The Effect of Acetaldehyde on Red Wine Color Stability and Astringency
By: Marlena Sheridan
Acetaldehyde is a compound found in wine that has a profound effect on color stability and astringency. Acetaldehyde reacts directly with red wine tannins and anthocyanins to form polymeric pigments and modified tannins. Denise has already provided a great review of polymeric pigment formation (see her post form March 6, 2015). Here, I’ll be focusing on the reactions involving acetaldehyde and research we’ve completed on the topic.
Acetaldehyde reacts with tannins and anthocyanins to form irreversible, covalent bridges. When these reactions are between tannins, they can alter the structure so that its shape and activity are changed. This affects wine astringency, a mechanism based on the interaction of tannins and salivary proteins. Modified tannins, including acetaldehyde-bridged tannins, have been shown to have lower astringency because of their structural changes (Gambuti 2013). A model of these reactions is shown in Figure 1. These changes contribute to the shift from drying or puckering mouthfeel to a more velvety mouthfeel in aged wines.
Similar reactions take place between anthocyanins and tannins as well as between anthocyanins themselves. These reactions form polymeric pigments: pyranoanthocyanins and vitisins (see Denise’s previous post; Cheynier 2006). A model for these reactions is shown in Figure 2. As Denise has covered, these polymeric pigments have increased stability to sulfite bleaching and pH changes compared to monomeric anthocyanins.
As has hopefully been made clear, using these reactions of acetaldehyde with tannins and anthocyanins is an important tool for winemakers. Winemakers can use several techniques to get the benefits of acetaldehyde on pigment and tannin structure. These use oxygen incorporation to form acetaldehyde through a series of metal-catalyzed reactions (Danilewicz 2003). Along this pathway, there is the possibility to form detrimental oxidation products instead of the desired acetaldehyde. A simplified version of this mechanism is shown in Figure 3. These other oxidation pathways can lead to some of the risks of oxygen exposure including the loss of desirable aromas, browning, and the formation of off odors.
Ideally, winemakers would be able to get the benefits of acetaldehyde without the risks of oxygen exposure. By adding acetaldehyde directly, we could avoid the problems of oxidation while simultaneously controlling the beneficial oxidation reactions we’re hoping for. With this in mind, we conducted an experiment where exogenous acetaldehyde was added during red wine fermentation using Cabernet Franc grapes from North East, PA.
The must was separated into three groups – control, low acetaldehyde, and high acetaldehyde. The acetaldehyde groups received four doses during the fermentation, 4×25 mg/L acetaldehyde in the low group and 4×250 mg/L acetaldehyde in the high group. Fermentations were performed in quadruplicate in microfermenters (Figure 4). Wines were fermented to dryness and pressed prior to analysis.
Wines were analyzed for color stability by the modified Somers assay (Mercurio 2007) with measures of sulfite-resistant pigments, or polymeric pigments, shown here. Astringency was measured using a model protein precipitation, where wines were mixed with bovine serum albumin (BSA) and the amount of tannin precipitated was quantified (Mercurio 2008). Higher tannin precipitation implies higher astringency. Data from this experiment is shown in Figure 5.
As shown in Figure 5, there was a statistically significant decrease in the measure of astringency and increase in color stability with high acetaldehyde treatment. This data provides evidence that exogenous acetaldehyde can be used in red wines to get beneficial effects on color and astringency without oxygen exposure. A more detailed discussion of the results can be found in the published manuscript (Sheridan 2015).
Based on this work, we are continuing to examine the reaction of acetaldehyde with tannins and anthocyanins. We are currently working with model wine experiments to further understand the chemistry of these reactions including characterizing the final products and the effect of wine composition.
Cheynier, V. & Dueñas-Paton, M. Structure and properties of wine pigments and tannins. Am. J. Enol. Vitic. 2006, 57, 298–305.
Danilewicz, J. C. Review of Reaction Mechanisms of Oxygen and Proposed Intermediate Reduction Products in Wine : Central Role of Iron and Copper. Am. J. Enol. Vitic. 2003, 54, 73–85.
Gambuti, A., Rinaldi, A., Ugliano, M. & Moio, L. Evolution of phenolic compounds and astringency during aging of red wine: effect of oxygen exposure before and after bottling. J. Agric. Food Chem. 2013, 61, 1618–27.
Mercurio, M. D., Dambergs, R. G., Herderich, M. J. & Smith, P. A. High throughput analysis of red wine and grape phenolics-adaptation and validation of methyl cellulose precipitable tannin assay and modified Somers color assay to a rapid 96 well plate format. J. Agric. Food Chem. 2007, 55, 4651–7.
Mercurio, M. D. & Smith, P. A. Tannin quantification in red grapes and wine: comparison of polysaccharide- and protein-based tannin precipitation techniques and their ability to model wine astringency. J. Agric. Food Chem. 2008, 56, 5528–37.
Sheridan, M.K. & R.J. Elias. 2015. Exogenous acetaldehyde as a tool for modulating wine color and astringency during fermentation. Food Chem. 2015, 177, 17-22.