. Moreover, mold can cause animal and human diseases.
How does mold affect the health of people? Most people have no reaction to mold, according to Dr. Eisenback. However, some people have allergic reactions — similar to other allergies — that are irritating but not life threatening. They may develop flu-like symptoms and a skin rash. Mold can also make asthma more severe. Fungal infection mold may become systemic in people with serious immune disease. Also, some molds are known to produce mycotoxins that can affect animal and human health and are known carcinogens.
People who are most affected by mold include the elderly, infants and children, pregnant women, individuals with respiratory conditions or allergies and asthma, and people with weakened immune systems (for example, chemotherapy patients, organ or bone marrow transplant recipients, and those with HIV infections or auto-immune diseases). Clearly, mold can present a human health hazard.
Impact of Mold in Food Processing Environment
Dr. Rob Williams at Virginia Tech’s department of food science and technology offers valuable insight from the food processing industry perspective as to why moldy pallets cannot be tolerated. Similar characteristics apply to the pharmaceutical industry.
First of all, the goals of any food processor are to produce safe food, achieve the highest quality for the price, prevent product loss, keep customers satisfied, and avoid regulatory enforcement. From the perspective of the food industry, mold is a contaminant that limits shelf life, threatens food safety and imposes significant sanitation costs.
According to U.S. Economic Research Service estimates, 96 billion lbs. of food are lost annually in the U.S. (27% of available food). Among the key reasons for this loss is spoilage caused by mold. Therefore, it is critical for food processors to extend shelf life, ensure quality and prevent loss.
The four factors of shelf life include the formulation (ingredients), processing, packaging, and storage conditions. Mold can contaminate and inhibit the shelf life at every level of this environment.
It is important to appreciate that federal regulations were created to 1) protect public health, 2) protect the consumer from fraud, and 3) to enforce the law. The Department of Agriculture and the Food and Drug Administration are primarily responsible for inspecting food processing facilities. Surprisingly, there are no specific regulations regarding mold although highly dangerous mycotoxins are regulated specifically. The regulations associated with food inspections and mold pertain more to the fact that, due to mold, food can be rendered unwholesome and unfit for human consumption. For example, an inspector may determine that a given product has become adulterated due to the presence of mold in the food or in the storage environment (emphasis added). Given this assessment, the inspector can immediately and temporarily shut down production. Regulatory agencies may take action when “food has been manufactured under such conditions that it is unfit for food” and when “food has been prepared, packed, or held under unsanitary conditions whereby it may have become contaminated with filth or whereby it may have been rendered injurious to health” (emphasis added). Similar regulations apply to the pharmaceutical industry.
In summary, mold is generally detrimental to food quality, and sanitation depends on eliminating sources and reservoirs of spoilage microorganisms. Processors wish to avoid violations of government regulations to keep the inspectors away and avoid shut downs and lost production. They cannot afford the risk that moldy pallets impose.
How to Control Mold on Pallets
Effective mold control on wood pallets must begin with a management commitment to a continuous, comprehensive mold control program. The key components of a mold control program are: 1) reducing airborne spore density around the manufacturing facility, 2) reducing wood surface moisture levels to below 20% moisture content (average pallet part moisture content about 25%), and 3) maintaining low wood surface moisture levels. Responsibility for managing a mold control program should be centralized and assigned to one person.
1. Reduce airborne mold spore density.
A regular inspection of all premises — building, grounds, raw materials, and finished goods — should be performed at least twice weekly. Mold can occur on any type of surface — wood, plastic, metal, etc. Regular inspections will identify mold spore breeding grounds and incidences of mold occurring on raw materials, finished goods, or other possible sources of mold.
Spot treat to kill any existing mold with a 2:1 ratio of water to household beach solution. The bleach kills existing mold and prevents additional mold spores from being released, but it does not prevent mold from reoccurring.
When possible, eliminate excess piles of wood debris and any other unnecessary materials that can serve as breeding grounds for mold spore. All storage areas should be well ventilated and well drained (to eliminate standing water).
2. Dry pallets to an average moisture content below 25% before shipping.
In general, dry pallets are superior to wet pallets in just about every way. Dry pallets are stronger, more durable and weigh much less than green pallets. They also prohibit germination of mold spores and mold growth.
Although pallet manufacturers traditionally have not included a pallet drying process in their operations, it would be overwhelmingly beneficial.
Where mold is a concern, wood that has an average moisture content of around 25% should have a surface moisture content of 20% or less to prevent mold spore germination.
A fan drying system is an extremely effective and cost efficient method to dramatically accelerate the rate of drying for wood pallets. Inside a shed, pallets should be arranged in neat rows and columns with space between to allow air flow across all wood surfaces. This air flow allows moisture in the wood to wick away from the surface and enables a steady rate of drying. The fans should be reversed periodically to foster even drying throughout the shed. Stringers should be aligned parallel to the direction of air flow.
A fan shed will not only protect the pallets from the elements (i.e. rain) but will more than double the rate of drying. It is common for green hardwood pallets stored in fan sheds to reach 20% moisture content in 10-21 days, depending on ambient air temperature and relative humidity. This will significantly limit mold growth on pallets.
Fan sheds in combination with chemical fungicide dip treatments have proven very effective at preventing mold growth on pallets (as long as they are kept dry during shipment, future storage, and use). A chemical fungicide dip treatment for pallet parts will provide an effective barrier to temporarily prevent mold growth and allow time for drying. The chemical treatment itself does not provide permanent mold protection; dry pallets ultimately prevent mold from growing. Fungicide treatments are effective for around three weeks to a couple months—depending on concentration levels and environmental conditions. They can also be used in combination with wood brighteners to achieve a bright and natural wood color.
The most efficient chemical treatment technique is to dip treat dead-stacked parts (lightly banded to allow chemical penetration into the stack). The parts only need to be submerged for a couple minutes, then allowed to drip dry for one to two days before pallet assembly. Spraying with fungicide may be necessary to maintain vigilance against mold growth as pallets dry in the storage area.
Mold will not grow on pallets that are frozen during the winter. However, if pallets are stored in heated buildings, they will support mold growth; under these conditions, the dip treatment, followed by a fan drying or kiln drying strategy, must be used all year.
The moisture content of wood pallet parts should be monitored and recorded regularly to determine the effectiveness of the moisture control program and the suitability of mold-free pallets ready for shipment. Electric resistance meters can monitor moisture levels below wood fiber saturation point (25% to 30% moisture content). Random samples of each production run should be measured twice weekly. The samples should be randomly located within the drying area. Stringers and deck boards should be measured according to the instructions of the meter manufacturer (species, etc).
It is common for heat-treatment systems to exacerbate the occurrence of mold on wood pallets if they not properly ventilated at the end of each cycle. This is caused by a build-up of humidity in the chamber; the humidity condenses and re-wets pallet surfaces when cooled down. The warm, wet wood surfaces provide ideal conditions for mold spores to germinate.
For pallets that are heat-treated, a fully vented blow down cycle (at least 30 to 60 minutes) is recommended at the end of the heat-treating cycle to dry the wood surfaces. After the cycle, pallets should be moved immediately to a fan drying area. Heat treatment in combination with additional kiln drying is an effective way to both meet international phyto sanitation requirements and provide significant drying in one step.
3. Maintain low wood surface moisture levels.
Maintain a sufficient inventory of mold-free pallets that have been dried to an average of 25% moisture content. The best option for shipping mold-free pallets is ventilated trailer vans. These kind of trailer vans prevent humidity from rising and condensation from occurring inside; ventilated vans will prevent rain contamination but will also allow pallets to continue to dry during shipping. Alternatively, pallets should not be left in closed trailers for more than 24 hours.
Upon delivery, pallets should be stored and protected from rain in a well ventilated area with good drainage (to eliminate standing water). If pallet surfaces become wet, it is an opportunity for mold spores to germinate and grow. Remember: mold spores are always present.
Ideally, these procedures should encompass the entire scope of the manufacturing operations to reduce mold spore density and mold growth as they relate to all pallets and lumber. However, if this is not feasible, apply the procedures for pallets with mold limitations only, and keep them segregated from other pallets during storage, treatment, and shipment.
Finally, it is important to note that no wood pallet can be guaranteed mold-free. By following the above requirements, wood pallets can be manufactured and delivered free of mold to the extent that it will not be visible or create potential problems for the customer. Yet, it is impossible to declare that these or any pallets are free of mold. As previously noted, mold spores are present everywhere in the environment — even on pallets that have been manufactured in accordance with a sound mold prevention strategy. Chemical fungicide treatments do not last forever, and if the pallets get wet again later, mold spores can germinate and grow. The customer must take responsibility for keeping wood pallets stored in a dry place.
Virginia Tech and the U.S. Forest Service Wood Education & Resource Center recently launched a Web site dedicated to helping pallet manufacturers stay informed about mold issues; the site is located at www.palletmold.com.
The Web site currently includes all speaker presentations given during the June mold conference and links to numerous drying and chemical treatment companies. Additional information will be posted as it becomes available.
For more information, contact Peter Hamner at (540) 231-3043 or e-mail phamner@vt.edu.
