Process for producing D-mannitol
High concentration of free cells of heterofermentative lactic acid bacteria (LAB) in a resting or slowly growing state are used to convert fructose into mannitol. Efficient volumetric mannitol productivities and mannitol yields from fructose are achieved in a process applying cell-recycle, continuous stirred tank reactor and/or circulation techniques with native LAB cells or with LAB cells with inactivated fructokinase gene(s). Mannitol is recovered in high yield and purity with the aid of evaporation and cooling crystallization.
1. A process for the production of mannitol by bioconversion, comprising the steps of
bringing an initial concentration of at least 5 g dry cell weight per liter of free, mannitol-producing fructokinase-inactivated lactic acid bacterial cells into contact with a nutrient medium supplemented with a substrate convertible into mannitol, and a cosubstrate, in a bioreactor system,
performing the bioconversion under conditions suitable for converting said substrate into mannitol including keeping the cells in a resting or a slow growing state such that the average volumetric productivity of mannitol is at least 10 g/L/h,
separating the bacterial cells from the medium by filtration to obtain a cell-free solution,
recovering from the cell-free solution the mannitol produced, and
reusing the separated bacterial cells in the bioreactor system.
2. The process according to claim 1 , wherein the bacterial cells are native lactic acid bacterial cells.
3. The process according to claim 2 , wherein the bacterial cells are of the strain Leuconostoc mesenteroides ATCC-9135.
4. The process according to claim 1 , wherein the bacterial cells are of the strain Leuconostoc pseudomesenteroides BPT143 deposited under accession number DSM 14613 at the Deutsche Sammlung von Mikroorganismen and Zellkulturen GmbH.
5. The process according to claim 1 , wherein the substrate is fructose.
6. The process according to claim 1 , wherein the cosubstrate is glucose.
7. The process according to claim 1 , wherein the bioconversion is performed until at least 70% of said substrate has been consumed.
8. The process according to claim 1 , wherein the bioconversion is performed as a batch process in a bioreactor system comprising a bioconversion reactor unit and a filtration unit.
9. The process according to claim 1 , wherein the bioconversion is performed as a circulation process in a bioreactor system comprising a bioconversion reactor unit, a filtration unit and a mixing reactor unit.
10. The process according to claim 9 , wherein the bacterial cells are circulated between the bioconversion reactor unit and the filtration unit.
11. The process according to claim 9 , comprising the steps of leading the cell-free solution obtained by the filtration into the mixing reactor, and transferring the solution from the mixing reactor back to the bioconversion reactor unit.
12. The process according to claim 9 , comprising emptying the mixing reactor unit after the bioconversion step, and re-filling it with said nutrient medium supplemented with said substrate and co-substrate, to run successive bioconversions reusing the bacterial cells.
13. The process according to claim 9 , wherein said bioconversion is run in two or more bioconversion reactor units in series or in parallel.
14. The process according to claim 1 , wherein the bioconversion is performed as a continuous process in a bioreactor system comprising a mixing tank, a feed tank, a bioconversion reactor unit, a filtration unit and a recovery tank.
15. The process according to claim 14 , wherein the bacterial cells are circulated between the bioconversion reactor unit and the filtration unit.
16. The process according to claim 14 , comprising the steps of feeding the bioconversion reactor unit continuously with said nutrient medium supplemented with said substrate and co-substrate, and removing the cell-free solution gained by filtration from the bioreactor system to withhold a constant volume in the bioconversion reactor unit.