2. Gluconic acid is used as a mild
acidulant in metal processing,
leather tanning and foods.
Sodium gluconate is widely used
as a sequestering agent to prevent
the precipitation of lime soap scums
on cleaned products.
Calcium gluconate is widely used
in calcium therapy.
3. Gluconic acid is manufactured only by
submerged fermentation process.
The fungus used is a strain of Aspergilllus
niger. Inoculum consist of either a sporulated
culture or spores germinated in seed tank.
Each type of inoculum has special advantages
not seen in others.
The acid is quickly neutralised upon
excretion by the fungus. Therefore, the
fermented broth contains either calcium or
sodium gluconate.
4. The raw materials are D-glucose and oxygen.
The glucose substrate may be supplied as a solution
of crystalline glucose. Alternatively ,it may be
added as a syrup obtained from starch or crude
starchy matrials by the action of the α-amylase
followed by the amyloglucosidase.
The glucose concentration that can be handled
depends on whether the sodium or the calcium salt
is to be prepared.
5. It is necessary to maintain the maximum
concentration of oxygen dissolved in the solution.
This is achived by the application of vigorous
agitation with a turbomixer or by action of a
cavitator.
The greater the consumption of power by the
agitator motor,the more dissolution of oxygen.
7. Operating optimum fermentation conditions in the
manufacture of calcium or sodium gluconate are:
Temperature = 28-300C
Aeration = 1 – 1.5 volumes air/volume
solution/minute
Agitation = vigorous
Initial pH = ~6.5
8. At the end of the fermentation, fungal
mycelium is filtered off from the solution.
The mycelium is used for the recovery of
glucose aerodehydrogenase. On the other
hand ,the filtrate is used to recover calcium
or sodium gluconate, gluconic acid, and
glucono-δ-lactone.
9. Recovery of calcium gluconate is achieved by heating
the filtrate with a slight excess of Ca(OH)2 , then
decolourizing with carbon and filtering. On cooling
to a temperature below 200C and seeding with
calcium gluconate crystals, the compound readily
gets crystallized. A second crop of crystals of
calcium gluconate is obtained by the evaporation of
mother liquors through heating about 10-15 per cent
volume followed by treatment with carbon, filtration
and chilling.
10. To recover gluconic acid and its delta lactone,calcium
is precipitated by the addition of a stoichiometric
quantity of sulfuric acid.
Calcium sulfate is filtered off and the filtrateis
decolourized with carbon.
After the removal of carbon, the acid solution is
subjected to a concentration of 50 per cent acid
strength. The product, thus obtained is a mixture of
free gluconic acid and its γ- and δ-lactones.
11. Crystals separating from a gluconic acid solution at a
temperature below 300C and preferably near 00C, are
principally of gluconic acid.
Crystals separating from solution at a temperature
between 30-700C are of δ-lactone. Finally , γ-lactone
crystallizes out at a temperature above 700C.