There are many factors that help determine when it is time to harvest, but all winegrowers use a mixture of science and intuition to make their decision. Achieving the proper balance of sugar, acidity and tannins during the ripening process of the grapes will result in a wine with perfect balance.
From a scientific perspective, sugar and pH levels should be optimal before harvest. Yeasts need the right amount of glucose to turn juice into wine. Brix degrees are a measure of the sugar content of grapes. Ideally, Brix levels should be between 24°- 26° for red grapes and 22°-23° for white.
We also measure the acidity to see if the grapes are ready. As the fruit ripens and the sugar levels rise, the pH rises and acidity drops. We have to maintain certain levels of acidity for the wine to be balanced, so we strive to achieve a pH of around 3.2-3.4.
One must also consider the physiological changes of the vines. Fernando checks the color and texture of the grapes, stems and seeds. Grapes that are ripe will have bright, plump fruit. The stems and seeds will be brown. The seeds should also be tough but easy to chew.
Lastly, intuition comes into play throughout this entire process, but especially when we taste the grapes. When the grapes are ripe, they will taste sweet, be slightly acidic, and the tannins will be noticeable. The characteristics of each variety will also be easily distinguished.
In short, determining the ideal time to harvest requires science and intuition. Taking all this into account and making a precise decision about when to harvest allows us to bring out all the qualities of our terroir and produce the best quality wine.