PLANTS - EXTRACTION OF FLAVOR
Plants contain a lot and a big variety of flavors so they are often used to set the basic flavor of many dishes, if they are not used as the main ingredients. The prime example is sweating an onion in oil before adding other ingredients. This will extract some of the onion juices into the oil and then the onion-flavored oil will coat and diffuse into the other ingredients. That's a basic technique of cooking and using vegetables. However, more exciting things can be done.
Oil can be used to extract flavors from veggies and herbs and then used as a garnish or an extra seasoning on top of some dishes. Basically, some very tasty herbs (e.g. mint, thyme, basil, etc.) can be finely cut and immersed in some oil and left for extraction over a course of a day or more. Then, the oil is filtered or decanted to remove the herbs/veggies and stored in a refrigerator for later use. Such a procedure is simple and gives an impression of professionalism on people (who seldom do such things).
The most basic technique of extracting flavors from vegetables however has to be stock making. Here, water is used to extract flavors from vegetables and herbs by simmering them until soft and possibly until the water volume is sufficiently reduced. Stocks are usually made of cheaper, aromatic vegetables like onions, carrots, celery, leeks, garlic. Also some special treatments with mushrooms or tomatoes are possible. Vegetable stock can then be used in sauces, risotto, stew, and a number of other dishes, and they can also form a base for making stocks of beef, chicken, fish, or shellfish. The field of usage is tremendous, and nothing is as good as a home-made fresh stock. Sadly, most people rely on the salty non-fresh stock cubes as the main substitute for a home-made stock. In some cases, they can be used, although not for dishes where the real stock flavor is crucial for the outcome of the dish. (For more on stock making take a look at our HOW TO section.)
Vegetables can be made into purées as well. This is simply a procedure where you put the veggies into blenders and mix them so they become more or less liquefied. Some purées make use of a gentler form of mixing. Several dishes are literally based on vegetable purées, e.g. potato mash or guacamole. What makes purées so easy to make is the high content of water in a vegetable, but we may also add some liquid, often a flavorful one, to further enhance the texture of a purée. usually though, the texture is too simmery and the purée has to be slightly cooked so to reduce the water content. Regardless of the technique, purées can be used as stand-alone dishes, or to flavor other dishes, e.g. in risotto, pasta, or to make sauces. Purées can also be frozen into icy sorbets.
If we separate the purée from its solid components we get a juice. This has of course high water content and contains many of the flavor molecules of the purée. The big difference is the texture, and whether we decide to use purée or juice in our dish depends on the desirable outcome. The juices contain lots of the plant cell's enzymes that can become involved in actually degrading the flavor and color molecules, which means - use the juice fresh! The same would go for purées, if they are prepared cold. In most cases though, purées are made from pre-cooked vegetables so the digestive enzymes are inactivated and a purée can be stored for a while. Juices sold in shops are often pasteurized (quick heating) which has a similar stabilizing effect.