One of the primary reasons for the change in color when green vegetables are cooked is the change in chlorophyll. Chlorophyll has a chemical structure that is quite similar to hemoglobin, which is found within our red blood cells. A basic difference is that chlorophyll contains magnesium at its center, while hemoglobin contains iron. When plants (e.g., green vegetables) are heated and/or exposed to acid, the magnesium gets removed from the center of this ring structure and replaced by an atom of hydrogen.
In biochemical terms, the chlorophyll a gets turned into a molecule called pheophytin a, and the chlorophyll b gets turned into pheophytin b. With this one simple change, the color of the vegetable changes from bright green to olive-gray as the pheophytin a provides a green-gray color, and the pheophytin b provides an olive-green color. This color change is one of the reason.
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