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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.