APPLES
Apples belong to the pome fruit family that also features pears and quinces. The sweet and slightly sour tasting flesh of an apple develops by thickening and accumulation of nutrients in the flower stem, just below the flower itself. This part also contains the seeds of the apple. The small black threads at the bottom of an apple are remnants of the flower base that carries the colorful flower petals. Apples are good when harvested fully mature, but they can also be pre-harvested and ripened on their own (as can be done with e.g. bananas). The sweet taste will develop by itself from the vast amount of starch that is contained within the unripened apples. As a rule, apples are harvested when fully ripe, as they can be stored in a cold room for a long time without getting bad. This cannot be done with e.g. pears, even though they belong to the same pome fruit family. (See how well apples keep their shape compared with other fruits here: http://www.youtube.com/watch?v=gvRtA4qVMLY)
There are several kinds of apple trees cultivated all around the world, and they all differ in flavors. The ordinary eating apples have a good ratio between sweetness and sourness, but they lose much of their elegancy when cooked. The remedy for this is to buy another kind of apples - the cooking apples - which are the ones that have a not as nice taste when raw, but become better when cooked, without excessive loss of texture. Cooking apples remain fairly firm even when hot. Some apples can be used for both purposes, depending on which stage of maturation they've reached. These apples are common in the stores and include Golden Delicious and Granny Smith. If you are not sure if your apples are suited for cooking you can always test them in the heat of the oven and see how they taste and feel after a while of baking.
From the nutritional point of view, the main goodies of an apple include vitamin C and phenolic antioxidants. They also provide dietary fibers, both insoluble (the ones that remove bad cholesterol from our intestines) and the soluble pectins (that decrease bad cholesterol production in the liver). And if we reduce bad cholesterol in our bodies, our cardiovascular system is more healthy. The cardiovascular diseases also are prevented by the apple's flavonoids, and of course by vitamin C that is an antioxidant. The apple's phenolic antioxidants are by the way also protective against the damaging sun radiation UV-B, which should be remembered during the hot summer days. And, apples and apple juice have been found to be very beneficial in treating and preventing kidney stones.
One thing that should be noticed though is that not all apples provide the high amount of all these beneficial health molecules. Of the better ranked health promoters are the Fuji apples, although even Red Delicious are good. And, when eating apples, eat the skin as well - it contains lots of special phytonutrients for your health that you don't find that much in the flesh.
Flavor of the apple is mainly produced by an ester compound, and can be also produced artificially by a simple reaction between an organic acid and an alcohol - something that candy makers are fond of. The reason that the artificial flavor does not come very close to the natural apple flavor is that apples (as well as other fruits) contain more than one ester, all of which contribute to the complex flavor of the fruit. However, in apples, one ester is pre-dominant and is called ethyl acetate. You may wonder what the alcohol comes from in the apple? It is usually a biproduct of the ordinary cell metabolism, and together with the acidic substances contained within the plant, there exist many possibilities for ester production.
When buying apples, there shouldn't be a problem to select the ones that are the best in flavor and ripeness. The fruits should be firm and have lots of nice colors, if they are not naturally green when ripened. The sweet eating apples are Red Delicious and also Golden Delicious (that can also be used in cooking if they're still tart). The typical cooking apples include the tart Granny Smith, Pippin, and Gravenstein. The best season for apples is from late summer to winter in the northern part of our planet. Other times of the year, the apples come from cold storage rooms, or from the southern hemisphere.
A few things need to be mentioned about apples and cooking. One characteristic of apples is their high amount of air (especially the overripened apples) and pectins (the jelly-forming sugars). If you cook an apple, its interior will expand since the water will start to evaporate from the cells and fill in the air pockets. The expansion is so big that if you bake whole apples, you need to remove a part of the skin at the top of the apple, or cut off the skin in some other place. When it comes to pectins, they will give you a jelly-like consistency if you cook the mashed apples for a longer time - as the water evaporates, pectins become more concentrated and give the apple this characteristic apple "mousse" texture.
Apples go well together with cinnamon, vanilla, rose pepper, star anise, black pepper, cloves, Calvados, brown sugar, caramel, lemon, raisins, and butter. This is just to mention a few possible food combinations you can try. Of course, use everything in moderation so e.g. cloves don't get to dominate over the apples but just season them. Other possible pairings for apples include Cognac, Cointreau, Grand Marnier, Kirsch, vermouth - to mention the liqueurs; almonds, blackberries, black currants, pears, and oranges - to add some fruits.