How does applying heat take away the nutritive values in the cooking, baking process?

Like humans, all plants and animals thrive within certain temperature ranges and experience problems if they are exposed for prolonged periods of time to temperatures outside of their normal range. When we consume plants and animals as foods, however, they are no longer living on their own, and they become even more susceptible to temperature changes. Many plants that would do just fine and retain their nutrients when growing at room temperature need to be stored in the refrigerator after being picked because many of their nutrients become more temperature sensitive.

Relatively small amounts of heat can take away nutrients from harvested foods in four basic ways. First, heat can speed up chemical reactions, especially reactions involving enzymes. Some of these enzyme-based reactions can transform nutrients into less desirable or non-usable forms. Second, heat can increase the solubility of certain nutrients. This process is related to any water-based heating method, including boiling, steaming, simmering, or sautéing/stir-frying in a water-based liquid. The increased energy provided by the heat can cause nutrient molecules to break apart when they would otherwise remain held together, and these nutrient molecules can then “escape” into the cooking water or very moist surrounding air. Third, heat can disrupt the chemical bonds in a nutrient molecule and change its structure. (This type of change commonly occurs in proteins and is called “protein denaturation.”) Finally, cooking heats can speed up the rate of oxidation for many nutrients (with vitamin C serving as a great example here.) In this situation, oxygen-based reactions can happen more easily and more quickly with the help of heat, and in the case of a nutrient like vitamin C, oxidative changes can take place that transform the nutrient into a different form.