1. Gluconic Acid
1.Introduction
Gluconic acid is an organic compound with molecular formula C6H12O7 and condensed
structural formula HOCH2(CHOH)4COOH.
In aqueous solution at neutral pH, gluconic acid forms the gluconate ion. The salts of gluconic
acid are known as "gluconates". Gluconic acid, gluconate salts and gluconate esters occur
widely naturally in fruit, honey, kombucha tea and wine because such species arise from the
oxidation of glucose.
Annual worldwide production capacity for gluconic acid and its derivatives is estimated to be
60 000 t. The bulk of production (85 %) is in the form of sodium gluconate and other alkali
gluconate salts.
2. Productions
For commercial purposes, d-gluconic acid and its salts are prepared exclusively by the
oxidation of glucose or glucose-containing raw materials. The oxidation method may be
chemical, electrolytic, catalytic or biochemical.
D-Gluconic acid results from the oxidation of D-glucose. The synthesis can be mediated by a
precious metal catalyst, catalysed electrochemically in the presence of bromide ions or
synthesised by microorganisms. The synthesis of D-gluconic acid by microorganisms was
discovered in 1880 by L. Boutroux, who observed the formation of a "sugar acid” by
Mycoderma aceti, an acetic acid bacterium. In 1922, M. Molliard detected gluconic acid in
cultures of the filamentous fungus Sterigmatocystis nigra, now known as Aspergillus niger.
Subsequently the formation of gluconic acid was demonstrated with a number of other fungi
and bacteria, including species of the genera Aspergillus, Endomycopsis, Penicillium,
Pullularia and Scopulariopsis as well as Acetobacter, Acinetobacter, Gluconobacter,
Enterobacter, Micrococcus, Moraxella and Pseudomonas. The biological (fermentation-)
method is fully competitive with chemical techniques and is at present the method of choice
by industry.
The economic viability of the processes depends largely on the activity, selectivity, lifetime
and cost of the catalyst as well as on the measures required for product purification and the
energy demands.
The principal organisms employed today for large-scale biological preparation of gluconic
acid are Aspergillus niger, Gluconobacter suboxydans and Acetobacter methanolicus.
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2. The biochemistry of gluconic acid formation by Aspergillus niger
The fate of glucose and formation of gluconic acid in bacteria
The production of gluconic acid and its sodium and calcium salt with Acetobacter
methanolicus the following process steps must be realised:
1. Fermentation
2. Gluconic Acid Isolation and Purification
3. Evaporation
4. Crystallisation of Sodium Gluconate & Calcium Gluconate
Parameters of production of gluconic acid and its sodium and calcium salt are dextrose syrup,
methanol, lime hydrate, hydrochloric acid, caustic soda, liquid ammonia, process water,
demineralized water, cooling water, electric energy and steam.
3. Utilizations and Applications
Due to some outstanding properties – extremely low toxicity, low corrosivity and the
capability of forming water-soluble complexes with divalent and trivalent metal ions -
gluconic acid has found applications, e.g., in the dairy industry to prevent the deposition of
milkstone or to remove it. In beverages it prevents cloudiness and scaling by calcium
compounds. In various foods it produces and improves a mild sour taste and complexes traces
of heavy metals.
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3. Gluconic acid, which removes alkali and protein films through sequestering power without a
toxic effect and may be used as a water conditioner.
The remarkable non corrosiveness of gluconic acid may generally be utilized in gentle metal
cleaning operations, e.g., in cleaning aluminum cans and other equipment.
Gluconate is an essential ingredient in cleaning and/or degreasing ferrous and some non-
ferrous metals, especially where mineral deposits dissolve in alkaline solution. The gluconate
anion chelates Ca2+, Fe2+, Al3+ and other heavy metals. The sequestering function of
gluconate in most metal washing operations is to prevent hard water ions, such as, calcium
and magnesium from complexing with the alkali in the cleaner. If these hard water ions are
allowed to react with alkali components, an insoluble complex will be precipitated. This
undesirable precipitate can build up and deposit itself causing damage to the cleaning vessel
and surface to be cleaned.
Sodium gluconate is a component of commercial cleansers. This is due to the outstanding
property of forming stable complexes with various metal ions, especially in alkaline (i.e.,
sodium hydroxide or carbonate) solutions (its sequestering properties with respect to
hardening agents in water) and its absolute stability to hydrolysis at high temperature and high
pH. For example, sodium gluconate is used in bottle washing and in cleaning aluminum
surfaces (e.g., building facades, aircraft and containers) and to scale-off oxides of heavy
metals from metal surfaces, to remove zinc from metal surfaces or in removing paints and
lacquers from various objects.
Sodium gluconate is also recommended as an additive to concrete acting as a plastifier and
retarding the setting process.
Sodium gluconate is often included in shampoo formulation as a chelator to control water
hardness minerals. Calcium and magnesium form deposits on hair, which adversely effect the
texture and sheen. Sodium gluconate is also used in toothpaste as a chelator to assist in the
solubilization of the fluoride carrier.
There has been a progressive trend to more effective environmentally friendly dishwashing
detergents. Phosphate-free detergents are becoming more significant because of
environmental and legislation concerns. The European market has recognized the damaging
effects of heavy phosphate use and has imposed very strict legislation. These regulations have
provided the impetus for the development of several phosphate-free, gluconate containing
products. The chelation properties combined with their low corrosion and ease of
biodegradation have allowed gluconates to become an essential ingredient in the “green” dish
washing compounds. But it is not used for “personal care” due to high pH levels.
Sodium gluconate is used as a sequestrant in liquid soap products such as laundry detergent to
prevent discoloration caused by metallic ions.
As a food additive (E574), it is an acidity regulator. But according to German law gluconic
acid is considered as food and thus not as an additive. It has been utilized as a partial
replacement for sodium chloride in sausage products. It is sometimes used as a debitterant
ingredient in sugar replacement packets and diet beverages too. It can improve the taste of
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4. sweetness when it is used with other artificial sweeter. Food technologists and formulators
have shown great interest in gluconate salts because of their prebiotic effect.
Several other salts of gluconic acid have gained importance: Gluconates of calcium and iron
are the preferred carriers used in calcium and iron therapy because of the extremely low
toxicity of gluconate and its favorable biodegradability. Various gluconates together with
gluconic acid are used in the tanning and textile industry.
Sodium gluconate in low concentrations is an effective corrosion inhibitor used in
recirculating water systems, such as cooling towers and heat exchangers. These systems are
composed of pipes, vessels, … etc. constructed of copper and carbon steels which are
particularly prone to oxidative corrosion. This occurs because air from the atmosphere will
dissolve in the cooling water. The dissolved oxygen will diffuse into the metal inducing
corrosion. The corrosion can be inhibited by dissolving sodium gluconate in a certain
concentration.
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