I was asked whether there's any truth to the assertion that high alcohol wines don't age very well. To summarize what we know about ageworthiness, we should recognize two distinct realms where observations have been made. One is that, on the whole, wines with high alcohol tend to have higher overall ripeness. There is much experience and scientific understanding to verify that these wines age poorly. But this tendency is not universal, because brix and ripeness are only loosely connected. Secondly, we have a small body of experience and research which suggests that alcohol itself somewhat hastens ageing in otherwise identical wines.
A key distinction must be made between a wine's alcohol level and the state of ripeness of the grapes, which leads to the compositional status of the resulting wine. Alcohol level is closely related to the brix at harvest unless the wine has been adjusted: add any additional alcohol from chaptalization or fortification, minus any alcohol which may have been removed by the various existing mechanical processes such as reverse osmosis or spinning cone.
Fruit ripeness is only loosely related to the grape's sugar content. Depending on the climate in which grapes are grown, and particularly the weather at harvest, grapes may reach the same state of compositional ripeness (color, flavor, tannin status) at anywhere from 20 to 30 degrees brix. The low numbers are common in France, and in fact in many parts of Germany, 20 brix is considered late harvest for riesling. In California, which lacks autumnal rainfall, riesling often doesn't reach the same degree of ripeness until the high 20's, and cabernet is typically also picked between 25 and 27 brix. But in a cool year like 1999 or 2005, much California fruit got overripe by hanging too long while winemakers were waiting for these numbers.
Overripe red wines lack fresh aromas, have low reductive strength, develop pruney aromas and fail to age well. In technical terms, the degree of oxidative polymerization of the tannins has proceeded on the vine to an excessive extent. It might be said that the wine has run down the chemical battery it normally uses to protect itself from oxygen during ageing in barrel and in bottle and to defend itself from oxygen-loving microbes like Acetobacter, or vinegar bacteria. High ripeness also tends to be associated with high pH, which is the "gas pedal" of ageing, and controls the rate of oxidation of many wine constituents. Such "over the hill" wines tend to brown early and their tannins dry out -- that is, they become grainy and move from the top of the tongue to under the tongue and into the cheeks, giving a dirty impression which obscures flavor perception. The tannins have essentially curdled, and just like a botched bearnaise sauce, they fail to integrate aromas. As a consequence these wines show, along with oxidative notes of caramel and prune, also disjointed aromas of oak, vegetal notes and microbial smells, which then protrude in the nose in unpleasant disarray.
On the other hand, sometimes grapes achieve very high brix without these problems. A case in point is the 1999 CSU Fresno Syrah in our study, which came in on September 17th at 31 brix, but possessed fresh blueberry aromas and fine, firm tannin, and is still drinking quite well today. So high alcohol per se does not necessarily indicate overripeness. This wine was, however, quite hot on the palate, and the high level of alcohol caused a bitterness in the finish and also exacerbated the astringency of the tannins. When we adjusted the alcohol down to the normal range, these imbalances disappeared, and the wine did well in competitions and aged well.
Not so the unadjusted 18% wine, which more rapidly developed raisiny notes, browning, and oxidation. A dozen or so trials over the years have shown us that high alcohol wines develop differently than their counterparts in which we have reduced the alcohol, wines with exactly the same composition of color, flavor and tannin. We often see zinfandels developing raisiny notes more rapidly at higher alcohol, for example.
We don't know why this is happening. A possible explanation may stem from the fact that at higher alcohol, wine has a greatly diminished dialectic constant -- the driving force of water to "herd" phenolics into the colloids. These tiny tarry bead-like gobs hold most of the color and tannin in red wine. They also provide a protective zone for many aromatic compounds to insinuate themselves inside. I can only speculate that the decreased stability of these colloids at higher alcohol exposes these hidden aromatics to oxidative attack.