GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

22. MICROORGANISMS AS OBJECTS OF BIOTECHNOLOGY

22.5. MICROBIAL TRANSFORMATION OF SUBSTANCES

22.5.2. Microbiological transformations of carbohydrates

Carbohydrate transformations carried out by microorganisms can be divided into several main types: oxidative transformations (oxidation of polyols, production of aldonic acids), reduction, and isomerization.

The oxidation of polyols was first described in the classic works of G. Bertrand (1896): this is how the oxidation of mannitol to fructose and sorbitol to sorbose was carried out using the Bacteria Acetobacter aceti and Acetobacter xylinum. Later, cultures that perform this process more intensively were discovered (Acetobacter suboxydans).

Conversion of glycerol to dihydroxyacetone. Dihydroxyacetone is a valuable organic compound used in medicine, perfumery, and pharmaceutical industries. It is obtained from glycerol by microbial oxidation using Acetobacter suboxydans Cells. To obtain this product, a method of repeated use of non-growing immobilized cells has been developed. The medium contains glycerol, monopotassium phosphate, 10% Yeast Water, yeast autolysate, corn steep liquor, and CaCO₂ for neutralization. Temperature — 28 ºC, process duration — 36 hours. The main requirement is a high oxygen content in the medium.

The ability of immobilized Gluconobacter oxydans cells to oxidize glycerol to dihydroxyacetone has also been established.

Oxidation of D-sorbitol to L-sorbose. The oxidation of sorbitol to sorbose is a key step in the synthesis of L-ascorbic acid. For this conversion, submerged cultivation of

Acetobacter suboxydans is used. The inoculum is grown on a medium containing sorbitol, glucose, yeast extract, and CaCO₂. Air is passed through sparging systems into 15-20% sorbitol solutions containing the transforming strain and Vitamins required for its growth. After 24 hours at 30 ºC, sorbose is obtained with a 93% yield.

The culture broth containing sorbose is decolorized with active charcoal, filtered, and the filtrate is concentrated in vacuo to a syrupy mass that crystallizes at 15 ºC. The crystals are separated by centrifugation, washed with ice water, and dried.

Reduction of xylose to xylitol. Xylitol is used in the food industry as a sugar substitute (in diabetic products). To convert xylose to xylitol, the yeast Candida utilis is used. Xylitol production can be carried out in two ways: 1) the yeast is grown on a mineral medium containing xylose as a carbon and energy source; during growth, xylose is reduced to xylitol, which accumulates in the culture broth; 2) xylose is reduced by washed yeast cells previously grown on a medium with xylose and incubated in phosphate buffer (pH 7.0). The maximum yield of xylitol is 60%.