Jorman Heflin | 08-21-2020
Tags: chalk talk
An explanation of the chemicals in grape berries that stimulate the senses.
There are many components that make up the flavors, colors and aromas of grape berries. To keep this post simple, we will not touch on the ways that sugar and minerals (K, N, Ca, P Mg, S) influence berry flavor or color.
Acids create the sour ‘zing’ in wines that balances residual sugar. There are 23 different organic acids found in grape berries. These acids lower berry pH, which helps to reduce microbial spoilage and oxidation in the grape growing and winemaking processes. The two main acids in berries are tartaric and malic. Both of these are synthesized in the berry, and together they account for about 90% of total berry acidity.
Tartaric acid is 3x more acidic than malic acid, making it the main compound that affects berry pH. It tastes mineral and citrus-like, and is sometimes added artificially during winemaking to control wine pH. Synthesis of tartaric acid occurs mainly during the first 28 days after anthesis, and once formed, it is resistant to degradation. As berries increase in size after veraison, the tartaric acid gets diluted down to a final concentration of about 7.5g/L in grape juice. Light and temperature changes have little effect on tartaric acid levels in ripe berries.
Malic acid has a harsh, metallic, green apple taste. Winemakers sometimes use malolactic fermentation to convert malic acid into lactic acid, which is softer on the palate1. Unlike tartaric acid, a rapid breakdown of malic acid occurs around veraison. This breakdown is achieved mainly via gluconeogenesis and respiration. In hotter climates, berries respire more, and more malic acid is broken down. This will be the most immediate negative impact of climate change on grape composition. The final concentration of malic acid in berry juice is 1-9g/L depending on the variety.
Phenolics come from the metabolism of carbohydrates (like sucrose) in the berry. These carbohydrates are made into the amino acid phenylalanine via the shikimate pathway, which is then made into phenolic compounds via the phenylpropanoid and flavonoid pathways. Flavonoids are the group of phenolic compounds made via the flavonoid pathway2. Flavonoids make up the main flavor and color compounds in grape berries. The flavonoids are divided into three structural classes, flavonols, flavan-3-ols and anthocyanins3.
Flavanols are synthesized only in berry skin, and protect berries from damaging UV rays. They are yellow pigments that contribute to the color of white wines, and in reds they conjugate to anthocyanins (copigmentation), reinforcing the stability of red wine color. Flavanol levels peak a few weeks after veraison, but their levels vary wildly depending on variety, ranging from 1-80mg/kg of berry fresh weight. Flavanols make up 1-10% of total berry flavonoids, and their levels increase with sun exposure.
The majority of flavan-3-ols bond together via interflavan bonds to form polymeric flavan-3-ols, also known as proanthocyanidins. Proanthocyanidins are the most abundant class of phenolic compounds in grape berries. They make up about 3g/kg of berry fresh weight, and accumulate mostly in berry seeds and skin. These polymers come in many shapes and sizes, depending on the number and type of flavan-3-ol subunits4. Synthesis of these compounds is completed by veraison, and they are not easily broken down. Proanthocyanidins form complexes with polysaccharides in the berries, and these complexes give wines silky mouth-feel and smooth, round sensory attributes. For this reason, proanthocyanidins are also often called condensed tannins. Heat increases the polymerization rate of flavan-3-ols thereby increasing the concentration of proanthocyanidins in berries. This makes hot climates conducive to smoother tannin profiles. Like flavanols, proanthocyanidins also conjugate to anthocyanins (copigmentation), reinforcing the stability of wine color.
Flavan-3-ols not polymerized into proanthocyanidins are converted into anthocyanidins by the enzyme anthocyanidin synthase. These anthocyanidins are then stabilized by the addition of a glucose molecule (glycosylation) to form anthocyanins. Anthocyanins are responsible for the red, blue and purple pigmentation of grape berries. Each anthocyanin has a unique absorbance spectrum which gives the compound a particular hue ranging from red to blue5. Which particular anthocyanins are present in a berry is variety specific, but anthocyanin synthesis requires cool nights (15°C) and warm days (25°C). Temperatures over 30°C degrade anthocyanin concentration in ripe berries at a rate of about 7%/day. Post-veraison sun exposure also induces anthocyanin synthesis. Anthocyanins accumulate in the skins of red grapes from veraison until maturity. These compounds range from 26-6300mg/kg of berry fresh weight.
Aroma compounds: Methoxypyrazines, Terpenes, Thiols
Grape berries accumulate mostly non-volatile aroma precursor molecules, that become volatile during alcoholic fermentation and wine aging. There can be several hundred aroma compounds in the headspace of a wineglass, and they represent a large diversity of flavors. These compounds are teased apart from the wine in labs using gas chromatography. Compound characteristics and chemical makeup are then identified using mass spectrometry and olfactometry. Grape aroma potential is highest in vines with cluster sun exposure, moderate water deficit and non-excessive nitrogen supply. Here we will briefly discuss the most important groups of aroma compounds.
Methoxypyrazines give wines a vegetable-like odor. Think pea pods, green pepper and earthy tones. The methoxypyrazine IBMP (IBMP) is the most abundant odor compound in grapes, grape juice and wine6. It contributes an undesirable, herbaceous aroma to wines. IBMP is stable, and not able to be oxidized, but, the settling of must during fermentation (whites/rose) and thermovinification (reds) can significantly reduce the amount of IBMP in wines. In grapes, IBMP levels increase until several weeks before veraison, and then decrease steadily during ripening. It accumulates in berry skins and seeds. IBMP is degraded in UV light, so sun exposure may reduce the levels of this compound in berries. IBMP is present 0.5-100ng/L range in grape juice. If not removed during vinification, it is thought to have a negative effect on wine flavor at around 15ng/L.
There are over forty monoterpene compounds present in grape berry skins. The important ones are linalool, geraniol, citronellol, hotrienol, rose oxide and nerol. These monoterpene alcohols and oxides play a major role in contributing to the floral aromas found in varieties like Muscat. These chemicals generally have very low detection thresholds in the ~10-200ug/L range. Many of these compounds may be transformed into other monoterpenes (chemical rearrangement) or degraded into non-volatile compounds during fermentation and wine aging. This is why the floral notes in Muscat wines often decrease after 2-3 years of aging. Monoterpene synthesis begins at berry set, but it is not known how long these compounds continue to be made after veraison. Sun exposure has been shown to increase volatile monoterpene levels in berries.
Sesquiterpenoids are terpenes produced as secondary metabolites in grape berries, and are usually not present in high enough concentrations to contribute to grape juice aromas. One exception may be rotundone, which contributes a black pepper aroma to Syrah wine at around 50-600ng/L.
C13-Norisoprenoid terpenes arise in berry skins from the oxidative degradation of carotenoids. They contribute a wide variety of aromas and are separated into three groups: megastigmane (ex. β-damascenone, apple sauce, tropical fruit and β-ionone, violet), non-oxygenated megastigmane (ex. TPB, geranium leaf) and non-megastigmane (ex. TDN, kerosene). Most of these compounds have detection thresholds in the ng/L range. TDN has a higher detection threshold of about 20ug/L. Non-oxygenated megastigmanes and non- megastigmanes are not present in ripe berries, and are formed only during wine aging.
Adding one or more thiol groups (-SH) to a molecule can convert it into a highly-potent aroma compound. This change often occurs when precursor molecules present in the grape skins are thiolated during fermentation. Accumulation of these precursors can be maximized with moderate water stress after veraison, and moderate (non-excessive) nitrogen supply. Sulfur compounds like thiols often contribute to olfactory defects in wine, but compounds in the thiol family also make great contributions to the fruity aromas of wine. Some important thiol compounds in wine are: 3SH (passion fruit, grapefruit), 3SHA (boxwood, passion fruit), 4MSP (boxwood, broom), 4MSPOH (citrus zest), BM (gunflint, smoke), 3MSB (cooked leeks), 2M3F (meaty), 2FM (coffee), 3SP (citrus), 3SHp (grapefruit) and 2M3SB (raw onion). Like terpenes, most of these compounds have a detection threshold in the ng/L range.
Beware, lactic acid is a weaker acid than malic acid, and the resultant rise in pH during malolactic fermentation increases the risks of microbial spoilage and oxidation.
All flavonoids are constructed from tetrahydroxychalcone which is synthesized by the enzyme chalcone synthase.
Flavonoids are C6-C3-C6 polyphenolic compounds, and are classified into these three groups according to oxidation state of the interconnecting C3 ring.
There are five flavan-3-ols in berries: catechin, its isomer epicatechin, gallocatechin, epigallocatechin and epicatechin-3-O-gallate.
There are six anthocyanins that occur in grape berries, cyanidin (red), peonidin (blue), malvidin (blue), delphinidin (blue), petunidin, and pelargonidin. Pelargonidin only occurs in trace amounts, and only in the varieties Pinot Noir and Cabernet Sauvignon.