The
acids in
wine
Wine is an alcoholic drink typically made from fermented grapes. Yeast consumes the sugar in the grapes and converts it to ethanol and carbon dioxide, releasing heat in the process. Different varieties of grapes and strains of yeasts are ...
are an important component in both
winemaking
Winemaking or vinification is the production of wine, starting with the selection of the fruit, its fermentation into alcohol, and the bottling of the finished liquid. The history of wine-making stretches over millennia. The science of wine and ...
and the finished product of wine. They are present in both
grape
A grape is a fruit, botanically a berry (botany), berry, of the deciduous woody vines of the flowering plant genus ''Vitis''. Grapes are a non-Climacteric (botany), climacteric type of fruit, generally occurring in clusters.
The cultivation of ...
s and wine, having direct influences on the color,
balance and taste of the wine as well as the growth and vitality of
yeast
Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. They are estimated to consti ...
during
fermentation and protecting the wine from
bacteria
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...
. The measure of the amount of acidity in wine is known as the “
titratable acidity In chemistry, titratable acid generally refers to any acid that can lose one or more protons in an acid–base reaction.
The term is used slightly differently in other fields. For example, in renal physiology, titratable acid is a term to describe ...
” or “total acidity”, which refers to the test that yields the total of all acids present, while strength of acidity is measured according to
pH, with most wines having a pH between 2.9 and 3.9. Generally, the lower the pH, the higher the acidity in the wine. There is no direct connection between total acidity and pH (it is possible to find wines with a high pH for wine and high acidity). In
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 onward. Modern, professional w ...
, the term “acidity” refers to the fresh, tart and sour attributes of the wine which are evaluated in relation to how well the acidity balances out the
sweetness and bitter components of the wine such as
tannins
Tannins (or tannoids) are a class of astringent, polyphenolic biomolecules that bind to and precipitate proteins and various other organic compounds including amino acids and alkaloids.
The term ''tannin'' (from Anglo-Norman ''tanner' ...
. Three primary acids are found in wine grapes:
tartaric,
malic, and
citric acids. During the course of winemaking and in the finished wines,
acetic
Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main component ...
,
butyric
Butyric acid (; from grc, βούτῡρον, meaning "butter"), also known under the systematic name butanoic acid, is a straight-chain alkyl carboxylic acid with the chemical formula CH3CH2CH2CO2H. It is an oily, colorless liquid with an unpl ...
,
lactic, and
succinic acids can play significant roles. Most of the acids involved with wine are
fixed acids with the notable exception of
acetic acid
Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main componen ...
, mostly found in vinegar, which is
volatile and can contribute to the
wine fault
A wine fault or defect is an unpleasant characteristic of a wine often resulting from poor winemaking practices or storage conditions, and leading to wine spoilage. Many of the compounds that cause wine faults are already naturally present in wine ...
known as
volatile acidity
A wine fault or defect is an unpleasant characteristic of a wine often resulting from poor winemaking practices or storage conditions, and leading to wine spoilage. Many of the compounds that cause wine faults are already naturally present in wine ...
. Sometimes, additional acids, such as
ascorbic,
sorbic and
sulfurous
Sulfur (or sulphur in British English) is a chemical element with the Symbol (chemistry), symbol S and atomic number 16. It is abundance of the chemical elements, abundant, Polyvalency (chemistry), multivalent and nonmetallic. Under standard c ...
acids, are used in winemaking.
[J. Robinson (ed) ''“The Oxford Companion to Wine”'' Third Edition pg 2–3 Oxford University Press 2006 ]
Tartaric
Tartaric acid
Tartaric acid is a white, crystalline organic acid that occurs naturally in many fruits, most notably in grapes, but also in bananas, tamarinds, and citrus. Its salt
Salt is a mineral composed primarily of sodium chloride (NaCl), a c ...
is, from a winemaking perspective, the most important in wine due to the prominent role it plays in maintaining the chemical
stability
Stability may refer to:
Mathematics
* Stability theory, the study of the stability of solutions to differential equations and dynamical systems
** Asymptotic stability
** Linear stability
** Lyapunov stability
** Orbital stability
** Structural st ...
of the wine and its color and finally in influencing the taste of the finished wine. In most plants, this
organic acid
An organic acid is an organic compound with acidic properties. The most common organic acids are the carboxylic acids, whose acidity is associated with their carboxyl group –COOH. Sulfonic acids, containing the group –SO2OH, are ...
is rare, but it is found in significant concentrations in
grape vines. Along with malic acid, and to a lesser extent citric acid, tartaric is one of the fixed acids found in wine grapes. The concentration varies depending on
grape variety
This list of grape varieties includes cultivated grapes, whether used for wine, or eating as a table grape, fresh or dried (raisin, currant, sultana). For a complete list of all grape species including those unimportant to agriculture, see Vit ...
and the
soil content of the vineyard. Some varieties, such as
Palomino
Palomino is a genetic color in horses, consisting of a gold coat and white mane and tail; the degree of whiteness can vary from bright white to yellow. Genetically, the palomino color is created by a single allele of a dilution gene called the ...
, are naturally disposed to having high levels of tartaric acids, while
Malbec
Malbec () is a purple grape variety used in making red wine. The grapes tend to have an inky dark color and robust tannins, and are known as one of the six grapes allowed in the blend of red Bordeaux wine. In France, plantations of Malbec are n ...
and
Pinot noir
Pinot Noir () is a red-wine grape variety of the species ''Vitis vinifera''. The name may also refer to wines created predominantly from pinot noir grapes. The name is derived from the French language, French words for ''pine'' and ''black.' ...
generally have lower levels. During
flowering, high levels of tartaric acid are concentrated in the grape flowers and then young berries. As the vine progresses through
ripening
Ripening is a process in fruits that causes them to become more palatable. In general, 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 th ...
, tartaric does not get metabolized through
respiration like malic acid, so the levels of tartaric acid in the grape vines remain relatively consistent throughout the ripening process.
[J. Robinson (ed) ''“The Oxford Companion to Wine”'' Third Edition pg 681 Oxford University Press 2006 ]
Less than half of the tartaric acid found in grapes is free standing, with the majority of the concentration present as
potassium acid salt. During fermentation, these
tartrates bind with the
lees, pulp debris and precipitated tannins and pigments. While some variance among grape varieties and wine regions exists, generally about half of the deposits are soluble in the
alcoholic
Alcoholism is, broadly, any drinking of alcohol that results in significant mental or physical health problems. Because there is disagreement on the definition of the word ''alcoholism'', it is not a recognized diagnostic entity. Predomina ...
mixture of wine. The crystallization of these tartrates can happen at unpredictable times, and in a wine bottle may appear like broken glass, though they are in fact harmless. Winemakers will often put the wine through
cold stabilization
In winemaking, clarification and stabilization are the processes by which insoluble matter suspended in the wine is removed before bottling. This matter may include dead yeast cells ( lees), bacteria, tartrates, proteins, pectins, various tannins ...
, where it is exposed to temperatures below freezing to encourage the tartrates to crystallize and precipitate out of the wine,
or
electrodialysis
Electrodialysis (ED) is used to transport salt ions from one solution through ion-exchange membranes to another solution under the influence of an applied electric potential difference. This is done in a configuration called an electrodialys ...
which removes the tartrates via a membrane process.
Malic
Malic acid, along with tartaric acid, is one of the principal organic acids found in wine grapes. It is found in nearly every
fruit
In botany, a fruit is the seed-bearing structure in flowering plants that is formed from the ovary after flowering.
Fruits are the means by which flowering plants (also known as angiosperms) disseminate their seeds. Edible fruits in partic ...
and
berry
A berry is a small, pulpy, and often edible fruit. Typically, berries are juicy, rounded, brightly colored, sweet, sour or tart, and do not have a stone or pit, although many pips or seeds may be present. Common examples are strawberries, rasp ...
plant, but is most often associated with green (unripe)
apple
An apple is an edible fruit produced by an apple tree (''Malus domestica''). Apple trees are cultivated worldwide and are the most widely grown species in the genus '' Malus''. The tree originated in Central Asia, where its wild ances ...
s, the flavor it most readily projects in wine. Its name comes from the
Latin
Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power ...
''malum'' meaning “apple”. In the grape vine, malic acid is involved in several processes which are essential for the health and sustainability of the vine. Its chemical structure allows it to participate in
enzymatic reactions that transport energy throughout the vine. Its concentration varies depending on the grape variety, with some varieties, such as
Barbera
Barbera is a red Italian wine grape variety that, as of 2000, was the third most-planted red grape variety in Italy (after Sangiovese and Montepulciano). It produces good yields and is known for deep color, full body, low tannins and high le ...
,
Carignan
Carignan (also known as Mazuelo, Bovale Grande, Cariñena, Carinyena, Samsó, Carignane, and Carignano) is a red grape variety of Spanish origin that is more commonly found in French wine but is widely planted throughout the western Medite ...
and
Sylvaner, being naturally disposed to high levels. The levels of malic acid in grape berries are at their peak just before
veraison, when they can be found in concentrations as high as 20
g/
L. As the vine progresses through the ripening stage, malic acid is metabolized in the process of respiration, and by
harvest
Harvesting is the process of gathering a ripe crop from the fields. Reaping is the cutting of grain or pulse for harvest, typically using a scythe, sickle, or reaper. On smaller farms with minimal mechanization, harvesting is the most lab ...
, its concentration could be as low as 1 to 9 g/L. The respiratory loss of malic acid is more pronounced in warmer climates. When all the malic acid is used up in the grape, it is considered “over-ripe” or
senescent. Winemakers must compensate for this loss by adding extraneous acid at the winery in a process known as acidification.
[J. Robinson (ed) ''“The Oxford Companion to Wine”'' Third Edition pg 421–422 Oxford University Press 2006 ]
Malic acid can be further reduced during the winemaking process through
malolactic fermentation or MLF. In this process, bacteria convert the stronger malic acid into the softer lactic acid; formally, malic acid is
polyprotic (contributes multiple protons, here two), while lactic acid is
monoprotic
In computer science, ACID (Atomicity (database systems), atomicity, Consistency (database systems), consistency, Isolation (database systems), isolation, Durability (database systems), durability) is a set of properties of database transactions i ...
(contributes one proton), and thus has only half the effect on acidity (pH); also, the first
acidity constant (pK
a) of malic acid (3.4 at room temperature) is lower than the (single) acidity constant of lactic acid (3.86 at room temperature), indicating stronger acidity. Thus after MLF, wine has a higher pH (less acidic), and a different mouthfeel.
The bacteria behind this process can be found naturally in the winery, in
cooperage
A cooper is a person trained to make wooden casks, barrels, vats, buckets, tubs, troughs and other similar containers from timber staves that were usually heated or steamed to make them pliable.
Journeymen coopers also traditionally made ...
s, which make
oak wine barrels that will house a population of the bacteria or they can be introduced by the winemaker with a cultured specimen. For some wines, the conversion of malic into lactic acid can be beneficial, especially if the wine has excessive levels of malic acid. For other wines, such as
Chenin blanc and
Riesling
Riesling (, ; ) is a white grape variety that originated in the Rhine region. 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 ...
, it produces off flavors in the wine (such as the
butter
Butter is a dairy product made from the fat and protein components of churned cream. It is a semi-solid emulsion at room temperature, consisting of approximately 80% butterfat. It is used at room temperature as a spread (food), spread, melted a ...
y smell of
diacetyl) that would not be appealing for that variety. In general, red wines are more often put through MLF than whites, which means a higher likelihood of finding malic acid in white wines (though notable exceptions, such as oaked
Chardonnay
Chardonnay (, , ) is a green-skinned grape variety used in the production of white wine. The variety originated in the Burgundy wine region of eastern France, but is now grown wherever wine is produced, from England to New Zealand. For new ...
, are often put through MLF).
Lactic
A much milder acid than tartaric and malic,
lactic acid
Lactic acid is an organic acid. It has a molecular formula . It is white in the solid state and it is miscible with water. When in the dissolved state, it forms a colorless solution. Production includes both artificial synthesis as well as natur ...
is often associated with “
milk
Milk is a white liquid food produced by the mammary glands of mammals. It is the primary source of nutrition for young mammals (including breastfed human infants) before they are able to digest solid food. Immune factors and immune-modulati ...
y” flavors in wine and is the primary acid of
yogurt
Yogurt (; , from tr, yoğurt, also spelled yoghurt, yogourt or yoghourt) is a food produced by bacterial fermentation of milk. The bacteria used to make yogurt are known as ''yogurt cultures''. Fermentation of sugars in the milk by these bact ...
and
sauerkraut
Sauerkraut (; , "sour cabbage") is finely cut raw cabbage that has been fermented by various lactic acid bacteria. It has a long shelf life and a distinctive sour flavor, both of which result from the lactic acid formed when the bacteria fe ...
. It is produced during winemaking by lactic acid bacteria (LAB), which includes three
genera
Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial ...
: ''
Oenococcus,
Pediococcus'' and ''
Lactobacillus''. These bacteria convert both sugar and malic acid into lactic acid, the latter through MLF. This process can be beneficial for some wines, adding complexity and softening the harshness of malic acidity, but it can generate off flavors and
turbidity
Turbidity is the cloudiness or haziness of a fluid caused by large numbers of individual particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of water quality.
Fluids ...
in others. Some strains of LAB can produce
biogenic amines
In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen atoms have been replaced by a substituent such ...
, such as
histamine
Histamine is an organic nitrogenous compound involved in local immune responses, as well as regulating physiological functions in the gut and acting as a neurotransmitter for the brain, spinal cord, and uterus. Since histamine was discovered in ...
,
tyramine
Tyramine ( ) (also spelled tyramin), also known under several other names, is a naturally occurring trace amine derived from the amino acid tyrosine. Tyramine acts as a catecholamine releasing agent. Notably, it is unable to cross the blood ...
and
putrescine, which may be a cause of
red wine headache
Red wine headache ("RWH") is a headache often accompanied by nausea and flushing that occurs after consuming red wine in susceptible individuals. White wine headaches have been less commonly reported.
Sulfites
Many wines contain a warning ...
s in some wine drinkers. Winemakers wishing to control or prevent MLF can use
sulfur dioxide
Sulfur dioxide ( IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a toxic gas responsible for the odor of burnt matches. It is released naturally by volcanic ...
to stun the bacteria. Racking the wine quickly off its
lees will also help control the bacteria, since lees are a vital food source for them. The winemakers must also be very careful of what wine barrels and winemaking equipment to which the wine is exposed, because of the bacteria's ability to deeply embed themselves within wood fibers. A wine barrel that has completed one successful malolactic fermentation will almost always induce MLF in every wine stored in it from then on.
[J. Robinson (ed) '' The Oxford Companion to Wine'' Third Edition pg 387 ]Oxford University Press
Oxford University Press (OUP) is the university press of the University of Oxford. It is the largest university press in the world, and its printing history dates back to the 1480s. Having been officially granted the legal right to print book ...
2006
Citric
While very common in
citrus fruits, such as
limes
Limes may refer to:
* the plural form of lime (disambiguation)
* the Latin word for ''limit'' which refers to:
** Limes (Roman Empire)
(Latin, singular; plural: ) is a modern term used primarily for the Germanic border defence or delimitin ...
,
citric acid
Citric acid is an organic compound with the chemical formula HOC(CO2H)(CH2CO2H)2. It is a colorless weak organic acid. It occurs naturally in citrus fruits. In biochemistry, it is an intermediate in the citric acid cycle, which occurs in t ...
is found only in very minute quantities in wine grapes. It often has a concentration about 1/20 that of tartaric acid. The citric acid most commonly found in wine is commercially produced acid supplements derived from
fermenting
Fermentation is a metabolic process that produces chemical changes in organic substrates through the action of enzymes. In biochemistry, it is narrowly defined as the extraction of energy from carbohydrates in the absence of oxygen. In foo ...
sucrose
Sucrose, a disaccharide, is a sugar composed of glucose and fructose subunits. It is produced naturally in plants and is the main constituent of white sugar. It has the molecular formula .
For human consumption, sucrose is extracted and refi ...
solutions. These inexpensive supplements can be used by winemakers in acidification to boost the wine's total acidity. It is used less frequently than tartaric and malic due to the aggressive citric flavors it can add to the wine. When citric acid is added, it is always done after primary alcohol fermentation has been completed due to the tendency of yeast to convert citric into acetic acid. In the
European Union
The European Union (EU) is a supranational political and economic union of member states that are located primarily in Europe. The union has a total area of and an estimated total population of about 447million. The EU has often been ...
, use of citric acid for acidification is prohibited, but limited use of citric acid is permitted for removing excess
iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
and
copper
Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish ...
from the wine if
potassium ferrocyanide is not available.
[J. Robinson (ed) ''“The Oxford Companion to Wine”'' Third Edition pg 171 Oxford University Press 2006 ]
Acetic
Acetic acid
Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main componen ...
is a two-
carbon
Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon makes ...
organic acid produced in wine during or after the fermentation period. It is the most volatile of the primary acids associated with wine and is responsible for the sour taste of
vinegar
Vinegar is an aqueous solution of acetic acid and trace compounds that may include flavorings. Vinegar typically contains 5–8% acetic acid by volume. Usually, the acetic acid is produced by a double fermentation, converting simple sugars to ...
. During fermentation, activity by yeast cells naturally produces a small amount of acetic acid. If the wine is exposed to
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as we ...
, ''Acetobacter'' bacteria will convert the
ethanol
Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an alcohol with the chemical formula . Its formula can be also written as or (an ethyl group linked to a h ...
into acetic acid. This process is known as the “acetification” of wine and is the primary process behind wine degradation into vinegar. An excessive amount of acetic acid is also considered a
wine fault
A wine fault or defect is an unpleasant characteristic of a wine often resulting from poor winemaking practices or storage conditions, and leading to wine spoilage. Many of the compounds that cause wine faults are already naturally present in wine ...
. A taster's sensitivity to acetic acid will vary, but most people can detect excessive amounts at around 600
mg/L.
Ascorbic
Ascorbic acid
Vitamin C (also known as ascorbic acid and ascorbate) is a water-soluble vitamin found in citrus and other fruits and vegetables, also sold as a dietary supplement and as a topical 'serum' ingredient to treat melasma (dark pigment spots) a ...
, also known as
vitamin C
Vitamin C (also known as ascorbic acid and ascorbate) is a water-soluble vitamin found in citrus and other fruits and vegetables, also sold as a dietary supplement and as a topical 'serum' ingredient to treat melasma (dark pigment spots) a ...
, is found in young wine grapes prior to
veraison, but is rapidly lost throughout the ripening process. In winemaking, it is used with sulfur dioxide as an
antioxidant
Antioxidants are compounds that inhibit oxidation, a chemical reaction that can produce free radicals. This can lead to polymerization and other chain reactions. They are frequently added to industrial products, such as fuels and lubricants ...
, often added during the bottling process for white wines. In the European Union, use of ascorbic acid as an additive is limited to 150 mg/L.
[J. Robinson (ed) ''“The Oxford Companion to Wine”'' Third Edition pg 35–36 Oxford University Press 2006 ]
Butyric
Butyric acid
Butyric acid (; from grc, βούτῡρον, meaning "butter"), also known under the systematic name butanoic acid, is a straight-chain alkyl carboxylic acid with the chemical formula CH3CH2CH2CO2H. It is an oily, colorless liquid with an unpl ...
is a bacteria-induced wine fault that can cause a wine to smell of spoiled
Camembert or rancid butter.
[International Sommelie]
October, 2003
, pg 10. Accessed 10/4/2008
Sorbic
Sorbic acid
Sorbic acid, or 2,4-hexadienoic acid, is a natural organic compound used as a food preservative. It has the chemical formula and the structure . It is a colourless solid that is slightly soluble in water and sublimes readily. It was first iso ...
is a winemaking additive used often in sweet wines as a preservative against
fungi
A fungus (plural, : fungi or funguses) is any member of the group of Eukaryote, eukaryotic organisms that includes microorganisms such as yeasts and Mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified ...
, bacteria and yeast growth. Unlike sulfur dioxide, it does not hinder the growth of the lactic acid bacteria. In the European Union, the amount of sorbic acid that can be added is limited — no more than 200 mg/L. Most humans have a detection threshold of 135 mg/L, with some having a sensitivity to detect its presence at 50 mg/L. Sorbic acid can produce off flavors and aromas which can be described as “rancid”. When lactic acid bacteria metabolize sorbates in the wine, it creates a wine fault that is most recognizable by an aroma of crushed ''
Pelargonium
''Pelargonium'' () is a genus of flowering plants that includes about 280 species of perennials, succulents, and shrubs, commonly called geraniums, pelargoniums, or storksbills. ''Geranium'' is also the botanical name and common name of a sepa ...
'' geranium leaves.
[J. Robinson (ed) ''“The Oxford Companion to Wine”'' Third Edition pg 644 Oxford University Press 2006 ]
Succinic
Succinic acid is most commonly found in wine, but can also be present in trace amounts in ripened grapes. While concentration varies among grape varieties, it is usually found in higher levels with red wine grapes. The acid is created as a
byproduct of the
metabolization of nitrogen by yeast cells during fermentation. The combination of succinic acid with one molecule of ethanol will create the
ester
In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides ...
ethyl succinate
Ethyl may refer to:
Arts and entertainment
* Cold Ethyl, a Swedish rock band
*Ethyl Sinclair, a character in the ''Dinosaurs'' television show
Science and technology
* Ethyl group, an organic chemistry moiety
* Ethyl alcohol (or ethanol)
* Et ...
which contributes to a mild fruity aroma in wines.
[J. Robinson (ed) ''“The Oxford Companion to Wine”'' Third Edition pg 665 Oxford University Press 2006 ]
Effects
Making
Acidity is highest in wine grapes just before the start of ''
veraison'', which ushers in the ripening period of the
annual cycle of grape vine
The annual growth cycle of grapevines is the process that takes place in the vineyard each year, beginning with bud break in the spring and culminating in leaf fall in autumn followed by winter dormancy. From a winemaking perspective, each step ...
s. As the grapes
ripen, their sugar levels increase and their acidity levels decrease. Through the process of
respiration, malic acid is
metabolized by the grape vine. Grapes from cooler climate
wine regions generally have higher levels of acidity due to the slower ripening process. The level of acidity still present in the grape is an important consideration for winemakers in deciding when to begin
harvest
Harvesting is the process of gathering a ripe crop from the fields. Reaping is the cutting of grain or pulse for harvest, typically using a scythe, sickle, or reaper. On smaller farms with minimal mechanization, harvesting is the most lab ...
. For wines such as
Champagne
Champagne (, ) is a sparkling wine originated and produced in the Champagne wine region of France under the rules of the appellation, that demand specific vineyard practices, sourcing of grapes exclusively from designated places within it, ...
and other
sparkling wine
Sparkling wine is a wine with significant levels of carbon dioxide in it, making it fizzy. While the phrase commonly refers to champagne, European Union countries legally reserve that term for products exclusively produced in the Champagne regi ...
s, having high levels of acidity is even more vital to the
winemaking process, so grapes are often picked under-ripe and at higher acid levels.
In the winemaking process, acids aid in enhancing the effectiveness of
sulfur dioxide
Sulfur dioxide ( IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a toxic gas responsible for the odor of burnt matches. It is released naturally by volcanic ...
to protect the wines from spoilage and can also protect the wine from bacteria due to the inability of most bacteria to survive in low pH solutions. Two notable exceptions to this are ''
Acetobacter'' and the lactic acid bacteria. In red wines, acidity helps preserve and stabilize the color of the wine. The
ionization
Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive Electric charge, charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged a ...
of
anthocyanins
Anthocyanins (), also called anthocyans, are water-soluble vacuolar pigments that, depending on their pH, may appear red, purple, blue, or black. In 1835, the German pharmacist Ludwig Clamor Marquart gave the name Anthokyan to a chemical com ...
is affected by pH, so wines with lower pH (such as
Sangiovese
Sangiovese (, also , , ) is a red Italian wine grape variety that derives its name from the Latin ''sanguis Jovis'', "the blood of Jupiter". Though it is the grape of most of central Italy from Romagna down to Lazio (the most widespread grape ...
-based wines) have redder, more stable colors. Wines with higher pH (such as
Syrah
Syrah (), also known as Shiraz, is a dark-skinned grape variety grown throughout the world and used primarily to produce red wine. In 1999, Syrah was found to be the offspring of two obscure grapes from southeastern France, Dureza and Mondeus ...
-based wines) have higher levels of less stable blue pigments, eventually taking on a muddy grey hue. These wines can also develop a brownish tinge. In white wines, higher pH (lower acidity) causes the
phenolics in the wine to darken and eventually
polymerize as brown deposits.
Winemakers will sometimes add acids to the wine (acidification) to make the wine more acidic, most commonly in warm climate regions where grapes are often harvested at advanced stages of ripeness with high levels of sugars, but very low levels of acid. Tartaric acid is most often added, but winemakers will sometimes add citric or malic acid. Acids can be added either before or after primary fermentation. They can be added during blending or aging, but the increased acidity will become more noticeable to wine tasters if added at this point.
Tasting
The acidity in wine is an important component in the quality and taste of the wine. It adds a sharpness to the flavors and is detected most readily by a prickling sensation on the sides of the tongue and a mouth-watering aftertaste. Of particular importance is the balance of acidity versus the sweetness of the wine (the leftover residual sugar) and the more bitter components of the wine (most notably tannins but also includes other
phenolics). A wine with too much acidity will taste excessively sour and sharp. A wine with too little acidity will taste flabby and flat, with less defined flavors.
See also
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Wine chemistry
Wine is a complex mixture of chemical compounds in a hydro-alcoholic solution with a pH around 4.
The chemistry of wine and its resultant quality depend on achieving a balance between three aspects of the berries used to make the wine: their suga ...
References
{{Wines
Organic acids