This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
The Service and Regulatory Announcements define rye flour as "the fine-ground product made by bolting rye meal, and contains not more than 13.5 per cent moisture, not less than 1.36 per cent nitrogen, and not more than 1.25 per cent ash."
Flour contains pigments, enzymes, cellulose, moisture, fat, mineral salts, carbohydrates, and proteins. All the constituents of flour are important in that all affect the baking qualities of the flour and thus the product in which the flour is used.
Bailey states that the color of flour is due to five factors: (1) Granulation or size of flour particles, which affects the appearance of the flour and not that of the dough. Large granules appear darker than small ones.
(2) Color changes due to mixing with water in contact with air at elevated temperatures are caused by enzymes found principally in the bran.
(3) Dirt and foreign matter, as weed seeds, if highly pigmented, may become pulverized and color the flour. (4) A reddish-brown pigment is present in small quantities in flour made from "red" wheats. Highly refined flours from those wheats show less of these small bran particles containing the pigments.
(5) Carotinoid pigments. Flour contains the pigment carotene which gives it a creamy or yellow tint, the depth of color depending upon the amount of the pigment present. Because of the fineness of division of the flour particles its bulk includes a large proportion of air. The carotene is bleached by oxidation; thus, in aging, natural bleaching of the flour occurs. Markley and Bailey have reported that flour contains the following fat-soluble pigments: carotene, xanthophyll, and an unknown pigment.
(6) Flavones. The flavones are water-soluble pigments. Markley and Bailey state that flavones have long been known to exist as glucosides and from their study it appears possible that they will be found as esters of the fatty acids. The flavones are colorless in acid or neutral mediums but develop a greenish yellow tint in alkaline reactions.