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Citric
Acid
Fermentations -Citric acid, which is a key intermediate of the TCA cycle is produced by fungi, yeast and bacteria as an overflow product due to a faulty operation of the citric acid cycle. The ability of fungi to produce citric acid was first discovered by Wehmer in 1893 and today all the citric acid commercially produced comes from the mold fermentation. Among the organisms used for citric acid production, A. niger has been the mold of choice for several decades.
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A
variety
of
carbohydrate
sources
such
as
beet
molasses,
cane
molasses,
sucrose,
commercial
glucose,
starch
hydrolysates
etc.,
have
been
used
for
citric
acid
production.
Among
these,
sucrose,
cane
and
beet
molasses
have
been
found
to
be
the
best.
For
citric
acid
production
the
raw
material
is
diluted
to
20-25
per
cent
sugar
concentration
and
mixed
with
a
nitrogen
source
and
other
salts.
The
pH
of
the
medium
is
maintained
around
five
when
molasses
is
used
and
at
a
lower
level
(pH
3.0)
when
sucrose
is
used
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The
fermentations
are
carried
out
either
under
surface,
submerged,
or
solid
state
conditions.
In
the
surface
culture
method,
shallow
aluminium
or
stainless
steel
pans
are
filled
with
the
growth
medium,
inoculated
with
the
fungal
spores
and
allowed
to
ferment.
In
the
submerged
culture
method
the
mold
is
cultured
in
fermentors
under
vigorous
stirring
and
mixing,
while
in
solid
state
fermentation,
the
mold
is
grown
over
carrier
material
such
as
bagasse
etc.,
which
is
impregnated
with
the
fermentation
medium.
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The
production
of
citric
acid
by
A.niger
is
largely
influenced
by
the
concentration
of
trace
metals
such
as
iron,
manganese,
copper
and
zinc
in
the
medium.
An
appropriate
concentration
of
these
elements
is
essential
for
good
acid
production.
However,
an
excess
is
detrimental.
To
optimize
the
level
of
these
trace
metals,
the
raw
materials
are
treated
with
either
ferrocyanide,
charcoal,
chelating
agents
or
catien
exchange
resins.
Addition
of
methanol
at
3-4
%
concentration
has
been
found
to
enhance
the
yield
of
citric
acid.
This
fermentation
is
an
aerobic
fermentation
and,
therefore,
adequate
aeration
is
essential
for
successful
citric,
acid
production
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In
recent
years,
the
production
of
citric
add
by
yeast
is
gaining
importance
because
a
variety
of
yeasts
such
as
Candida,
Hansenula
etc.
have
been
found
to
produce
citric
acid
from
carbohydrates
and
hydrocarbons.
Strains
of
Candida
lipolytica
appear
promising
and
good
yields
of
citric
acid
from
various
raw
materials
has
been
reported.
The
mechanism
by
which
these
yeasts
produce
citric
acid
appears
to
be
slightly
different
from
the
mechanism
by
which
the
fungi
produce
citric
acid.
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After
the
fermentation
is
over,
calcium
citrate
is,
precipitated
from
the
fermented
broth
by
the
addition
of
calcium
hydroxide.
It
is
then
filtered
washed
and
treated
with
sulphuric
acid
to
precipitate
calcium
sulphate.
The
solution
containing
citric
acid
is
then
purified
by
treatment
with
ion
exchange
resins,
charcoal
etc.,
and
finally
crystalized.
Citric
acid
is
used
in
food,
beverage,
textile,
pharmaceutical
and
detergent
industries.
It
is
also
increasingly
used
in
the
removal
of
toxic
and
corrosive
gases
in
air.
The
variety
of
uses
that
it
has,
have
increased
the
demand
for
this
organic
acid.
Besides
fungi
and
yeast,
the
possibilities
of
using
bacteria
to
produce
citric
acid
is
also
being
explored.
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