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

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.

Automated summary

Oak heartwood darkens during drying, and non-colored areas on wood boards are caused by heterogeneous distribution of tannins. Raman and fluorescence microscopy revealed crystalline ellagic acid precipitations in non-colored areas, especially in ray cells with higher density. Dense areas accumulate hydrolyzed tannins during drying, leading to irreversible precipitation of non-colored ellagic acid and preventing chemical reactions causing wood darkening. Adjusting drying and processing parameters is necessary to avoid water domains and ellagic acid precipitations in wood boards with pronounced density fluctuations.