Astringency

Astringency is a sensory attribute that is described as a drying-out, roughening, and puckery sensation felt in the mouth. Foods that are often astringent include red wine, green and black teas, soy-based foods, and certain fruits, especially when they’re not yet ripe. In these foods, astringency is caused by the polyphenolic compounds they contain. Polyphenols are the most common cause of astringency in foods, though acids, metal salts such as alum , and alcohols are known to also cause astringency1.
 

Sensory assessment of astringency

Astringency tends to be a difficult sensory attribute to assess because it takes over 15 seconds to fully develop, and it builds in intensity and becomes increasingly difficult to clear from the mouth over repeated exposures2,3. A few approaches can be employed to overcome these issues: one suggested approach is the use of statistics to unearth “true” ratings without the carryover artifact, but this does nothing to reduce the cumulative effects of the sensation that can lead to taster fatigue4. A second option is to delay subsequent tasting of astringent samples until the oral environment has returned to a baseline state, although the strength of the sample and the number of sample repetitions will impact the interstimulus period needed for this to occur. A third option is to use a palate cleanser between samples. Several cleansers have been identified for their ability to alleviate astringency including various viscous polysaccharides and gums (e.g., pectin or carboxymethylcelluslose), oils, and also crackers5,6,7,8; however, there is recent evidence suggesting that palate cleansers capable of reducing astringency may be masking the sensation in subsequent tastings9.
 

The cause of astringency

The majority of studies on astringency support the notion that astringency is primarily a tactile sensation rather than a taste6. The mechanism of astringency is not fully understood, although it is known that polyphenols bind and precipitate a class of salivary proteins called the proline-rich proteins (PRPs)10. One theory is that the precipitation of PRPs from saliva reduces its ability to lubricate, and this loss of lubricity is perceived as an increase in oral friction11. A second theory proposes that the sensation is caused by a direct effect of astringents on the oral epithelium and that PRPs play a protective role and prevent astringency by binding the astringent compounds before they can interact with the oral mucosa12. Both theories predict that people with high salivary flow rates and protein/PRP concentrations will report lower ratings of astringency. Several studies support this prediction12,13,14,15.
 

References

1 Green, B.G. 1993. Oral astringency: a tactile component of flavor. Acta Psychol 84(1):119-25.
2 Guinard, J.X., Pangborn, R.M., Lewis, M.J. 1986. The time-course of astringency in wine upon repeated ingestion. American Journal of Enology & Viticulture 37(3):184-9.
3 Lyman, B.J., Green, B.G. 1990. Oral astringency: effects of repeated exposure and interactions with sweeteners. Chem Senses 15(2):151-64.
4 Arnold, G. 1983. A tasting procedure for assessing bitterness and astringency. In: Anonymous Sensory Quality in Foods and Beverages: Definition, Measurement and Control. American ed ed. Chichester, West Sussex; Deerfield Beach, Fla.: E. Horwood Ltd; Verlag Chemie International. P109—114.
5 Brannan, G.D., Setser, C.S., Kemp, K.E. 2001. Effectiveness of rinses in alleviating bitterness and astringency residuals in model solutions. J Sens Stud 16(3):261-75.
6 Breslin, P.A.S., Gilmore, M.M., Beauchamp, G.K., Green, B.G. 1993. Psychophysical evidence that oral astringency is a tactile sensation. Chem Senses 18(4):405-17.
7 Colonna, A.E., Adams, D.O., Noble, A.C. 2004. Comparison of procedures for reducing astringency carry-over effects in evaluation of red wines. Australian Journal of Grape & Wine Research 10(1):26-31.
8 Ross, C.F., Hinken, C.,Weller, K. 2007. Efficacy of palate cleansers for reduction of astringency carryover during repeated ingestions of red wine. J Sens Stud 22(3):293-312.
9 Lee, C.A., Vickers, Z. M. 2010. Discrimination among astringent samples is affected by choice of palate cleanser. Food Quality & Preference 21(1):93-99.
10 Baxter, N.J., Lilley, T.H., Haslam, E., Williamson, M.P. 1997. Multiple interactions between polyphenols and a salivary proline-rich protein repeat result in complexation and precipitation. Biochemistry (NY) 36(18):5566-77.
11 Clifford, M.N. 1997. Astringency. Proceedings of the Phytochemical Society of Europe 41(Phytochemistry of Fruit and Vegetables):87-107.
12 Horne, J., Hayes, J., Lawless, H.T. 2002. Turbidity as a measure of salivary protein reactions with astringent substances. Chem Senses 27(7):653-9.
13 Fischer, U., Boulton, R.B., Noble, A.C. 1994. Physiological factors contributing to the variability of sensory assessments: relationship between salivary flow rate and temporal perception of gustatory stimuli. Food Quality & Preference 5(1, 2):55-64.
14 Imm, B., Lawless, H.T. 1996. Relationships between salivary responses and astringency, bitterness and sourness responses to aluminum ammonium sulfate. Chem Senses 21(5):618.
15 Kallithraka, S., Bakker, J., Clifford, M.N., Vallis, L. 2001. Correlations between saliva protein composition and some T-I parameters of astringency. Food Quality & Preference 12(2):145-52.