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Anthocyanins are pigments with taste!

Anthocyanins are the pigments in the skin of grapes and determine the color of the wine. The contribution of these anthocyanins to other sensory aspects of wine has not been widely investigated. Do these pigments also affect taste and mouthfeel?

A sip of wine causes an explosion of taste (sweet, sour, bitter, salt) and a mouthfeel that can be greasy, tight, round or astringent. The tastes are recognized by the taste receptors on the tongue, and the mouthfeel depends on the interaction with saliva. A greasy or round mouthfeel is due to a film layer that remains behind, which makes the saliva thicker. A tight mouthfeel is caused by a dilution of the saliva, and an astringent or wry feeling in the mouth is caused by molecules such as tannins that bind to proteins in the saliva. As a result, the saliva no longer works as a good lubricant and it feels like your tongue sticks to your cheeks. It is unclear what the precise role of anthocyanins is in this whole interplay between wine, taste receptors and saliva. It is therefore investigated whether anthocyanins react with proteins in the saliva, and if it is also possible to taste these pigments.

Chromatogram of anthocyanins in Barbera grapes
Figure 1. Chromatogram of the anthocyanins in Barbera grapes. The structural formula for each class is shown on the right. Peak 1-5 are glucosides, peak 6-10 acetyl glucosides and 11-16 are cinnamoylides.
Adapted from Paissoni et al 2018 via CC BY 4.0

Three classes of anthocyanins

To obtain the anthocyanins, fifty pounds of Barbera and Nebbiolo grapes were peeled (!). The peels were freeze-dried and ground into powder to subsequently extract the pigments. The anthocyanins obtained in the powder have been purified by chromatography techniques and divided into three different classes of anthocyanins; glucosides, acetyl glucosides and cinnamoylides. The acetyl glucosides and cinnamoylides have bound an additional acetyl or cinnamoyl group in comparison with the glucosides (see the structural formulas in Figure 1). Each anthocyanin also binds a residual group, the rest of the molecule. The combination of the class with the residual group determines the identity of each individual anthocyanin, and this also determines the speed at which they are detected with the chromatography techniques. The glucosides are detected first, followed by the acetyl glucosides and finally the cinnamoylides. Figure 1 shows a chromatogram – the overview of all measured anthocyanins – in which each peak represents a different anthocyanin in the skin of the Barbera grapes. The peaks per class are also shown separately and enlarged in the figure. Each peak within a class is an anthocyanin with a different residual group.

Interaction with saliva proteins

The reaction of the anthocyanins with proteins has been tested in the saliva of 18 subjects and in a protein solution made from bovine serum albumin (BSA), a non-reactive protein that is often used in laboratory experiments. The anthocyanins are dissolved per class in a wine-like solution (12% alcohol, 4 g/L tartaric acid and a pH of 3.5). BSA or saliva was then added to these solutions. Reaction of the proteins with the anthocyanins ensures that they precipitate and can be filtered out of the solution. By measuring the amount of anthocyanins in the filtered solution it is possible to determine how many proteins were previously bound by the BSA or saliva (Figure 2).

BSA binds to two cinnamoylides, but saliva proteins bind to different anthocyanins from all three classes. This stronger reaction is probably due to the fact that saliva proteins contain more proline – an amino acid, and ‘building block’ of a protein. This enables them to make connections with the anthocyanins much more easily than BSA. This also shows that a BSA solution is not a good model for future experiments on saliva-protein interaction.

Anthocyanin reaction with protein
Figure 2. Reaction of anthocyanins with proteins. The reaction of BSA (a, c, e) and saliva (b, d, f) with glucoside (a, b), acetyl glucoside (c, d) and cinnamoylides (e, f). *, **, *** and ns indicate a significance of p <0.05, 0.01, 0.001 and not significant. The numbers above the bars indicate the percentage difference with the control (without protein).
Paissoni et al 2018 via CC BY 4.0

Tasting anthocyanins

Proteins in the saliva bind to the anthocyanins. The question remains however, if this also influences the perception of taste and mouthfeel of the wine? This can only be tested by tasting the anthocyanins! A panel of 18 experienced wine tasters were asked in multiple trials to taste three glasses of the wine-like solution of which only one was spiked with anthocyanins. The spiked glass contained different concentrations of either all anthocyanins, or one of the three groups of anthocyanins. The used concentrations were all within the range that can normally be present in wine. To prevent that the tasters were influenced by the color of the solution, all tasting glasses (and spitting glass) were black. The tasters were asked to pick out the spiked glass and specify one or more descriptors of in-mouth properties that set it apart from the other two glasses.

All anthocyanins, glucosides, acetyl glucosides and cinnamoylides, were described as astringent and bitter, and glucosides were in addition also described as salty. However, the observation threshold of glucosides was with 297 mg/L, much higher than the observation threshold for acetyl glucosides (68 mg/L) or the cinnamoylides (58 mg/L). The latter two can therefore be tasted much earlier, but are present in lower quantities in the wine.

Conclusions

Anthocyanins have a bitter taste and provide an astringent mouthfeel by binding to proteins in the saliva (similarily to tannins). Extra color extraction during vinification – for example by a cold maceration  – can therefore also have an effect on the other sensory properties of the wine. In this research the anthocyanins were only tasted in a wine-like solution. Further research must show whether variation in pH, alcohol percentage and the presence of other components (such as tannins) in the wine influence these observations.

Reference:
Paissoni MA, Waffo-Teguo P, Ma W, Jourdes M, Rolle L, Teissedre P-. Chemical and sensorial investigation of in-mouth sensory properties of grape anthocyanins. Sci Rep. 2018 Nov 20;8(1):17098. https://doi.org/10.1038/s41598-018-35355-x

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