Astringency and Bitterness of Flavonoid Phenols - American Chemical

Chapter 15

Astringency and Bitterness of Flavonoid Phenols Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 25, 2015 | http://pubs.acs.org Publication Date: July 19, 2002 | doi: 10.1021/bk-2002-0825.ch015

A. C. Noble Department of Viticulture and Enology, University of California, One Shields Avenue, Davis, CA 95616

In beverages and fruits the lingering tactile sensation of astringency, as well as the persistent taste of bitterness, are primarily elicited by flavonoid phenols. Chemically, relative astringency of a compound is defined by its effectiveness in precipitating protein. A s the degree of polymerization of tannin increases, bitterness decreases, while astringency increases. The astringent flavanol polymers or condensed tannins have a strong affinity for binding with proline rich proteins, such as those found in saliva. Sensorially astringency is a drying or rough mouthfeel thought to result from decreased oral lubrication, following binding of salivary proteins by tannins. This may explain the increase in intensity of astringency over several sips or wine or tea. Also consistent with this hypothesis, subjects with low salivary flow rates perceive astringency more intensely than high-flow individuals.

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© 2002 American Chemical Society In Chemistry of Taste; Given, Peter, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

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Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 25, 2015 | http://pubs.acs.org Publication Date: July 19, 2002 | doi: 10.1021/bk-2002-0825.ch015

Introduction In beverages such as tea, cider or red wine, as well as in several fruits, the tactile sensation of astringency and the taste of bitterness are elicited by the flavonoid phenols which include anthocyanins, flavanols and flavonols. O f these, the flavan-3-ol monomers, (catechin,epicatechin, epigallocatechin, epicatechin gallate, epigallocatechin gallate), and their oligomers and polymers, which are called proanthocyanidins or condensed tannins are the most abundant in wine and tea. Both procyanidins (polymers of epicatechin and catechin) and prodelphinidins (polymers of epigallocatechin) have been reported in grapes (1,2), while tea contains monomers and polymers all 5 flavanols (3) With the exception of bitterness of caffeine in tea, the flavanols are the primary source of bitterness and astringency in tea and red wine.

Bitterness

Bitterness is a characteristic taste associated with many food products including cider tea, and wine. Bitter taste is elicited by structurally diverse compounds, but no clear definition of the molecular properties which confer bitterness has yet been proposed (4,5). Several transduction mechanisms for perception of bitterness have been identified and appear to be compound specific (6), however the bitterness transduction mechanisms of flavonoid phenols have never been investigated.

Astringency

The tactile sensation of astringency is thought to be perceived by touch via mechanoreceptors (7). Sensorially, astringency is described as a puckering, rough or drying mouthfeel, whereas chemically an astringent is defined as a compound which precipitates proteins. For water soluble phenols, molecular weights between 500 and 3000 were reported to be required (8). Consistent with this definition, the sensitive assay for tannins developed by Adams and Harbertson (9) can only detect tannins which have more than three flavan-3-ol units. Despite the inability to precipitate proteins in chemical assays, flavan-3-ol monomers (10-12), and flavan-3-ol dimers and trimers (13) have been shown to elicit the sensation of astringency. Astringency of these smaller phenols may arise from formation of unprecipitated complexes with proteins (14) or by cross linking of proteins with simple phenols which have 1,2 dihydroxy or 1,2,3 trihydroxy groups as proposed by McManus et al. (15).

In Chemistry of Taste; Given, Peter, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

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Effect of tannin composition


Astringency and bitterness are very persistent sensations. The intensity and duration of both attributes increase as concentrations of polyphenols are raised in model solutions (10,16,17). Similarly, bitterness and astringency increase in wine as flavonoid phenols are extracted from grape seeds and skins during

Fig. 1. Maximum intensity of bitterness (left) and astringency (right) offlavanol monomers (CAT, EPI), dimers (B3, B4, Β 5) and trimers (C2 andC) (0.9 g/L). (n=18 judges χ 2 reps). Adaptedfrom Peleg et al (13). fermentation. Small differences in flavonoid configuration can produce significant differences in sensory properties. The chiral isomers, epicatechin and catechin, differ only in the absolute stereochemistry of the hydroxyl group at position 3 of the heterocyclic C ring. Yet, epicatechin is more bitter and astringent than catechin (11,12). The degree of polymerization also affects the relative bitterness and astringency. Monomers of flavonoid (and nonflavonoid) phenolics are more bitter than astringent, whereas polymer fractions are more astringent than bitter (10,17-19). Comparison of individual dimers and trimers synthesized from catechin and epicatechin showed the same trend (13). A s the molecular weight increased, maximum bitterness intensity (and total duration) decreased whereas both parameters increased for astringency (Fig. 1). The bond linking the monomeric units had an influence on both sensory properties. The catechincatechin dimer linked by a 4 ^ 6 bond was more bitter than both the 4->8 linked dimers, catechin-catechin and catechin-epicatechin. Astringency was also


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affected by the identity of the monomeric units; catechin-(4->8)-catechin, was lower in astringency than catechin-(4-»8)-epicatechin. The increase in astringency intensity with molecular size is consistent with the reported increase in effectiveness of tannins to precipitate protein from dimers to higher oligomers (20). No studies have addressed the contribution of anthocyanins to wine flavor. Anthocyanins interact strongly with other phenolic compounds (21,22) and these pigments have been reported to increase tannin solubilization (23). However, whether these interactions interfere with the binding of tannins with proteins is unknown. Tannins in red wines are extracted during fermentation from grape seeds and skins. Grape seed tannins are procyanidins (24), whereas skin tannins also contain prodelphinidins (25) and flavonols, such as the glycoside and glucuronide of quercetin. Very little sensory work has studied flavonols but their levels are so low that they are unlikely to contribute significantly to bitterness or astringency of red wines. Seed tannins typically have lower average degree of polymerization (DP) and larger proportions of galloylated units than skin tannins (24,25). Whereas the lower D P of the seed tannins would suggest that they may be less astringent than skin tannin, no sensory studies have addressed the effect of galloylation. The amount of galloylation may increase astringency since it has been shown to increase the interaction of tannin with proteins (26,27). There is considerable controversy in the wine industry as to the role of skin vs seed tannins in affecting taste and mouthfeel of wine. To compare their sensory properties, tannin fractions were extracted from ripe Cabernet franc grapes from the Loire Valley and from varying ripeness levels of Cabernet Sauvignon in Napa Valley, C A . The average D P of the seed and skin extracts were, respectively, 6 and 40 for the Cabernet Franc (28) and 5.5 and 33 for the Cabernet Sauvignon (29). Despite this large difference in composition, the astringency of the Cabernet franc skin and seed tannins were virtually identical (28,30) as shown by the average astringency curves in Fig. 2 (left). Conflicting with these results, as shown in Fig 2 (Right), skin tannin extracted from ripe Cabernet Sauvignon grapes (26.5°B) was significantly more astringent than seed tannin (extracted from less ripe 21.5°B grapes) (30). The difference between these two studies may arise from the variation in ripeness of the grapes from which the extracts were isolated, in extraction procedures, in the varieties or in the location of origin.

Effect of sensory methodology Evaluation of astringent products such as red wine or tea cannot be made in typical side by side comparisons. Intensity of astringency, as well as that of bitterness, builds up when several samples are tasted. Correspondingly, astringency increases upon taking many sips of the same sample. Single sip TI



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Fig. 2. Average time-intensity curves for astringency of skin (Solid line) and seed (Dashed line) tannin fractions. Left: Cabernetfranc(1500 mg/L) (n =12 judges χ 2 reps)(28). Right: Cabernet Sauvignon (1000 mg/L) n= 10 judges χ 2 reps) (30). protocols such as that shown in Fig. 2 minimize "carry over" effects, but do not reflect perception during normal consumption of beverages. This phenomenon has been illustrated in studies in which judges repeatedly sip astringent solutions at defined intervals, while continuously rating astringency. In Fig. 3 (Top), maximum intensity of astringency increased upon repeated sipping of red wine at 25 second intervals (57). A t each sip, astringency increased rapidly, reaching a maximum 6-8 s after the wine was swallowed. Intensity then decreased until the next sip was taken, whereupon another rapid increase occurred. Increasing the time between sips to 30 seconds considerably reduced the enhancement of astringency of the second sip as illustrated in Fig. 3 (Bottom) (32). A similar trend was observed when a significant increase in astringency occurred when red wines were sipped at 20 s, whereas the increase in astringency with 40 s intervals was not significant (33). Astringency can continue to build over further sips as shown for two strengths of tea (Fig. 4). Similar to the red wines, maximum intensity for each sip and the minimum value of astringency between sips increased as the number of sips increased. However, after the third sip the increase between sips was not significant suggesting a plateauing effect may have occurred by sip 3 in contrast to red wines where the increase between each of 4 sips was significant (34).

Effect of salivaryflowrate Saliva flow rate is increased dramatically by sour or astringent stimuli. Monitoring flow of the parotid salivary gland, while subjects sipped wine and expectorated at 10 seconds, revealed that the increase in flow-rate is almost instantaneous, reaches a maximum approximately 20 seconds later and rapidly decreases until further stimulation occurs (35). When subjects were partitioned into groups based on their salivary flow-rates and their data analyzed separately,



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Fig. 3. Average intensity of astringency over time of red wine. Top: Sipping a Merlot wine at 25 s intervals. (n=18 judges χ 2 reps) (31); Bottom: sipping another Merlot wine at 30 s intervals (n= 16 judges χ 2 reps) (32). Intensity

Tea

— 5g/L — 12g/L 0

25

50

75

100

125

150

175

200

Time (s)

Figure 4. Average intensity of astringency of 2 concentrations of black tea sipped at 25 s intervals (n=16 judges χ 3 reps) (34).



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low-flow subjects perceived maximum intensity of astringency later, rated it more intensely and for a longer duration than high-flow subjects both in white (35) and red wine (36). The same results occurred over multiple sips for the astringency ratings of red wines (37) and tea (34). A s shown for tea in Figure 5, low flow subjects rated astringency higher than the high-flow subjects over 8 successive sips (34). Conflicting with these results, no effect of salivary flow on perception of astringency was observed for flavanol monomers, dimers and trimers(ii), hydroxy benzoic acids (38) or soymilk (13,39). When the flow of saliva is stimulated, salivary pH rises and the protein concentration decreases although the total protein content remains fairly constant (40). Over twenty percent of saliva proteins are basic, proline-rich proteins (PRPs) (41) for which polyphenols have a strong affinity (42). The oral sensation of astringency, elicited by polyphenolic compounds, is thought to be linked to precipitation of these salivary proteins. It has been speculated that astringency is the friction perceived when oral lubrication is reduced upon binding of astringent compounds with salivary proteins (43-46). If this is true, then the lower astringency ratings by individuals with high salivary flow rates may reflect the greater ability of the higher volume of saliva to restore oral lubrication. Similarly, the reduced buildup of astringency observed when the interval between sips is increased by 5 sec (Fig. 3) may be due to the additional amount of secreted saliva. Correspondingly, the progressive buildup of astringency over successive sips may be the result the inability of saliva to restore lubrication during prolonged ingestion of astringent fluids.

Fig. 5. Average maximum intensity of astringency for successive sips of black tea (averaged over 3 tea concentrations) as a fucntion of salivaryflow-ratefor 7 subjects with high salivaryflowrates (Hi-Flow) and 3 subjects with low flow (34).


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Summary Astringency is evoked by compounds which precipitate or complex proteins, whereas the taste of bitterness is elicted by very diverse compounds. Flavonoid phenols are both bitter and astringent, and the intensity on an equal weight basis varies inversely with increasing molecular size. It is thought that the drying sensation of astringency is the friction resulting from decreased salivary lubrication upon binding of astringents with salivary proteins. However interactions with oral surfaces have not been studied and may play a role in the buildup of astringency upon continued ingestion.

Acknowledgement This research was supported in part by grants from the American Vineyard Foundation.

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Astringency and Bitterness of Flavonoid Phenols - American Chemical

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