Methods for propagating microorganisms for fermentation and related methods and systems

1. A method of propagating a microorganism, the method comprising:

providing a lignocellulosic hydrolysate comprising at least one monosaccharide and at least one inhibitor, where the at least one inhibitor is at a first concentration, wherein the at least one inhibitor is a byproduct of hydrolysis of lignocellulosic biomass, and wherein the at least one inhibitor is an inhibitor of fermentation by the microorganism;

reducing the first concentration of the at least one inhibitor in a portion of the lignocellulosic hydrolysate to form a treated lignocellulosic hydrolysate having a second concentration of the at least one inhibitor;

providing a propagation medium comprising at least a portion of the treated lignocellulosic hydrolysate, wherein the at least one monosaccharide is at a concentration up to 2.0 wt % of the propagation medium;

growing a first cell mass of the microorganism on the propagation medium to form a second cell mass of the microorganism, wherein the second cell mass of microorganism is greater than the first cell mass of the microorganism; and

adding a portion of the lignocellulosic hydrolysate to the propagation medium and growing the second cell mass of microorganism to form a third cell mass of the microorganism, wherein the portion of the lignocellulosic hydrolysate comprises the at least one inhibitor at the first concentration.

2. The method of claim 1 , wherein providing the lignocellulosic hydrolysate comprises:

providing lignocellulosic biomass comprising hemicellulose, cellulose, and lignin; and

hydrolyzing at least a portion of the lignocellulosic biomass to form the at least one inhibitor at the first concentration and a saccharide chosen from one or more oligosaccharides, the one or more monosaccharides, and combinations thereof, wherein the one or more monosaccharides are chosen from one or more pentoses, one or more hexoses, and combinations thereof.

3. The method of claim 2 , wherein providing the lignocellulosic hydrolysate further comprises enzymatically hydrolyzing at least a portion of the cellulose in the lignocellulosic biomass.

4. The method of claim 2 , wherein providing the lignocellulosic hydrolysate further comprises subjecting the lignocellulosic biomass to steam explosion.

5. The method of claim 1 , wherein the hydrolyzing comprises contacting the lignocellulosic biomass with hot water, and wherein the at least one inhibitor is a byproduct of contacting the lignocellulosic biomass with the hot water.

6. The method of claim 1 , wherein the at least one inhibitor is a byproduct of contacting the lignocellulosic biomass with a treatment composition comprising an acid, and wherein the at least one inhibitor is a byproduct of contacting the lignocellulosic biomass with the treatment composition comprising the acid.

7. The method of claim 1 , wherein the at least one inhibitor is a byproduct of contacting the lignocellulosic biomass with a treatment composition comprising a basic, and wherein the at least one inhibitor is a byproduct of contacting the lignocellulosic biomass with the treatment composition comprising the base.

8. The method of claim 1 , wherein the at least one inhibitor is chosen from acetic acid, furfural, 5-hydroxymethylfurfural, phenolic compounds produced from lignin hydrolysis, and combinations thereof.

9. The method of claim 1 , wherein the second concentration of the at least one inhibitor is 500 ppm or less.

10. The method of claim 1 , wherein the second concentration of the at least one inhibitor is 200 ppm or less.

11. The method of claim 1 , wherein the first concentration of the at least one inhibitor is greater than 1000 ppm.

12. The method of claim 1 , wherein the first concentration of the at least one inhibitor is from 600 ppm to 4200 ppm.

13. The method of claim 1 , wherein reducing the first concentration of the at least one inhibitor in a portion of the lignocellulosic hydrolysate comprises a process chosen from dilution, gas stripping, and combinations thereof.

14. The method of claim 13 , wherein the gas stripping is performed with a gas chosen from air, nitrogen, ozone, and combinations thereof.

15. The method of claim 1 , wherein the at least one monosaccharide is at a concentration from about 0.1 wt % to 2.0 wt % of the propagation medium.

16. The method of claim 1 , wherein the propagation medium further comprises a stillage component, wherein the stillage component is chosen from whole stillage, thin stillage, wet cake, syrup, and combinations thereof.

17. The method of claim 1 , wherein the first cell mass of the microorganism is from about 0.001 wt % to 0.05 wt % of propagation medium, wherein the second cell mass of the microorganism comprises 100 to 1000 times the first cell mass of microorganisms, and wherein substantially no ethanol is produced by the first cell mass of the microorganism or second cell mass of the microorganism during the growing.

18. The method as in claim 1 , wherein the microorganism comprises yeast, wherein the yeast is Saccharomyces cerevisiae , and wherein the yeast is capable of growing on a carbon source chosen from glucose, xylose, arabinose, and combinations thereof.

19. The method of claim 1 , further comprising providing cell mass from the third cell mass of the microorganism to fermentation to produce a fermentation product.

20. The method of claim 19 , wherein the fermentation product comprises ethanol.