Use of acetaldehyde in the fermentative production of ethanol

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1 Use of acetaldehyde in the fermentative production of ethanol AIM: Considerably reduce growth-coupled formation of glycerol during alcoholic fermentation and concomitantly increase output of ethanol per amount of sugar Moralcobv.com November

2 Glycerol production during ethanol fermentation by yeast Basically two mechanisms of glycerol formation: 1. Production of glycerol directly coupled to growth of yeast (redox balance of NAD + /NADH) 2. Glycerol as osmoprotectant to counterbalance unfavorable medium compositions 2

3 Production of glycerol directly coupled to growth of yeast for NADH neutral situation Sugar NADH oxidation NADH surplus NADH neutral Glycerol Yeast biomass Ethanol + CO 2 3

4 Use of acetaldehyde in reaching an NADH neutral situation; no glycerol produced Ethanol Acetaldehyde Ethanol Chemically NADH oxidation Biologically Sugar NADH surplus Yeast biomass NADH neutral Ethanol + CO 2 Please note that 1 molecule of ethanol oxidized chemically, results biologically in 2 molecules of ethanol. NADH-oxidation by reducing 1 acetaldehyde results not only in 1 ethanol. It also evades the formation of 1 molecule of glycerol. The sugar that was transformed into glycerol, now can be converted into ethanol. The ratio glycerol not formed to ethanol produced is 1:1 at a molar basis. 4

5 Example calculation increase ethanol as based on the acetaldehyde technology If no yeast growth would occur during ethanol production, then the redox balance of NAD + /NADH would be neutral. Then, as an example: 100 glucose = 200 ethanol CO 2 (with no excess of NADH) In practice, yeast growth occurs resulting in excess NADH which has to be reoxidized. Yeast typically reoxidizes NADH by the formation of glycerol. As example for an industrial fermentation: 100 glucose = 184 ethanol CO glycerol + yeast biomass With the new acetaldehyde technology, yeast does not have to rely on the formation of glycerol for reoxidizing NADH. Instead it will reduce externally supplied acetaldehyde into ethanol. In this new situation: 100 glucose + 7 acetaldehyde = 198 ethanol CO 2 + yeast biomass Acetaldehyde is obtained by chemically oxidizing ethanol into acetaldehyde with air in a separate unit: 7 ethanol O 2 = 7 acetaldehyde + 7 H 2 O Hence, in the new situation the overall balance is: 100 glucose + 7 acetaldehyde = 198 ethanol CO 2 + yeast biomass 7 ethanol O 2 = 7 acetaldehyde + 7 H 2 O 100 glucose O 2 = 191 ethanol + 7 H 2 O CO 2 + yeast biomass Increase ethanol per glucose: ( )/184 * 100 = 3.8 % 5

6 Integrated acetaldehyde on-site production Carbon dioxide Detection device acetaldehyde Dosage controller acetaldehyde Ethanol Acetaldehyde Fermenters Ethanol (gas) Condensor and acetaldehyde in water Conversion column Saturator Air 6

7 Integrated acetaldehyde on-site production Carbon dioxide Process Control Detection device acetaldehyde Dosage controller acetaldehyde Microbiology Ethanol Acetaldehyde Fermenters Ethanol (gas) Condensor and acetaldehyde in water Conversion column Chemical Technology Saturator Air 7

8 Chemical Technology The chemical oxidation of ethanol to acetaldehyde is a standard procedure More simple on-site than the standard acetaldehyde production procedure, since no distillation column is required to separate acetaldehyde from any remaining ethanol 8

9 Process Control Realistic assumptions are: Henry s law constants for acetaldehyde is 15 M/atm Yeast is not inhibited by acetaldehyde concentrations below 0.1 kg/m 3 The maximal rate of acetaldehyde consumption during the fermentation process is 0,5 kg/m 3 /h It can be calculated on the basis of the above assumptions that acetaldehyde can be supplied conveniently into the fermenter either by: Addition to the mash. Addition to the external cooling recirculation loop. High turbulence in this loop makes the contact time to possibly toxic concentrations very short. Addition as a gas. This procedure is safe due to the high solubility of acetaldehyde and extreme short contact times around the moving bubble. 9

10 Microbiology It has been determined experimentally that yeast regulation indeed favors reduction of acetaldehyde over glycerol formation in reoxidizing NADH. Complete consumption of externally added acetaldehyde has been demonstrated Excess ethanol production was indeed equivalent to the sum of glycerol reduction + acetaldehyde addition (molar basis) 10

11 In conclusion Relatively simple integrated process Not based on genetic engineering of yeast but rather on process engineering Bench scale continuous and batch experiments have proven the concept: consumption of acetaldehyde, reduction in glycerol, and increase in ethanol 11

12 Additional benefit Acetaldehyde as produced on site may be applied locally at relatively high concentrations (1-2 kg/m 3 ) and thus be employed as disinfectant: At certain locations in the mash supply Prior to the start of a run in the fermenter 12