Analysis of Degradation Products from Wood and Methods for Detoxification of Lignocellulose Hydrolysates

Abstract: In order to produce fuel ethanol from lignocellulose, it is necessary to hydrolyse the sugar-containing constituents hemicellulose and cellulose. During acid hydrolysis, a number of compounds are formed or liberated in addition to the fermentable monomeric sugars. Several of these compounds might act as inhibitors during the fermentation of the sugars to ethanol. Thus, in order to obtain high ethanol yields and productivities it is necessary to remove these inhibiting compounds prior to fermentation. A previously well-known detoxification method, alkali detoxification, was carefully investigated with the intention to explain the mechanism behind the detoxification effects as well as to elucidate the optimal conditions during treatment. The effect of treatments with different forms of alkali on 21 compounds in the hydrolysate was quantified after separation using reversed phase high-performance liquid chromatography (RP-HPLC) and capillary electrophoresis. It was discovered that the inhibitors are not physically removed, but rather chemically converted by this treatment. It was also concluded that compounds added or formed during the treatment might have positive effects on the yeast during fermentation. Furthermore, it was realised that the alkali methods need careful optimisation to obtain maximum detoxification effect in unison with minimum degradation of fermentable sugars. In addition, a novel method for detoxification was developed, namely countercurrent supercritical fluid extraction. This method relies on the partitioning of inhibitors between the hydrolysate and a supercritical carbon dioxide phase in a pressurised countercurrent flow system. It was discovered that this method decreased the concentration of inhibitors more than the alkali treatments. Moreover, as the extracted inhibitors were trapped and concentrated subsequent to the extraction, this method facilitated the analysis of inhibitors. A relationship between retention in RP-HPLC and extractability was established for the aromatic compounds identified in the hydrolysate. Furthermore, the enzymatic degradation and simultaneous product formation of glucomannan and O-acetyl-galactoglucomannan, two major hemicellulose constituents, were studied.