Ripeness in viticulture
Encyclopedia
The term ripeness in viticulture can mean different things to different viticulturists and winemaker
s. At its broadest definition it refers to the completion of the ripening
process of wine grapes on the vine which signals the beginning of harvest
. But what exactly constitute ripeness will vary depending on what style of wine
is being produced (sparkling, still, 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 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 (as well as the potential alcohol level) 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
.
s. 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, replace 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 build up, the concentration of acids decrease 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 build up of potassium
, triggers a rise in the pH level of the grape juice.
In addition to the change in sugar, acids and pH levels 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 begin 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.
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 in 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 intoxication. 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 reduces 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 aging 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 decrease 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 often been under ripe and produce table wines that many wine drinkers would consider unpalatable yet the balance of sugars and acids is well suited for sparkling wine production
.
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 can not 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 a 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.
is a good indicator of the amount of sugars in the wine. Rather than measure the actual "weight" of the must, the density
or specific gravity
of the juice is measured in relation to the specific gravity of distilled water
. Viticulturists and winemakers can use a refractometer
which uses a refractive index
to indirectly measure the must weight from the juice of a single grape or they can use a hydrometer
in the winery with the juice from several dozen or hundreds of grape berries. Different countries around the world use various scales to measure the must weight of grape juice. In the United States and parts of Australia it is measured in degrees brix
(symbol °Bx); in Germany (wine) it is degrees Oechsle (°Oe); in France and most of Europe the Baumé scale
is used and in Austria the Klosterneuburger Mostwaage (°KMW) scale is used.
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. 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.
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.
The major acids in wine
are tartaric and malic acid with citric and succinic acid
s playing a small role. The titratable acidity or "TA" (also referred to as "total acidity") is the measure of the tartaric acid in the grapes. This is the most abundant acid and also the one acid that has the most pronounced and long lasting impact on the taste of the wine. The TA is often measured by neutralizing some grape juice with a standard 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.
ions. It is related to the titratable acidity level of a wine but differs in significant ways. Low pH numbers indicate a high concentration of acids in a solution. While pure water is neutral with a pH of 7, wine tends to be more acidic with a pH between 3 and 4. As the acid levels in ripening grapes fall, the concentration of acids are lessening which means the pH level is rising. Yeasts, bacteria, phenolic compounds such as anthocyanins which influence color all have varying tolerance to wines with high pH levels. In general, wines with high pH value tend to have duller colors and less developed flavors and be more prone to wine faults caused by spoilage organisms which makes monitoring the pH levels of grapes during ripening a priority for viticulturists and winemakers.
While the rudimentary method of testing pH is to expose the grape juice to a pH indicator
such as the strips used for a standard 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.
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.
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) 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.
to determine the concentration of color producing anthocyanins in the skins of grapes. A sizable amount of research as 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, has 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.
Scientist 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.
Winemaker
A winemaker or vintner is a person engaged in winemaking. They are generally employed by wineries or wine companies, where their work includes:*Cooperating with viticulturists...
s. At its broadest definition it refers to the completion of the ripening
Ripening
Ripening is a process in fruits that causes them to become more palatable. In general, a fruit becomes sweeter, less green, and softer as it ripens. Even though the acidity of fruit increases as it ripens, the higher acidity level does not make the fruit seem tarter, which can lead to the...
process of wine grapes on the vine which signals the beginning of harvest
Harvest (wine)
The harvesting of wine grapes is one of the most crucial steps in the process of winemaking. The time of harvest is determined primarily by the ripeness of the grape as measured by sugar, acid and tannin levels with winemakers basing their decision to pick based on the style of wine they wish to...
. But what exactly constitute ripeness will vary depending on what style of wine
Wine
Wine is an alcoholic beverage, made of fermented fruit juice, usually from grapes. The natural chemical balance of grapes lets them ferment without the addition of sugars, acids, enzymes, or other nutrients. Grape wine is produced by fermenting crushed grapes using various types of yeast. Yeast...
is being produced (sparkling, still, fortified, rosé
Rosé
A rosé is a type of wine that has some of the color typical of a red wine, but only enough to turn it pink. The pink color can range from a pale orange to a vivid near-purple, depending on the grapes and wine making techniques.- Production techniques :There are three major ways to produce rosé...
, dessert wine
Dessert wine
Dessert wines are sweet wines typically served with dessert.There is no simple definition of a dessert wine. In the UK, a dessert wine is considered to be any sweet wine drunk with a meal, as opposed to the white fortified wines drunk before the meal, and the red fortified wines drunk after it...
, 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 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
Veraison
Véraison is a viticulture term meaning "the onset of ripening". It is originally French, but has been adopted into English use...
, sugars in the grapes will continue to rise as acid levels fall. The balance between sugar (as well as the potential alcohol level) and acids is considered one of the most critical aspects of producing quality wine so both the must weight
Must weight
Must weight is a measure of the amount of sugar in grape juice , and hence indicates the amount of alcohol that could be produced if it is all fermented to alcohol, rather than left as residual sugar...
and "total acidity", as well as the pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
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
Aroma of wine
It is through the aromas of wine that wine is actually tasted. The human tongue is limited to the primary tastes perceived by taste receptors on the tongue-acidity, bitterness, saltiness, sweetness and umami. The wide array of fruit, earthy, floral, herbal, mineral and woodsy flavor perceived in...
.
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 (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 acidMalic acid
Malic acid is an organic compound with the formula HO2CCH2CHOHCO2H. It is a dicarboxylic acid which is made by all living organisms, contributes to the pleasantly sour taste of fruits, and is used as a food additive. Malic acid has two stereoisomeric forms , though only the L-isomer exists...
s. 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, replace the green color of chlorophyll
Chlorophyll
Chlorophyll is a green pigment found in almost all plants, algae, and cyanobacteria. Its name is derived from the Greek words χλωρος, chloros and φύλλον, phyllon . Chlorophyll is an extremely important biomolecule, critical in photosynthesis, which allows plants to obtain energy from light...
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
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
. Sucrose
Sucrose
Sucrose is the organic compound commonly known as table sugar and sometimes called saccharose. A white, odorless, crystalline powder with a sweet taste, it is best known for its role in human nutrition. The molecule is a disaccharide composed of glucose and fructose with the molecular formula...
produced by photosynthesis is transferred from the leaves to the berries as it is broken down into glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
and fructose
Fructose
Fructose, or fruit sugar, is a simple monosaccharide found in many plants. It is one of the three dietary monosaccharides, along with glucose and galactose, that are absorbed directly into the bloodstream during digestion. Fructose was discovered by French chemist Augustin-Pierre Dubrunfaut in 1847...
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
Yield (wine)
In viticulture, the yield is a measure of the amount of grapes or wine that is produced per unit surface of vineyard, and is therefore a type of crop yield...
size of grape clusters and young vine shoot tips which compete for the resources of the mother grapevine. As the concentration of sugars build up, the concentration of acids decrease due, in part, to simple dilution
Dilution
Dilution may refer to:* Reducing the concentration of a chemical* Serial dilution, a common way of going about this reduction of concentration* Homeopathic dilution* Dilution , an equation to calculate the rate a gas dilutes...
but also to the consumption of acids in the process of plant respiration. The decrease in free acids, as well as the build up of potassium
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
, triggers a rise in the pH level of the grape juice.
In addition to the change in sugar, acids and pH levels other components of the grapes are building up during the ripening process. The mineral components of potassium, calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
, magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
and sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
increase in concentration as they are disseminated among the skin of the grapes and its fleshy pulp. The color of the grape berries begin to change due to the building up of phenolic compounds such as anthocyanin in the skins. Flavonoids and volatile compounds
Volatility (chemistry)
In chemistry and physics, volatility is the tendency of a substance to vaporize. Volatility is directly related to a substance's vapor pressure. At a given temperature, a substance with higher vapor pressure vaporizes more readily than a substance with a lower vapor pressure.The term is primarily...
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 under ripe 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 ValleyLoire Valley (wine)
The Loire Valley wine region includes the French wine regions situated along the Loire River from the Muscadet region near the city of Nantes on the Atlantic coast to the region of Sancerre and Pouilly-Fumé just southeast of the city of Orléans in north central France. In between are the regions of...
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
Cabernet Sauvignon
Cabernet Sauvignon is one of the world's most widely recognized red wine grape varieties. It is grown in nearly every major wine producing country among a diverse spectrum of climates from Canada's Okanagan Valley to Lebanon's Beqaa Valley...
taking much longer to ripen compared to early ripening varieties such as Chardonnay
Chardonnay
Chardonnay is a green-skinned grape variety used to make white wine. It is originated from the Burgundy wine region of eastern France but is now grown wherever wine is produced, from England to New Zealand...
and Pinot noir
Pinot Noir
Pinot noir is a black wine grape variety of the species Vitis vinifera. The name may also refer to wines created predominantly from Pinot noir grapes...
.
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
Yeast
Yeasts are eukaryotic micro-organisms classified in the kingdom Fungi, with 1,500 species currently described estimated to be only 1% of all fungal species. Most reproduce asexually by mitosis, and many do so by an asymmetric division process called budding...
into alcohol by the process of fermentation
Fermentation (wine)
The process of fermentation in wine turns grape juice into an alcoholic beverage. During fermentation, yeast interact with sugars in the juice to create ethanol, commonly known as ethyl alcohol, and carbon dioxide...
. 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
Alcohol by volume
Alcohol by volume is a standard measure of how much alcohol is contained in an alcoholic beverage .The ABV standard is used worldwide....
(ABV) and cease fermentation before all the sugar is converted in 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
Ethanol
Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...
) in the wine contributes much more than just intoxication. It has an immense impact of the weight and mouthfeel
Mouthfeel
Mouthfeel is a product's physical and chemical interaction in the mouth, an aspect of food rheology. It is a concept used in many areas related to the testing and evaluating of foodstuffs, such as wine-tasting and rheology. It is evaluated from initial perception on the palate, to first bite,...
of the wine as well as the balance of sweetness, tannins and acids. In wine tasting
Wine tasting
Wine tasting is the sensory examination and evaluation of wine. While the practice of wine tasting is as ancient as its production, a more formalized methodology has slowly become established from the 14th century onwards...
, the anaesthetic qualities of ethanol reduces the sensitivity of the palate
Palate
The palate is the roof of the mouth in humans and other mammals. It separates the oral cavity from the nasal cavity. A similar structure is found in crocodilians, but, in most other tetrapods, the oral and nasal cavities are not truly separate. The palate is divided into two parts, the anterior...
to the harsh effects of acids and tannins, making the wine seem softer. It also plays a role during the aging of wine
Aging of wine
The aging of wine, and its ability to potentially improve wine quality, distinguishes wine from most other consumable goods. While wine is perishable and capable of deteriorating, complex chemical reactions involving a wine's sugars, acids and phenolic compounds can alter the aroma, color,...
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 decrease 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
Vintage
Vintage, in wine-making, is the process of picking grapes and creating the finished product . A vintage wine is one made from grapes that were all, or primarily, grown and harvested in a single specified year. In certain wines, it can denote quality, as in Port wine, where Port houses make and...
. With their high acidity and low sugar levels, these grapes would often been under ripe and produce table wines that many wine drinkers would consider unpalatable yet the balance of sugars and acids is well suited for sparkling wine production
Sparkling wine production
There are four main methods of sparkling wine production. The first is simple injection of carbon dioxide , the process used in soft drinks, but this produces big bubbles that dissipate quickly in the glass. The second is the Metodo Italiano – Charmat process, in which the wine undergoes a...
.
Factors influencing when ripeness occurs
One of the primary factors influencing the ripening process of grapevine is the climate and weather. SunlightSunlight
Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.When the direct solar radiation is not blocked...
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
Growing season
In botany, horticulture, and agriculture the growing season is the period of each year when native plants and ornamental plants grow; and when crops can be grown....
, 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
Tartaric acid
Tartaric acid is a white crystalline diprotic organic acid. It occurs naturally in many plants, particularly grapes, bananas, and tamarinds; is commonly combined with baking soda to function as a leavening agent in recipes, and is one of the main acids found in wine. It is added to other foods to...
. 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 can not be overstated, but it is not the only factor. Vineyard management such as pruning
Pruning
Pruning is a horticultural practice involving the selective removal of parts of a plant, such as branches, buds, or roots. Reasons to prune plants include deadwood removal, shaping , improving or maintaining health, reducing risk from falling branches, preparing nursery specimens for...
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
Powdery mildew
Powdery mildew is a fungal disease that affects a wide range of plants. Powdery mildew diseases are caused by many different species of fungi in the order Erysiphales. It is one of the easier diseases to spot, as its symptoms are quite distinctive. Infected plants display white powdery spots on the...
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 a 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
Millerandage
Millerandage or shot berries is a French term referring to an viticultural problem in which grape bunches contain berries of greatly different size and, most important, different levels of maturity. Its most common cause is too cold or otherwise bad weather during the flowering stage of the vines....
, 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
Boron
Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...
, an attack of various grapevine ailments such as the grapevine fanleaf virus
Grapevine fanleaf virus
Grapevine fanleaf virus is a plant pathogenic virus of the family Secoviridae.-External links:**...
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 wine with the purpose of harvesting at point when each number reaches its most ideal range for the type of wine being produced. In recent years, viticulturists and winemakers have shifted away from focusing purely on those numbers towards considering other factors including the ripeness of tannins, the development of flavor precursors and the potential for glycosides to development. A combination of these factors apart from sugar, acid and pH are considered "physiological" ripeness of the grape.Must weight
Since more than 90% of all the dissolved solids in grape juice are sugars, measuring the must weightMust weight
Must weight is a measure of the amount of sugar in grape juice , and hence indicates the amount of alcohol that could be produced if it is all fermented to alcohol, rather than left as residual sugar...
is a good indicator of the amount of sugars in the wine. Rather than measure the actual "weight" of the must, the density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
or specific gravity
Specific gravity
Specific gravity is the ratio of the density of a substance to the density of a reference substance. Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. The reference substance is nearly always water for...
of the juice is measured in relation to the specific gravity of distilled water
Distilled water
Distilled water is water that has many of its impurities removed through distillation. Distillation involves boiling the water and then condensing the steam into a clean container.-History:...
. Viticulturists and winemakers can use a refractometer
Refractometer
A refractometer is a laboratory or field device for the measurement of an index of refraction . The index of refraction is calculated from Snell's law and can be calculated from the composition of the material using the Gladstone-Dale relation....
which uses a refractive index
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
to indirectly measure the must weight from the juice of a single grape or they can use a hydrometer
Hydrometer
A hydrometer is an instrument used to measure the specific gravity of liquids; that is, the ratio of the density of the liquid to the density of water....
in the winery with the juice from several dozen or hundreds of grape berries. Different countries around the world use various scales to measure the must weight of grape juice. In the United States and parts of Australia it is measured in degrees brix
Brix
Degrees Brix is the sugar content of an aqueous solution.One degree Brix is 1 gram of sucrose in 100 grams of solution and represents the strength of the solution as percentage by weight . If the solution contains dissolved solids other than pure sucrose, then the °Bx is only approximate the...
(symbol °Bx); in Germany (wine) it is degrees Oechsle (°Oe); in France and most of Europe the Baumé scale
Baumé scale
The Baumé scale is a pair of hydrometer scales developed by French pharmacist Antoine Baumé in 1768 to measure density of various liquids. The unit of the Baumé scale has been notated variously as degrees Baumé, B°, Bé° and simply Baumé . One scale measures the density of liquids heavier than water...
is used and in Austria the Klosterneuburger Mostwaage (°KMW) scale is used.
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. 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.
Acid level
Chaptalization
Chaptalization is the process of adding sugar to unfermented grape must in order to increase the alcohol content after fermentation. The technique is named after its developer, the French chemist Jean-Antoine-Claude Chaptal...
). 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.
The major acids in wine
Acids in wine
The acids in wine are an important component in both winemaking and the finished product of wine. They are present in both grapes and wine, having direct influences on the color, balance and taste of the wine as well as the growth and vitality of yeasts during fermentation and protecting the wine...
are tartaric and malic acid with citric and succinic acid
Succinic acid
Succinic acid is a dicarboxylic acid. Succinate plays a biochemical role in the citric acid cycle. The name derives from Latin succinum, meaning amber, from which the acid may be obtained....
s playing a small role. The titratable acidity or "TA" (also referred to as "total acidity") is the measure of the tartaric acid in the grapes. This is the most abundant acid and also the one acid that has the most pronounced and long lasting impact on the taste of the wine. The TA is often measured by neutralizing some grape juice with a standard alkaline solution (such as sodium hydroxide) and then using an indicator (such as phenolphthalein
Phenolphthalein
Phenolphthalein is a chemical compound with the formula C20H14O4 and is often written as "HIn" or "phph" in shorthand notation. Often used in titrations, it turns colorless in acidic solutions and pink in basic solutions...
) 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.
pH level
The pH level of a wine is the measurement of the amount of free (H+) hydrogenHydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
ions. It is related to the titratable acidity level of a wine but differs in significant ways. Low pH numbers indicate a high concentration of acids in a solution. While pure water is neutral with a pH of 7, wine tends to be more acidic with a pH between 3 and 4. As the acid levels in ripening grapes fall, the concentration of acids are lessening which means the pH level is rising. Yeasts, bacteria, phenolic compounds such as anthocyanins which influence color all have varying tolerance to wines with high pH levels. In general, wines with high pH value tend to have duller colors and less developed flavors and be more prone to wine faults caused by spoilage organisms which makes monitoring the pH levels of grapes during ripening a priority for viticulturists and winemakers.
While the rudimentary method of testing pH is to expose the grape juice to a pH indicator
PH indicator
A pH indicator is a halochromic chemical compound that is added in small amounts to a solution so that the pH of the solution can be determined visually. Hence a pH indicator is a chemical detector for hydronium ions or hydrogen ions in the Arrhenius model. Normally, the indicator causes the...
such as the strips used for a standard litmus test
Litmus test (chemistry)
Litmus is a water-soluble mixture of different dyes extracted from lichens, especially Roccella tinctoria. It is often absorbed onto filter paper to produce one of the oldest forms of pH indicator, used to test materials for acidity. Blue litmus paper turns red under acidic conditions and red...
, the results are usually not as detailed and accurate as what is needed to evaluate ripeness. Therefore, most wineries will us a pH meter
PH meter
A pH meter is an electronic instrument used for measuring the pH of a liquid...
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.
Balancing sugar, acidity and pH
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 "royal flush" poker hand of winemaking that is rarely dealt to winemakers. With all the variables of climate, 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.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
Ratio
In mathematics, a ratio is a relationship between two numbers of the same kind , usually expressed as "a to b" or a:b, sometimes expressed arithmetically as a dimensionless quotient of the two which explicitly indicates how many times the first number contains the second In mathematics, a ratio is...
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.
Physiological ripeness
The term physiological ripeness (or physiological maturity) of grapes is a relatively recent addition to the discussion of ripeness in viticulture and winemaking. The phrase has become a "catch all" term used to describe other 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 includes 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 term Engustment (from the Latin root gustis or taste) which denotes 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 build up 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 characteristic a complex or quality wine.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) 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) spectroscopySpectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...
to determine the concentration of color producing anthocyanins in the skins of grapes. A sizable amount of research as 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, has 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
Riesling
Riesling is a white grape variety which originated in the Rhine region of Germany. Riesling is an aromatic grape variety displaying flowery, almost perfumed, aromas as well as high acidity. It is used to make dry, semi-sweet, sweet and sparkling white wines. Riesling wines are usually varietally...
and Muscat
Muscat (grape and wine)
The Muscat variety of grapes of the species Vitis vinifera is widely grown for wine, raisins and table grapes. Their color ranges from white to near black. Muscat almost always has a pronounced sweet floral aroma. Muscat grapes are grown around the world...
, and methoxypyrazine, which contributes to the "green-bell pepper" aroma associated with Cabernet Sauvignon
Cabernet Sauvignon
Cabernet Sauvignon is one of the world's most widely recognized red wine grape varieties. It is grown in nearly every major wine producing country among a diverse spectrum of climates from Canada's Okanagan Valley to Lebanon's Beqaa Valley...
and Sauvignon blanc
Sauvignon blanc
Sauvignon Blanc is a green-skinned grape variety which originates from the Bordeaux region of France. The grape most likely gets its name from the French word sauvage and blanc due to its early origins as an indigenous grape in South West France., a possible descendant of savagnin...
. 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
Hydrolysis
Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
, 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.
Scientist 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.
External links
- M. Greenspan "Assessing Ripeness Through Sensory Evaluation" Wine Business Monthly, November 15, 2006
- Fruit Maturity Evaluation of Wine Grapes for Harvest Planning, information from Cooperative Extension