Growing grapes entails learning to recognize when grapes are ready to pick. During veraison red grapes turn from green to red and white grapes turn an opaque yellow-green. Normally veraison precedes grape harvest by about six weeks.
Ripening Grapes
With access to more air and light, grape bunches highest in the vine will ripen first. Bunches lower under the shady leaf canopy ripen later. As grapes ripen, desirable sugar content and acidity depend on healthy well-placed leaves that take in plenty of sunshine. Weather also plays an important role in grape ripening. Cloudy, wet weather can adversely affect sugar levels, keeping them low. Cool nights enhance the vine's ability to make color pigment while too warm nights can impair the color process in some varieties.
Evaluating grape ripeness for eating fresh from the vine relies mainly on aroma, flavor and texture. Wine grapes require a little more precision in determining ripeness. Depending on the variety, grapes ripen early, mid-season or late in the growing season. Three basic ripening indicators - sugar, acid and pH - should be measured and recorded weekly to note the approach of ripeness and be ready to harvest grapes for making wine.
Sugar Content of Grapes
An instrument called a hydrometer is used to measure the sugar content of grape juice. The percentage of sugar in grapes is known as Brix or degrees Balling. As all sugar is converted to alcohol the potential alcohol content of fermented grapes can also be determined. A Brix Table aids in making the calculation.
Acid Content of Grapes
In his book, From Vines to Wines, Jeff Cox says, "Acids give crispness, brightness and thirst-quenching qualities to wines and are essential components of the balance in a fine wine."
Grapes contain two major types of acid - malic and tartaric. Together they are referred to as total acid or tetratable acid. Acid-testing kits may be purchased form home winemaking supply stores. The optimum level for acid is from 5 to 9 grams per litre of juice. To determine tetratable acidity the grape juice is neutralized with an alkaline solution and the point of neutralization identified.
pH Levels in Grapes
Hydrogen ion potential or pH refers to the strength of the acidity in grapes. pH is a measure of how many hydrogen ions are combined as acids versus how many are free floating. The more free floating hydrogen ions there are, the lower the pH and the more tart the juice tastes. A portable pH pen can be used to assess grape ripeness.'
The Time to Harvest Grapes
When sugar comes closest to its ideal for a given grape variety at the same time that the acid comes closest to the ideal, grapes are ready to harvest.
Ripe grapes will pull away from the stem easily while unripe grapes will not.
Grape berries soften as they ripen, the skin of fully ripe grapes collapses easily when bitten into and the pulp is thick but not watery.
Fully mature grapes have brown seeds. If the seeds are beige or tan in color but not brown, the grapes are not quite ripe.
Resources:
From Vines to Wines by Jeff Cox
Storey Publishing, 1999
The Backyard Vintner by Jim Law
Quarry Books, 2005
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