Annual Growth Cycle of the Grapevine
Harvest
As harvest approaches, the grape is considered ripe when the sugar and remaining acids are well balanced. A grower may choose to harvest the grapes before they are ripe, at their point of ideal ripeness, or when they are overripe; it all depends on the style of wine desired by the vintner. For example, Pinot Noir grapes for sparkling wine production are harvested much earlier, at lower sugar and higher acid content, than Pinot Noir for non-sparkling red wine.
Chemical Composition of the Fruit
Berry development includes accumulation and metabolism of Berry development includes accumulation and metabolism of hundreds of compounds, many in tiny quantities, which may contribute to fruit quality attributes. The single largest component of berries is water, followed by the sugars (predominantly fructose and glucose), then the acids tartaric and malic. Other important classes of chemical compounds found in grape berries include phenolic compounds, nitrogenous compounds, aroma compounds, minerals, and pectins.
Sugars
The majority of soluble solids in grape juice are sugars, mostly glucose and fructose. Sugars increase in concentration during ripening, and may reach 25 percent or more of berry fresh weight by the time of harvest. At veraison, glucose exceeds the fructose concentration but at the ripening stage, glucose and fructose are usually present in equal amounts (1:1 ratio). In overripe grapes, the concentration of fructose exceeds that of glucose.
Organic Acids
Next to sugars, organic acids are the most abundant solids present in grape juice. Acids give crispness, brightness, and thirst-quenching qualities to wines and are essential components of the balance in a fine wine. They have a marked influence on wine stability, color, and pH. Excessive grape acidity can lead to tart, acidic wines.
Phenolic Compounds
Phenolic compounds are often referred to as polyphenolics, polyphenols or simply phenols. Phenolic compounds are important constituents of grapes because they play a key role in determining fruit color and also provide most of the characteristic taste and aging properties in wine.
Nitrogenous Compounds
Grapes contain various nitrogenous compounds such as amino acids, peptides, and proteins. Nitrogen containing compounds are important because they serve as the nutrient for yeast and lactic acid bacteria.
Aroma and Flavor Compounds
Grapes contain numerous aroma and flavor compounds, which increase in complexity during the later stages of ripening. Some varieties have specific aroma compounds that give a variety its distinct varietal character. Examples of these include:
Minerals
Minerals too play a role and generally include potassium, sodium, iron, phosphates, sulfate, and chloride.
Pectic Substances
Pectin substances are cementing agents present in the cell wall composed of polymers of galacturonic acids.
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