How the term "ripe" is used in viticulture and winemaking
Grapes ripening on the vine.
In viticulture, ripeness is the completion of the
fortified, rosé, dessert wine, etc.) 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.[1]
There are several factors that contribute to the ripeness of the grape. As the grapes go through
Pinot noir grapes in the early stages of veraison. As the grapes ripen, the concentration of phenolic compounds like anthocyanins replaces the green color of chlorophyll in the grape berries which makes them black instead.
If ripening is broadly defined as the development of wine grapes, then it could be said that ripening is happening throughout the continuous
anthocyanins for red wine grapes, replaces the green color of chlorophyll as the grape berries themselves change color.[2][3]
The increase of sugars in the grapes comes from the storage 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.[2]
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,
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.[2] 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.[4]
Varying ripeness levels for different wines
Pinot noir grapes that are destined for sparkling wine will be considered ripe much earlier than Pinot noir destined for still red wine.
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
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.[2]
Since over the course of ripening sugars in the grapes increase, the
late harvest wines because they are harvested at extreme points of ripeness much later than when regular table wine grapes have been harvested.[1]
The presence of alcohol (particularly
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.[4]
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.[2]
Factors influencing when ripeness occurs
Vineyard management techniques such as canopy management can influence the ripening process of grapes by balancing the amount of foliage needed for photosynthesis versus excessive foliage that shades the grapes and competes for the grapevine's resources.
One of the primary factors influencing the ripening process of grapevine is the climate and weather.
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.[1]
The role that climate plays in influencing the ripening process cannot be overstated, but it is not the only factor. Vineyard management 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.[2]
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
Grapes that have been left on the vine too long may become over ripe and dehydrated.
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.
glycosides to development. A combination of these factors apart from sugar, acid and pH are considered "physiological" ripeness of the grape.[2]
Must weight
Since more than 90% of all the dissolved solids in grape juice are sugars, measuring the
Klosterneuburger Mostwaage (°KMW) scale is used.[2]
After veraison has begun, viticulturists will test several hundred individual berries picked from clusters throughout the vineyard in increasing intervals as the harvest draws closers. The berries will usually be taken from the middle of the cluster bunch, avoiding vines on the end of rows that tend to be exposed to the most unusual elements. The must weight is then plotted on a chart to see the increasing ripeness and sugar levels of the grape.[1] What must weight reading is most desirable will depend on the winemaker's personal goal for ripeness. A wine with the intended potential alcohol level of 12% will need to be harvested at around 21.7°Bx/12 degree Baumé/93°Oe. A wine with the intended potential alcohol level of 15% will need to be harvested at around 27.1°Bx/15 degree Baumé/119°Oe. The desired ripeness for most table wines tend to fall somewhere between those two must weight measurements.[2]
Acid level
The principle acids found in wine grapes are tartaric and malic acids.
As sugar levels in the grape rise, acid levels fall. All wines need some degree of acidity in order to be balanced and avoid tasting flabby or dull. Acidity is also a key component in
food and wine pairing so its presence in wine is important with winemakers trying to harvest grapes before acid levels fall too low. The stress to maintain acid levels is not as bearing due to the fact that winemakers can rectify the situation somewhat by later adding acids during the winemaking process (winemakers can also rectify deficiencies in sugar levels by chaptalization). However, natural acids in the grape play other roles in the development of flavor and aroma compounds as well as fighting against the effects of spoilage organisms so the most ideal situation for winemakers is to try and harvest while acid levels are acceptable.[2]
The major
alkaline solution (such as sodium hydroxide) and then using an indicator (such as phenolphthalein) which changes color depending on the acid levels of the solution. The indicator is added to the grape juice followed by incremental amounts of the alkaline solution as the wine changes color until adding more of the solution ceases to promote a color change. At this point the wine has been neutralized with the amount of the alkaline solution needed to neutralize calculated in a formula to give an indication of how much tartaric acid was in the wine. The TA level is then expressed in a percentage of grams per 100 milliliter. As with must weight, the ideal levels for ripeness will vary according to wine style and winemaking preference. For still table wines, TA levels often fall between 0.60-0.80% for red wine grapes and 0.65-0.85 for whites.[1]
pH level
The pH levels for most wine fall between 3 and 4 on the pH scale.
The pH level of a wine is the measurement of the amount of free (H+)
wine faults caused by spoilage organisms which makes monitoring the pH levels of grapes during ripening a priority for viticulturists and winemakers.[2]
While the rudimentary method of testing pH is to expose the grape juice to a
litmus test, the results are usually not as detailed and accurate as what is needed to evaluate ripeness. Therefore, most wineries will us a pH meter that can give readings to an accuracy of plus or minus 0.1. As with sugars and acids, the ideal pH levels to determine ripeness will vary. For white wines, winemakers often look for pH readings between 3.1 and 3.2, while would be a maximum of 3.4. If the pH is too high, it may be a sign that the grapes are overripe (or that the soil has too much potassium which will also influence pH readings). While there are risks to letting the pH go too high, winemakers can counter high pH by adding more tartaric or malic acid during the winemaking. However, many viticulturists and winemakers uses pH readings as a strong boundary line for when to start the harvest.[1]
Balancing sugar, acidity and pH
Winemakers use a refractometer on samples of grapes picked in the vineyard to measure sugar levels while determining ripeness.
The most ideal situation for a viticulturist or winemaker is to have the sugar, acidity and pH levels to be perfectly balanced at the time of harvesting. One hypothetical ideal for still red table wine is to have grape measurements reading 22 Brix, 0.75 TA and 3.4 pH. As author and winemaker Jeff Cox notes, these numbers are the "
vineyard soils, grape varieties, vineyard management and the general characteristics of the vintage, winemakers learn to find a compromise between all these component readings and select the point of ripeness that is most align with their vision for the end product wine.[1]
There are several formulas that viticulturist and winemakers can use that utilize the various measurements of sugar, acid and pH level. One method developed by researchers at the
University of California-Davis is the Brix:TA ratio which uses the ratio of brix degrees to the TA measurements. For example, a wine with 22°Bx and .75 TA will have almost a 30:1 Brix:TA ratio. According to the Davis researchers, the most balanced table wines tend to have a Brix to TA ratio between 30:1 - 35:1. Another method is to multiply the pH reading by itself and then multiply that number by the Brix reading. Using this method, when white wine grapes gets close to 200 and red wine grapes close to 260, it can be a good rule of thumb of when to harvest. For example, white wine grapes have a pH of 3.3 and Brix of 20, after going through that formula they will have a finally number of 217.80 which is well within an acceptable harvest range for some winemakers.[1]
Physiological ripeness
In determining physiological ripeness, winemakers will observe the lignification of the grape stems as they turn from being flexible and green to hard, woody and brown.
The idea of physiological ripeness (or physiological maturity) of grapes is a relatively recent addition to the discussion of ripeness in viticulture and winemaking. It is a broad category of factors in the development of ripening grapes that affect a wine's quality beyond the standard measurements of sugars, acids, and pH. These factors generally include evaluating the ripeness of tannins as well as the development of other phenolic compounds that contribute to the color, flavor, and aroma of wine. In many ways, the concept of physiological ripeness is similar to the
secondary metabolites which occur late in ripening as the buildup of sugars have leveled. This stage is distinct from the sugar/acid interactions of ripening because it is possible for a grape to be "ripe" in the context of sugar and acid levels but still be very immature when it comes to the development of tannins, aromas and flavor that are characteristic of a complex or quality wine.[2][4]
For the most part, many of these qualities are difficult to objectively measure so evaluation of the physiological ripeness of grapes is centered around observing and physically sampling the grapes. With experience winemakers and viticulturists learn to associate certain taste and characteristics with different stages of development. They evaluate the skin and pulp texture of the berry as well as the color of skins, seeds and stems. If the seeds are still green, the tannins inside the grape are more likely to be harsh and bitter. As the tannins continue to develop, the seeds start darkening in color. They will observe the
lignification of the stems as they turn from being flexible and green to hard, woody and brown (for many varieties but not all[5]) 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.[2]
Flavor precursors and glycosides
Researchers in the wine industry are developing new ways to objectively measure ripeness.
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.[2]
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
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.[2]
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
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.[2]
