PLANT FLAVOR SCIENCE
Plants in general are composed of a lot of many different flavor molecules, and they all make an impression on our tongue and in our nose. Packed with carbohydrates, minerals, some amino acids, and some bitter compounds, plants vary in taste more than any other type of food. Sugars and amino acids can be used by plants in their own metabolism, and this happens even after they have been harvested. For this reason, the longer you wait with cooking your vegetables the less intensive flavor you will get. Some vegetables however contain relatively high amounts of savory amino acids, e.g. glutamate, and these can be enjoyed even a while after harvest - tomatoes, oranges, are just some examples, and they also provide an appreciative amount of sweet sugars and acidity.
Some vegetables or unripened fruits contain lots of tannins - molecules that create a strangely dry and biting feel on our tongue. Tannins can often be felt in red wines or nuts as you probably have noticed. These molecules are used by plants against bacterial or parasitic aggression since they are able to cross-link and thus inactivate enzymes and other proteins that may threaten the life of the plant. In our mouth, tannins cross-link the lubricating mucins contained in saliva, which leads to a higher stickiness of food and consequently the dry feel on the tongue. This special feeling is known as astringency and is, among other applications, used to describe and assess quality of a red wine (they shouldn't have too much of it). Sometimes, astringency may be desirable in a dish, but just to a certain extent, as each new dose of tannins will multiply the effect of tannins in all preceding doses of food. In red wines, tannin sensation can be controlled by simply serving the wine well tempered, that is not too cold, since coldness amplifies the astringency.
Vegetables and fruits also possess inherent aromas, that is molecules that are more complex than the simple sugars and amino acids, and create a special sensation in our mouths. The most aromatic plants are often used in sauces, stocks, and to flavor dishes - carrots, onions, garlic, aromatic herbs, seasonings, and a heap of other plants are the aromatic flavor basics in every kitchen. The interesting thing in the world of aromas is that there are so many of them, and they all ought to be experimented with and tasted, just to create some new modern variations of the traditional food, or simply to create new trends in cooking. Many unusual combinations of ingredients are used by top chefs of the world, for example vanilla and vegetable stock, or cardemum and milk. Such combinations work because the flavor molecules complement each other well and shape a nice sensation in our mouths.
Besides the basic flavors, aromas, and astringency, the description of flavor usually also includes hotness or spiciness - that is, the hot feeling that is created in the mouth when eating for example a chili pepper or garlic. Ironically, one may say, these hotness molecules are used by plants to defend themselves against parasites and other animals. We humans have learned to use these plants in cooking by simply using them in low or moderate amounts, just to make an addition to the "ordinary" taste of a dish. Some vegetables, e.g. onions, also lose their irritable feel when they are cooked.