Phenolic compounds—natural phenol andpolyphenols—occur naturally inwine. These include a large group of several hundredchemical compounds that affect thetaste,color andmouthfeel of wine. These compounds includephenolic acids,stilbenoids,flavonols,dihydroflavonols,anthocyanins,flavanol monomers (catechins) andflavanol polymers (proanthocyanidins). This large group of natural phenols can be broadly separated into two categories,flavonoids and non-flavonoids. Flavonoids include theanthocyanins andtannins which contribute to the color and mouthfeel of the wine.[1] The non-flavonoids include thestilbenoids such asresveratrol andphenolic acids such asbenzoic,caffeic andcinnamic acids.
The natural phenols are not evenly distributed within the grape. Phenolic acids are largely present in the pulp,anthocyanins andstilbenoids in the skin, and other phenols (catechins,proanthocyanidins andflavonols) in the skin and the seeds.[2] During thegrowth cycle of the grapevine, sunlight will increase the concentration of phenolics in the grape berries, their development being an important component ofcanopy management. The proportion of the different phenols in any one wine will therefore vary according to the type ofvinification.Red wine will be richer in phenols abundant in the skin and seeds, such asanthocyanin,proanthocyanidins andflavonols, whereas the phenols inwhite wine will essentially originate from the pulp, and these will be the phenolic acids together with lower amounts ofcatechins andstilbenes. Red wines will also have the phenols found in white wines.
Wine simple phenols are further transformed duringwine aging into complex molecules formed notably by the condensation of proanthocyanidins and anthocyanins, which explains the modification in the color. Anthocyanins react with catechins, proanthocyanidins and other wine components during wine aging to form new polymeric pigments resulting in a modification of the wine color and a lowerastringency.[3][4] Average total polyphenol content measured by theFolin method is 216 mg/100 ml for red wine and 32 mg/100 ml for white wine. The content of phenols in rosé wine (82 mg/100 ml) is intermediate between that in red and white wines.
Inwinemaking, the process ofmaceration or "skin contact" is used to increase the concentration of phenols in wine. Phenolic acids are found in the pulp or juice of the wine and can be commonly found in white wines which usually do not go through a maceration period. The process ofoak aging can also introduce phenolic compounds into wine, most notablyvanillin which addsvanillaaroma to wines.[5]
Most wine phenols are classified assecondary metabolites and were not thought to be active in the primarymetabolism and function of the grapevine. However, there is evidence that in some plantsflavonoids play a role as endogenous regulators ofauxin transport.[6] They arewater-soluble and are usually secreted into thevacuole of the grapevine asglycosides.
Vitis vinifera, the common grape vine, from which European style wines are made the world over, produces many phenolic compounds. There is a varietal effect on the relative composition.
In red wine, up to 90% of the wine's phenolic content falls under the classification offlavonoids. These phenols, mainly derived from the stems, seeds and skins are often leached out of the grape during the maceration period of winemaking. The amount of phenols leached is known asextraction. These compounds contribute to theastringency, color and mouthfeel of the wine. In white wines the number of flavonoids is reduced due to the lesser contact with the skins that they receive during winemaking. There is on-going study into thehealth benefits of wine derived from theantioxidant andchemopreventive properties of flavonoids.[7]
Within the flavonoid category is a subcategory known asflavonols, which includes the yellowpigment -quercetin. Like other flavonoids, the concentration of flavonols in the grape berries increases as they are exposed to sunlight. Wine grapes facing too much sun exposure can see an accelerated ripening period, leading to a lessened ability for the synthesis of flavonols.[8] Someviticulturalists will use measurement of flavonols such as quercetin as an indication of a vineyard's sun exposure and the effectiveness of canopy management techniques.
Anthocyanins are phenolic compounds found throughout theplant kingdom, being frequently responsible for the blue to red colors found inflowers,fruits andleaves. In wine grapes, they develop during the stage ofveraison, when the skin of red wine grapes changes color from green to red to black. As thesugars in the grape increase duringripening so does the concentration of anthocyanins. An issue associated with climate change has been the accumulation of sugars within the grape accelerating rapidly and outpacing the accumulation of anthocyanins.[8] This leaves viticulturists with the choice of harvesting grapes with too high sugar content or with too low anthocyanin content. In most grapes anthocyanins are found only in the outer cell layers of the skin, leaving the grape juice inside virtually colorless. Therefore, to get color pigmentation in the wine, thefermentingmust needs to be in contact with the grape skins in order for the anthocyanins to be extracted. Hence, white wine can be made from red wine grapes in the same way that many whitesparkling wines are made from the red wine grapes ofPinot noir andPinot Meunier. The exception to this is the small class of grapes known asteinturiers, such asAlicante Bouschet, which have a small amount of anthocyanins in the pulp that produces pigmented juice.[9]
There are several types of anthocyanins (as theglycoside) found in wine grapes which are responsible for the vast range of coloring from ruby red through to dark black found in wine grapes.Ampelographers can use this observation to assist in the identification of differentgrape varieties. The European vine familyVitis vinifera is characterized by anthocyanins that are composed of only one molecule ofglucose while non-vinifera vines such ashybrids and the AmericanVitis labrusca will have anthocyanins with two molecules. This phenomenon is due to a double mutation in theanthocyanin 5-O-glucosyltransferase gene ofV. vinifera.[10] In the mid-20th century, French ampelographers used this knowledge to test the various vine varieties throughout France to identify which vineyards still contained non-vinifera plantings.[9]
Red-berriedPinot grape varieties are also known to not synthesizepara-coumaroylated oracetylated anthocyanins as other varieties do.[11]
The color variation in the finished red wine is partly derived from theionization of anthocyanin pigments caused by theacidity of the wine. In this case, the three types of anthocyanin pigments are red, blue and colorless with the concentration of those various pigments dictating the color of the wine. A wine with lowpH (and such greater acidity) will have a higher occurrence of ionized anthocyanins which will increase the amount of bright red pigments. Wines with a higher pH will have a higher concentration of blue and colorless pigments. As thewine ages, anthocyanins will react with other acids and compounds in wines such as tannins,pyruvic acid andacetaldehyde which will change the color of the wine, causing it to develop more "brick red" hues. These molecules will link up to createpolymers that eventually exceed theirsolubility and become sediment at the bottom of wine bottles.[9]Pyranoanthocyanins are chemical compounds formed in redwines byyeast duringfermentation processes[12] or duringcontrolled oxygenation processes[13] during theaging of wine.[14]
Tannins refer to the diverse group of chemical compounds in wine that can affect the color, aging ability and texture of the wine. While tannins cannot be smelled or tasted, they can be perceived duringwine tasting by thetactile sensation ofastringency and sense of bitterness that they can leave in the mouth. This is due to the tendency of tannins to react withproteins, such as the ones found insaliva.[15] Infood and wine pairing, foods that are high in proteins (such asred meat) are often paired with tannic wines to minimize the astringency of tannins. However, many wine drinkers find the perception of tannins to be a positive trait—especially as it relates to mouthfeel. The management of tannins in the winemaking process is a key component in the resulting quality.[16]
Tannins are found in the skin, stems, and seeds of wine grapes but can also be introduced to the wine through the use of oak barrels and chips or with the addition of tannin powder. The natural tannins found in grapes are known asproanthocyanidins due to their ability to release red anthocyanin pigments when they are heated in an acidic solution. Grape extracts are mainly rich in monomers and small oligomers (mean degree of polymerization < 8). Grape seed extracts contain three monomers (catechin, epicatechin and epicatechin gallate) and procyanidin oligomers. Grape skin extracts contain four monomers (catechin, epicatechin, gallocatechin and epigallocatechin), as well asprocyanidins andprodelphinidins oligomers.[17] The tannins are formed byenzymes during metabolic processes of the grapevine. The amount of tannins found naturally in grapes varies depending on the variety withCabernet Sauvignon,Nebbiolo,Syrah andTannat being 4 of the most tannic grape varieties. The reaction of tannins and anthocyanins with the phenolic compoundcatechins creates another class of tannins known aspigmented tannins which influence the color of red wine.[18] Commercial preparations of tannins, known asenological tannins, made fromoak wood,grape seed and skin, plantgall,chestnut,quebracho,gambier[19] andmyrobalan fruits,[20] can be added at different stages of the wine production to improve color durability. The tannins derived from oak influence are known as "hydrolysable tannins" being created from theellagic andgallic acid found in the wood.[16]
In the vineyards, there is also a growing distinction being made between "ripe" and "unripe" tannins present in the grape. This "physiological ripeness", which is roughly determined by tasting the grapes off the vines, is being used along with sugar levels as a determination of when toharvest. The idea is that "riper" tannins will taste softer but still impart some of the texture components found favorable in wine. In winemaking, the amount of the time that the must spends in contact with the grape skins, stems and seeds will influence the amount of tannins that are present in the wine with wines subjected to longer maceration period having more tannin extract. Following harvest, stems are normally picked out and discarded prior to fermentation but some winemakers may intentionally leave in a few stems for varieties low in tannins (like Pinot noir) in order to increase the tannic extract in the wine. If there is an excess in the amount of tannins in the wine, winemakers can use variousfining agents likealbumin,casein andgelatin that can bind to tannins molecule andprecipitate them out as sediments. As a wine ages, tannins will form long polymerized chains which come across to a taster as "softer" and less tannic. This process can be accelerated by exposing the wine tooxygen, which oxidize tannins to quinone-like compounds that are polymerization-prone. The winemaking technique ofmicro-oxygenation anddecanting wine use oxygen to partially mimic the effect of aging on tannins.[16]
A study in wine production and consumption has shown that tannins, in the form ofproanthocyanidins, have a beneficial effect on vascular health. The study showed that tannins suppressed production of the peptide responsible for hardening arteries. To support their findings, the study also points out that wines from the regions of southwest France and Sardinia are particularly rich in proanthocyanidins, and that these regions also produce populations with longer life spans.[21]
Reactions of tannins with the phenolic compoundanthocyanidins creates another class of tannins known aspigmented tannins which influences the color of red wine.[18]
Commercial preparations of tannins, known asenological tannins, made fromoak wood,grape seed and skin, plantgall,chestnut,quebracho,gambier[19] andmyrobalan fruits,[20] can be added at different stages of the wine production to improve color durability.
Tannins are a natural preservative in wine. Un-aged wines with high tannin content can be less palatable than wines with a lower level of tannins. Tannins can be described as leaving a dry and puckered feeling with a "furriness" in the mouth that can be compared to a stewed tea, which is also very tannic. This effect is particularly profound when drinking tannic wines without the benefit of food.
Manywine lovers see natural tannins (found particularly in varietals such asCabernet Sauvignon and often accentuated by heavyoak barrel aging) as a sign of potential longevity andageability. Tannins impart a mouth-puckering astringency when the wine is young but "resolve" (through a chemical process calledpolymerization) into delicious and complex elements of "bottlebouquet" when the wine is cellared under appropriate temperature conditions, preferably in the range of a constant 55 to 60 °F (13 to 16 °C).[22] Such wines mellow and improve with age with the tannic "backbone" helping the wine survive for as long as 40 years or more.[23] In many regions (such as inBordeaux), tannic grapes such asCabernet Sauvignon are blended with lower-tannin grapes such asMerlot orCabernet Franc, diluting the tannic characteristics. White wines and wines that are vinified to be drunk young (for examples, seenouveau wines) typically have lower tannin levels.
Flavan-3-ols (catechins) are flavonoids that contribute to the construction of various tannins and contribute to the perception of bitterness in wine. They are found in highest concentrations in grape seeds but are also in the skin and stems. Catechins play a role in themicrobial defense of the grape berry, being produced in higher concentrations by the grape vines when it is being attacked bygrape diseases such asdowny mildew. Because of that grape vines in cool, damp climates produce catechins at high levels than vines in dry, hot climates. Together with anthocyanins and tannins they increase the stability of a wines color-meaning that a wine will be able to maintain its coloring for a longer period of time. The amount of catechins present varies among grape varieties with varietals like Pinot noir having high concentrations whileMerlot and especially Syrah have very low levels.[17] As an antioxidant, there are some studies into the health benefits of moderate consumption of wines high in catechins.[24]
In red grapes, the main flavonol is on averagequercetin, followed bymyricetin,kaempferol,laricitrin,isorhamnetin, andsyringetin.[25] In white grapes, the main flavonol is quercetin, followed by kaempferol and isorhamnetin. The delphinidin-like flavonols myricetin, laricitrin, and syringetin are missing in all white varieties, indicating that the enzyme flavonoid 3',5'-hydroxylase is not expressed in white grape varieties.[25]
Myricetin,laricitrin[26] andsyringetin,[27] flavonols which are present in red grape varieties only, can be found in red wine.[28]
Hydroxycinnamic acids are the most important group of nonflavonoid phenols in wine. The four mostabundant ones are thetartaric acid esterstrans-caftaric,cis- andtrans-coutaric, andtrans-fertaric acids. In wine they are present also in the free form (trans-caffeic,trans-p-coumaric, andtrans-ferulic acids).[29]
V. vinifera also producesstilbenoids.
Resveratrol is found in highest concentration in the skins of wine grapes. The accumulation in ripe berries of different concentrations of both bound and free resveratrols depends on the maturity level and is highly variable according to the genotype.[30] Both red and white wine grape varieties contain resveratrol, but more frequent skin contact and maceration leads to red wines normally having ten times more resveratrol than white wines.[31] Resveratrol produced by grape vines provides defense against microbes, and production can be further artificially stimulated byultraviolet radiation. Grapevines in cool, damp regions with higher risk of grape diseases, such asBordeaux andBurgundy, tend to produce grapes with higher levels of resveratrol than warmer, drier wine regions such asCalifornia andAustralia. Different grape varieties tend to have differing levels, withMuscadines and the Pinot family having high levels while theCabernet family has lower levels of resveratrol. In the late 20th century interest in the possible health benefits of resveratrol in wine was spurred by discussion of theFrench paradox involving the health of wine drinkers in France.[32]
Piceatannol is also present in grape[33] from where it can be extracted and found in red wine.[28]
Vanillin is a phenolicaldehyde most commonly associated with the vanilla notes in wines that have been aged in oak. Trace amounts of vanillin are found naturally in grapes, but they are most prominent in thelignin structure of oak barrels. Newer barrels will impart more vanillin, with the concentration present decreasing with each subsequent usage.[34]
Oak barrel will add compounds such asvanillin and hydrolysable tannins (ellagitannins). Thehydrolyzable tannins present in oak are derived fromlignin structures in the wood. They help protect the wine from oxidation andreduction.[35]
4-Ethylphenol and4-ethylguaiacol are produced during ageing of red wine in oak barrels that are infected bybrettanomyces .[36]
Low molecular weight polyphenols, as well as ellagitannins, are susceptible to be extracted fromcork stoppers into the wine.[37] The identified polyphenols are gallic,protocatechuic,vanillic, caffeic,ferulic, and ellagic acids;protocatechuic,vanillic,coniferyl, andsinapic aldehydes; the coumarinsaesculetin andscopoletin; the ellagitannins are roburinsA andE,grandinin,vescalagin andcastalagin.[38]
Guaiacol is one of the molecules responsible for thecork taint wine fault.[39]
Flash release is a technique used in winepressing.[40] The technique allows for a better extraction of phenolic compounds.[41]
The exposure of wine tooxygen in limited quantities affects phenolic content.[42]
Depending on the methods of production, wine type, grape varieties, ageing processes, the following phenolics can be found in wine. The list, sorted in alphabetical order of common names, is not exhaustive.
Polyphenol compounds may interact withvolatiles and contribute to the aromas in wine.[49] Although wine polyphenols are speculated to provideantioxidant or other benefits, there is little evidence that wine polyphenols actually have any effect in humans.[50][51][52][53] Limited preliminary research indicates that wine polyphenols may decreaseplatelet aggregation, enhancefibrinolysis, and increaseHDL cholesterol, but high-qualityclinical trials have not confirmed such effects, as of 2017.[50]