A multitude of different pretreatment methods have been suggested during the past few decades. They can loosely be divided into different categories: physical (e. g. milling, grinding and irradiation), chemical (e. g. alkali, dilute acid, oxidizing agents and organic solvents), physicochemical (e. g. steam pre — treatment/autohydrolysis, hydrothermolysis and wet oxidation) and biologi­cal, or combinations of these. In general, it is difficult to place the methods into one category only.

A rough classification of the pretreatment methods can also be made ac­cording to the following:

• Acid-based methods, i. e. pretreatment at low pH, result in hydrolysis of the hemicellulose to monomer sugars and minimize the need for hemicel — lulases.

• Methods working close to neutral conditions, e. g. steam pretreatment and hydrothermolysis, solubilize most of the hemicellulose due to the acids re­leased from the hemicellulose, e. g. acetic acid, but do not usually result in total conversion to monomer sugars. This thus requires hemicellulases acting on soluble oligomer fractions of the hemicellulose.

• Alkaline methods leave a part of the hemicellulose, or in the case of ammonia fibre explosion (AFEX), almost all hemicellulose in the solid fraction. This then requires hemicellulases acting both on solid and on dissolved hemicellulose. An alternative is to perform an acid hydrolysis of this fraction, for instance after removal of the cellulose by enzymatic hydrolysis.

This affects, of course, not only the method that should be used for assess­ment of the pretreatment but also the cost of the overall hydrolysis of the carbohydrates.

3.1