Gluconic acid (pentahydroxycaproic acid, Fig. 1) is
produced from glucose through a simple dehydrogenation reaction catalysed by glucose oxidase. Oxidation of
the aldehyde group on the C-1 of b-D-glucose to a carboxyl group results in the production of glucono-d-lactone (C6H10O6, Fig.1) and hydrogen peroxide. Glucono-d-lactone is further hydrolysed to gluconic acid either
spontaneously or by lactone hydrolysing enzyme, while
hydrogen peroxide is decomposed to water and oxygen
by peroxidase. The gluconate pathway is detailed in Fig.
2. The conversion process could be purely chemical too,
but the most commonly involved method is the fermentation process. The enzymatic process could also be conducted, where the conversion takes place in the absence
of cells with glucose oxidase and catalase derived from
A. niger. Nearly 100 % of the glucose is converted to gluconic acid under the appropriate conditions. This method is an FDA approved process. Production of gluconic acid using the enzyme has the potential advantage
that no product purification steps are required (1) if the
enzyme is immobilised, e.g. the use of a polymer membrane adjacent to anion-exchange membrane of low-density polyethylene grafted with 4-vinylpyridine (1).
However, this approach is not yet common in the industry, and it will not be considered in the review.