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Comparison of Chemical Analysis and Bioassay as Measures of Vitamin A Value: Yellow Corn Meal

One Figure

Bureau of Human Nutrition and Home Economics, Agricultural Research Administration, U. S. Department of Agriculture, Washington, D. C.

The provitamin A carotenoids of yellow old process water-ground corn meal were separated by chromatographic techniques and measured spectrophotometrically. The vitamin A values of the corn meal and of its carotenoid extract were determined by rat growth bioassay, and availability of the carotene calculated.

The "ß-carotene" fraction was found to include significant amounts of the isomers neo-ß-carotene B, neo-ß-carotene U, and a carotenoid which may well be identical with a pigment isolated from dehydrated alfalfa by other investigators, but which, to our knowledge, has not been reported previously in corn. It was necessary to chromatograph the ß-carotene fraction on Ca(OH)₂ in order to distinguish between these isomers.

Another pigment, which may be either "contaminated" -carotene or another carotene closely related to it, occurred in considerable quantities, and was separated from ß-carotene both by MgO Hy-flo Supercel and by Ca(OH)₂. These last two pigments were compared with similar carotenoids which have been described by other investigators, and some attempt was made to characterize them further.

It was found that the bioassay vitamin A value of the corn meal extract, 324 I.U. per 100 gm meal, agreed with the vitamin A potency calculated from chemical analysis, 317 I.U. per 100 gm meal, when hydrated lime was employed as adsorbent to separate an inactive pigment and the partially active isomers of ß-carotene from all-trans-ß-carotene, all of which compounds remain together on another common adsorbent, MgO Hy-flo Supercel. The biologically active carotenes, as they occur in corn meal, proved to have a biological vitamin A value of 264 I.U. per 100 gm and therefore to be 82% available to the rat. It is suggested that the poor availability reported by other investigators may be accounted for by the inclusion of considerable amounts of inactive pigments with the ß-carotene fraction.

² Present address: Division of Research Grants, National Institutes of Health, U. S. Public Health Service, Bethesda, Maryland.

Manuscript received 3 November 1952.