A team led by Lingjun Ma and Andrew Waterhouse of the University of California-Davis examined the phenomenon of wine-aging by analyzing the tannin profile of three wines produced using different amounts of bisulfite.
The research, published in the Journal of Agricultural and Food Chemistry, suggests that wine indeed improves with age.
Humans have appreciated the benefits of wine-aging since the days of the ancient Greeks, who produced straw wine that was capable of aging due to its high sugar content. The early Romans prized Falernian and Surrentine because these wines could be stored for decades. Even the Bible mentions the superiority of aged wine -- in Luke 5:39, it reads: “And no one after drinking old wine desires new, for he says, ‘The old is good.’”
Wine tastes better with age because of a complex chemical reaction occurring among sugars, acids, and certain substances known as phenolic compounds. In time, this chemical reaction affects the taste of wine in a way that gives it a pleasing flavor. The chemical reaction can also change the color and aroma of the spirit, in addition to altering the way wines feel in the mouth. Examples of wines with high phenol levels are Cabernet Sauvignon, Nebbiolo, and Syrah.
How Does Wine Taste Better with Age?
One group of phenolic compounds, tannins, has particular importance in determining how wines taste after aging. Chemically, tannins are large molecules made up of linked subunits. Molecules such as tannins are known as polymers, with the subunits termed 'monomers.'
The monomers in wines are phenolic compounds that are joined together in a bewildering array of combinations and can be further chemically modified in a myriad of different permutations.
Tannins, found in the stems, seeds, and skins of the grape, give the wine its dry texture. Both white and red wine contain tannins, but the reds contain significantly more of this phenolic compound.
White wines get a small number of tannins from the type of grapes, and also from being aged in wooden barrels. In addition, white wines have natural acidity that helps improve their own flavor over time. Wines with a low pH, such as Pinot Noir and Sangiovese, are more capable of tasting better with age than the less acidic wines.
The research study, led by Lingjun Ma and team, suggests that during the process of aging, individual tannin molecules no longer bind together because of a chemical called bisulfite (HSO3-) that is added during the winemaking process as a preservative.
Bisulfite ions presumably attack the polymerized tannins, breaking them apart and creating a byproduct - a sulfonated tannin. This sulfonated tannin gives aged wine its smooth taste. The number of overall tannins was seen to dramatically decrease as the bisulfite increased.
The molecules on the left represent a polymerized tannin, while the molecules on the right represent the breakdown products following a reaction with bisulfite. (Source: Acsh.org)
How Wine Color Changes with Age
One of the most visible processes in an evolving wine is the slow oxidation. Color is the most obvious indicator of this.
As white wines age, they often evolve from pale lemon or golden to amber and even brown. Vivid salmon-hued rosés can take on onion skin tones as they age. As reds develop, oxidation often moves these wines from the purple end of the spectrum toward tawny or brown hues. While young reds can be opaque when held against a white background, mature reds often show a lighter color around the edges. This phenomenon is known as “rim.”
The rate of oxidation depends on the amount of air left in the neck of the bottle after it was sealed, and how permeable the closure is. Traditionally, natural cork has allowed minimal oxygen exchange, which is why most wines deemed 'age-worthy' are still bottled under cork. However, since cork is a natural product, there is no such thing as uniformity. This can cause considerable bottle variation within the same case of wines.
Meanwhile, sophisticated synthetic closures like Nomacorc mimic this oxygen exchange in a more predictable fashion. Even the liners of screw caps allow for a certain amount of oxygen exchange, and it is perfectly possible to age and cellar these wines.
Color changes in wine with age. (Source: www.winemag.com)
Researchers are now beginning to understand the nature of color in red wines, and the emerging picture is challenging traditional understanding in this area.
Color in red wines actually falls into three categories. First, we have the anthocyanins, the primary pool of color from the grape. Young wines are packed with anthocyanins, which are very reactive: they interact with both sulfur dioxide and oxygen, which bleaches them.
Color of wines is also influenced by pH. At lower (more acidic) pHs, wines are redder; at higher (less acidic) pHs, they appear bluer. It turns out that anthocyanins are unstable, and are not that important for the long-term color of red wines.
In addition to anthocyanins, there are two major fermentation-derived color groups:
The first of these is the pigmented polymers. They are formed by the chemical linkage between tannins and anthocyanins. The polymers are covalently (strong) linked and are very important in forming stable color in wines. Evidence suggests that most of the pigmented-polymer formation occurs during fermentation.
The second (third including the anthocyanins group) group is called the anthocyanin-derived pigments, which arise from reactions between anthocyanins and other phenolics and aldehydes. This group is a massive, complicated class of non-bleachable pigments and an area of intense current research with new members being added all the time. The anthocyanin-derived pigments are still quite reactive, and they can further form combinations with tannins to form pigmented polymers.
Certain Phenomena - Co-Pigmentation - Need More Research
Currently, there is also interest in a phenomenon known as co-pigmentation.
This is the stable combination of anthocyanins with phenolic ‘co-pigments,’ which are colorless molecules combined with the anthocyanins to increase color intensity. It is a very intricate phenomenon that has still not been fully understood but forms the basis for co-fermenting small proportions of white grapes, such as Viognier, which are rich in co-pigments, and with red grapes, most particularly Shiraz.
Co-pigmentation is becoming increasingly trendy in Australia. Although, since it is difficult to achieve, some winemakers miss the point by combining portions of white and red wine after fermentation where the window for co-pigmentation may have passed.
Top Image: Different types of wines. (Source: WTSO)
Lingjun M.A. et al. (2018), ‘Condensed Tannin Reacts with SO2 during Wine Aging, Yielding Flavan-3-ol Sulfonates,’ J. Agric. Food Chem, 66 (35), Pp 9259-9268
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