Ann C. Noble

Bitterness and astringency of flavan-3-ol monomers, dimers and trimers†

Intensity of astringency and bitterness of seven flavonoid compounds was evaluated by a time-intensity (TI) procedure. Eighteen trained judges rated intensity continuously from ingestion, through expectoration at 10 s until extinction of the sensation. The seven stimuli included two flavan-3-ol monomers, (+)-catechin and (−)-epicatechin, three dimers and two trimers synthesised from catechin or epicatechin by condensation with (+)-dihydroquercitin. As the degree of polymerisation increased, maximum bitterness intensity (Imax) and total duration (Ttot) decreased whereas astringency Imax increased. The monomers were significantly higher in bitterness at Imax than the dimers, which were significantly higher than the trimers. Astringency Imax of the monomers was lower than the dimers or trimers, although no significant difference was found in Ttot among the polymer classes. The bond linking the monomeric units had an influence on both sensory properties. The catechin-catechin dimer linked by a 4→6 bond was more bitter than both catechin-(4→8)-catechin and catechin-(4→8)-epicatechin. Astringency was affected by both the specific linkage and the identity of the monomeric units with the dimer, catechin-(4→8)-catechin, being lower in astringency than either catechin-(4→6)-catechin or catechin-(4→8)-epicatechin. © 1999 Society of Chemical Industry

Taste-aroma interactions

Taste and aroma interactions occur each time we eat or drink. In the laboratory, smell and taste stimuli can be physically separated, whereas under normal conditions the consumption of foods and beverages results in the simultaneous perception of aroma and taste coupled with tactile sensations, all of which contribute to an overall impression of flavor. Tastes can increase the apparent intensity of aromas; conversely, the perceived intensity of tastes is increased when we taste flavored solutions, especially when there is a logical association between them, such as between sweetness and fruitiness. Recent investigations of factors that affect aroma and taste interactions are summarized in this brief review.

Physiological factors contributing to the variability of sensory assessments: relationship between salivary flow rate and temporal perception of gustatory stimuli

Abstract Stimulation by oral manipulation or ingestion of stimuli causes the salivary flow rate to increase. Not only do gustatory stimuli affect salivary response, but saliva in turn can affect perception of taste by titration, dilution, or precipitation of stimuli. Average bitterness and astringency time-intensity (TI) curves and individual TI parameters generated in response to wines varying in ethanol, pH and phenolic composition revealed differences in temporal perception among the salivary flow groups. For both attributes, low-flow (LF) subjects took a longer time to reach maximum intensity and had a longer duration than high-flow (HF) subjects. For wines with tannic acid at pH 3·0 and pH 3·6, LF subjects recorded a higher intensity at maximum of bitterness and astringency than HF subjects. This perceptual difference was more pronounced at pH 3·6, where a greater difference in salivary flow rate was also elicited between flow groups. The log intensity decay curves for bitterness and astringency were linear, suggesting that a first-order decay process governed the perception function. Desorption is thus considered to be the primary phenomenon, rather than diffusion through a boundary layer. For all flow groups, the decay constants for bitterness were higher than those for astringency. However, the decay constants within one modality were similar for all flow groups, indicating similar desorption rates. Reciprocal plots of salivary flow rate were linear, suggesting that a second-order process governed salivary flow.

Temporal perception of astringency and sweetness in red wine

Abstract The interaction between astringency and sweetness was investigated in red wine using time-intensity (T-I) methodology. Maximum intensity and total duration for astringency decreased significantly with increasing sucrose concentration, whereas no sweetness T-I parameter was significantly affected by astringency level. Although no differences in perception of astringency or sweetness were found as a function of PROP (6-n-propylthiouracil) taster status, there was a significant difference in intensity and persistence of astringency as a function of salivary flow status. Low -flow subjects rated astringency higher and recorded longer duration of astringent aftertaste than high -flow subjects, although there was no difference in sweetness responses as a function of salivary flow status.

Effects of viscosity on the bitterness and astringency of grape seed tannin

Abstract To examine the separate effects of viscosity and sweetness on astringency, aqueous solutions of grape seed tannin (GST) were thickened with carboxymethyl cellulose (CMC) from 2 to 45 cP (experiment 1) or sweetened with 0 to 1.8 g L aspartame (experiment 2). Trained subjects continuously rated astringency and bitterness in duplicate. Subjects were categorized by the salivary flow induced by citric acid and ability to taste n-propyl thiouracil (PROP). In experiment 1, maximum intensity and total duration of astringency were significantly decreased as viscosity rose, although time to maximum intensity of astringency was not affected. Maximum intensity and total duration of bitterness were not significantly affected by increasing viscosity; however, the onset of bitterness was significantly delayed. In experiment 2, increasing sweetness had no affect on any astringency parameter, although maximum intensity of bitterness was significantly decreased. Neither PROP nor salivary flow-status had any effect on perception of bitterness or astringency in either experiment.

Bitterness in wine

Bitterness in wine is elicited primarily by flavonoid phenols, which are bitter and astringent, and by ethanol. Monomeric flavonoid phenols are primarily bitter but as the molecular weight increases upon polymerization, astringency increases more rapidly than bitterness. The chiral difference between the two wine flavan-3-ol monomers produces a significant difference in temporal perception of bitterness: (-)-epicatechin is significantly more bitter and had significantly longer duration of bitterness than (+)-catechin. Ethanol enhances bitterness intensity and duration, whereas varying wine pH has little or no effect on perceived bitterness. Whereas PROP status had no significant effect on temporal perception of bitterness or astringency, subjects with low salivary flow rates took longer to reach maximum bitterness and astringency intensity and reported longer persistence of both attributes than high-flow subjects.

Human saliva and taste responses to acids varying in anions, titratable acidity, and pH

Twenty subjects recorded perceived sourness of solutions of citric + fumaric and of citric + tartaric acids, at pH 3.5 and titratable acidity (TiA) of 4.0 g/l on a moving chart, while parotid saliva flow was recorded via a sialometer . Sourness intensity and flow were greater when citric was the minor acid than when it was dominant. Subjects varied widely in calculated volume of saliva reservoir, but not flow rate (time to 2/3 reservoir vol.). In tartaric-fumaric acid mixtures varying in pH (3.0-3.75) at a constant TiA of 4.0 g/l, and varying in TiA (3.7-4.6 g/l) at a constant pH of 3.5, sourness intensity and parotid flow increased with acidity and decreased with pH. However, eight subjects with a high flow (HF = 1.2 +/- 0.28 g/2 min) and nine subjects with a low flow (LF = 0.43 +/- 0.11 g/2 min) differed widely: (a) In response to variation in stimulus pH and TiA, HF demonstrated marked alteration in flow, but little change in sourness ; LF responded at a lower absolute level, but showed marked changes in sourness and little change in flow; (b) Salivary pH was higher and Na+ was three times greater for the HF than for the LF subjects; and (c) Salivary Ca++ showed a direct relationship with flow and pH among the HF, but an inverse relationship for the LF subjects.

Automated HPLC analysis of glutathione and thiol-containing compounds in grape juice and wine using pre-column derivatization with fluorescence detection

Abstract An easy and sensitive method for the analysis of glutathione (GSH) and other thiol-containing compounds in grape juice and wine has been developed and optimized. Following a pre-column derivatization of thiols with o-phthalaldehyde (OPA) and 2 aminoethanol, isoindole derivatives are separated on reversed-phase HPLC column and quantified by a fluorescence detector. The minimum detection limits for thiols are: GSH, 3.3 nmol/l (1 μg/l); cysteine, 22 μmol/l (2.7 mg/L); methanethiol, 0.27 μmol/l (12.8 μg/l); ethanethiol, 0.65 μmol/l (11 μg/l). The method yields linear responses up to 40 and 21 mg/l for GSH and cysteine, respectively. GSH levels in two varietal grape juices during fermentation varied from 0 (starting juice) to 2.1 mg/l (wine) in Sauvignon blanc, while the GSH in a Palomino sample with 1.28 mg/l in the juice increased to 5.1 mg/l in the wine. This automated pre-column derivatization of thiols followed by an automatic injection procedure is sensitive, reproducible and rapid, with a run time of 35 min.

Using repeated ingestion to determine the effect of sweetness, viscosity and oiliness on temporal perception of soymilk astringency

Abstract Astringency is a persistent sensation which increases upon repeated ingestion. To evaluate the effect of viscosity, sucrose and oil on perception of astringency during consumption of soymilk, a sequential sipping time–intensity (TI) procedure was utilized. For each soymilk, judges sipped the first of four ingestions and initiated the continuous recording of astringency intensity. Each sip was expectorated at 10 s after ingestion, and sipped 10 s after expectoration of the previous stimulus. After the fourth sample, judges rated astringency for 30 s. Traditional TI parameters, as well as rate of onset for each sip and increase in maximum intensity per sip were extracted from the TI curves. Maximum astringency (IMAX) increased significantly with successive sips as did the astringency at the time of sipping. Time to IMAX decreased from sip 1 to 3, but was longer for sip 4, which may be an artifact of the rapid test pace. Although addition of 60 g l −1 canola oil had no affect on astringency, adding 40 g l −1 sucrose or increasing viscosity by 5 cp with CMC significantly lowered all astringency parameters. The reduction in astringency by CMC may result from restoration of salivary lubrication and in part by chelation or hydrogen bonding of CMC to the astringents reducing their ability to bind to salivary proteins. The reduction in astringency produced by sucrose is more probably due to a cognitive process. ©

Effect of viscosity, temperature and pH on astringency in cranberry juice

Abstract This study explored the interactive effects of temperature, pH, viscosity and quinic acid in modifying astringency of cranberry juice. A panel of 17 trained judges rated astringency of nine samples, in duplicate, at 10 and 20 s after ingestion. Astringency intensity at both times showed the same trends, although the ratings were lower at 20 s. Addition of 1.5 g/l quinic acid lowered the pH to 2.59 and failed to produce a difference in astringency from the base juice (pH 2.65) at 5°C and 25°C. Astringency of juice adjusted to pH 3 was significantly lower than either the base or quinic acid juices at both temperatures. Decreasing temperature caused an increase of about 2 cp in viscosity and lowered the perceived astringency at all pH levels. Increasing the viscosity by 1.5 cp using CMC (carboxymethylcellulose, medium viscosity, Sigma Chemical) lowered the perceived astringency for all pHs but the effect was only significant at 25°C. Thus, without varying the concentration of astringent compounds, astringency intensity could be significantly modified by altering viscosity or pH. ©