Bioconversion of d-galactose into d-tagatose

d -Tagatose has potential food uses as a humectant and/or non-calorific sweetener. A variety of different microbial strains were tested for ability to produce d -tagatose from d -galactose. As a result, we have found that d -tagatose can be produced from d -galactose by the lactic acid bacterium Lactobacillus gayonii . Surprisingly, this bioconversion of a d -hexose sugar is probably carried out by an l -pentose isomerase ( l -arabinose-ketol-isomerase) enzyme activity of the cells. Thus this enzyme reacts with the d -hexose isomer of its normal l -pentose substrate. Commercially available d -glucose (xylose)-ketol-isomerases did not carry out this bioconversion. Options for the optimization of this bioconversion are proposed.