Wine Color Chemistry: Anthocyanins, Browning, and Stability

The Science of Wine Color

Wine color is one of the first things we notice when we pour a glass, and it provides a wealth of information about the wine's grape variety, age, winemaking style, and condition. The chemistry behind wine color is remarkably complex, involving dozens of different pigment molecules, their interactions with other wine components, and their transformations over time.

Understanding color chemistry helps winemakers make informed decisions about maceration, pressing, sulfite management, and aging that optimize color extraction, stability, and evolution. For the scientifically curious winemaker, few topics better illustrate how chemistry translates directly into sensory experience.

Color in Red, White, and Rose Wines

Red wine owes its color primarily to anthocyanins, pigment molecules extracted from grape skins during maceration. A deeply colored young red wine may contain 200-500 mg/L of anthocyanins, though this concentration decreases dramatically during aging as anthocyanins participate in polymerization reactions and degradation.

White wine contains very low levels of anthocyanins (since white grapes have minimal skin pigmentation) and derives its yellow to gold color from flavonols, hydroxycinnamic acids, and their oxidation products. The intensity of color in white wine is strongly influenced by phenolic content and oxidation state.

Rose wine contains lower concentrations of anthocyanins than red wine, typically 20-100 mg/L, extracted during brief skin contact (typically 2-24 hours). The pale pink to salmon color of rose reflects this limited extraction.

Anthocyanins: The Red Pigments

Chemical Structure

Anthocyanins are glycosylated forms of anthocyanidins, which are flavonoid compounds with a characteristic C6-C3-C6 skeleton. The five major anthocyanins found in Vitis vinifera grapes are based on five different anthocyanidin aglycones: malvidin, petunidin, delphinidin, peonidin, and cyanidin. Malvidin-3-glucoside is typically the most abundant, accounting for 40-90% of total anthocyanins depending on the grape variety.

These molecules are attached to a glucose sugar, and some may be further modified by acylation with acetic, p-coumaric, or caffeic acid. Acylated anthocyanins are more stable than their non-acylated counterparts, which has implications for color longevity.

pH-Dependent Color Changes

Anthocyanins exist in several structural forms that are in equilibrium, and the dominant form depends on pH. This pH-dependent behavior is one of the most important aspects of wine color chemistry.

At very low pH (below 2), anthocyanins exist primarily as the red flavylium cation. At wine pH (3.0-3.8), most anthocyanin molecules exist as colorless forms, the carbinol pseudobase and the chalcone. Only a small fraction remains in the red flavylium form. At higher pH (above 6), the blue quinoidal base predominates.

This means that at typical wine pH, the majority of anthocyanins are actually colorless. The visible red color of young wine comes from the relatively small fraction in the flavylium form, augmented by copigmentation effects that shift the equilibrium toward colored forms.

Copigmentation

Copigmentation is the phenomenon where colorless or weakly colored molecules associate with anthocyanins in non-covalent complexes, shifting the color equilibrium toward the red flavylium form and enhancing color intensity by 30-50% in young wines.

Common copigments include flavonols (like quercetin), hydroxycinnamic acids (like caffeic acid), and even other anthocyanins. The copigmentation complex absorbs light at slightly longer wavelengths than free anthocyanins, producing a subtle bathochromic shift toward more purple-blue hues.

Copigmentation is most important in young wines and decreases during aging as anthocyanins are consumed by polymerization reactions. The "bright purple" color of a young red wine gradually shifts to garnet and then brick-orange as copigmented and free anthocyanins give way to polymeric pigments.

Polymeric Pigments and Color Stability

Formation of Stable Pigments

As wine ages, free anthocyanins gradually react with tannins and other compounds to form polymeric pigments. These reactions are crucial for long-term color stability because polymeric pigments are resistant to pH changes, SO₂ bleaching, and oxidative degradation, unlike free anthocyanins.

The major pathways for polymeric pigment formation include direct condensation between anthocyanins and tannins, acetaldehyde-mediated bridging that links anthocyanins to tannins through an ethylidene bridge, and reactions involving pyruvic acid that produce vitisins (pyranoanthocyanins).

Vitisins are a particularly interesting class of polymeric pigments. They form a fourth ring on the anthocyanin structure, making them exceptionally stable. Vitisins contribute orange-red hues and are important color components in aged wines, Port wines, and wines that have undergone micro-oxygenation.

The Color Evolution Timeline

The color trajectory of a red wine over its lifetime follows a predictable pattern driven by the chemistry described above.

In young wines (0-1 year), color is dominated by free anthocyanins and copigmentation complexes. The wine appears deep purple to ruby. Color intensity is at its maximum but is relatively unstable, susceptible to bleaching by SO₂ and pH changes.

In developing wines (1-5 years), free anthocyanins gradually decrease as they form polymeric pigments. Color shifts from purple toward ruby to garnet. Color intensity may decrease slightly, but stability increases as polymeric pigments become a larger proportion of total color.

In mature wines (5-15+ years), polymeric pigments dominate color. The wine appears garnet to brick-orange at the rim. Color intensity continues to decline slowly as some polymeric pigments precipitate as sediment. The remaining color is highly stable.

Browning in Wine

Enzymatic Browning

Enzymatic browning occurs in grape must before and during fermentation. The enzyme polyphenol oxidase (PPO), also called tyrosinase, catalyzes the oxidation of phenolic compounds (particularly caftaric acid and coutaric acid) to quinones. These quinones are highly reactive and rapidly polymerize to form brown pigments.

Enzymatic browning is most problematic in white wines, where even small amounts of brown pigment are visible against the pale background. Winemakers combat enzymatic browning through SO₂ additions at crush (which inhibit PPO), rapid pressing to minimize oxygen exposure, and working at cool temperatures (which slow enzyme activity).

The fungal enzyme laccase, produced by Botrytis cinerea, is even more problematic than grape PPO because it is resistant to SO₂ inhibition and can oxidize a wider range of substrates. Grapes affected by gray rot (unwanted Botrytis infection) produce must that is extremely prone to browning.

Non-Enzymatic Browning

Non-enzymatic browning occurs during aging through several chemical pathways. Phenolic oxidation is the most important: oxygen reacts with phenolic compounds in a cascade of reactions that ultimately produces brown-colored polymers. Iron and copper act as catalysts in these reactions.

In white wines, the primary substrates for oxidative browning are flavanols (catechin and epicatechin), hydroxycinnamates (caffeic and coutaric acids), and flavonols. The resulting brown pigments are poorly characterized but are believed to be complex polymeric structures.

Maillard reactions between amino acids and sugars can also contribute to browning, particularly in wines that have been heated or stored at elevated temperatures. These reactions are more significant in fortified wines and wines with residual sugar.

Preventing Browning

For white wines, the keys to preventing browning are minimizing phenolic extraction (gentle pressing, limited skin contact), protecting against oxygen exposure, maintaining adequate free SO₂, and storing at cool temperatures. Some winemakers practice hyperoxidation, deliberately exposing must to oxygen before fermentation to oxidize and precipitate the most easily oxidized phenolics, producing a more stable wine.

For red wines, moderate oxidation during aging actually contributes positively to color stability by promoting polymeric pigment formation. However, excessive oxidation degrades anthocyanins faster than they can form stable polymeric pigments, leading to premature browning. Maintaining appropriate SO₂ levels and limiting excessive oxygen exposure during aging protects red wine color.

Measuring Wine Color

Spectrophotometric Analysis

Wine color is quantified using a spectrophotometer, which measures light absorption at specific wavelengths. The standard measurements for red wine color are:

• Absorbance at 420 nm (A420): measures yellow-brown pigments
• Absorbance at 520 nm (A520): measures red pigments
• Absorbance at 620 nm (A620): measures blue-violet pigments

Color intensity is calculated as A420 + A520 + A620 (or sometimes just A420 + A520). Hue is calculated as A420/A520 and indicates whether the wine leans more yellow-brown (higher hue, typical of aged wines) or red-purple (lower hue, typical of young wines).

Visual Assessment

For home winemakers without a spectrophotometer, visual assessment against a white background provides useful information. Tilt the glass at a 45-degree angle over white paper and observe the color at the center (indicating depth and concentration) and at the rim (indicating age and evolution). A young red wine shows purple-red at the rim, while an older wine shows orange-brown.

Practical Implications for Winemakers

Maximizing color extraction in red wines requires adequate maceration time (at least 7-10 days for most varieties), appropriate temperature (25-30°C for active fermentation), and effective cap management. Cold soaking before fermentation extracts anthocyanins without excessive tannin, since alcohol is needed for efficient tannin extraction.

Protecting color during aging requires maintaining adequate free SO₂ (which prevents oxidative degradation of anthocyanins), appropriate storage temperature (cool and stable), and minimal light exposure (ultraviolet light degrades anthocyanins). For white wines, all of these factors also help prevent browning.

Frequently Asked Questions

Why does red wine change color with age?

Red wine changes color because free anthocyanins (which provide the vibrant purple-red color of young wine) gradually react with tannins and other compounds to form polymeric pigments. These polymeric pigments absorb light at slightly different wavelengths, shifting the color from purple toward garnet and eventually brick-orange. Simultaneously, some pigments precipitate as sediment, reducing overall color intensity.

What causes white wine to turn brown?

White wine browning is caused by oxidation of phenolic compounds. Oxygen reacts with flavanols, hydroxycinnamic acids, and other phenolics in a chain of reactions catalyzed by trace metals (iron and copper) that produce brown-colored polymers. Insufficient SO₂, excessive oxygen exposure, warm storage temperatures, and high phenolic content all accelerate browning.

Why are some grape varieties darker than others?

Darker grape varieties contain higher concentrations of anthocyanins in their skins. Varieties like Petit Verdot, Petite Sirah, and Tannat can have two to three times the anthocyanin content of lighter-skinned varieties like Pinot Noir or Grenache. Skin thickness, the number of cell layers containing pigment, and the specific anthocyanin profile all contribute to varietal color differences.

Does copigmentation affect how I should make red wine?

Yes. Copigmentation is enhanced by higher ratios of cofactors (flavonols, hydroxycinnamic acids) to anthocyanins. Practices like whole cluster fermentation and co-fermentation with small amounts of white grapes (as in Cote-Rotie tradition) can increase copigmentation. Cold soaking in the presence of grape solids also promotes copigmentation complexes that boost early color intensity.

Can I fix a wine that has turned brown?

Mild browning in white wine can sometimes be reduced by fining with PVPP (polyvinylpolypyrrolidone), which selectively adsorbs oxidized phenolics. Casein fining can also help. However, severe browning is generally irreversible. Prevention through proper SO₂ management, oxygen exclusion, and cool storage is far more effective than any corrective treatment.