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.

中文翻译：

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橡木干燥：结晶鞣花酸沉淀导致变色

橡木心材在干燥过程中和干燥后通常会变暗。这种变暗可能是不均匀的，在木板上留下无色区域。这些轻微的变色与单宁的不均匀分布有关，但缺乏令人信服的微观证据。在这项研究中，拉曼和荧光显微镜揭示了结晶鞣花酸的沉淀，尤其是在射线细胞中，而且在密度较高的非着色区域 (NCA) 的腔、细胞角和细胞壁中也有沉淀。在这些更密集的区域，干燥过程中游离水的存在时间更长，并导致水解单宁的积累。当最终变干时，这些单宁会以无色鞣花酸的形式不可逆地沉淀，并且不能用于导致木材变黑的化学反应。所以，