Process for simultaneous saccharification and fermentation for production of ethanol

1. A method for the production of ethanol comprising:

a) providing pretreated biomass comprising insoluble solids and polysaccharides;

b) providing at least one saccharification enzyme for the conversion of polysaccharides to fermentable sugars;

c) providing a prokaryotic ethanologen;

d) preparing, in a bioreactor comprising agitation using an impeller, a saccharification-fermentation mixture comprising the pretreated biomass of a), the saccharification enzyme of b), and the prokaryotic ethanologen of c); and

e) growing the prokaryotic ethanologen in the saccharification-fermentation mixture wherein, the concentration of total input insoluble solids in the saccharification-fermentation mixture is at least about 16% based on dry weight per liter, wherein the agitation provides an impeller power to mass radio of no more than about 0.2 watt/kg of total saccharification-fermentation mixture in the presence of biomass particles of size between 100 μm and 600 μm, and wherein the prokaryotic ethanologen produces ethanol.

2. The method of claim 1 wherein the prokaryotic ethanologen is a member of a genus selected from the group consisting of Zymomonas, Zymobacter, Clostridium, Escherichia, Klebsiella , and Geobacillus.

3. The method of claim 1 wherein the agitation using a rotary stirrer comprises at least one impeller.

4. The method of claim 1 wherein the ethanologen of c) is added at a time after addition of the saccharification enzyme of b) when partial saccharification has occurred.

5. The method of claim 1 wherein pretreated biomass is added in at least two portions that in combination give a total input insoluble solids concentration of at least about 16% based on dry weight per liter.

6. The method of claim 1 wherein the concentration of total input insoluble solids in the saccharification-fermentation mixture is at least about 20% based on dry weight per liter.

7. The method of claim 1 wherein the prokaryotic ethanologen is tolerant to the acetate concentration in the saccharification-fermentation mixture.

8. The method of claim 1 wherein biomass is selected from the group consisting of switchgrass, waste paper, sludge from paper manufacture, corn cobs, corn husks, corn stover, grasses, wheat, wheat straw, hay, barley straw, rice straw, sugar cane bagasse, sorghum, components obtained from processing of grains, trees, branches, roots, leaves, wood chips, sawdust, shrubs and bushes, vegetables, fruits, flowers and animal manure.

9. The method of claim 1 wherein the pretreated biomass is produced by treating cellulosic biomass with ammonia.

10. The method of claim 9 wherein the ammonia is less than about 12 weight percent relative to dry weight of biomass.

11. The method of claim 1 wherein the at least one saccharification enzyme is selected from the group consisting of cellulose-hydrolyzing glycosidases and hemicellulose-hydrolyzing glycosidases

12. The method of claim 1 wherein the at least one saccharification enzyme is a member of an enzyme consortium.

13. The method of claim 12 wherein the saccharification enzyme consortium comprises enzymes selected from the group consisting of cellulases, endoglucanases, exoglucanases, cellobiohydrolases, β-glucosidases, xylanases, endoxylanases, exoxylanases, β-xylosidases, arabino-xylanases, mannases, galactases, pectinases, glucuronidases, amylases, α-amylases, β-amylases, glucoamylases, α-glucosidases, isoamylases, peptidases, lipases, ligninases, and feruloyl esterases.

14. The method of claim 1 wherein the polysaccharides comprise xylan and glucan.

15. The method of claim 1 wherein the fermentable sugars comprise xylose and glucose.

16. The method of claim 1 wherein the concentration of ethanol produced is at least about 40 g/L.

17. A saccharification-fermentation system comprising:

a) a pretreated biomass comprising insoluble solids and polysaccharides;

b) at least one saccharification enzyme for the conversion of polysaccharides to fermentable sugars; and

c) a prokaryotic ethanologen;

wherein the biomass of a), enzyme of b), and ethanologen of (c) are combined in a saccharification-fermentation mixture, having a concentration of total input insoluble solids that is at least about 16% based on dry weight per liter in a bioreactor comprising agitation using a rotary stirrer, and wherein the agitation provides no more power than about 0.2 watt/kg of total saccharification-fermentation mixture in the presence of biomass particles of size between 100 μm and 600 μm.

18. The saccharification-fermentation system of claim 17 wherein the prokaryotic ethanologen is selected from the group consisting of Zymomonas, Zymobacter, Clostridium, Escherichia, Klebsiella , and Geobacillus.