Felhofer Martin, Bock Peter, Xiao Nannan, Preimesberger Christoph, Lindemann Martin, Hansmann Christian, Gierlinger Notburga
Department of Nanobiotechnology (DNBT), Institute for Biophysics, University of Natural Resources and Life Sciences, Muthgasse 11-II, 1190 Vienna, Austria.
Institute of Wood Technology and Renewable Materials, Konrad Lorenz-Straße 24,3430 Tulln, Austria; Wood K plus - Competence Centre for Wood Composites and Wood Chemistry, Konrad-Lorenz-Straße 24, 3430 Tulln, Austria.
Holzforschung. 2021 Jan 7;75(8):712-720. doi: 10.1515/hf-2020-0170.
Oak heartwood usually darkens during and after drying. This darkening can be heterogeneous, leaving noncolored 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.
橡木心材在干燥过程中及干燥后通常会变深。这种变深可能是不均匀的，会在木板上留下未变色区域。这些浅色变色与单宁的不均匀分布有关，但在微观尺度上缺乏确凿证据。在本研究中，拉曼显微镜和荧光显微镜显示，结晶鞣花酸沉淀，特别是在射线细胞中，但也在未染色区域（NCA）的管腔、细胞角和细胞壁中，这些区域的密度也较高。在这些密度较高的区域，干燥过程中自由水存在的时间更长，导致水解单宁积累。当最终变干时，这些单宁不可逆地沉淀为无色鞣花酸，无法参与导致木材变黑的化学反应。因此，木板中明显的密度波动需要调整干燥和加工参数，以便在干燥过程中避免水域和鞣花酸沉淀。
