PLANT COOKING SCIENCE
Vegetables and fruits have the advantage of being relatively easy to cook, thanks to their chemistry and texture - this is certainly not the case with fish and meat, as you may already have noticed. What's good about veggies is that they are high on sugars and low on proteins, so you don't need to watch the possible overcooking (overcoagulation and drying) of proteins. Heating vegetables and also immersing them in water actually makes them feel better and more homoogenous in the mouth, as the sugars become evenly distributed. However, we have to still watch the cooking procedure so that we achieve the result that we want, that is in terms of texture (do we want a crispy veggie or not?), and color (we don't want discolored or scorched veggies I guess?). So, let's go through some of the basics of cooking vegetables and fruits, and how the heat and water affects their color, texture, and nutritional value.
Nutritional content of plants makes up an important part of our daily diet. As you probably know already, cooking vegetables makes them leak out some of their vitamins and minerals. This can be prevented to some degree by cooking the veggies as quickly as possible, and to use cooking methods that retain most of the nutrition inside the veggie. Steaming is good, as is baking. And when cooking in water and for a long time, try to use as much of the liquid for eating.
Cooking can also be good for a vegetable. Potatoes and other starchy vegetables fare well from cooking (from our point of view) because their starch becomes more unfolded and easy to digest in our intestines.
The obvious change that happens quickly when we throw in some green vegetables into boiling water is that their color becomes more green. This happens due to the escape of gas pockets in plants - gas normally obscures the green color as it covers the green chlorophyll by just being positioned in between all the plant cells. So, simply removing the gas by heat, a simple physical rule, makes the vegetable more green. This nice green color can be discolored though, and this happens if you use an acidic or alkaline ingredient in your dish, or otherwise overexpose the veggie for its own acid juices that are normally trapped within the plant cells. The big problem here is the susceptibility of magnesium - a component of chlorophyll that makes it shiny green. Magnesium is easily dislodged from its position by acids, whether they come from your ingredients or from their own cells that become disrupted by cooking. There is also the famous temperature range 66-77 C/150-170 F in which the plants own enzyme destroys part of the green chlorophyll and makes it lose its shiny green color and become more dull.
Prevention of this problem is quite simple though. First of all, make sure to cook the veggies in plenty of water just to dilute the plants' own acids. Also, protect them from acids, and if you have to use them (e.g. lemon juice, vinegar) then wait to add them until at the end of the dish, and most preferably just as it is served. Of course, this is not always desirable, for the sake of flavor, so then cook the veggies for as short time as possible and if possible - stop the cooking sharp by plunging the hot veggies into icy water for a while, so their temperature reduces a bit below the critical 60 C/140 F. And lastly, don't use the ancient methods of cooking veggies together with copper coins or in copper bowls - even though it works in preventing the discoloration, too much copper or other metals is toxic for you!
A different kind of problem comes with veggies and fruits that are colored red, blue, or orange. They contain different kind of colorful molecules and they therefore behave differently in heat. They can change color completely! For example, red cabbage gets purple when braised. To avoid this, you need to use something that you wouldn't use with green veggies - acids! Adding some lemon juice or vinegar or some other acidic ingredient to your dish will prevent the discoloration of the veggies or fruits.
Texture of vegetables is also affected by cooking. In general, all veggies become soft by heat at above 60 C/140 F because their cell walls (the ones forming the rigid crispy structure) become softened and the water in the plant cells flows out and becomes more evenly distributed within the veggie. Vegetables also become soft quickly if you add some salt or alkaline ingredient to them (remember that drinking tap water is slightly alkaline in many countries). However, if you reduce the alkalinity by adding some acid (lemon juice, vinegar, etc.) or forget about the salt until the end of cooking (alternatively, reduce the salt amount in the beginning and then add more at the end) then the crispiness will remain for a longer time.