Grape Anatomy
The Skin
Even if the skin is only six to ten cells thick, it is full of Aromas, Tannins responsible for the bitter / astringent taste of wine, and pigments responsible for the wine Color.
The skin plays an important role when making red wines.
The Pulp/Flesh
The pulp beneath the skin contains the juice: Sugar, Water, Aromas, and Acid.
The pulp plays an important role for the flavor and acidity in a wine.
The Seeds
Seeds are full of Tannins and winemakers will not crush them during the pressing.
The Stems
Stems are full of Tannins and winemakers will remove them before the pressing.
The Sugars
Grapes contain a combination of different sugars.
Fructose is the sweetest and the predominant when berries are overripe.
Glucose and Sucrose are less sweet.
Not all sugars are fermentable by yeast, that´s why some wines taste sweet even after fermentation to "dryness".
The Acids
Tartaric and Malic acid are the primary ones.
They vary with varietal, climate and ripeness.
Tartaric acid tends to bind with potassium if stored too cool and is responsible for creating the "crystals" or "wine diamonds". Despite what people think, "wine diamonds" (tartrates) are not a sign of bad quality. Actually it can be seen as a sign of good quality, indicating that the wine was not overprocessed.
Wine crystals never impart an unpleasnt taste.
Lactic comes from latin "Lactis" and it means milk, in fact Lactic acid is found in milk and diary products, and gives wine a buttery, creamy taste.
Malic acid can convert to Lactic acid in presence of proper bacteria. This is called Malolactic Fermentation (MLF). This is an optional fermentation.
Winemakers do not want to risk a spontaneous MLF occurring in the bottle with consequential gas release and corks blowing or bubbly wines (if the gas stays in the bottle).
The Aromas
Primary aromas are in the skin and in the grape varietal.
Secondary aromas come from fermentation where different yeasts produce different aromas. Some yeasts are known to be more fruity.
Tertiary aromas come from aging and maturation. The main factors are: the type of oak, toasting levels and bottle maturation.
Aromas and flavors vary also with climate, varietal, ripeness level at harvest, age and struggle of the vines.