234 VITAMINS 



The structural formula of riboflavin is: 



H CHj— (CH0H)3— CH2OH 

 CHa— C^ \r ^C^ ^C=0 



I A II B I C I 



J II 



H O 



Riboflavin, C17H20N4O6 



Note the benzene A and pyrimidine C rings, as well as the sugar-like side 

 chain, which is related to the rare pentose sugar, ribose. 



Riboflavin is very stable to heat, acids, and oxidizing agents, but is 

 easily destroyed by light and by alkalies. Losses attendant on the cook- 

 ing of food may range from to 60 per cent. A quart of milk in an 

 ordinary clear glass bottle, if set in direct sunlight, may lose as much 

 as 50 per cent of its riboflavin content in two hours. Brown glass bottles 

 or paper cartons prevent this destruction. Although riboflavin is a water 

 soluble vitamin, surprisingly little — perhaps 10 to 20 per cent — is removed 

 by boiling vegetables in water. This is probably attributable to the 

 fact that in tissues it exists largely combined with various proteins. 



Occurrence 



Riboflavin is formed primarily in green leaves of actively growing 

 plants; hence, green leafy vegetables constitute a good source of this 

 vitamin. Brewer's yeast is a higlily potent source. Of the various meats, 

 liver contains the greatest concentration of the vitamin and is followed 

 closely by kidney. Tlie muscle meats likewise contain appreciable 

 amounts. Milk and eggs are quite satisfactory sources, and in the well- 

 planned diet the former contributes largely to the total riboflavin 

 intake. 



At present several fairly reliable methods other than animal assay are 

 available for determining the amount of riboflavin in foodstuffs. Chem- 

 ical methods depend on the measurement of the characteristic color or 

 fluorescence in suitably clarified extracts of the food. Still another 

 method of assay has been based on the fact that certain lactic acid bac- 

 teria require riboflavin for normal growth and acid production. An 

 aqueous suspension of the sample is fermented by the organism, and the 

 amount of lactic acid produced (as determined by titration of the entire 

 culture) serves as a measure of the riboflavin present. Similar assay 

 methods are used for most of the other B vitamins and for amino 

 acids. 



