December 15, 2019

The Color Of Red (Or Purple) . . . by Joel Mann

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One of the least understood aspects of wine, even by industry professionals, is the science of wine color. Whether it’s the crystal clear of a light white wine, the pale garnet of Burgundy, or the inky dark of Petite Sirah, the physics behind wine color are not straightforward, and often confusing without an understanding of biochemistry. I want to give a few ABC’s on wine color. I’ll try to keep the advanced chemistry to a minimum.

One aspect of wine color that novices don’t understand is that for the most part wine color comes from the grape’s skin. It’s the major distinction between a white and red wine. Both red and white grape varieties have clear juice without pigment. Otherwise, the color of red wine comes from the process of maceration, where the skins and seeds of the grapes are kept in contact with the clear juice, leaching the pigments from the skins into the juice. Of course, every rule has exceptions. There are a handful of red grapes known as teinturiers which have pigmented flesh and juice.

White wines typically range from clear to pale straw. White wine color basically comes from two sources: oak barrels and oxidation. White wine juice is essentially clear. Char from oak barrels or other oak products will provide a small amount of color to the wine, adding a golden/straw hue to the finished wine typically seen in common oak-aged whites such as Chardonnay.

The other source of color comes from age and air. Over time, or with exposure to oxygen, white wines will develop a yellow-gold/brownish hue from oxidation. While the color development from oak char gives positive flavor characteristics, the flavor profile from oxidation is rarely desired.

Red wine color comes from anthocyanin molecules present in the grape skins, giving them their characteristic red color. There are five principle anthocyanins in grapes: malvidin, cyanidin, delphinidin, peonidin, and petunidin. Malvidin by far is the dominant molecule of the five in grapes. Anthocyanins behave differently in solution as to the color they express depending on the pH of the solution.

Anthocyanins will express color from bright red, to purple, blue, and though the green range of the color spectrum as the pH of solution goes from acidic to basic. At the pH ranges common to wine (3.0 to 4.0) the color is red to purple.

The presence of anthocyanins does not determine the intensity of color in the finished wine. Grapes that are pitch black in color can at times produce light rosé wines. The equilibrium of the anthocyanins leaching into solution favors the compounds remaining on the skins. There are additional factors though that can shift the equilibrium towards more color molecules in the juice and wine. These compounds, called cofactors, cause a supersaturation effect known as copigmentation to occur by stabilizing stacks of anthocyanin molecules in solution, allowing greater concentrations.

The effect is most prevalent in the presence of potassium, which wine has in abundance. The effect is moderated somewhat over the course of fermentation as copigmentation behaves differently in aqueous versus alcoholic solution. Certain wines are known for developing a deeper color than others. One of the reasons is some grapes are very low in the cofactors needed to extract more wine color. A common practice is to co-ferment grapes that have a high concentration of cofactors with ones that have a dark skin pigment but low cofactors to extract more anthocyanins then the dark colored grapes could by themselves.

Chianti wines are a classic example, mixing the often poor coloring Sangiovese with a small portion of Canaiolo historically. Copigmentation is also a temporary effect that fades over the first year of a red wine’s aging process, progressing towards stable color pigmentation in aged wines.

As red wine ages, its tannin molecules begin to condense into longer chains. The anthocyanin molecules in the wine will become part of the tannin chains, making the tannins pigmented, and the wine color very stable in solution. The color appearance also changes from brighter red/purple, to a more brick red/orange color. This conversion happens in all red wines, and is known as the transition from young to stable red wine color.

While color has no effect on wine flavor, we first taste food with our eyes, so the appeal of a wine begins with its appearance. A nice rich color in the glass is inviting to the taster, and enhances enjoyment of the experience. So go forth, and impress the tasting rooms with your new knowledge of the color in the glass. Drink responsibly.

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