FIELD OF THE INVENTION

The present invention relates to a method for extracting sugars and derivates thereof and the corresponding polysaccharides especially hemicelluloses from lignocellulosic biomass with hot water. In particular the present innovation concerns a method to produce high concentration of high purity hemicellulose extract with minimal use of water. The extract is circulated through the biomass several times and as a result the concentration of the hemicelluloses in the water phase is increased. The circulating loop is vital in order for the non-hemicellulose material, such as lignin, to be adsorbed back to the biomass. The circulation also enables gentle extraction conditions regarding temperature, pressure and pH as the method does not require maximum extraction during the first extraction loop. Minimal oxidation and degradation of the hemicelluloses is achieved by removing virtually all of the air and especially oxygen gas from inside of the pores of the biomass during a impregnation step assisted by reduced pressure. The concentration of the extract is increased by using the same extract in several extractions of new and previously un-extracted biomass.

DESCRIPTION OF RELATED ART

Wood based biomass contains about 25% of hemicelluloses. Extraction of biomass is a very well known process and has traditionally been extracted with alkali. During this kind of treatment hemicelluloses are degraded and the fibrous material is decomposed.

Traditional extraction with only hot water degrades the sugars into toxic compounds such as furfural.

Steam explosion is another well know method for extracting carbohydrates from biomass. However, even this method involves the formation of toxic compounds.

A recent method for extracting hemicelluloses from biomass involves continuous flow of hot water extraction. The problem with this method is, that it produces a very diluted extract containing up to 30% lignin impurities (WO 2009/122018 and Leppanen et al.: Pressurized hot water extraction of Norway spruce hemicelluloses using a flow-through system, Wood Sci Technol (2011) 45:223-236)

All the known extraction methods of hemicelluloses results in extremely low solids content of the extract, and require expensive concentration of the extract. This results in an economically unsound product for transport and further treatment. (WO0034568, US2008029233, U.S. Pat. No. 8,136,747, DE3225074).

A recent patent publication WO2007090926 describes a method for recovering hemicelluloses with higher concentration through steam hydrolysis. The drawback with this approach is that hydrolysis always involves deacetylation of the hemicelluloses which effectively lowers their water solubility and also degrades the polymers by lowering the molar mass. Chemical and food industry are interested in polysaccharides with high molar mass which the aforementioned method is unable to provide.

Publication WO0061276 discloses a method for hydrolysis of hemicelluloses primarily into monomeric sugars which is a drawback especially if polymers are desired.

The main problem with known methods is the fact that they do not efficiently prevent oxidation, hydrolysis, degradation and deacetylation of the biomass. Especially the presence of air and oxygen gas inside of the pores of the biomass has prevented an efficient and gentle extraction procedure. Traditionally biomass impregnation and air removal from inside of biomass capillaries has been for example achieved through presteaming of biomass with high temperature steam and high pressure. However, this procedure causes oxidation and hydrolysis of hemicelluloses and is therefore unsuited for extracting hemicelluloses without degradation.

SUMMARY OF THE INVENTION

It is an aim of this invention it to overcome the problems related to known art and to provide an improved method for extracting biomass with water.

In particular, it is an aim of the invention to provide a method which effectively minimizes oxidation and degradation of the biomass extract.

It is further an aim to provide a method to concentrate and purify the extract with minimal water consumption, without the need for expensive evaporation and purification steps.

More specifically, the method according to the invention is characterized by what is stated in claim1.

