Determination of the lignin - and carbohydrate composition contained in the woods

The cell walls of wood essentially consist of three layers: the primary wall, a multi-layered secondary wall and the tertiary wall. The main components of wood, lignin, hemicellulose and cellulose are heterogeneously distributed in the cell walls. The largest proportion is cellulose, which accounts for an average of 40-50% of the total wood mass. Cellulose is a polysaccharide, which consists of a large number of glycosidically linked glucose molecules. When the long, unbranched cellulose molecules are arranged in parallel to form larger When the long, unbranched cellulose molecules are arranged in parallel in larger bundles, fibrous structures (fibrils) are formed, which in turn can form similar, larger structural units. They give the wood the necessary longitudinal stability and tensile strength.

Hemicelluloses make up 27-40% of hardwoods and 25-30% of softwoods. The plural ending already indicates that this is a group of polysaccharides consisting of different sugars, primarily xylose and mannose, but also arabinose, galactose, glucose and rhamnose, as well as uronic acid (4-O-methylglucoronic acid → sugar acid). In contrast to cellulose, they form much shorter and slightly branched macromolecules. Together with lignin, they strengthen the cellulose skeleton.

Lignin is present on average at 18-25% in hardwoods and 22-35% in softwoods. The three-dimensionally cross-linked, amorphous biopolymer consists of phenylpropane units linked together in a variety of (chemical) forms. This rigid matrix causes the compressive strength of the cells. In addition, lignin is much more resistant to chemical or biological degradation than polysaccharides. or biological degradation processes.

As a simplified image of the interaction of these three components, a comparison with a fibre composite material (e.g. GRP) can serve. In this composite material, a synthetic resin provides a rigid, pressure-resistant structure in which fibres/fabrics are inserted to absorb bending and tensile forces. bending and tensile forces. In wood, the cellulose fibrils would correspond to fibres embedded in a solid matrix of lignin and hemicellulose.

If wood survives over archaeologically relevant periods of time, this is usually due to an anaerobic, water-saturated environment of the site. Under such circumstances, the degradation of the wood substance is usually carried out by anaerobic bacteria, which preferentially break down the polysaccharides (cellulose, hemicellulose) enzymatically and hydrolyse them. enzymatically and hydrolytic reactions. These processes proceed much more slowly than degradation by fungi, which are even able to decompose lignin, but can only thrive in aerobic environments. The extent to which wood is degraded can therefore also be read from the altered quantity ratio of the main components forming the cell wall: lignin, hemicellulose and cellulose. If the polysaccharides degrade, the lignin content increases as a percentage of the remaining amount.

To determine the quantity ratios, a carbohydrate analysis was carried out by the University of Hamburg, Centre for Wood Economics. Only small quantities (1 g dry matter) are required as sample material, so that in advance from the test woods (with an average water content) was cut in advance from the test wood (with an average water content for the respective test series). In the laboratory, the wood samples were In the laboratory, the wood samples were dried in a vacuum drying oven at 40°C and then ground to a fine powder using a cryogenic mill under liquid nitrogen. After a two-stage acid hydrolysis, in which the polysaccharides are split into their monomers, the hydrolysate is qualitatively and quantitatively analysed by HPLC (in this case, borate anion exchange chromatography) for the for the monomeric sugars typically found in wood. The filtered hydrolysis residue was weighed and can be used as a measure of the lignin contained in the sample.

The detailed test results can be accessed under the menu item "Test series - Unpreserved reference sample - Cellulose/Lignin content". In the tables, one finds the results of three hydrolyses carried out for each sample, the resulting mean value, and the absolute and relative deviation of the individual results from the mean value. Under the table "Original data" are the percentage the total sample quantity, whereby the missing difference to 100% is acid-soluble lignin, degradation products (which were not detected by chromatography) or extractives which were not determined separately. Under "Carbohydrates normalised to 100%" the values are the values are converted exclusively to a consideration of the monomeric sugars. The last table, "Total analysis..." corresponds more or less to the "Original data" table with the difference that the non-analysable residue is ignored and the values are normalised to 100%.

The distribution calculated from these results for the three components cellulose, hemicellulose and lignin, taking into account the decomposed wood mass, can be found as a graph in the menu item "Test series - cellulose/lignin content". Average quantity ratios of recent woods of the same species serve as a reference.

The graph is intended to make two things quickly apparent: firstly, the loss of mass, which is shown as a percentage reduction in area in accordance with the different densities of space, and secondly, the main substances still remaining in the wood. The first is the mass loss, which is shown as a reduced area in percent according to the different densities, and the second is the main cell wall-forming substances still remaining in the wood, whose altered quantity ratio is characterised by the different degrees of degradation.

What is striking about all the results is the almost expected high value of the hydrolysis residues, i.e. the lignin content. The loss of the more easily degradable carbohydrates naturally increases the value of the lignin content in relation to the dwindling cellulose values.