Читать книгу Foods and Their Adulteration - Harvey Washington Wiley - Страница 109

—Carbon disulfid, containing about one percent of sulfur in solution, is mixed with an equal volume of amyl alcohol. Mix equal volumes of this reagent and the oil under examination and heat in a bath of boiling brine for fifteen minutes. In the presence of as little as one percent of cottonseed oil an orange or red color is produced, which is characteristic.

Lard and lard oil from animals fed on cottonseed meal will give a faint reaction; also the fatty acids thereof.

This test is more sensitive than the Bechi test (nitrate of silver) and less liable to give unsatisfactory results in the hands of an inexperienced person. It is not affected by rancidity. The depth of color is proportional, to a certain extent, to the amount of oil present, and by making comparative tests with cottonseed oil some idea as to the amount present can be obtained, but it must be remembered that different oils react with different intensities, and oils which have been heated from 200° to 210° C. react with greatly diminished intensity. Heating ten minutes at 250° renders cottonseed oil incapable of giving the reaction.

Cottonseed oil also has the property of reducing silver in silver nitrate to a metallic state. When mixed with a solution of silver nitrate under proper conditions a blackening or precipitation of black metallic silver is observed. This is known as the Bechi test which is conducted as follows: