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1 Department of Viticulture and Enology, University of California, One Shields Avenue,
Davis, CA 95616-8749 [Fax: 530-752-0382; email: rbboulton@ucdavis.edu]
Copigmentation is a solution phenomenon in which pigments and other noncolored organic components form molecular associations or complexes. It generally results in an enhancement in the absorbance and in some cases, a shift in the wavelength of the maximum absorbance of the pigment. Copigmentation has not previously been taken into account in traditional wine color measures, in the relationship between color and pigment analysis, or in spectrophotometric assays for anthocyanin content. It is now apparent that copigmentation can account for between 30 and 50% of the color in young wines and that it is primarily influenced by the levels of several specific, noncolored phenolic components or cofactors. Copigmentation is of critical importance in understanding the relationship between grape composition and wine color, the variation in color and pigment concentration between wines, and in all reactions involving the anthocyanins during wine aging. This review focuses on the importance of the individual pigments and cofactors, the strength of their interactions, and their relative abundance in grapes and wines. A simple mathematical analysis of the solution equilibrium is developed to explain the nonlinear deviation from Beer’s law. When solved for typical wines, this function provides estimates of the apparent association constant, K, and the apparent molar extinction of the copigmented form, Ec, in natural mixtures. These measures allow the fraction of the anthocyanins which is in the copigmented form to be estimated. The significance of this phenomenon on pigment extraction and color retention during fermentations, on the rate of subsequent pigment polymerization, on the possible protection of anthocyanins from oxidation, and in the possible involvement on perceived mouthfeel and astringency of wines are suggested. Aspects of the copigmentation phenomenon that are poorly understood are identified and some research directions are suggested.
Note:
Acknowledgments: The author gratefully acknowledges the thoughts and discussions provided
by Professors Vernon Singleton and Ted Molinski and graduate students Ranuccio Neri, Joanne
Levengood, and Micha Vaadia during the formative stages of this review. The author also appreciates the cooperation of Professor Molinski in obtaining the circular dichroism spectra of the red
wine presented in Figure 1.
Key words: Copigmentation, anthocyanins, red wine color, pigment complexes, cofactors
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