208 VITAMINS 



and palmitate 







II 



Ci9H27CH20C(CH2)l4CH3 



have also been prepared artificially. They are fully active, and yet con- 

 siderably more stable than the free vitamin. 



The vitamin A value of a given food sample may be due either to the 

 actual vitamin itself, an ester of it, one of the carotenoids listed above, 

 or to a mixture of these. The carotenoids which are capable of conversion 

 to vitamin A are often designated as provitamins. Unless otherwise 

 specified, the collective meaning, that is, precursors as well as the true 

 vitamin, is implied in the subsequent discussion. 



Preparations of vitamin A rapidly lose their potency in the presence 

 of oxidizing agents. Atmospheric oxygen at room temperature or above 

 brings about noticeable destruction of vitamin A unless the vitamin is 

 protected by antioxidants. These antioxidants, or inhibitors, are asso- 

 ciated with lipidcs in nature and hence stabilize the vitamin in food- 

 stuffs. Vitamin E is one of the most important of these antioxidants, 

 which act to minimize the destruction of vitamin A by oxidation. Cooked 

 foods retain the greater part of their original vitamin A potency. Since 

 vitamin A is fat-soluble, it is extracted from foodstuffs along with the 

 lipides by the common fat solvents. Saponification of these lipides does 

 not destroy the vitamin. 



The vitamin A value of biological samples may be determined by 

 animal feeding tests, but this laborious method has been almost entirely 

 supplanted by quicker chemical methods. However, the animal assay 

 measures the precursors as well as true vitamin A, whereas each chemical 

 test measures either one or the other, but not both. The chemical meas- 

 urement of true vitamin A depends on reacting it with antimony trichlo- 

 ride in dry chloroform (Carr-Price reaction). A clear, deep blue color 

 is produced which is compared with the intensity of the color from known 

 amounts of the pure vitamin to estimate the potency of the sample. 

 The carotenoid pigments, many of which have no vitamin A value, all 

 respond to some extent to this same test, and so interfere unless a suitable 

 correction is applied. This method is widely used for determining the 

 true vitamin A content of butter, cheese, fish oils, pharmaceuticals, and 

 many animal tissues such as blood and liver. 



The vitamin A value of plant samples is usually determined by measur- 

 ing the carotene content. This is accomplished by extracting with a fat 

 solvent, purifying the extract to remove interfering pigments, and then 

 comparing the yellow color of the solution with that of known concen- 

 trations of carotene in the same solvent. 



