Process for producing a sugar solution by combined chemical and enzymatic saccharification of polysaccharide enriched biomass

1. A method of producing a concentrated sugar solution from biomass, the method comprising:

a) delignifying biomass comprising the substeps of

i) contacting with water and at least one nucleophilic base, a biomass comprising lignin and having a glucan/xylan weight ratio G 1 /X 1 to form a biomass slurry having a pH of about 12.5 to about 13.0; and

ii) maintaining the biomass slurry under reaction conditions such that the slurry attains a pH of about 9.5 to about 10.0 and has a glucan/xylan weight ratio G 2 /X 2 within about 15% of the value of G 1 /X 1 , and wherein the slurry comprises a lignin-containing liquid fraction and a solid fraction comprising a polysaccharide enriched biomass;

wherein G 1 and G 2 are grams of glucan per 100 grams of biomass and biomass slurry respectively, and X 1 and X 2 are grams of xylan per 100 grams of biomass and biomass slurry respectively;

b) contacting with an aqueous acid solution comprising at least one mineral acid the solid fraction of the polysaccharide enriched biomass at reaction conditions sufficient to produce an intermediate saccharification product comprising xylose, xylan, and glucan, wherein the concentration of the solid fraction in the aqueous acid solution is about 13 weight percent to about 20 weight percent; and

c) contacting with a saccharification enzyme consortium at a pH of from about 4.5 to about 5.5 the intermediate saccharification product at reaction conditions sufficient to produce a final saccharification product comprising at least about 7 percent by weight fermentable sugars, based on the total weight of the saccharification product, in 24 hours of contact with the saccharification enzyme consortium.

2. The method of claim 1 , wherein the at least one nucleophilic base comprises a water soluble metal hydroxide, optionally in combination with a metal carbonate or an organic hydroxide.

3. The method of claim 1 , wherein the reaction conditions sufficient to produce a polysaccharide enriched biomass include a temperature from about 20° C. to about 110° C. and a reaction time from about 4 hours to about 30 days.

4. The method of claim 1 , wherein the value of G 2 /X 2 is within 10% of the value of G 1 /X 1 .

5. The method of claim 1 , further comprising isolating at least a portion of the polysaccharide enriched biomass solid fraction.

6. The method of claim 1 , wherein the at least one mineral acid is selected from the group consisting of sulfuric acid, phosphoric acid, hydrochloric acid, nitric acid, and a combination of these.

7. The method of claim 6 , wherein the concentration of the mineral acid in the aqueous acid solution is about 0.1 weight percent to about 5 weight percent.

8. The method of claim 1 , wherein the reaction conditions sufficient to produce an intermediate saccharification product include a temperature from about 70° C. to about 160° C.

9. The method of claim 1 , wherein the reaction conditions sufficient to produce an intermediate saccharification product include a reaction time from about 10 minutes to about 200 minutes.

10. The method of claim 1 or 5 , wherein at least about 50 percent of the xylan in the solid fraction of the polysaccharide enriched biomass is hydrolyzed in the intermediate saccharification product.

11. The method of claim 1 , wherein the final saccharification product comprises at least about 12 percent by weight sugars in 72 hours.

12. The method of claim 1 or 5 , wherein the composition of the solid fraction of the polysaccharide enriched biomass, on a dry weight basis, is greater than about 80% polysaccharide.

13. The method of claim 1 , wherein the final saccharification product comprises at least one sugar monomer selected from the group consisting of glucose, arabinose, xylose, mannose, galactose, and a combination of these.