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.

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.

[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, 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.

[1] 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.

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.

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.

The most favorable vintages allow a slow, steady ripening without drastic jumps in heats or the threat of excessive rain fall.

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.

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.

[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.

Yeasts, bacteria, phenolic compounds such as anthocyanins which influence color all have varying tolerance to wines with high pH levels.

[1] 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.

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.

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 French notion of engustment (from the Latin root gustus or taste), the stage of ripening when aroma and flavor become apparent.

Research has shown that most aroma compounds develop in the berry in glycosylated form as 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.

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] 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.

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.

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.

Grapes ripening on the vine.
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.
Pinot noir grapes that are destined for sparkling wine will be considered ripe much earlier than Pinot noir destined for still red wine.
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.
Grapes that have been left on the vine too long may become over ripe and dehydrated.
The principle acids found in wine grapes are tartaric and malic acids.
The pH levels for most wine fall between 3 and 4 on the pH scale.
Winemakers use a refractometer on samples of grapes picked in the vineyard to measure sugar levels while determining 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.
Researchers in the wine industry are developing new ways to objectively measure ripeness.