Using dissolved oxygen to inhibit lactic acid production during propagation of yeast and/or hydrolysis of lignocellulosic biomass

1. A method of propagating yeast that can convert one or more monosaccharides into a biochemical, the method comprising:

a) providing a first cell mass of the yeast in an aqueous propagation medium, wherein the aqueous propagation medium comprises a cellulosic hydrolysate that includes Lactobacillus bacteria that can produce lactic acid; and

b) propagating the first cell mass of the yeast in the aqueous propagation medium in the presence of an amount of dissolved oxygen that inhibits the production of lactic acid by the Lactobacillus bacteria and for a time period to form a second cell mass of the yeast that is greater than the first cell mass of the yeast, wherein lactic acid is present in the aqueous propagation medium during the time period in an amount from 0 to 150 milligrams lactic acid per liter of aqueous propagation medium.

2. The method of claim 1 , wherein the dissolved oxygen is present in the aqueous propagation medium in an amount of at least 11 milligrams of dissolved oxygen per liter of aqueous propagation medium.

3. The method of claim 1 , further comprising

a) measuring a sample of the aqueous propagation medium to determine the presence and amount of lactic acid in the aqueous propagation medium;

b) determining an amount of oxygen to add to the aqueous propagation medium based on the amount of lactic acid measured in step (a); and

c) adding a gas to the aqueous propagation medium so that the aqueous propagation medium comprises dissolved oxygen in an amount that inhibits the production of lactic acid by the Lactobacillus bacteria, wherein the gas comprises oxygen.

4. The method of claim 1 , wherein lactic acid is present in the aqueous propagation medium during the time period in an amount from 0 to 100 milligrams lactic acid per liter of aqueous propagation medium.

5. The method of claim 1 , wherein the dissolved oxygen is present in the aqueous propagation medium in an amount of at least 15 milligrams of dissolved oxygen per liter of aqueous propagation medium.

6. The method of claim 1 , wherein the dissolved oxygen is present in the aqueous propagation medium in an amount of at least 30 milligrams of dissolved oxygen per liter of aqueous propagation medium.

7. The method of claim 1 , wherein the dissolved oxygen is present in the aqueous propagation medium in an amount from 15 to 200 milligrams of dissolved oxygen per liter of aqueous propagation medium.

8. The method of claim 1 , wherein the propagation medium comprises:

a) a carbon source that can support growth of the first cell mass of the yeast, wherein the carbon source comprises xylose and/or glucose; and

b) a nutrient source that can support growth of the first cell mass of the yeast, wherein the nutrient source comprises a stillage component and/or yeast extract.

9. The method of claim 3 , wherein adding gas comprises introducing the gas into a headspace of a vessel that contains the aqueous propagation medium so that the gas diffuses into the aqueous propagation medium.

10. The method of claim 3 , wherein adding gas comprises sparging the gas into the aqueous propagation medium so that the gas transfers into the aqueous propagation medium.

11. The method of claim 3 , further comprising exposing the second cell mass of the yeast to anaerobic conditions to convert monosaccharide into a biochemical.

12. The method of claim 11 , wherein the second cell mass of yeast is combined with the monosaccharide in a fermentation system to convert monosaccharide into a biochemical under anaerobic conditions.

13. The method of claim 11 , wherein the monosaccharide is derived from a method of treating lignocellulosic biomass in an aqueous slurry, wherein the method comprises converting cellulose in the lignocellulosic biomass into the monosaccharide in the presence of an amount of dissolved oxygen that inhibits the production of lactic acid by Lactobacillus bacteria, wherein lactic acid is present in an amount from 0 to 150 milligrams lactic acid per liter of aqueous slurry.

14. The method of claim 13 , wherein converting cellulose in the lignocellulosic biomass into the monosaccharide comprises providing the aqueous slurry comprising:

a) the lignocellulosic biomass that comprises the cellulose;

b) one or more enzymes that can convert the cellulose into the monosaccharide; and

c) dissolved oxygen in an amount that inhibits the production of lactic acid by the Lactobacillus bacteria.

15. The method of claim 13 , wherein the dissolved oxygen is present in the aqueous slurry in an amount of at least 11 milligrams of dissolved oxygen per liter of aqueous slurry.

16. The method of claim 13 , wherein converting cellulose in the lignocellulosic biomass into the monosaccharide comprises:

a) providing the aqueous slurry comprising the lignocellulosic biomass that comprises the cellulose and one or more enzymes that can convert the cellulose into the monosaccharide;

b) adding a gas to the aqueous slurry so that aqueous slurry comprises dissolved oxygen in an amount that inhibits the production of lactic acid by the Lactobacillus bacteria, wherein the gas comprises oxygen;

c) maintaining the aqueous slurry at a pH and temperature for a time period to convert at least a portion of the cellulose in the lignocellulosic biomass into the monosaccharide, wherein the dissolved oxygen inhibits the production of lactic acid by the Lactobacillus bacteria.

17. The method of claim 13 , wherein converting cellulose in the lignocellulosic biomass into the monosaccharide comprises:

a) maintaining an aqueous slurry at a pH and temperature for a time period to convert at least a portion of the cellulose in the lignocellulosic biomass into the monosaccharide, wherein the aqueous slurry comprises the lignocellulosic biomass that comprises the cellulose and one or more enzymes that can convert the cellulose into the monosaccharide;

b) after step (a), measuring a sample of the aqueous slurry to determine the presence and amount of lactic acid in the aqueous slurry;

c) determining an amount of oxygen to add to the aqueous slurry based on the amount of lactic acid measured in step (b).

18. The method of claim 17 , further comprising adding a gas to the aqueous slurry so that the aqueous slurry comprises dissolved oxygen in an amount that inhibits the production of lactic acid by the Lactobacillus bacteria, wherein the gas comprises oxygen.