White clouds - Undesired discolouration of oak wood during kiln drying

The issue of inhomogeneous colour changes of oak wood during drying processes is well known in wood industry and can cause drastic losses in quality. These colour inhomogeneities, namely darkening from the edges, are frequently also called "white clouds". Although the phenomenon is well known, the reasons and conditions how and when the colour change occurs are not fully understood or controversially discussed. Own investigations show the connection between discoloration and moisture distribution. To further investigate the background of the colour differences, extractions with hexane, acetate and formate were first carried out and analysed by GC-MS. However, no differences in detected substances could be found. Also, ATR/FTIR spectra of light and dark areas showed no differences in the spectrum. By means of Raman measurements on sections of light and dark areas, indications could be detected. The hypothesis is that tannins in the more humid regions hydrolyse to ellagic acid. Ellagic acid was found preferentially in the lighter areas.