Vitamins and Coenzymes
The vitamin riboflavin, which occurs as a yellow pigment in egg yolk and milk become

colorless on reduction with Zn in acid and regained its color on re-oxidation
colorless on oxidation and regained its color on reduction with Zn in acid
more deep in color on reduction with Zn in acid and regained its color on re-oxidation
more deep in color on oxidation and regained its color on reduction with Zn in acid

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