Can HT pallets mold?

Question: Can a heat treated pallet mold and why?


Short answer:

Yes, Mold can begin growing on already heat treated pallets.

Although heat treating pallets will kill existing mold (it is dead but will not remove it physically), it will not prevent mold from attacking the pallet after heat treating. Actually, based on our current experiences heat treating pallets (without any drying) increases the likeliness of mold. The only way to reduce or nearly eliminate mold is to either dry the pallets or some kind of chemical surface treatment (by dipping or spraying the pallets). Kiln-direct pallet kilns and controls are always designed to both heat treat and dry pallets. A cool down cycle after the heat treatment process can also be very beneficial, some of our customers spend as much time cooling down as heating up to lower the risk.


Detailed answer:

Let us first warn you that this detailed answer will is a little bias to our own system and we will be blowing our own horn a little bit. Since we feel very fortunate that we have designed kilns that help reduce and eliminate this problem from the beginning.

Mold can attack lumber that is above a certain moisture content (most experts believe this critical moisture level to be between 22% to 30%). Although most heat treatment processes (except direct steam heat treating) removes some moisture from the pallets it is not sufficient to get the pallets sufficiently dry to eliminate or minimize the conditions for mold to attack and grow. Actually, based on our current experiences the heat treatment process may push some of the moisture and sugar to the surface. This provides mold with both a good conditions to develop and grow and may even make mold a little more likely on heat treated pallets than non-heat treated pallets.

How to solve this problem:

Cool down the pallets properly:

This is probably the most common method we see to significantly reduce the potential of mold. Cool down can either happen by leaving the pallets for 4-8 hours in a well ventilated area of fresh air. If you have purchased a Kiln-direct pallet kiln you also have the option of making the cool down part of the heat treatment cycle. All our kilns have high capacity power exhaust that allow for efficient cool down of the pallets and remove surface moisture as part of the HT cycle.
The cooling down has two significant benefits for reducing the potential of mold. First, if a load of hot pallets (not dried) is placed directly into a container or van they will cool down by releasing moisture and creating a water saturated climate, during the warmer weather this makes mold almost certain if the pallets are left in the container/van for several days. Secondly, when you cool down the pallets while still in the kiln for up to 2-3 hours you will be removing a significant amount of surface moisture and latent heat energy. This means they do not "steam" up the container or van in the same way as before. Furthermore, most of Kiln-direct pallet kilns use a direct gas heating system with also creates a drying climate during the whole process = a drier surface = lower chance of mold.

Kiln drying pallets

You can kiln dry pallets and crating components in about 15-20 hours in most situations (Some lumber drying experts still claim that it will take 4-7 days, but we have proven otherwise). This drying bring the average moisture content to 15-25%. This should be sufficient to minimize the change of mold (not eliminate totally, but minimize the change). Not all kilns systems can accomblish this in the mentioned time frame. But nearly all Kiln-direct pallet kilns have been designed to drying standard stringer (also oak) pallets in the mentioned time frames. Learn more about drying pallet here.

Chemical treating

A few of our customers use an anti-fungicide to eliminate or reduce mold on both heat treated and non-heat treated pallets. This can either be done as part of the production line with a spray booth or in a separate dip tank.