252 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1941 



the vitamin D in such milk will be vitamin D2 with the small quantity 

 of natural vitamin D normally present. Eggs of high vitamin-D 

 activity are obtained by including cod-liver oil in the hen's feed so 

 that eggs generally contain only natural vitamin D. 



Milk may also be enriched in vitamin D by irradiating the cow, 

 by irradiating the milk, or by adding concentrates of the vitamin 

 directly to the milk. Only the last two methods have been used 

 to any extent commercially. 



RIBOFLAVIN (VITAMIN G) 



Properties. — Pure riboflavin is a yellow crystalline material readily 

 soluble in water, giving a yellow green-fluorescent solution. Ribo- 

 flavin is not readily destroyed by heating but is less stable in alkaline 

 than in acid solution. 



As it occurs in nature, riboflavin forms part of a protein phos- 

 phoric acid complex that must be broken down before the pure 

 vitamin can be obtained. 



Food sowces. — Food sources of riboflavin are less completely 

 known than are sources of the other vitamins so far discussed. This 

 is due partly to its later discovery but largely to the lack of a 

 satisfactory method of measurement. 



Milk, eggs, and lean meats are the richest sources. The yolk and 

 the white of eggs contain it in about the same concentration. As 

 riboflavin occurs associated with protein, it is present in milk in the 

 skimmed milk and not in the butterf at. 



In plants, riboflavin seems to be concentrated in the green parts. 

 Thin green leaves are especially rich sources. Green stems are much 

 richer than the flower or the root. Although the vitamin is more 

 concentrated in the green parts, the bleached parts of plants are not 

 devoid of it, as they are of vitamin A. Most root vegetables and 

 tubers contain some riboflavin. In fact, riboflavin is present in 

 practically all vegetables of one sort or another. 



Seeds vary considerably in the amounts of riboflavin they con- 

 tain. Legumes, peas, beans, and especially soybeans are good sources, 

 while nuts and cereal grains are not so rich. The germ portion of 

 the seed usually contains a high concentration of riboflavin, as it 

 does of vitamin Bi. 



In general, fruits are low in their content of riboflavin. The ma- 

 jority can be rated only fair and some fruits such as grapes, lemons, 

 oranges, and grapefruit, contain little more than a trace. If there 

 is a basis for classifying fruits as to riboflavin content, it is not 

 apparent in the few data now available. 



Fats and oils have already been described as not containing the 

 water-soluble vitamins Bi and C. They are also about the only 

 foods that do not contain at least traces of riboflavin. 



