Stem
The stem contains high amounts of bitter tannins as well as acids of various types which if left in contact with the grape itself, would increase the bitterness and acidity of the juice... And the wine.
Seed
The seeds contains the greatest amount of bitter tannins (65% of the total); care should be taken not to break them during crushing and/or pressing.
Skin
The skin contains 12% of the total tannin as well as the necessary coloring agents, or pigments (where needed). Since most of the wine grapes' juice is colorlesss, this pigmentation in the skin, if left in contact with the fermenting juice determines the color (hue) of the resulting wines.
Pulp
The pulp contains almost 80% water; in most cases it yields a colorless juice. It contains most of the acids, sugars, pectins (Yes, the same agents used for making jellies), extracts, etc. necessary for desirable fermentation.
Waxy Coating
It is to this film, sometimes called the "bloom" on the skins of the grape that the natural "wild" yeasts in the vineyard areas of the world adhere. In some instances these natural yeasts are used for fermenting the juice into wine. In most instances, in developed wine regions, artificial or "cultured" yeasts are used, their composition depending upon their usage.
Yeast
(not diagrammed, too small) clings to the waxy coating. Essentially, the yeast is a fungus, similar in family (thallophytes) to algae, lichens, etc.
*Tannin or tannins are responsible for the bitter taste of plain tea or young red wines made for ageing a while.
DO NOT FORGET
The Soil
(its composition, its structure, its aspect, etc.) affects to varying degrees the flavor of the wine.
The Climate
(sunshine, moisture, temperature, cloud cover, etc.) greatly affects the final beverage's nature.
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