Vines produce energy through photosynthesis and convert extra energy into simple sugars which are stored in the grape berries. Hot, dry weather can cause a brix reading to rise as a result of dehydration. Brix is a reading of the concentration of sugar in a solution, not a reading of the total amount of sugar. So, when a berry loses water, the concentration of sugar remaining goes up. A good winemaker watches carefully to know if the vines are producing new sugars or simply dehydrating. Likewise, an excess amount of water in the soil can cause the vines to take on more water in the berries, producing plump fruit with a lower concentration of sugars (lower brix).
Grape berries start out with a low pH and a lot of natural acids in their juice. As the berries ripen, they lose acid, and their pH rises. Natural acidity is critical to producing balanced and long-aging wines, so we watch this transformation carefully as the grapes ripen. Picking too early, regardless of brix, can produce wines with too much acid, while picking too late can produce wines that taste “flabby” due to a lack of natural acidity to support and to lift the palate.
Compounds called phenols influence the final color, the mouthfeel and texture, and the taste of wines in big, big ways. For instance, tannins are phenols. These compounds are produced in the skins, the stems, and the pips of the grapes, and they take lots of sunlight, heat, and time to develop fully. There is no reliable measurement scale, like brix or pH, to monitor the phenolic ripeness of a grape. (Phenolic ripeness is also referred to as physiological ripeness). Instead, a winemaker has to taste the grapes and has to be able to know when they are finally a peak ripeness. This is probably the single most important and most influential decision that a winemaker makes during the entire winemaking process. Some of the most interesting, ethereal, long-lasting, and unique pieces of a wine’s final flavor are a result of these phenolic compounds. Pick too early, and they taste harsh and “green.” But pick too late, and they change and even disappear altogether.