Summary Gluconic acid is a mild organic acid derived from glucose by a simple oxidation reaction. The reaction is facilitated by the enzyme glucose oxidase (fungi) and glucose dehydrogenase (bacteria such as Gluconobacter). Microbial production of gluconic acid is the preferred method and it dates back to several decades. The most studied and widely used fermentation process involves the fungus Aspergillus niger. Gluconic acid and its derivatives, the principal being sodium gluconate, have wide applications in food and pharmaceutical industry. This article gives a review of microbial gluconic acid production, its properties and applications.

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Gluconic Acid: Properties, Applications and Microbial Production

Statistical media optimization studies for growth and PHB production by Ralstonia eutropha

The use of chemically defined media is gaining popularity in some commercial fermentations, particularly for the preparation of biological products. Although these media are still not frequently developed for industrial processes, they do exhibit favorable characteristics at large scale that are not observed with traditional complex media. This review focuses on the application, development, and practical considerations, especially process economics, of fermentations in chemically defined media in an industrial environment.

Chemically defined media for commercial fermentations

Gluconic acid (GA) is a multifunctional carbonic acid regarded as a bulk chemical in the food, feed, beverage, textile, pharmaceutical, and construction industries. The favored production process is submerged fermentation by Aspergillus niger utilizing glucose as a major carbohydrate source, which accompanied product yield of 98%. However, use of GA and its derivatives is currently restricted because of high prices: about US$ 1.20–8.50/kg. Advancements in biotechnology such as screening of microorganisms, immobilization techniques, and modifications in fermentation process for continuous fermentation, including genetic engineering programmes, could lead to cost-effective production of GA. Among alternative carbohydrate sources, sugarcane molasses, grape must show highest GA yield of 95.8%, and banana must may assist reducing the overall cost of GA production. These methodologies would open new markets and increase applications of GA.

Biotechnological production of gluconic acid: future implications

A mathematical model for gluconic acid fermentation by Aspergillus niger

Gluconic acid fermentation by Aspergillus niger has been investigated using untreated and treated Indian cane molasses. The yield of gluconic acid was found to be reduced using an untreated molasses medium compared to a defined medium. Hence, molasses was subjected to various pretreatment techniques. Pretreatment reduced the levels of various cations and anions. As the synthesis of gluconic acid has been observed to be influenced more by cations than anions, the effect of various metal ions, viz., copper, iron, zinc, manganese, calcium, and magnesium on the yield of gluconic acid has been critically examined in both untreated and treated cane molasses. These results have been compared with a defined medium. The yield of gluconic acid was influenced more by a combination of metal ions rather than individual ions. Potassium ferrocyanide treatment gave the most promising results compared to other treatment techniques.

Critical analysis of the effect of metal ions on gluconic acid production by Aspergillus niger using a treated Indian cane molasses

Sodium gluconate and calcium gluconate methods are important techniques available for gluconic acid fermentation. The comparative analysis of these fermentations has been addressed using Aspergillus niger. The techniques are equally influenced by the spores age in slant growth, inoculum level in germination and production media, different levels of Fe, Cu, Zn and Mn. Sodium gluconate method is promising with respect to lesser time for slant age (3 d) and lesser time of fermentation (6 d) compared to the calcium gluconate method (slant age — 6 d, and time of fermentation — 7 d).

Comparative analysis of calcium gluconate and sodium gluconate techniques for the production of gluconic acid by Aspergillus niger

To compare the efficiency of various whole cell immobilization techniques for the production of gluconic acid by Aspergillus niger were investigated using potassium ferrocyanide-treated cane molasses as the substrate. The techniques followed were:

(1)

Calcium alginate entrapment,




(2)

cross-linking with glutaraldehyde after cell permeabilization with (a) acetone, (b) toluene and (c) isopropanol and




(3)

development of granular catalyst.

D. Subba Rao

Papers

Critical analysis of the effect of metal ions on gluconic acid production by Aspergillus niger using a treated Indian cane molasses

Comparative analysis of calcium gluconate and sodium gluconate techniques for the production of gluconic acid by Aspergillus niger

Comparative analysis of different whole cell immobilized aspergillus niger catalysts for gluconic acid fermentation using pretreated cane molasses