Irradiation

Vitamin D was rarely found in food. But it was known that sunshine converted an inactive precursor in skin to vitamin D. Steenbock showed that some food could be exposed to sunlight in order to produce vitamin D, making irradiated food a common occurrence.
 Harry Steenbock United States 1924 Irradiation

Vitamin D was rarely found in food. But it was known that sunshine converted an inactive precursor in skin to vitamin D. Steenbock showed that some food could be exposed to sunlight in order to produce vitamin D, making irradiated food a common occurrence.