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How is the vitamin content in food determined
Most labs determine the vitamin content of foods using a combination of two technologies —chromotography, which sorts food molecules based on their molecular weight, and a light emission detector that identifies molecules based on their interaction with light。
To test the vitamin A content in an apple, for example, laboratories first grind the fruit to a pulpy liquid and drop a small sample through a device known as a vertical chromotography column. As the sample falls through the column, smaller molecules move faster while heavier molecules move more slowly. As each molecule drops from the bottom of the column, a monitor sounds out like the blips on an electrocardiogram that ring out each time the heart beats. All molecules pass through the column at specific speeds— vitamin A typically takes 10 minutes — a characteristic that has been previously determined.
When 10 minutes have elapsed, a machine shoots a beam of light through the bottom end of the column and records the wavelength and intensity of light the molecule reemits. If vitamin A is present, it should have reemitted the light at a characteristic 325-nanometer wavelength. The intensity of the light given off is a measure of how much vitamin A is present. So by comparing the intensity of the light given off in the apple sample to the intensity of a known amount of vitamin A, scientists can calculate the amount present in the sample.