In July’s blog, I discussed how grapes are pollinated and explained the terms “bloom” and “set.” Since August is the month in which grapes begin their final push toward maturity, I thought it appropriate to discuss how they develop, grow, and ripen.
Once past berry set the grapes enter a period of accelerated growth. From May onward, they enter the “green stage.” Regardless of the specific variety the berries are high in chlorophyll, and are very green and very hard in composition. Much like green apples, they are sour to the taste being high in natural acidity. As they grow and “mature,” they become softer and sweeter, much less sour in taste due to a decrease dilution of their total acid content.
By July, if all has gone well, the vines have developed a fairly full leaf system or foliage. The leaves are absolutely vital to the development of wine grapes. First, we train the vine so the leaf surface provides a canopy affect to shade the grape clusters. The leaves are even more important to us because they function as solar receptors capturing the sun’s energy. The sunlight reacts with the leaves’ chlorophyll forming carbohydrates which will eventually be transmitted to the berries where it converts into sugar.
So, after the initial growth, there comes a stage usually in July when the leaves are luxurious, but the berries themselves show little development. However, certain complex internal changes are taking place. Then sometime in August in Sonoma Valley, our grapes begin a second stage of rapid growth by means of cell division and enlargement. Each grape becomes softer and develops color in the skins, if a red variety. The fruit sugar starts to form within the structure of the berry.
This maturation process is the stretch run for grapes and continues for roughly the next 30 to 60 days. The exact length of time depends on the grape varietal. The vine has in a way changed gears, moving on to fruit or berry maturation. It is accompanied by a decrease, actually, a dilution, of acidity as the sugar content increases in each berry. In the beginning of this stage, the concentration of the tartaric and malic acids, the two most prevalent acids in grapes, were at their maximum level of about 3%. The sugar content of the grapes are low, usually under 4%. But the leaves are now working hard to photosynthesize fruit sugar which is transferred from leaves to grapes in incremental stages.
Just when the grapes should be harvested depends on both the winemaker and the style of wine in mind. Winemakers consider the ratio between grape sugar and acid, and just about every winemaker in the world has a definite concept of the “ideal” ratio for each wine variety. Old timers, I should point out, firmly believed from experience that the harvest should begin 100 days after the flowering stage. In many European wine regions, vintners still follow this theory which, strangely, is fairly reliable. Both my father and grandfather placed much emphasis on tasting each variety periodically in the vineyards as harvest approached.
Today, the maturity of the grapes is carefully and regularly monitored. We test for sugar development and acidity by using very precise instruments. The finest wines worldwide are made by winemakers in tune with the cycle of the vine and its fruit. We “listen” to the grapes for their signs as to when they should be harvested and adjust our schedule to their internal calendar.
Wine grapes come to vintners is all size, shapes and, yes, even colors. The color of every wine made comes from the skin of the grapes. The juice of every major wine variety is white, and red wines are red or colored because of the skins which are used when they are fermented. Certain red varietals, such as Alicante Bouschet and Carignan do have lightly colored red juice but most grape juice is color-less,
Chemically, the grape is one of the most complex creations of Nature. It, to generalize, includes at least 13 major acids, 4 sugars, and over 75 minerals. So far, well over 412 constituents or components of grapes have been identified and isolated. I do not want to imply that wine lovers need to have a degree in chemistry or in plant physiology to understand and appreciate wine and the grapes they are made from. However, some general understanding should help account for the diversity that exists in wine.
Let’s look at a wine grape as it nears full maturity. The juice of the grape, which is mostly water accounts for 80% of the grape. In addition, we have the skins, pulp and seeds. The skins make up roughly 10% of the grape and are nearly as important to winemakers and consumers as the juice. For it is the skins which not only give each wine its color from pale straw to gold for white wines to red and purple for red wines, but also the favor and aromatic compounds found in wine. Incidentally, the finest Vitis vinifera grapes tend 
to be smaller in size than freshly enjoyed table grapes; and the advantage for quality wines, the smaller the grape is at maturity the greater the ratio between skin and juice. Small grapes such as Cabernet Sauvignon can yield more complex, more flavorful wines than would be possible with grapes that are large, very juicy. Tannins, the compound responsible for the astringent, bitter character of wine comes from the skins.
Finally, wine grapes are comprised of seed which represent about 5% of the grape. Most wine grapes contain 2 to 4 seeds per berry, if the set has been good. The seeds are often very high in tannins and astringent oils. For this reason, great care is taken not to “crush” the seeds during the winemaking. White wine juice is removed as quickly as possible from the seeds to avoid undue bitterness.
Of the four primary tastes – sweet, sour, bitter, and salty – the first three are basic to wine. Sweetness is a measure of fruit maturity or fruitiness and that derives from the degree of ripeness achieved by the grape in the form of its sugar content. Sour is a flavor that relates directly to the fruit acids in grapes, flat tasting wines are low in acidity. Bitterness comes from the skins of the grapes and sometimes the seeds. Thus, the basic flavors of all wines are there in varying degrees in the grapes.
The grape vine is a remarkable plant. One question frequently asked me is how long does a grape vine live? Well, the Sonoma Valley has some vineyards with vines around 100 years-old, still producing grapes. On average, vintners consider 40 to 50 years as the time to replace grape vines, because around then their productivity has considerably diminished. We don’t automatically replace them on a set schedule. For some wines, old vines provide rich flavored grapes of concentrated character because the amount of grapes per vine is small. Whether to replant or not is another one of the annual decisions vintners must deal with as we work the soil, tend the vineyards and become involved with the vine’s annual cycle.
A presto!
