The evolotion of components during red-wine aging

According to winecurmudgeon.com, "as much as 90% of the wine bought in the U.S. is drunk within 24 hours" of purchase and "some 95 percent of all wine purchased in the U.S. is consumed within a week." And that result maps closely to the production dynamic as, according to Jancis Robinson MW, only small percentages of red and white wines produced can improve such that they provide greater drinking enjoyment at 5 years of age than they did at 1 year of age.

For those wines with aging potential, complex chemical reactions between their components -- and over an extended period -- will alter the aroma, color, mouthfeel, and taste such that they become more pleasing to the taster. Table 1 below captures the factors which contribute to the ageability of wine.

Table 1. Factors contributing to the Ageability of Wines

Factor	Selected Potential Impacts
Grape Variety	Tannin and acidity are great preservatives; varieties with medium-to-high levels of one or both of these components are good candidates for aging
Vintage	High-heat vintages may result in low acid levels and damaged skins Too-wet vintages introduce disease risk and increases the ratio of water to acid/phenolics
Viticultural Practices	Overly long hang times may negatively affect acidity levels Picking too early may yield phonetically unripe grapes
Wine Region	Hot climates yield lower-acid fruit
Winemaking Style	Malolactic fermentation  on a low-acid wine will lead to even lower acid levels Shorter post-fermentation maceration will yield lower tannin levels and the potential aging time of the wines
Bottle Storage	Poor storage conditions will negatively affect the life and quality of the aged wine

In the foregoing table, the role of acidity and tannins in preserving the wine against attack by microbes is highlighted but the ratio of these two components, plus sugar, to water is a key determinant as to how well a wine will age. The more diluted these components, the less well the wine will age. In addition, maceration length, and, hence, the increased volume of phenolic extraction, will improve the ageability of the wine. Acid and tannin levels of major grape varieties are provided in the figure below.

While a lot of attention on aging is focused on residence in the buyer's cellar, the aging process actually begins in the winemaker's cellar. The figure below is a block diagram which provides an overarching view of the aging process.

Table 2 below summarizes the changes that each of the wine components undergo from the freshly fermented stage until the bottle is opened many years down the road.

Table 2. Summary of the changes that wine components undergo during aging.

Component	Young Red Wine	Maturation	Cellar Aging
Color	Inky dark color due to monomeric anthocyanin pigments extracted from skin	Monomers replaced by polymeric form (anthocyanin pigments + tannins)	Color loss due to continued polymerization and precipitation of tannin-anthocyanin complexes     - Light brick red after 5 - 10 years      - Light orange-red color further out
Aroma	Varietal, grape, and yeasty aromas Alcohols, esters, fatty acids, aldehydes, and ketones	Acquisition of toasted aromas to include     - Smoky     - Spicy     - Cocoa     - Vanilla     - Roasted coffee     - Toasted bread	Loss of certain grape and yeasty aromas retention of varietal aromas  Formation of new aromas (Savory, for example Synthesis of new esters Integration of all flavors to produce a harmonious and pleasing fragrance
Alcohol	Amount of alcohol in wine = ºBrix x ).55 Residual sugar should range between 0.2 - 0.3 g/l	- Oxidized to acetaldehyde
Acidity	- Red wines should range between 0.6 and 0.8% TA	- Malolactic fermentation changes malic acid to the softer lactic acid	Acid precipitation Ester formation     - Loss of acidity makes wine taste less astringent and mellower
Bitterness	Grape tannins are largely responsible for the bitter taste in wine	Oxidative and non-oxidative polymerization of phenolic compounds     - Reduced astringency     - Smoother, softer taste	Flavonoid phenols polymerize and become less bitter, more astringent Further polymerization leads to eventual precipitation     - Reduction in phenolic compounds     - reduction in astringency
Astringency	- Grape tannins are responsible for the astringency in wine	- Conversely, astringency is increased by the phenolic products absorbed into the wine from barrel - Astringency can be reduced by fining	Loss of astringency due to polymerization and precipitation of tannins     - Wine becomes mellower and smoother

Wine Maturation
Wine is matured in wooden barrels to: (i) enhance its flavor, aroma, and complexity through transfer of substances from the wood to the wine; and (ii) allow gradual oxidation of the wine.

In the first instance, many of the wood's native aromatic compounds, as well as the aromatic compounds created during seasoning and toasting, are absorbed, and integrated, into the wine, thus contributing to wine richness and aromatic complexity.  For example, hemicellulose will hydrolyze upon exposure to wine, creating, as a result, sugars and acetyl groups.  The sugars are further converted to furanaldehydes and ketones while the acetyl groups are converted to acetic acid during maturation.  A small proportion of lignin will dissolve in wine (these are called native lignins) while some undergo ethanolysis and are oxidized to aromatic compounds.  These compounds have low olfactory thresholds and will, therefore, impact the wine's aromatic profile. As noted by Dr. Murli Dharmadikari, common descriptors of oak-aged wines are oaky, vanilla, smoky, toasty, spicy, and coconut.

In terms of gradual oxidation, wine loss from barrels amount to approximately 2% per year, resulting from the fact that water and ethanol are smaller molecules and will diffuse into the wood and, ultimately, escape as vapor.  If the air in the cellar is dry, more water is lost and the wine is more concentrated in terms of alcohol.  If the environment is too humid then more alcohol is lost, reducing the ethanol content in the remaining wine.  This loss of liquid opens up a space between the wine surface and the barrel which the winemaker generally "tops up" in order to prevent oxidation and acetic spoilage.  During this "topping-up" process, small amounts of oxygen are dissolved in the wine.  Oxygen is also introduced into the wine during winery operations such as filtering and racking.

The oxygen which is now in the wine reacts with resident phenolic compounds in a manner such that: (i) tannins are softened (polymerization and precipitation as well as tannin-polysaccharide combinations); (ii) complex aromas develop; and (iii) there is improvement in the wine's body and mouthfeel.

In a Herjavec, et al., study, the authors found that the sensorial characteristics of barrel-aged wines were modified, due to the wood-derived compounds. These wines manifested roundness in taste with a complex retro nasal aroma." Barrel toast also affected flavor perception: aging in medium-toast barrels yielded a smoky, roasted, and raw oak flavor while light toast resulted in a more fruity aroma.

Cellar Aging
As the wine continues to age under ideal conditions, chemical oxidation and polymerization and precipitation will result in a continued loss of astringency and color. The loss of astringency results in a mellower wine while the loss of color yields a light orange-red colored wine.

The perception of acidity will decrease -- and with it the perception of astringency -- due to formation of esters resulting from chemical interactions between acids and alcohol and some precipitation of acids.

Beatty discusses the formation of savory aromas (beef broth, cured, smoked and grilled meats, mushrooms, vegetables and herbs, roasted nuts, caramel, and soy sauce) resulting from Maillard reactions ("a series of chemical reactions between reduced sugars and compounds with free ammonia acids") which can occur under wine-like conditions (relatively low pH, relatively low storage temperatures, an aqueous solution) at very slow reaction rates to "produce savory aromas over the course of years or even decades of aging."

And finally, the aged wine will exhibit a bouquet that is more developed and layered and should also show a lengthened finish.

Bibliography
Alyssa Mae Beatty, Characterization of "savory" aroma compounds in aged red wines via gas chromatography-olfactometry and descriptive analysis. Dissertation, Purdue University.
Seth Cohen, Managing Wine Aroma, Presentation, North Carolina Wine Growers Association, 2/5/2012.
Dr. Murli Dharmadhikari, Wine Aging, extension.iastate.edu
Denis Dubourdieu and Takatasgi Tominaga, Evolution of Toasted Aromas  in red wine during barrel maturation.


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