Chapter 31

Evaluation of Wine Grape Maturity

Assessing Wine Grape Ripeness by 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 cultural and environmental influences that come into play. The maturity of grapes is usually based on three parameters: sugar content, titratable acidity, and pH.

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 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. Acids contribute sourness, while both acidity and pH influence tartness.

Ratio of Malate to Tartrate

In some cases it is desirable to have some malate in the juice at the time of harvest so that malolactic conversion will occur.

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.

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.

Brix-acid Ratio

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

Brix x pH

Van Rooyen et al. (1984) found an index of °Brix x pH to be a better measurement of optimum ripeness in Cabernet Sauvignon and Pinotage than Brix-acid ratio.

Brix x pH2

Coombe et al. (1980) found °Brix x pH2 to be an even better indicator of optimum ripeness.

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.

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