O-11
On the mechanism of astringency. The astringency of polyphenols results from their combination with and
precipitation of salivary proline-rich proteins (PRP), which reduces lubrication
in the mouth. This is a tactile sensation perceived by the trigeminal nerve
rather than a taste. Acids have long been known to enhance polyphenol
astringency. Many acids in water without polyphenol have been described as more
astringent than sour. A mechanism suggested for the effect of acid is reduction
of saliva viscosity, increasing the diffusion rate. The object this study was to
investigate an alternate explanation for the influence of acids on astringency,
that they intensify protein-polyphenol interaction, resulting in increased
astringent sensation. Evidence has recently been published reporting the
presence of polyphenols in saliva for extended times. Polyphenols could normally
be present in saliva at concentrations that are below the threshold for
astringency. When acid is ingested, however, the PRP-polyphenol interaction is
greatly intensified, which could lead to perception of astringency. Saliva was
collected, adjusted in pH, and light scattering measurements (an indication of
the strength of protein-polyphenol interaction) were carried out with and
without added tannic acid (TA). In both cases, maximum scattering was observed
near pH 4.4, which is well below the range of saliva pH (6.5 - 7).
Determinations of the total polyphenol content of the saliva of a single
individual were carried out using the Folin-Ciocalteu method. This showed 53
mg/L when sampled more than eight hours after the last polyphenol ingestion and
93 mg/L soon after consuming coffee. A sensory panel employed magnitude
estimation to rate the relative intensities of astringency of solutions
containing dilute HCl, TA, and both combined. Sensory experiments showed similar
astringency sensations from 0.005N HCl and 0.5 g/L tannic acid. When
combined, these produced a significantly greater astringent sensation than
either alone. It appears likely that the sensation of astringency produced by
acids results from intensification of interactions between salivary polyphenols
and salivary PRP. This accounts for the effects of ingestion of acids whether or
not polyphenols are in the sample.
Karl Siebert received a Ph.D. in biochemistry from Penn State in 1970. He
joined the Stroh Brewery Company in Detroit where he spent 18 years and held
positions from Research Associate to Director of Research. In 1990, Dr. Siebert
joined Cornell University as Professor of Biochemistry in the Department of Food
Science and Technology. He served five years as department chairman and now has
a 100% research commitment. Dr. Siebert served on ASBC technical subcommittees
and was a member and chairman of the Technical Committee. He is serving his
second stint on the journal editorial board (1980-1992; 1996-).
KARL J. SIEBERT and Alexander W. Chassy. Dept. of Food Science & Technology,
Cornell University, Geneva, NY 14456-0462.