Ripeness in viticulture

In viticulture, ripeness is the completion of the ripening process of wine grapes on the vine which signals the beginning of harvest. What exactly constitutes ripeness will vary depending on what style of wine is being produced and what the winemaker and viticulturist personally believe constitutes ripeness. Once the grapes are harvested, the physical and chemical components of the grape which will influence a wine's quality are essentially set so determining the optimal moment of ripeness for harvest may be considered the most crucial decision in winemaking.
There are several factors that contribute to the ripeness of the grape. As the grapes go through veraison, sugars in the grapes will continue to rise as acid levels fall. The balance between sugar and acids is considered one of the most critical aspects of producing quality wine so both the must weight and "total acidity", as well as the pH of the grapes, are evaluated to determine ripeness. Towards the end of the 20th century, winemakers and viticulturists began focusing on the concept of achieving "physiological" ripeness in the grapes-described as a more complete ripeness of tannins and other phenolic compounds in the grapes that contribute to the color, flavor and aroma of wine.

What happens to a grape as it ripens

If ripening is broadly defined as the development of wine grapes, then it could be said that ripening is happening throughout the continuous annual cycle of the grapevine. More narrowly defined, ripening begins at the inception of veraison. At this point, the grapes are hard and green with low sugar levels and very high levels of mostly malic acids. During veraison, which may last from 30–70 days depending on the climate and other factors, the grapes go through several changes which impact their sugar, acid, tannin and mineral composition. The concentration of phenolic compounds in the skin, most notably anthocyanins for red wine grapes, replaces the green color of chlorophyll as the grape berries themselves change color.
The increase of sugars in the grapes comes from the storage of carbohydrates in the roots and trunk of the grapevines as well as through the process of photosynthesis. Sucrose produced by photosynthesis is transferred from the leaves to the berries as it is broken down into glucose and fructose molecules. The rate of this build up will depend on several factors including the climate as well as the potential yield size of grape clusters and young vine shoot tips which compete for the resources of the mother grapevine. As the concentration of sugars builds up, the concentration of acids decreases due, in part, to simple dilution but also to the consumption of acids in the process of plant respiration. The decrease in free acids, as well as the buildup of potassium, triggers a rise in the pH level of the grape juice.
In addition to the change in sugar, acids and pH levels of other components of the grapes are building up during the ripening process. The mineral components of potassium, calcium, magnesium and sodium increase in concentration as they are disseminated among the skin of the grapes and its fleshy pulp. The color of the grape berries begins to change due to the building up of phenolic compounds such as anthocyanin in the skins. Flavonoids and volatile compounds known as "flavor precursors" which contribute to the eventual flavor and aroma of the wine also begin to build up in the skins and pulp. Additionally the concentration of tannins in the grape increases in several areas of the grape including the skin, seeds and stem. Early in the ripening process these tannins are very bitter and "green". Exposure to the warmth and sunlight during the ripening period ushers in chemical changes to the tannins that when processed into wine makes the tannins feel softer in the mouth.

Varying ripeness levels for different wines

What constitutes "ripeness" will vary according to what style of wine is being produced as well as the particular views of winemakers and viticulturists on what optimal ripeness is. The style of wine is usually dictated by the balance between sugars and acids. What may be considered "ripe" for one winemaker could be considered underripe to another winemaker or even overripe to yet a third winemaker. Climate and the particular grape variety will also play a role in determining ripeness and date of harvest. In very hot climates, such as certain areas in California and Australia, ripeness is usually achieved around 30 days after veraison starts while in much cooler climates, like the Loire Valley and parts of Germany, this may not occur until 70 days after veraison. The ripening periods for each individual grape variety will vary with grapes such as Cabernet Sauvignon taking much longer to ripen compared to early ripening varieties such as Chardonnay and Pinot noir.
Since over the course of ripening sugars in the grapes increase, the sweetness level as well as the potential alcohol level of the wine will play a considerable role in dictating when a grape is "ripe" enough. This is because sugars 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. However, most strains of winemaking yeast have difficulties surviving in an alcohol solution above 15% alcohol by volume and cease fermentation before all the sugar is converted into alcohol. This leaves a certain amount of residual sugar which influences the sweetness level of the wine. Wines that are destined to be sweet, such as dessert wines, are often called late harvest wines because they are harvested at extreme points of ripeness much later than when regular table wine grapes have been harvested.
The presence of alcohol in the wine contributes much more than just healthful benefits in moderation and minimal consumption, prudently applied, or, negative effects in excess. It has an immense impact of the weight and mouthfeel of the wine as well as the balance of sweetness, tannins and acids. In wine tasting, the anaesthetic qualities of ethanol reduce the sensitivity of the palate to the harsh effects of acids and tannins, making the wine seem softer. It also plays a role during the ageing of wine in its complex interaction with esters and phenolic compounds that produce various aromas in wine that contribute to a wine's flavor profile. For this reason, some winemakers will value having a higher potential alcohol level and delay harvesting until the grapes have a sufficiently high concentration of sugars.
For other types of wines, such as sparkling wines like Champagne, maintaining a certain amount of acidity in the grapes is important to the winemaking process. As the concentration of acids in the grapes decreases the further along the ripening process you go, grapes destined for sparkling wines are often some of the earliest grapes to be harvested in a vintage. With their high acidity and low sugar levels, these grapes would be underripe and would produce table wines that many wine drinkers would consider unpalatable, yet the balance of sugars and acids is well suited for sparkling wine production.

Factors influencing when ripeness occurs

One of the primary factors influencing the ripening process of grapevine is the climate and weather. Sunlight and temperature warmth are vital to the physiological functions of the grapevine. An absence of either, such as long periods of extensive cloud cover, will cause many functions of the vine to slow or even completely halt as the vine enters a type of "survival mode". As the grapevine funnels more resources to preserve its own survival, less resources are directed towards the ripening and development of the grape clusters. Excessive heat can also cause a grapevine to react adversely. The occurrence of heat waves during the growing season, particularly as it nears harvest, can cause the sugars in grapes to jump as acids fall dramatically. Some winemakers may decide to harvest early in order to maintain acid levels even though other components may not be at optimal ripening. For the winemakers that decide to "wait it out", a lack of acid can be partially rectify during the winemaking process with the addition of acids such as tartaric acid. It is much more difficult to remedy the effects of extensive rains during the ripening period. Steady rains before the harvest can cause the berries to swell with water which dilutes the flavors as well as causing cracking in the skin that creates openings for spoilage causing microorganism to propagate. Because of these risks, the threat of prolong rainfall during a vintage may cause an early harvest before the grapes have fully ripened. The most favorable vintages allow a slow, steady ripening without drastic jumps in heats or the threat of excessive rain fall.
The role that climate plays in influencing the ripening process cannot be overstated, but it is not the only factor. Vineyard management such as pruning and canopy management can also play a significant role as it not only influences the physiological processes of the grapevine but also how the vine responds in sharing its limited resources of energy and nutrients. The leaves of a grapevine produce energy via the process of photosynthesis. A certain amount of foliage is needed to ensure that the grapevine can produce enough energy to support all its physiological functions, but too much leaf cover will shade the grape clusters, limiting the direct exposure of sunlight and warmth needed for some chemical components of the grapes to develop. An excessive amount of foliage and shading may also promote the development of various vine diseases and ailments such as bunch rot and powdery mildew which can hamper the ripening process. A very vigorous vine with many clusters and vine shoots will have several parties competing for the same resources, with the overall development of an individual clusters thus slowed. Through the process of canopy management, viticulturists try to balance not only the amount of clusters and vine shoots on the vine but also try to achieve an optimal balance of needed foliage for photosynthesis without excessive shading that could hamper the ripening process.
Even if climate and vineyard management has been ideal, other factors may prevent full and even ripeness. Among the clusters of a grapevine, individual berries may not all ripen at the same pace. This problem, commonly known as millerandage, could occur because of poor weather during the flowering period of the grape but can also be caused by soil deficient in various nutrients such as boron, an attack of various grapevine ailments such as the grapevine fanleaf virus or a number of other factors that may contribute to incomplete plant fertilization.