Chapter 32

Evaluation of Wine Grape Maturity

Determining Wine Grape Ripeness

Quantitative Parameters

The greatest potential of any wine grape variety is realized only when it is harvested at the right time in order for the wines to possess the characteristic varietal aroma, flavor, and balance intended for its use. The date of previous harvests can be used as a guide when trying to determine the projected harvest date. However, such dates alone should never be relied upon exclusively given management practices and environmental influences that come into play. The maturity of grapes is usually based on three parameters: sugar content, titratable acidity, and pH. All of these parameters change over time, and the rate at which they change depends on conditions during the growing season. Therefore it is critical to properly monitor and assess the fruit quality and maturity to make the appropriate management, harvesting, and winemaking decisions to produce the best quality grapes and wine possible.

Sugar Content

The criterion most commonly employed for determining when to harvest grapes is the sugar content, which is measured as total soluble solids (TSS) in ˚Brix. Sugar concentration is important due to its impact on fruit quality (sweet taste) and its role in alcohol formation during fermentation. During the course of fermentation, the yeast converts these sugars to alcohol and carbon dioxide. The amount of alcohol produced is related to the amount of sugar initially present in the juice; thus, by controlling the amount of sugar in the juice, one can control the amount of alcohol in the resulting wine.

Titratable Acidity

Titratable acidity (TA) measures the quantity of grape acids. The acidity level of the grapes at harvest is very important to the structure of the wine as well as to the composition of the finished wine.

pH

The pH of the juice at harvest is also an important variable. Wine pH will impact both tartrate and protein stability and affects the rates of key phenolic reactions. Equally important, juice and wine pH will impact the nature of microorganisms that can persist in the fermentation and subsequently in the wine.

Phenolic Compounds

Phenolic compounds are a group of substances that are structurally diverse and are present in various amounts. They play a vital role in determining the wine’s color and flavor. They are involved in browning reactions in grapes and wines, and also play a key role in the aging and maturation of wines. The two main substances included in this group of compounds are anthocyanins and tannins.

Other Indices

In an attempt to better estimate, the right time to harvest, the following combinations of sugar, acid, and pH for various wine types are often used.

Qualitative Parameters

The balance between sugar and acidity in the grape at harvest will reflect the balance between ethanol and acidity post fermentation.

Soft Berries

Berries dehydrate slightly, and the texture of the pulp softens when grapes ripen. When red grapes are fully ripe, the berries feel less, firm when squeezed and the skin becomes slightly slack but not wrinkled like a raisin.

Red Fruit Flavors

Under-ripe red grapes often have a green, herbaceous smell and taste reminiscent of asparagus or bell peppers. Ripe grapes have less of this green, herbaceous character and more plum and cherry characteristics.

Pulp Ripeness

Phenolic compounds are a group of substances that are structurally diverse and are present in various amounts. They play a vital role in determining the wine's color and flavor. They are involved in browning reactions in grapes and wines, and also play a key role in the aging and maturation of wines. The two main substances included in this group of compounds are anthocyanins and tannins.

Skin Ripeness

Early in the season, the tannin in the skins is harsh and astringent. Then, as the grapes ripen, the harsh, bitter astringency of the tannin diminishes. The quality and quantity of the tannins determines the structure, body, astringency, bitterness, dryness, and color intensity of the wine.

Brown Seeds

The color of grape seeds changes from green to brown as the berries ripen and should be brown, hard and have toasty (or at least nutty) aromas.

Clean Pedicel

When grapes are fully ripe, the pedicel (stem) can be pulled off the berry easily, and little or no pulp or skin tissue will be attached to the pedicel.

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