"Caramelizing" Issues when Drying Hard Maple

      Here's a technical explanation of the chemical process that makes some wood turn brown during kiln-drying. December 12, 2008

Question
We dry lots of hard maple and I have a question about caramelizing. Is it possible for this to occur while air drying, or does it just happen while in the kiln? We have not changed any of our schedules in the past decade and all of a sudden we are having this problem. Don't know how to explain it. The only thing I can think of is that we have probed easy to dry boards, so the temp goes up quicker, yet there are harder to dry boards in there drying too fast.

Forum Responses
(Sawing and Drying Forum)
From contributor M:
Caramelization and/or the maillard reaction, which both yield similar results (browning), need sugar to be present along with amino acids (proteins), both present in wood, and heat. The temps I have seen listed for the maillard reaction and caramelization are above the boiling point of water, so they could not happen during air drying. The other browning reaction that may occur is some sort of oxidation (think apple slices). I am not sure what could be causing that in maple though.



From contributor M:
After I posted this I saw they have info for both reactions listed on Wikipedia. You need about 230F before anything will happen with either one.


From Professor Gene Wengert, forum technical advisor:
Browning when drying hard maple lumber can occur at 130 F or hotter. In fact, the color change can be set up to occur at even lower temperatures. The information that contributor M provided is for the more technical definition of caramelization of a sugar, but it is not the correct definition for the term that you are using when talking about color change (browning) in lumber.

The browning is an enzymatic oxidation reaction. The starches and sugars in the ray parenchyma cells are going through a multi-step chemical reaction. This reaction can actually start in the log. Periods of wet weather, especially if rain is present, seem to accentuate the color change. The link below provides more information.

Stain Causes and Cures in Lumber



From contributor M:
Just as a theoretical idea, I wonder if a mild acid solution sprayed on the wood could prevent this (might damage equipment though). In my day job, where I am a PhD chemical engineer, we use acidic conditions and/or EDTA to prevent enzymatic degradation and browning.


From Professor Gene Wengert, forum technical advisor:
This has been tried, but the main problem to such an approach is that the acid must go into the wood when the wood is green. Further, as stated, the reaction can begin in the log so that techniques applied to the lumber are not effective. In fact, methyl bromide has been used on fresh, green wood to kill the ray parenchyma cells. Anti-oxidants also work but are not practical.

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Have you reviewed the related Knowledge Base areas below?
  • KnowledgeBase: Knowledge Base

  • KnowledgeBase: Primary Processing

  • KnowledgeBase: Primary Processing: Kiln Operation


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