A practical method for the production of calcium 2-keto-L-gulonate (an interme-
diate in the Reichstein synthesis of L-ascorbic acid) from D-glucose has been
established by using a two-stage fermentation system. D-Glucose was first
converted to calcium 2,5-diketo-D-gluconate by a mutant strain of Erwinia sp. in a
medium containing D-glucose, corn steep liquor, (NH4)2HP04, and CaCO3. After
a 26-h cultivation, 328.6 mg of calcium 2,5-diketo-D-gluconate per ml was
obtained, with a 94.5% yield from D-glucose. This broth was used directly for the
next conversion without removal of cells by treatment with sodium dodecyl
sulfate. The stereospecific reduction of calcium 2,5-diketo-D-gluconate to calcium
2-keto-L-gulonate was performed with a mutant strain of Corynebacterium sp.
When the cell growth reached a maximum (about 16 h) in a medium containing D-
glucose, corn steep liquor, NaNO3, KH2PO4, and trace elements, NaNO3 was
added to the culture, and then the calcium 2,5-diketo-D-gluconate broth was fed
over a period of about 50 h. Since the mutant strain requires a hydrogen donor for
reduction, the calcium 2,5-diketo-D-gluconate broth was mixed with D-glucose
before being fed. The results off our two-stage fermentations in 10-m3 convention-
al fermentors showed that an average of 106.3 mg of calcium 2-keto-L-gulonate per
ml was obtained, with a 84.6% yield from D-glucose, the starting material of
calcium 2,5-diketo-D-gluconate production. Calcium 2-keto-L-gulonate was stable
in the broth. Neither 2-keto-D-gluconic acid nor 5-keto-D-gluconic acid was
detected in the final broth.
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