Nutrient Leaching

In food preparation, nutrient leaching is the transfer of nutrients out of a food and into surrounding liquid, such as cooking water. Water-soluble vitamins and mineral constituents can leave the solid food this way. The process changes where the nutrients are located, without necessarily destroying them .¹

Leaching is distinct from chemical degradation, in which a nutrient is changed or broken down. Both processes can occur during heating, so a lower vitamin content in a cooked vegetable may reflect a combination of transfer into water and chemical change. Experiments that separate contact with water from heating alone can help distinguish their contributions .^1,2

The extent of leaching depends on the nutrient, the size of the food, and the conditions of preparation. In experiments with peas and Brussels sprouts, vitamin C and folate moved into water at different rates, and temperature and vegetable size affected the balance between leaching and degradation. One result therefore cannot be treated as a universal loss percentage for all foods or nutrients .²

Nutrients transferred to cooking liquid are no longer in the solid portion, but may still be present in the food and liquid taken together. Whether the liquid is discarded or included therefore matters when describing nutrient retention. A loss from the solid food alone does not establish an equivalent loss from everything served .¹