When making wine, it’s essential to harvest grapes at the right moment. Some say determining the optimal moment of ripeness is even the most crucial decision in winemaking. Fortunately science can help winemakers choose the time to be right.
After the onset of ripening has begun and the grapes start to change color, winemakers will test several hundred grapes picked from clusters throughout the vineyard as the harvest approaches.
To test the ripeness of the grapes, winemakers may use a hydrometer to measure the weight of grape must, the juice pressed from grapes before it has fermented into wine.
The hydrometer contains a scale inside the stem, so that the weight of the must can be read directly. The Oechsle Scale, for example, is a hydrometer scale widely used in German, Swiss and Luxembourgish wine-making industries.
Since more than 90% of all the dissolved solids in grape juice are sugars, measuring the must weight is a good indicator of the amount of sugars in the wine, which are converted by yeast into alcohol by the process of fermentation. The greater the concentration of sugars in the grape, the greater the potential alcohol level.
Rather than measure the actual “weight” of the must, the density of the juice is measured by the hydrometer in relation to the gravity of water. One liter of the must at 20 degrees Celsius is compared to the mass of one liter of water. Every gram of difference between the two masses is one degree Oeschle.
For example, must with a specific mass of 1084 grams per liter has 84° Oe. What must weight is most desirable will depend on the winemaker’s personal goal for ripeness.
Other countries around the world are known to use other scales to measure the must weight of grape juice than the Oechsle Scale. In the United States, New Zealand and parts of Australia it is measured in degrees brix (symbol °Bx), in France and most of Europe the Baumé is used and in Austria the Klosterneuburger Mostwaage (°KMW) scale.
Source: Popsci, Wikipedia
Photo: bby_’s / Flickr
Principles and Practices of Winemaking, by Roger B. Boulton.