During the heating and cooking process, green leafy vegetables undergo a variety of chemical changes. Many of these changes cause alterations in chlorophyll. Chlorophyll, the pigment in plants, has a complex structure with a magnesium atom at its molecular center, and it can undergo chemical changes in many ways.
The most common change in cooking is that the bright green color of vegetables disappears after heating, turning into a greenish-brown. This is because the acids produced during heat processing cause chlorophyll to be converted to pheophytin. Scientists have identified 10 acids in heat-processed green vegetables like spinach and peas, with acetic acid and pyrrolidone carboxylic acid being the main ones that destroy chlorophyll.
The active ingredient of vinegar is acetic acid, and its content varies depending on the quality of the vinegar, with commercially available vinegar generally containing 4% to 10% acetic acid. Therefore, adding vinegar when cooking green leafy vegetables will cause them to quickly turn yellowish-brown, which not only ruins the aesthetic appeal of the dish but also reduces its nutritional value. For this reason, vinegar should not be added when cooking green leafy vegetables.