A grape growing text from the 1880s said that you know when the grapes are ripe because the birds are enjoying a banquet. That is probably true, but growers and winemakers like to have a bit more information before they arrive at their decisions to pick. Measuring sugars, acid and PH will help them make the call.
From verasion, when the grapes begin to change color, sugars, acids and PH are recorded weekly to note the approach of ripeness and when to harvest the grapes for making wine. An instrument called a refractometer is used to measure the sugar content of grape juice.
The percentage of sugar in grapes is known as Brix. As all sugar is converted to alcohol, the potential alcohol content of fermented grapes can also be determined. For Brix ranges at harvest can range from 21 for whites to 26 for reds.
Acids give crispness, brightness and thirst quenching to wines and are essential components of balance in fine wine. Grapes contain two major types of acid, malic and tartaric. Together they are referred to as total acid or tetratable acid. The optimum level for acid is from 5 to 9 grams per liter of juice. To determine tetratable acidity the grape juice is neutralized with an alkaline solution and the point of neutralization identified.
Hydrogen ion potential of PH refers to the strength of acidity in grapes. PH is a measure of how many hydrogen ions are combined as acids and how many are free floating. The more free-floating hydrogen ions there are, the lower PH and the more tart the juice tastes. A portable PH pen can be used tp assess grape ripeness.
When sugar comes closest to its ideal for a given grape variety at the same time the acid comes closest to the ideal, grapes are ready to harvest.
There are other tests for ripeness that cannot be measured in the lab.
Ripe grapes pull away from the stem 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.
The quality of the finished wine depends on fully ripe grapes. Grapes that are not fully ripe can give vegetal flavors to wine. One can see how important the decision of when to pick can be and where the advantage of wine being made in the vineyard comes from.
