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Licensed Unlicensed Requires Authentication Published by De Gruyter January 7, 2021

Oak wood drying: precipitation of crystalline ellagic acid leads to discoloration

  • Martin Felhofer ORCID logo , Peter Bock ORCID logo EMAIL logo , Nannan Xiao ORCID logo , Christoph Preimesberger , Martin Lindemann ORCID logo , Christian Hansmann and Notburga Gierlinger ORCID logo
From the journal Holzforschung


Oak heartwood usually darkens during and after drying. This darkening can be heterogeneous, leaving non-colored areas in the wood board. These light discolorations have been linked to heterogeneous distribution of tannins, but compelling evidence on the microscale is lacking. In this study Raman and fluorescence microscopy revealed precipitations of crystalline ellagic acid, especially in the ray cells but also in lumina, cell corners and cell walls in the non-colored areas (NCA), which also had higher density. In these denser areas free water is longer present during drying and leads to accumulation of hydrolyzed tannins. When eventually falling dry, these tannins precipitate irreversible as non-colored ellagic acid and are not available for chemical reactions leading to darkening of the wood. Therefore, pronounced density fluctuations in wood boards require adjusting the drying and processing parameters so that water domains and ellagic acid precipitations are avoided during drying.

This paper is dedicated to Prof. Adya Singh who spent the last months of his long and fruitful scientific carrier with us. We are very grateful that he chose our working group for his last stay abroad and we profited a lot from his knowledge and insight. Besides that, we also enjoyed the time off-duty with him and wish him a beautiful retirement in New Zealand.

Corresponding author: Peter Bock, Department of Nanobiotechnology (DNBT), Institute for Biophysics, University of Natural Resources and Life Sciences, Muthgasse 11-II, 1190 Vienna, Austria, E-mail:

Award Identifier / Grant number: European Union’s Horizon 2020 research and innov

Funding source: Austrian Science Fund

Award Identifier / Grant number: START Project (Y-728-B16)

Award Identifier / Grant number: DOC-Fellowship (24763)


We thank the whole bionami research group for helpful comments ( and Wood K plus – Competence Centre for Wood Composites and Wood Chemistry ( for the collaboration. We especially thank Tayebeh Saghaei for the fruitful scientific discussions.

  1. Author contributions: M.F. carried out Raman microscopic imaging, stainings, density measurements, interpreted the data and wrote the manuscript. N.G. had the idea for the manuscript, conducted analysis of the Raman images and assisted in manuscript writing. N.X. conducted the fluorescence microscopic experiments. C.P. carried out the drying experiments. M.L. conducted the HPLC measurements. C.H. assisted in data interpretation and manuscript writing. P.B. measured Raman, IR and UV-spectra of all reference compounds, interpreted the data and wrote the manuscript.

  2. Research funding: This work is supported by a fellowship of the Austrian Academy of Science (ÖAW) [24763], the START Project [Y-728-B16] from the Austrian Science Fund (FWF) and from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program grant agreement no. 681885.

  3. Conflict of interest statement: The authors declare no conflicts of interest regarding this article.


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Supplementary Material

The online version of this article offers supplementary material (

Received: 2020-07-02
Accepted: 2020-11-27
Published Online: 2021-01-07
Published in Print: 2021-08-26

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