How does food processing remove vitamins from grains?

Food processing removes essential vitamins from grains primarily through the mechanical removal of nutrient-dense outer layers during milling and refining.

In the production of white flour, the grain is processed to remove the bran and the germ (embryo). Because B vitamins are concentrated in the scutellum—a covering of the germ—and the bran, this refining process significantly reduces the vitamin content of the resulting flour. Similarly, when brown rice is polished to create white rice, the removal of the outer layers leads to a substantial loss of thiamin (vitamin B1).12

To mitigate these losses, many countries mandate the enrichment of white flour and rice with synthetic vitamins. However, without such fortification, reliance on highly processed grains can lead to nutritional deficiencies, such as beriberi, which is caused by a lack of thiamin. Additionally, nutrient losses can be further exacerbated during cooking if excessive water is used or if the grains are not properly prepared.12