Odoriferous Compounds in Grape Berry and Wine - Varietal Aromas

Wine aromas are made up of several hundreds of volatile compounds, at concentrations ranging from several mg/l to a few ng/l, or even less.
The complexity of wine aromas is due to the diversity of the mechanisms involved in their development:

• (i) grape metabolism, depending on the variety, as well as climate, soil and vineyard management techniques
• (ii) biochemical phenomena (oxidation and hydrolysis) occurring prior to fermentation, triggered during extraction of the juice and maceration
• (iii) the fermentation metabolisms of the microorganisms responsible for alcoholic and malolactic fermentations
• (iv) chemical or enzymatic reactions occurring after fermentation, during aging of the wine in vat, barrel and bottle

Yeast Fermentation produces volatile metabolites that are derived from sugar and amino acid metabolism and comprise esters, higher alcohols, carbonyls, volatile fatty acids and sulfur compounds, giving wine its vinous character Grapes produce odorifeous compounds that reflect the particular grape variety and play a more decisive role in the quality and regional character of wines than any other aroma component

• these include mainly monoterpenes, C13-norisoprenoids, methoxypyrazines, and sulfur compounds, and are responsible for the varietal aroma of wines
• numerous studies have shown that the terpenoid compounds form the basis of the sensory expression of the wine bouquet, and they can be used to differentiate grape varieties
• terpene compounds belong to secondary plant constituents, whose biosynthesis begins with acetyl-coenzyme A (CoA), and are located in the berry skin
• at present, about 50 monoterpene compounds are known, dominating monoterpene alcohols, particularly from Muscat varieties, are linalool, geraniol, nerol, citronellol, α-terpineol and hotrienol
  

A general classification of grape varieties based on monoterpene concentration allows division into:

• (1) intensely-flavoured muscats, in which total free monoterpene concentrations can be as high as 6 mg/L
• (2) non-muscat but aromatic varieties with total monoterpene concentration of 1-4 mg/L, including Traminer, Huxel and Riesling varieties
• (3) more neutral varieties not dependent upon monoterpenes for their flavor, including numerous cultivars such as Cabernet Sauvignon, Sauvignon Blanc, Merlot, Shiraz and Chardonnay

There are three categories of monoterpenes with some interrelationships between them:
(1) free aroma compounds

• commonly dominated by linalool, geraniol, and nerol, together with the pyran and furan forms of the linalool oxides
• depending on how juice has been treated and on other factors, which may include climate, many additional monoterpenes can be found in this group, i.e. citronellol, α-terpineol, hotrienol, nerol oxide, myrcenol, the ocimenols plus several other oxides, aldehydes and hydrocarbons
• in wines, several monoterpene ethyl ethers and acetate esters have also been found among free aroma compounds
• the most odoriferous of the monoterpene alcohols are citronellol and linalool
• olfactory impact of terpene compounds is synergistic

(2) free odorless polyols (diols and tiols)

• most significant feature of the polyols is that some of them are reactive and can break down easily to give pleasant and potent volatiles, i.e. diendiol (3,7-dimethylocta-1,5-diene-3,7-diol) which can give hotrienol and nerol oxide
• these monoterpene polyols are present in grapes at concentrations up to 1 mg/L

(3) glycosidically conjugated monoterpenes

• make no direct contribution to the aroma of the grape
• glycosides are, in most cases, more abundant than the unglycosilated forms of individual mono- terpenes and polyols
• glycon moiety is composed by glucose or by a disaccharide constituted by glucose and rhamnose or arabinose or apiose
• enzyme hydrolysis has been used to enrich flavour by release of free aromatic compounds from natural glycoside precursors producing a more “natural” flavour in the wine

Other information:

• free and bonded forms of terpenols accumulate in ripening grapes from the colour change onwards
• free monoterpenes start to decrease before maximum sugar level is reached
• vineyard conditions during ripening, such as temperature and water supply, may influence aroma development during ripening
• methoxypyrazines contributes herbaceous aroma in certain grape varieties, such as Cabernet Sauvignon, Cabernet Franc and Sauvignon Blanc
• the compounds 2-isopropyl-3-methoxypyrazine, 2-sec-butyl-3-methoxypyrazine and 2-isobutyl-3-methoxypyrazine, have odors reminiscent of green pepper and asparagus, or even earthy overtones
• a characteristic component of the aroma of Sauvignon Blanc wines is 4-mercapto-4-methyl-pentan-2-one, with a marked smell of boxwood and broom and concentrations may reach 40 ng/l
• the excessive hydrocarbon smells (produced by the non-megastigmane C₁₃-norisoprenoid derivative TDN, 1,1,6-trimethyl-1,2-dihydronaphtalene) that sometimes develop as Riesling wines age are related to extremely high temperatures, especially during the grape ripening periods

References:
1. Carlos Conde, Paulo Silva, Natacha Fontes, Alberto C. P. Dias, Rui M. Tavares, Maria J. Sousa, Alice Agasse, Serge Delrot, Hernâni Gerós, Biochemical changes throughout Grape Berry development and fruit and wine quality, Food, 1(1), 1-22 ©2007 Global Science Books.

Vintage 2020: Got Rain Will Grow

The first weeks in May were unusually cold and the buds on the vines appeared to be in arrested development. We finally had budbreak on most of our varieties around May 11. The cool temperatures that continued allowed us to remove excess buds, in a process called disbudding.
On Saturday, May 23, we had showers that lasted all day long. A few days previous to that, I posted a blog that was about Rub Out Those Buds. In that blogpost, I showed an example of Chardonnay. After the rains on Saturday, most of the Chardonnay had grown like the proverbial weed!

Rain during this period of time is especially welcome because it helps the developing vine to grow. This period of growth will lead to long internodes and good spacing for the leaves and flowers, the eventual grape cluster. According to Ed Hellman's Grapevine Structure and Function, our vines are at Principle Stage 1, Leaf Development.

Our Chardonnay is textbook stage 15. What is interesting for us this year is that many of the Chardonnay have two inflorescences!

Phenolics in Wine

• the greatest concentration of phenolics occurs in the seeds and stems
• destemming and even removing seeds, minimizes the concentration of complex green tannins and astringency, which results from excessive extraction of these plant part
• destemming is generally practiced when one desires a fruitier wine with less complex astringency
• macerating before fermentation, leads to additional color and tannin extraction from the pomace
• fermentation conditions, in particular the temperature, and the choice of the yeast strain will confer different chemical and flavor characteristics to wine
• the presence of yeast and bacteria or other fungal organisms in wine directly impacts the wine quality and flavor and influences the metabolism of phenolics in wine

The Influence of Oak Barrels on Flavor:
Among the 250 species of the genus Quercus, two European species, Q. petraea (sessile oak) and Q. robur (pedunculate oak) and one American species, Q. alba (white oak) have been used in the past and continue to be widely employed in barrel making.
Research into the composition of the wood has shown that:

• Q. petraea sessile oaks are richer in aromatic substances such as vanillin and methyl octalactone (characterized by a distinct odor of coconut, celery, and fresh wood
• Q. robur pedunculate oaks primarily contain phenolic compounds such as ellagitannins or catechol tannins
• Q. alba American white oak, contains fewer tannins than French oak, but more aromatic compounds, particularly methyl octalactone
• differences in the composition of American white oak and European sessile oak indicate that wines are likely to develop differently according to the origin of the barrels used for aging
• European sessile oak and American white oak are perfectly suitable for aging fine wine
• the low aromatic potential and the high ellagitannins content of pedunculate oak indicate that it should only be used for aging brandies
• wine character is also influenced by the age and preparation of the oak
• smaller barrels result in more phenolic and tannin content, due to the increased surface exposure area and extraction
• anthocyanin-tannin complexes can be formed stabilizing the color of red wines and resulting in wines that taste less fruity but also less astringent after aging

The effects of bottle aging:

• there is also some degree of continuous oxygen exposure during wine aging in the bottle
• the interaction of tannins and anthocyanins with oxygen, which diffuses during barrel storage, further polymerizes these compounds making them taste less astringent
• proanthocyanidins and other polyphenolics eventually aggregate into larger molecules accumulating as sediment at the base of the bottle
• red wines become lighter in color
• white wines often deepen in color, turning darker honey colors as they oxidize and age
• widespread existence of cork taint is now universally acknowledged
• cork taint is mainly due to the presence of 2,4,6-trichloroanisole (TCA) in the wine
• TCA in wine leads to an unpleasant, musty off-flavor and can render an otherwise excellent wine completely useless and may cause economic losses
• range of alternative closure techniques, which might vary in such aspects as degree of market acceptability, oxygen permeation rate, ability to carry out “flavor scalping” and their propensity to contribute “taints” of varying character are now available and used by the industry
• alternative closures such as roll-on tamper evident screw cap closures (ROTE) have become a popular alternative for Australian and New Zealand white wines
• high temperature during wine aging can negatively affect its sensory properties and visual stability
• exposure of wine to heat increases the loss of individual and total anthocyanins, reducing color density
• high temperature can accelerate the reaction between urea and ethanol causing the formation of ethyl carbamate

References:
1. Carlos Conde, Paulo Silva, Natacha Fontes, Alberto C. P. Dias, Rui M. Tavares, Maria J. Sousa, Alice Agasse, Serge Delrot, Hernâni Gerós, Biochemical changes throughout Grape Berry development and fruit and wine quality, Food, 1(1), 1-22 ©2007 Global Science Books.

Rub Out Those Buds

We are currently in the process of disbudding the shoots that we laid down for this year's growth. We began with the Chardonnay because they were ahead in growth of all the other varieties. After the Chardonnay, we did the Barbera and the Cabernet Franc clone 214. Amazingly, this clone appears to be an early budder. The next to be disbudded was the Chenin Blanc, clone FPS-1. Currently, we are disbudding the Chenin Blanc, clone 982. Last to come to the stage at which we disbud are the Auxerrois, the Cabernet Sauvignon, and the Cabernet Franc, clone 327.
Here is a look at what our varieties look like as of May 20, 2020.

Variety	Budbreak Stage
Auxerrois
Chenin Blanc
Chardonnay
Cabernet Sauvignon
Cabernet Franc
Barbera

Phenolic Ripeness and Sugar Ripeness

Many factors influence the chemical composition of grapes and the quality of wines including:

• maturity stage
• genetic makeup
• climatic conditions
• management practices

To make a high quality wine, grapes must contain an adequate amount of sugars and acids, but also balanced levels of phenols. Sugar concentration is an indicator of berry maturity (“sugar ripeness”) and determines the ethanol content of the wine.
Changes in tannin content are used by the growers to control harvest, and this process is called “phenolic ripeness”. Sugar ripeness may affect phenolic ripeness because the expression of several enzymes of the phenylpropanoid pathway may be controlled by sugars.
Phenolic ripeness allows the selection of flavoured grapes and avoids the unpleasant unripe tastes in the wine.
Factors affecting phenolic ripeness:

• increased sunlight and moderate fertility support vine growth and cluster formation and lead to an increased formation of secondary plant products, including phenolics
• quercetin is an excellent indicator of sunlight available to the development of grape clusters
• ample but not excessive moisture
• adequate, but not excessive fertility will also result in higher concentrations of phenolics and tannins in red wine grapes
• nitrogen, present in the soil as nitrite and nitrate, affects polyophenols content and, consequently, the wine taste and flavor
• disease pressures can result in reduced berry quality, leading to reduced phenolic content over time in harvested grapes
• the enzyme laccase produced by B. cinerea can degrade complex polyphenols and thus modify wine flavour and colour
• esca disease results in significant changes in the phenolic composition of grapevine leaves

References:
1. Carlos Conde, Paulo Silva, Natacha Fontes, Alberto C. P. Dias, Rui M. Tavares, Maria J. Sousa, Alice Agasse, Serge Delrot, Hernâni Gerós, Biochemical changes throughout Grape Berry development and fruit and wine quality, Food, 1(1), 1-22 ©2007 Global Science Books.

Biosynthesis of Phenolic Compounds in Grape Berry

The biosynthesis of soluble phenolics begins with the Pentose Phosphate Pathway and/or from Glycolysis and results in the synthesis of the aromatic amino acid phenylalanine, a product of the shikimate pathway.
The first enzyme responsible for the phenolic synthesis is PAL (phenyl ammonia lysase), which converts phenylalanine into cinnamic acid. Cinnamic acid undergoes a series of transformations resulting in the formation of precursors of several simple phenolics, like phenolic acids, lignin precursors, etc.
On a per berry basis, total hydroxycinnamates in mesocarp tissues peak prior to véraison and then decline, leading to a constant amount (per berry) as the fruit ripens. In ripe fruit, the most abundant hydroxycinnamates are caftaric and coutaric acids. The level of hydroxycinnamates in the juice of different vinifera varieties is highly variable, ranging from 16 to 430 mg/L.
The incorporation of 3 molecules of malonyl-CoA, produced via the acetate pathway, with 4-coumaroyl-CoA starts the phenylpropanoid pathway. These precursors generate complex phenolic compounds, like the flavonoids or the stilbenes, depending on the intervening enzyme, chalcone synthase (CHS) for flavonoid synthesis and the stilbene synthase (SS) for stilbene synthesis.
Skin tannins, which are synthesized very early in berry development, change very little on a per berry basis from véraison to harvest, although their concentration declines along berry growth. Qualitative changes, such as the increase of the polymerization degree, can also take place from véraison to harvest.
Generally, there is a decline in seed tannins during ripening that accompanies seed browning, possibly due to tannin oxidation.
Skin anthocyanins, which show little turnover once formed, appear to behave like typical end products. Acquisition of the red/blue colour of red varieties along ripening is a visual indicator of the biochemical processes that occur in grape berries. This reflects more precisely the accumulation of anthocyanin pigments in the vacuoles of skin cells, absent in the white grape varieties.

References:
1. Carlos Conde, Paulo Silva, Natacha Fontes, Alberto C. P. Dias, Rui M. Tavares, Maria J. Sousa, Alice Agasse, Serge Delrot, Hernâni Gerós, Biochemical changes throughout Grape Berry development and fruit and wine quality, Food, 1(1), 1-22 ©2007 Global Science Books.
2. Yung-Fen Huang, Sandrine Vialet, Jean-Luc Guiraud, Laurent Torregrosa, Yves Bertrand,Veronique Cheynier, Patrice This and Nancy Terrier, A negative MYB regulator of proanthocyanidin accumulation, identified through expression quantitative locus mapping in the grape berry, New Phytologist, (2014) 201: 795–809 doi: 10.1111/nph.12557.
The illustration above is a composite from reference 1 (Pentose Phosphate Pathway and Glycolysis) and 2 (General Phenylpropanoid/flavonoid Pathway).

Phenolics in Grape Berry and Wine

Grape Phenolics
Many phenolic compounds are involved in plant protection as biologically active growth inhibitors of other living systems. They have strong antioxidant activities and play important beneficial roles in the mammalian systems.
Catechins, tannins, and anthocyanins are the most concentrated natural antioxidants present in red grapes and wine.
The berry skin contains tannins and pigments, the pulp contains juice but no pigments, and the seeds contain tannins. The insoluble cutin of the epidermis and the insoluble lignin of the hard seed coat are phenolics, which are as important as the skin tannins and pigments and seed tannins but only the soluble phenolics of the grape berry are important in winemaking.
Phenolic compounds of the grape are divided between the non-flavonoids and the flavonoids group:

Nonflavonoid phenolics (with a simple C6 backbone) are found in grapes and wine, with the exception of hydroxycinnamic acids, they are present at low concentrations

• hydroxycinnamates are present in hypodermal cells along with tannins and anthocyanins and in mesocarp and placental cells of the pulp
• hydroxycinnamates are the third most abundant class of soluble phenolics in grape berries, after tannins and anthocyanins
• caftaric acid is the most abundant hydroxycinnamate in free-run juice of white grapes and consequently in white wines

Flavonoids make up a significant portion of the phenolic material in grapes and include several classes:

• Flavan-3-ol monomers (catechins)
• flavan-3-ol monomers (catechins) are responsible for bitterness in wine and may also have some associated astringency
• the major flavan-3-ol monomers found in grapes and wine include (+)-catechin, (-)-epicatechin, and (-)-epi-catechin-3-O-gallate
• much of the flavan-3-ol monomers originate from seed material, although they are also present in grape skin hypodermal cells
• Proanthocyanidins (tannins)
• tannins or proanthocyanidins are polymers of flavan-3-ols and are the most abundant class of soluble polyphenolics in grape berries
• tannins confer astringency to red wines and are extracted from the hypodermal layers of the skin and the soft parenchyma of the seed between the cuticle and the hard seed coat, as well as, from the peduncle of the grape berry
• tannins are a very diverse set of biomolecules varying in size from dimers and trimers up to oligomers with more than 30 subunits
• Anthocyanins
• anthocyanins are responsible for red wine colour and are co-located with tannins in the thick-walled hypodermal cells of skin
• anthocyanins are anthocyanidins covalently associated with one or more sugar molecules
• anthocyanins can be esterificated by acids, such as acetic and coumaric acid, and substituted with hydroxyl or methyl groups that will confer a specific hue at light
• malvidin-3-O-glucoside and its acylated forms is the major anthocyanin present in grapes

References:
1. Carlos Conde, Paulo Silva, Natacha Fontes, Alberto C. P. Dias, Rui M. Tavares, Maria J. Sousa, Alice Agasse, Serge Delrot, Hernâni Gerós, Biochemical changes throughout Grape Berry development and fruit and wine quality, Food, 1(1), 1-22 ©2007 Global Science Books.
All structures were drawn by the freely available drawing program from ACD Labs called ACD/ChemSketch Freeware.

Vintage 2020: May Cold Temperatures

This weekend, we were crossing our fingers due to the unusually cold temperatures we were expected to have on Saturday morning and Sunday morning. My husband has a HOBO data logger and we went to the vineyard on Saturday and downloaded the temperatures as well as check on the buds that were coming out of dormancy and potentially susceptible to cold temperatures.
The HOBO showed that during the morning hours on May 9th, the temperature hovered between 30 - 32^o F between 2:30 and 6:00 a.m. The young buds seemed to have faired okay on Saturday morning, but there was still Sunday morning's forecasted cold temperatures to come. We did the same exercise on Sunday afternoon, downloaded the HOBO data and took photos of our various buds. My husband told me that on May 9th, from 8 p.m. into the morning hours of Sunday, May 10th, the temperatures varied between 34 and 32^o F, until dawn, when at 7:20 the temperatures climbed to 45^o F.
Our tender buds seemed to have withstood the cold onslaught.
Here are some photos taken on May 10th:

Variety	Budbreak Stage
Auxerrois
Chenin Blanc
Chardonnay
Cabernet Sauvignon
Cabernet Franc
Barbera

We were relieved that our young buds survived Mother Nature's late winter blast and then wondered what freeze does to a grapevine. One of the sources that I refer to time and again when there are freezing conditions is this: Understanding and Preventing Freeze Damage in Vineyards ~ Workshop Proceedings ~ December 5-6, 2007 University of Missouri-Columbia. On the right, you can see what happens when a freeze occurs during budbreak.
The vine was at E-L stage 7 which is described as "first leaf separated from shoot tip". The photo shows that the apical bud was affected but the next bud survived. Unfortunately, there was no information regarding what day the photo was taken and the temperatures that caused the bud to freeze.
Most of the information on freezing, refers to winter freeze in grape vines and I have written a few blogposts on that:

• March 9, 2018: Freezing Conditions During Spring Grapevine Deacclimation
• April 24, 2015 Spring Bud Deacclimation and Cold Weather
• February 6, 2015 Grapevine Cold Hardiness
• February 4, 2014 Cold Hardiness of Grapevines Through Cold Acclimation
• November 11, 2012 Winter Injury to Grapevines and Methods of Protection
• November 10, 2011 Cold Tolerance and Grapevine Phenology

We have two more days with low 30's morning temperature, so fingers still crossed!

Nitrogen in Grape Berry and Wine

Nitrogen in the Growing Grape Vine
The importance of nitrogen in the growing vine:

• lack of nitrogen can halt growth and development of the vine
• an excess can increase the vigour, prolong growth, delay maturation
• an excess can favour the development of mildew and rot and decrease the level of anthocyanins and tannins in the berry
• an excess is detrimental to sugar accumulation in the berries during ripening

Mineral nitrogen in the form of NH4+ can represent up to 80% of the total nitrogen before véraison but it decreases to 5-10% after maturation and decreases even further after fermentation of the must .
Nitrogen in the Must and Wine

• total nitrogen in the must can vary from 100 to 1200 mg/L, and usually red wines possess higher nitrogen content than white wines.
• nitrogen in the must is called fermentable nitrogen and used by yeast to carry on normal alcoholic fermentation
• total amino acid content can vary widely, from 300 to 5000 mg/L, and represents about 20-50% of the total nitrogen in the must
• the must contains about thirty amino acids, but only around seven are present in quantities above 100 mg/L: proline, arginine, glutamine, alanine, glutamate, serine and threonine.

• when fermentable nitrogen is below 150-200 mg/L, ammonium (in the form of phosphate, sulphate or sulphite salts) is added to the must to avoid “stuck” fermentations and formation of hydrogen sulfide and other sulphur odors
• ammonium consumption by yeasts results in a greater acidification of the media than amino acids consumption
• wine is generally less rich in amino acids than the initial must it is derived from
• by-products of yeast amino acid metabolism include precursors for the synthesis of aromatic compounds, such as isoamyl acetate, isovaleric acid and isobutyric acid and their ethyl esters, as well fusel alcohols and methionol
• urea originates from arginine and is a precursor of ethyl carbamate, a known carcinogen
• wine also contains biogenic amines, such as histamine, tyramine, putrescine, cadaverine, spermidine and spermine, which are formed mainly during the malolactic fermentation from amino acids present in the must
• biogenic amines can cause problems for some consumers: histamine can cause headaches, hypertension and digestive problems while tyramine can be associated with migraine and hypertension
Proteins:

• amino acids can also be present in polymerized form, such as small oligopeptides of 2-4 amino acids to large proteins of up to 150,000 Da in size
• proteins are present in widely varying quantities in the must and wines
• proteins are present in concentrations ranging from 15 to 230 mg/L
• proteins can also contribute to the development of problems in the fermentation of white wines due to exposure to cold, heat or cold-heat temperature variations
• proteins are are largely absent in red wines due to the precipitation of proteins with tannins
• the organoleptic effect of proteins on wine aroma can be considered negligible
• the protein content is also of economical importance because it greatly affects the clarity (translucency) of the wine
• proteins are usually precipitated from the wine by addition of bentonite

Other blogposts that I've written about nitrogen and proteins:

• The Importance of Nitrogen in Wine Fermentation
• Grape Berry Composition at Harvest

References:
1. Carlos Conde, Paulo Silva, Natacha Fontes, Alberto C. P. Dias, Rui M. Tavares, Maria J. Sousa, Alice Agasse, Serge Delrot, Hernâni Gerós, Biochemical changes throughout Grape Berry development and fruit and wine quality, Food, 1(1), 1-22 ©2007 Global Science Books.

The State of Our Grape Buds

April started off with many warm days and we were anticipating an early bud break but the latter half of April brought cooler weather which is still the case in May. In fact, this coming Saturday, we are forecasted to have low temperatures in the high 30's. So, what's a bud to do?
Here is where the buds of our various varieties are as of yesterday:

Variety	Budbreak Stage
Auxerrois
Chenin Blanc
Chardonnay
Cabernet Sauvignon
Cabernet Franc
Barbera

I compared the stage of our buds this year with the blogpost I wrote in 2019 Vintage 2019: May Budbreak in our Vineyard, showing that we are clearly behind the stage we were at last year.
According to the Modified Eichhorn Lorenz (E-L) System, our buds are currently between the E-L stage 3 wooly bud +/- green showing and stage 4 bud burst leaf tips visible.

Of all the varieties that we have planted in the vineyard, the Cabernet Sauvignon is the farthest behind, which is what we saw last year. This year, we have our first Barbera that we have trained in a single guyot and the buds look pretty far along!

Organic Acids in Wine

At harvest time, the main acids found in the berries are tartaric and malic, and, in minor amounts, citric acid. Succinic, lactic and acetic acids present in wine mainly result from alcoholic or malolatic fermentation. The acid content of grapes should fall in the range from 0.65 to 0.85 g/100 ml (%).
The pH of wine grapes is one of the most important and controversial quality parameters for grape quality and the wine-making industry. The pH value of the wine reflects the amount and the strength of the acids, and the effects of minerals and other materials present in the wine.
Wine pH depends upon three major factors:

• the total amount of acid present
• the ratio of malic acid to tartaric acid
• the quantity of potassium present

The importance of acidity in a wine:

• a wine too low in acid will taste flat and dull
• a wine too high in acid will taste too tart and sour
• acidity allows wine to maintain its freshness
• acidity has an impact on flavour components and color
• acid content in the berries and then in the must affect vinification and wine stability
• high acidity (pH 3-4) avoids the development of contaminating bacteria

The contribution of the various acids in wine:

• tartaric acid contributes the tart taste of wine, as well as to the biological stability and the longevity of wine
• malic acid is the most fragile wine acid and confers a green taste to the fruit and to the wine
• a decrease in malic acid content is achieved during malolactic fermentation (ML), when bacteria convert malic acid into lactic acid
• present in relatively small amounts, citric acid confers a fresh and slightly acid taste to wines
• the flavour of succinic acid is a complex mixture of sour, salty and bitter tastes
• lactic acid in wines confers a slightly sourish taste as it does in yogurts
• the high volatility of acetic acid makes wines less appealing to consumers, with unpleasant aroma and palate
• vinegar bacteria (Acetobacter) can produce large quantities of acetic acid from ethyl alcohol by an oxidation process, in the presence of large quantities of air

References:
1. Carlos Conde, Paulo Silva, Natacha Fontes, Alberto C. P. Dias, Rui M. Tavares, Maria J. Sousa, Alice Agasse, Serge Delrot, Hernâni Gerós, Biochemical changes throughout Grape Berry development and fruit and wine quality, Food, 1(1), 1-22 ©2007 Global Science Books.