Nonoxidative polymers have a soft, nonintrusive mouthfeel in young wine but tend to continue growing until they become harsh and eventually insoluble, falling out of the wine. We don’t like these polymers.
Fine colloidal structure depends on the promotion of early polymerization while at the same time preventing it from getting out of hand. It turns out that the key to good structure is a good concentration of red anthocyanin pigment. Color caps off tannins, leading to wines with more finesse. In effect, the more color that is present, the shorter the resulting polymers and the finer the colloids (figs. 2 and 3). Driven together by the polarity of water, these chains aggregate into colloids whose size is related to the chain length of its constituents.
If oxygen is delivered to a young red wine, a different kind of polymer results that is more expanded. In much the same way a wire whisk creates meringue from egg whites, skillful introduction of oxygen to young red wine creates a mouth-filling, light structure that is stable and can form a foundation for soulfulness and graceful longevity. That’s why the Aztecs taught the the Spanish explorer Cortés the use of oxygen (“conching”) to convert cocoa powder into chocolate, still a standard practice in the finest Belgian shops (yes, that chocolate waterfall in Willy Wonka’s Chocolate Factory really exists!).
In red wines, prompt action is critical, because color molecules (anthocyanins) are easily lost to precipitation, yeast adsorption, and enzymatic attack. Successful oxidative structuring is best begun within days of the completion of alcoholic fermentation, sometimes even under the cap.
FIGURE 2. Polymerization with poor color.
FIGURE 3. Polymerization with good color. High anthocyanin ratios result inshort, soft, stable oligomers.
The mechanism of oxidative polymerization was elucidated in 1987 by Vern Singleton, who found that certain phenols found in grape skins could take up an O2 molecule and become highly reactive, linking up to other phenols.1 Singleton discovered, bizarrely, that the starting structure gets re-created at the end of the reaction in an increasingly reactive form, available to react over and over, resulting in a cascading polymerization effect. The reaction is homeopathic: early introduction of oxygen actually increases the wine’s antioxidative power.
Understanding the ins and outs of the vicinal diphenol cascade is essential to a grasp of red wine’s fundamental chemistry, and I have dedicated chapter 6 to exploring its mechanism and implications. Here I’ll just touch on the high points.
The Golden Ratio
It has been empirically determined that a molar ratio of 4:1 total phenols to anthocyanins is ideal for good structure. Since anthocyanins are phenols too, this means the ideal polymer has about six units, with anthocyanins on each end, with a total molecular weight (MW) around 2,000. Yet these covalently bonded polymers aggregate into colloids that pass only with difficulty through a 100,000 MW ultrafilter, demonstrating that several dozen such oligomers (short polymers) are contained in a single colloid and hinting at the destructive potential for sterile filtration, which typically operates near this size range.2
Intentional encouragement of oxidative polymerization in nascent red wine is referred to as Phase 1 micro-oxygenation, which requires high-performance diffusion equipment that produces extremely small bubbles of pure oxygen that readily dissolve before reaching the wine’s surface. Splashing will not suffice. Since tannin polymerization is energetically favored over oxidative ring cleavage, it is critical to introduce oxygen at a rate slow enough to be entirely taken up by this reaction, thus preventing oxidation. Patrick Ducournau was the first to develop an oxygen diffuser that could regulate the extremely low flow rates necessary.
Oxygenation at this early stage does not shorten the wine’s life; paradoxically, it increases antioxidative power by stimulating latent phenolic reactivity. In fact, stopping abruptly will stimulate reductive behavior, causing the wine to close up aromatically and produce stinky sulfides. This is not a bad thing but rather a sign of longevity potential. Oxygen treatment may be extended to balance reductive strength as desired, depending on the intended aging trajectory. Tannins move from green to hard, lose their graininess, and gain volume in the mouth due to an expanded structure, eventually softening into a plush, stable mouthfeel.
While color is critical to creating refined texture, the winemaker should not be fooled by highly colored musts that have experienced field oxidation due to extensive hang time. These do not form stable structures. Only monomeric (unpolymerized) color is useful in refining structure.
When conducted prior to the addition of sulfur dioxide (SO2), Singleton’s cascade includes a second reaction, one that also stabilizes color. Hydrogen peroxide is a side product of the reaction, which, in the absence of sulfites, will oxidize a molecule of ethanol to acetaldehyde. This compound, responsible for the stale apple aroma in fino sherries, is problematic in mature wines but a godsend in young red wine. It bridges pigments to tannins, doubling the rate at which oxygen stabilizes color. Once incorporated into polymers, the anthocyanins become protected, also shedding their susceptibility to sulfite bleaching.
Even if oxygen is not employed, color will still improve structural finesse. Despite its high tannin levels, Syrah texture is dependably soft, while Pinot Noir, though much lighter, is notoriously susceptible to the coarse, dry mouthfeel associated with overpolymerized tannins.
Pinot is a tough town. Anthocyanins contain a glucose molecule that stabilizes their structure, and in most grape cultivars this is protected by an attached two-carbon acyl group that blocks attack by most glucose-loving enzymes. But Pinot Noir pigments lack acylation. Moreover, the grape’s weak tannins are insufficient to promote good yeast settling. Yeast and suspended grape solids not only adsorb pigment but also have a voracious appetite for oxygen, much greater than diphenols, thus thwarting polymerization and color stability.
A Season in Heaven or Hell
Although each vineyard has its own charms and virtues, it is a universal concern that red wines with low color/tannin ratios form coarse, grainy structures that lack integrative properties and shelf life. The path to sound, integrative structure and graceful longevity involves
1 balancing the vine;
2 picking at the proper moment;
3 facilitating effective coextraction; and
4 stabilizing structure.
Any misstep in this chain of events means that little can be done to enhance structure without remedial interventions in the winery such as component blending, lees incorporation, or even sugar addition.
Vineyard Enology
Within a growing season, efforts are generally focused on vine balance, a topic of great complexity that merits its own discussion (see chapter 5). For now, let’s focus on optimizing the development of flavor, tannin, and color.
Pigment and flavor elements are formed in grapes beginning at véraison (the onset of coloration) in order to attract birds to ingest mature seeds. This shift in the vine’s attention from green growth to reproduction is known as Cycle Two. The vineyard enologist strives to encourage a marked shift into Cycle Two by balancing crop load, judicious nutrient availability, and moderate water stress, thereby promoting light exposure, air movement, and moderate temperature in the fruit zone. If Cycle Two does not proceed enthusiastically, it is well to have highly colored components