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Dealing with moldy pallets

When mold happens, it isn’t pretty. Here’s what pallet users can do.
By Bob Trebilcock, Executive Editor
December 16, 2010

As a rule, pallets don’t often make national news. Then again, 2010 has been the exception to any number of rules. So it was that both Johnson & Johnson and Pfizer announced recalls of product that had a noxious moldy, musty smell. In both instances, the healthcare companies laid the blame on wooden pallets shipped from facilities in Puerto Rico. In both cases, the culprit was said to be a chemical treatment applied to wood to prevent the mold. 

The pallets were treated with a chemical called 2,4,6-tribromophenol, or TBP. TBP has been banned for use in the United States, but it is still used to treat lumber and lumber products against mold in some countries in South American and Caribbean, where the lumber and pallets at issue were produced. Under certain circumstances, TBP breaks down into another chemical, 2,4,6-tribromoanisole (TBA) and gives off a musty moldy smell. Both Johnson & Johnson and Pfizer have contended that their products were contaminated as a result.

While many pallet experts are skeptical that the odor could have penetrated a cardboard shipping container, consumer packaging and the foil seal and cotton ball inside a plastic bottle of Tylenol, the point is moot: Perception is reality and the perception is that mold and chemical treatments are a problem. The response by some large pallet users in the food, pharmaceutical and healthcare industries has been a major headache for pallet producers.

For instance, a medical device manufacturer shared with Modern a letter it had received from the chief procurement officer of one of the largest healthcare products distributors in the world requiring that it certify it’s pallets were chemical free. Other end users are mandating not only that their pallets are free of chemicals but also that they are heat treated to kill the mold or that the pallets have a certain minimum moisture level. Unfortunately, none of those approaches will necessarily solve the problem. “The issue is fungicides do work against mold,” says Edgar Deomano, technical director for the National Wooden Pallet & Container Association (http://www.palletcentral.org). “The problem is that pallet users in sensitive industries think that any chemical that comes into contact with wood can cause that reaction. So they want a non-chemical treatment or they want heat treatment, when in fact heat treating can encourage the growth of mold.”

Modern talked to Deomano and Brian Bond, an associate professor in the department of wood science and forest products at Virginia Tech to try to answer some of the questions around pallets and mold. Here’s what we learned.

What causes moldy pallets?
Mold spores are everywhere, looking for a place to land and propagate. That’s the reason a piece of bread develops mold after a while, even inside a closed refrigerator.  As it turns out, your basic green hardwood pallet is a pretty good source of moisture and sugars for mold spores. In part, that’s because the pallet industry today is a just-in-time industry. Pallet makers want to minimize the amount raw material they keep in inventory, which means that lumber doesn’t sit around long after it comes off the saw. Nor do they want to keep an inventory of finished pallets, which might otherwise air dry. “A typical green pallet will have an average moisture content of between 35% and 60%,” says Bond, who adds that a pallet with an average moisture content of 20% will resist mold growth if properly stored and handled. But, to get to that average moisture content requires some cost and effort. 

Does mold affect the performance of the pallet?
Not according to Bond. “Mold grows on the surface of the pallet,” he says. “It is not structurally degrading to the pallet.” Mold on a pallet does look bad and appearances count. There is also a concern that mold will contaminate the product, which could be a problem if it comes into direct contact with food products. For his part, Bond has consulted with companies on mold issues and has not seen product contamination, although he understands it’s possible. “Typically, what I’ve seen is the fear of mold contaminating a product,” he says.

Does heat treating kill mold?
Yes and no, say Bond and Deomano. Heat treating is typically done to kill insects or insect larvae that may be in the wood. “Yes, it will also kill mold, but heat treating is not designed to dry the pallet,” says Bond. The heat treatment process actually brings moisture to the surface of the wood. “That’s an ideal mold-growing environment if the pallet isn’t properly handled,” says Bond. A pallet manufacturer can start a core drying cycle at the end of the heat treatment, but that adds a cost to the pallet that most users are not willing to pay for.

What about air drying or kiln-dried lumber?
Bond and Deomano have seen recent pallet mandates that call for a pallet dried to an average 8% moisture content. According to Bond, a pallet dried to an average 20% moisture content won’t have mold growth, if it’s properly handled. The question is how to get to the 20% average moisture content.

Kiln drying hardwood lumber is an expensive proposition and could result in split boards. However, there is a market for kiln-dried pallets, and there are suppliers providing them. 

Air drying is less expensive than kiln drying, but it takes time. “Most pallet suppliers say they don’t have the time to air dry a pallet because their customers want them yesterday,” says Deomano.

Used pallets are more likely than new pallets to meet the average 20% moisture content advocated by Bond, simply because they have been in use for some time. Many users, however, will not accept a used pallet.

A pallet constructed from kiln-dried softwood lumber will meet that criteria and won’t have mold growth, says Deomano, if the pallet is properly handled. “However, if you store the pallet in the yard and it gets wet, you can get mold on a kiln-dried pallet too,” he adds. In fact, mold will develop on plastic, metal and corrugated pallets if they get wet and are stored in warm, damp places.

How then can I prevent mold?
The simplest way to avoid mold, say both Deomano and Bond, is to start with the basics.
·  Realize that where your pallet is going to be shipped and how it’s going to be used can impact whether you need to worry about mold. “Overall, I suggest that pallet suppliers talk to their customers about the differences between green wood, air dried wood, kiln dried wood and mold,” says Deomano. “To most end users, a pallet is just a pallet, but it’s carrying a valuable product.”

· Understand that how you store and handle your pallet can go a long way to preventing mold. Keeping them dry and clean is a start. Avoid storing them in the yard if possible, or storing a load of pallets in an enclosed trailer for an extended period of time. If you wash your pallets down for sanitary reasons, make sure they are dry before you put them back into use. Where possible, avoid storing pallets in closed trailers for any length of time. “I’ve been in facilities where the user keeps a load of green pallets in a closed trailer,” says Bond. “During the day, it gets hot and raises the relative humidity inside the trailer. At night, when the temperature drops, it hits the dew point, adds more water and promotes mold growth.” Storing palletized product in a trailer, at least for short time intervals, doesn’t have the same affect, Bond adds. “If you only have 20 or so pallets in the trailer, that’s not enough to create a problem,” he says.

· Consider chemicals when appropriate. “There are some very good products out there that will prevent mold and will not create a problem,” says Bond. “But, that’s really an issue of whether they will be accepted by the final customer.” 

· Consider an alternative pallet. Any pallet can develop mold if it’s not properly handled, but presswood pallets, corrugated pallets, plastic pallets and metal pallets may be less susceptible to mold than green hardwood pallets. Bear in mind, however, that alternative pallets are still niche products; they may not be accepted by your customer; may not work in your application; or, in the case of plastic and metal, may be more expensive than a comparable wooden pallet.

About the Author

Bob Trebilcock
Executive Editor

Bob Trebilcock, executive editor, has covered materials handling, technology and supply chain topics for Modern Materials Handling since 1984. More recently, Trebilcock became editorial director of Supply Chain Management Review. A graduate of Bowling Green State University, Trebilcock lives in Keene, NH. He can be reached at 603-357-0484.


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