| Popis: |
Summary Different techniques for determination of vitamin A have been discussed. The Evans-Bishop technique was found to have no advantage over those techniques which depend upon the reinitiation of growth or the cure of ophthalmia. Of these latter the ophthalmic method was preferred, because of the specific nature of its reaction, and its rapid responsiveness. The growth method was also found reliable and correlated closely in results obtained with the ophthalmic method when the animals were taken for test shortly after growth ceased. Furthermore, with animals of variable weight we experienced difficulty in deciding just when they should be taken for the growth test. Vitamin A is destroyed by ultraviolet treatment even when the irradiation is carried out in an atmosphere of CO 2 , nitrogen, or hydrogen. However, these gases apparently do retard the destructive reaction somewhat. It is still possible that our preparations of butter fat may have contained a sufficiency of oxygen in solution to have allowed destruction by oxidation. This hypothesis appears tenable when one takes into consideration the very small amounts of vitamin A which can produce cure of ophthalmia or reinitiation of growth. The destruction of vitamin A was found to be proportional to the time of exposure and inversely proportional to the thickness of the layer of fat exposed. The irradiation necessary to secure maximum activation was found to cause some destruction of vitamin A. Practically, however, maximum activation may not be desirable from the dietetic standpoint, so that the antirachitic activation of butter fat by direct irradiation is a practical possibility without danger of destroying vitamin A. Butter fat, antirachitically activated, was found to have retained most of its vitamin A even after storage for seven months at room temperature. |
