Vitamins and minerals can be very sensitive to the environment such as being exposed to light and heat. During processing of a food, vitamins and minerals can be destroyed; due to this sensitivity foods can be enriched. Enrichment is when they add a vitamin or mineral back to a product at equal or higher value than the amount that was in the product before processing.
An example of enrichment is adding thiamin, riboflavin, niacin, and iron back to bread after it has been processed. Another word that you may see with processed foods is fortification. Fortification is when they add a vitamin or mineral to a product that originally did not contain that vitamin or mineral. Some examples are adding vitamin D to milk and iodine to salt.