Ripeness in viticulture

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 (which is normally 40–60 days after fruit set though it may be longer in cooler climates), 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 (such as a string of cloudy weather which prohibits sunlight from reaching the vine) 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. 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 (ABV) 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 (particularly ethanol) 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 (such as photosynthesis). 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 (such as tannins and phenolic compounds) 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. ==Evaluating ripeness==

As "ripeness" constitutes a variety of factors, there are many methods that viticulturist and winemakers may use in order to determine when the grapes are sufficiently ripe to harvest. The most common method of determining ripeness involves measuring the sugar, acid and pH levels of the grapes with the purpose of harvesting at point when each number reaches its most ideal range for the type of wine being produced.) indicating that vine has completed its work in developing its "offspring" grape clusters and has started to store carbohydrates and resources for its next growing season. During the ripening period winemakers and viticulturists will continually sample grapes throughout the vineyard in the weeks and days leading up to harvest. Flavor precursors and glycosides While it is difficult to objectively measure the qualities of physiological ripeness, researchers in the wine industry have been continuing pursuing methods that give some indication of the grapes development in these areas. For instance, some wineries have started using near infrared (NIR) spectroscopy to determine the concentration of color producing anthocyanins in the skins of grapes. A sizable amount of research has gone into studying methods to determine the presence of flavor precursors and glycosides in the ripening grapes. Recently, similar methods to determine chlorophyll content in leaves non-destructively have been applied to measuring anthocyanin content. There are now a couple of optical absorbance instruments available commercially which are designed to measure and compute an index value that correlates highly with the actual amount of anthocyanin content in a sample. To use with grapes, the skin is removed and placed across the sensor of the meter. Measurements take only a second or two. These Anthocyanin Content Meters use an additional Near Infra-Red (NIR) signal, which takes into account the thickness of the sample, along with the absorbance wavelength to calculate a very accurate index value which is repeatable and consistent enough for comparative testing. A new method just being explored is to dip a piece of filter paper into a solution/sample to be measured and put that across the sensor head as the test sample. There have been positive reports on the second method, but they have not been published. Flavor precursors are flavorless compounds that occur naturally in grapes as the result of normal metabolic activity of the grape vine. They are more abundant in grapes than the phenolic compounds known as flavonoids, and include compounds such monoterpenes, which contributes to the floral aroma of Riesling and Muscat, and methoxypyrazine, which contributes to the "green-bell pepper" aroma associated with Cabernet Sauvignon and Sauvignon blanc. When these components are "free" they are known as "flavor compounds" but when they combine with sugars in the grapes, they become glycosides or "flavor precursors". These compounds are found in trace amounts, and measured in parts per trillion. Through the action of acids and enzymes, glucosides derived from the sugar in the grapes go through hydrolysis, creating glycosides. These compounds are released during the late stages of winemaking and aging, when they augment or enhance flavor compounds. Theoretically, grapes with more flavor precursors have the potential to produce higher quality wine. Scientists have discovered it is possible to determine, to some extent, the presence of these compounds in the grape before harvest. One way is to measured with gas chromatograph-mass spectrometers. Another method is through analysis of the glycosyl-glucose assay. Through this method glycosides from the grape juice are isolated and then hydrolysized to yield glucose. The amount of glucose produced is then quantified and tabulated in results that are expressed as amount of glycosides in micromoles per liter or per grape berry. The relationship between the presence of glycosides in wine grapes and the potential for quality in the resulting wine is not exact science but this remains an area of continuing research and development. ==References==