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WRITE DIFFERENT PROCESSESINVOLVED IN MICROBIAL CONVERSION

OF ETHANOL TO ACETIC ACID?

TUTORIAL 1: QUESTION 7


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MICROBIAL CONVERSION OF ETHANOL

TO ACETIC ACID

(involve a two-stage microbial process)

1st stage:

Alcohol is produced fromglucose in fruit or from

glucose/maltose derived fromstarch using plant or microbial

enzymes

2nd stage:

The prepared alcoholic productis then converted to acetic acid

by bacteria

INTRODUCTION


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ACETIC

ACIDBACTERIAObligateaerobes

A group inAlphaproteobacteria

More than 10 genera,including Acetobacter,

Gluconobacter andGluconacetobacter belong

to acetic acid bacteria(Yamada & Yukphan,

2008).

Oxidation of alcohol byAcetobacter aceti

(optimum temperature29C 34C),

Gluconobacter oxydans(optimum temperature

20C), and by A.Pasteurianus, found in

Japanese rice wine basedvinegar production

systems, results in theproduction of acetic acid

Biochemical event is

depicted byfollowing equation:


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Acetic acid formation in these species is anincomplete oxidation; oxygen is used as electron

acceptor

The metabolic process involves conversion of ethanolto acetaldehyde by alcohol dehydrogenase (Adh)and of hydrated acetaldehyde to acetic acid by

acetaldehyde dehydrogenase (Ald).

Thus, 1 mol of acetic acid is produced from 1 mol ofethanol; from 1 L of 12% (v/v) ethanol, 1 L of12.4% (v/v) acetic acid is produced.

CH3CH2OH

Ethanol Acetaldehyde Acetaldehyde hydrate Acetic acid

CH3CHO CH3CH(OH)2 CH3COOH

NAD NAD NADH2NADH2H2O

Adh Ald


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BIOTECHNOLOGICALPROCESSES FOR THEPRODUCTION OF ACETIC ACID

TRADITIONAL

SLOWORLEANSPROCESS

THE QUICKPROCESS

FRINGSACETATOR

ORSUBMERGEDCULTUREPROCESS


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Bacteria do not growthroughout the substrate

but at the air/liquid

interface

If the wine is leftundisturbed, a cell mat,or pellicle, forms on thesurface of the batch of

wine

As oxygen enters thewine through the mat,the bacteria use it andconvert the ethanol to

acetic acid

Therefore, acetic acid isproduced at the surfaceand diffuses throughout

the body of wine asshown in next figure

TRADITIONAL SLOW ORLEANS PROCESS


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The acid acetic production system depends on oxygen supply In the first production system, a microbial pellicle formed on the

surface of the wine

The oxygen and ethanol had to be transported by diffusion to

the microorganisms that occupied a relatively small space

TRADITIONAL SLOW

ORLEANS PROCESS


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THE

QUICK

PROCESS

Wine is trickled down

packed pumice stone,ceramic chips or wood

shavings, on whichacetic acid bacterial

cells adhere (cellimmobilization) so it isa fixed bed microbial

film reactor

volume of a typicalcommercial acetic acid

generator is 50 60 m3

Air is allowed to move freelythrough perforations in the bottom

of the packed container, or isforced through the trickling

generator to increase the oxygentransfer rate

alcohol is convertedrapidly to acetic acid

while the wine is tricklingdown the packed vinegargenerator

98% conversion ofethanol (12% v/v

wine) into aceticacid can be

achieved in 5days

temperature in thegenerator increases

from about 28C at thetop to about 35C atthe bottom due to the

metabolic activity of thebacteria


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THE QUICK PROCESS

The mass of bacteria is increased in this system and ethanol and oxygen are transported to

the cells.


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FRINGS

ACETATOROR

SUBMERGED

CULTURE

PROCESS

Submergedprocess, which

uses the so-calledFrings acetator

Widely used forcommercial aceticacid production bymeans of a baffled

bioreactor containinga bottom-driven

turbine for intensiveaeration

Typical commercialprocesses, involvingproduction of 12

15% acetic acid, arecarried out semi-

continuously Acetic acid andethanol concentrations

at the start of thecycle are 7 10%

and 5% respectively

Fermentation proceeds at 27C 32C until thealcohol concentration drops to 0.1 0.3%, atwhich point about one-third of the vinegar is

discharges and the vessel is filled with new mashcontaining 0 2% acetic acid and 12 -15%

ethanol, and the cycle is repeated

Both acetic acid andethanol must be

present for optimalgrowth of Acetobacter

The ethanol supplyis critical, and with

less than 0.2%ethanol, the death

rate increases

The efficiency of theprocess is high; 90 98% of the alcohol is

converted to acid


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THE SUBMERGED CULTURE PROCESS


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The advantages of submerged

cultivation over the trickling generator:

It permits 30 times faster

oxidation of alcohol,because of the highoxygen transfer rate

It requires a smallerreactor volume (16% of

that needed for thetrickling generator) to

produce an equivalentamount of vinegar,

because the dead spaceoccupied by the solidsupport in the tricklinggenerator is removed

It has greater efficiency

with a yield 5 8% higherand more 90% oftheoretical yield is

obtained

It can be highly automatedIt eliminates clogging of

packed wood shavings andinterruption of liquid flow

It allows an easy change inthe type of vinegar being

produced


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REFERENCES

Sahm, H. (1999). Prokaryotes in Industrial Production. In

Lengeler, J. W., Grews, G. And Hans, G. (Eds.). Biology

of the Prokaryotes. Germany: Thieme.

O Toole, D. K., Lee, Y. K. (2006). Fermented Foods. InLee, Y. K. (Ed.) Microbial Biotechnology: Principles and

Applications (2nd ed.). Singapore: World Scientific.

Yamada, Y. And Yukphan, P. (2008). Genera and

Species in Acetic Acid Bacteria. Int. J. Food ofMicrobiol.

125(1), 15-24.

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