Process for making ethanol by fermentation

1. A process for making ethanol by fermentation, with yeast recovery and recycling, comprising the steps:

a) fermenting a mash comprising a fermentable sugar with a yeast to form a fermented mash comprising ethanol,

b) separating a yeast concentrate from the fermentation mash obtained in step a),

c) adding a mineral acid to the yeast concentrate obtained in step b) to provide an acidified yeast concentrate having a pH between 1.8 and 3.0,

d) adding a peroxycarboxylic acid to the acidified yeast concentrate in an amount of from 5 to 80 ppm by weight to provide a treated yeast concentrate, and

e) fermenting a mash comprising a fermentable sugar with addition of the treated yeast concentrate obtained in step d) to form a fermented mash comprising ethanol.

2. The process of claim 1 , wherein in step b) a yeast suspension containing 60 to 80% by weight yeast cells is separated by centrifugation and said yeast suspension is diluted with water to provide said yeast concentrate.

3. The process of claim 1 , wherein the peroxycarboxylic acid is peroxyacetic acid.

4. The process of claim 1 , wherein the peroxycarboxylic acid is added intermittently in an amount of from 20 to 80 ppm by weight.

5. The process of claim 1 , wherein the peroxycarboxylic acid is added continuously in an amount of from 5 to 30 ppm by weight.

6. The process of claim 1 , wherein the peroxycarboxylic acid is used in combination with hydrogen peroxide at a molar ratio of peroxycarboxylic acid to hydrogen peroxide of from 0.02:1 to 5:1.

7. The process of claim 1 , wherein the mash comprising a fermentable sugar is selected from the group consisting of: sugar cane juice, sugar cane molasses and a combination of both.

8. The process of claim 1 , wherein the average time between adding a peroxycarboxylic acid to the acidified yeast concentrate in step d) and addition of treated yeast concentrate in step e) is at least 10 min.

9. The process of claim 1 , wherein steps a) and e) are carried out as batch fermentations or as fed-batch fermentations.

10. The process of claim 2 , wherein the peroxycarboxylic acid is peroxyacetic acid.

11. The process of claim 10 , wherein the peroxyacetic acid is added intermittently in an amount of from 20 to 80 ppm by weight.

12. The process of claim 11 , wherein the average time between adding a peroxycarboxylic acid to the acidified yeast concentrate in step d) and addition of treated yeast concentrate in step e) is at least 10 min.

13. The process of claim 12 , wherein steps a) and e) are carried out as batch fermentations or as fed-batch fermentations.

14. The process of claim 10 , wherein the peroxyacetic acid is added continuously in an amount of from 5 to 30 ppm by weight.

15. The process of claim 14 , wherein the average time between adding a peroxycarboxylic acid to the acidified yeast concentrate in step d) and addition of treated yeast concentrate in step e) is at least 10 min.

16. The process of claim 15 , wherein steps a) and e) are carried out as batch fermentations or as fed-batch fermentations.

17. A process for making ethanol by fermentation, with yeast recovery and recycling, comprising the steps:

a) fermenting a mash comprising a fermentable sugar with a yeast in batch to form a fermented mash comprising ethanol, operating fermenters in parallel and discharging fermenters in succession to provide an essentially constant flow of fermented mash;

b) separating a yeast cream from the fermentation mash obtained in step a) by centrifugation and diluting said yeast dream with water to provide a yeast concentrate containing 25 to 35% by weight yeast cells;

c) adding a mineral acid to the yeast concentrate obtained in step b) to provide an acidified yeast concentrate having a pH between 1.8 and 3.0;

d) adding an equilibrium peroxyacetic acid to the acidified yeast concentrate in an amount of from 20 to 80 ppm by weight to provide a treated yeast concentrate; and

e) fermenting a mash comprising a fermentable sugar with addition of treated yeast concentrate obtained in step d) to form a fermented mash comprising ethanol;

wherein the average time between adding peroxyacetic acid in step d) and adding the treated yeast concentrate obtained in step e) is from 1 to 4 hours.

18. The process of claim 17 , wherein steps c) and d) are carried out in a continuous stirred tank reactor, to which sulfuric acid and the equilibrium peroxyacetic acid are fed.

19. A process for making ethanol by fermentation, with yeast recovery and recycling, comprising the steps:

a) fermenting a mash comprising a fermentable sugar with a yeast in fed-batch to form a fermented mash comprising ethanol, operating fermenters in parallel and discharging fermenters in succession to provide an essentially constant flow of fermented mash;

b) separating a yeast cream from the fermentation mash obtained in step a) by centrifugation and diluting said yeast dream with water to provide a yeast concentrate containing 25 to 35% by weight yeast cells;

c) adding a mineral acid to the yeast concentrate obtained in step b) to provide an acidified yeast concentrate having a pH between 1.8 and 3.0;

d) adding an equilibrium peroxyacetic acid to the acidified yeast concentrate in an amount of from 5 to 80 ppm by weight to provide a treated yeast concentrate; and

e) fermenting a mash comprising a fermentable sugar in fed-batch with addition of treated yeast concentrate obtained in step d) before fed-batch fermentation is started to form a fermented mash comprising ethanol;

wherein the average time between adding peroxyacetic acid in step d) and adding the treated yeast concentrate obtained in step e) is from 1 to 4 hours.

20. The process of claim 19 , wherein steps c) and d) are carried out in a continuous stirred tank reactor, to which sulfuric acid and the equilibrium peroxyacetic acid are fed.