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One hundred ml aliquots of a nutrient culture medium, consisting of 2 w/v % trypton soya broth, one w/v % glycerol and deionized water, were distributed to ten 500-ml shaking flasks, followed by autoclaving the flasks at 120° C. for 20 min. Thereafter, the flasks were cooled and inoculated with a seed culture of Gluconobacter frateurii (IFO 3254) using a platinum loop, followed by incubation at 30° C. for 2 days under shaking conditions. After completion of the culture, the cells were collected by centrifugation, and about 10 g wet alive cells was mixed with 100 ml of 0.05 M Tris-HCl buffer (pH 7.0) containing 5 w/v % ribitol. The mixture solution in a volume of 100 ml was placed in a 500-ml shaking flask, and incubated at 30° C. for 20 hours under shaking conditions to convert ribitol into L-ribulose. Thereafter, the culture was centrifuged to remove cells, and the resulting supernatant was in a conventional manner decolored with activated charcoal, desalted with “DIAION SK1B (H-form)” and “DIAION WA30 (OH-form)”, both of which are cation exchangers of Mitsubishi Chemical Corporation, Tokyo, Japan, and concentrated in vacuo to obtain a transparent syrup with a concentration of about 60 w/w %. The syrup was fractionated by column chromatography using “DOWEX 50W-X4”, a cation exchanger in Ca++-form commercialized by The Dow Chemical Co., Midland, Mich., USA, to obtain high L-ribulose content fractions which were then concentrated into an about 70 w/w % syrup. High-performance liquid chromatographic (HPLC) analysis using a column, 8×300 mm, packed with “MCIGEL CK-08EC”, a gel in Ca++-form commercialized by Mitsubishi Chemical Corporation, Tokyo, Japan, revealed that the product contained at least 97 w/w % L-ribulose, d.s.b. The yield of L-ribulose to the material ribitol was about 90%, d.s.b.

1. customat 120° C.
2. customfor 20 min
3. temperatureThereafter, the flasks were cooled
4. customAfter completion of the culture, the cells were collected by centrifugation, and about 10 g wet alive cells
5. additionwas mixed with 100 ml of 0.05 M Tris-HCl buffer (pH 7.0)
6. additioncontaining 5 w/v % ribitol
7. stirringshaking flask
8. waitincubated at 30° C. for 20 hours under shaking conditions
9. customto remove cells
10. concentrationconcentrated in vacuo
11. customto obtain a transparent syrup with a concentration of about 60 w/w %