COMPARISON OF TIME-INTENSITY WITH CATEGORY SCALING OF BITTERNESS OF ISO-α-ACIDS IN MODEL SYSTEMS AND IN BEER*

A Timely Application-Temporal methods, past, present, and future

Eating is a dynamic experience, and temporal sensory methods have been proposed to document how products change over the course of consumption or use (nonfood). A search of online databases yielded approximately 170 sources related to temporal evaluation of food products that were compiled and reviewed. This review summarizes the evolution of temporal methodologies (past), offers guidance in selecting appropriate methods (present), and provides insights into the future of temporal methodologies in the sensory space. Temporal methods have evolved to document a variety of characteristics in food products including how the intensity of a specific attribute changes over time (Time-Intensity), which specific attribute is dominant at each time during evaluation (Temporal Dominance of Sensations), which attributes are present at each time point during evaluation (Temporal Check-All-That-Apply), and many others (Temporal Order of Sensations, Attack-Evolution-Finish, and Temporal Ranking). In addition to documenting the evolution of temporal methods, this review considers the selection of an appropriate temporal method based on the objective and scope of research. When choosing a temporal method, researchers should also consider the selection of panelists to perform the temporal evaluation. Future temporal research should focus on validation of new temporal methods and explore how methods can be implemented and improved to add to the usefulness of temporal techniques for researchers.

First objective evaluation of taste sensitivity to 6-n-propylthiouracil (PROP), a paradigm gustatory stimulus in humans

Practical and reliable methods for the objective measure of taste function are critically important for studying eating behavior and taste function impairment. Here, we present direct measures of human gustatory response to a prototypical bitter compound, 6-n-propyltiouracil (PROP), obtained by electrophysiological recordings from the tongue of subjects who were classified for taster status and genotyped for the specific receptor gene (TAS2R38), and in which taste papilla density was determined. PROP stimulation evoked negative slow potentials that represent the summated depolarization of taste cells. Depolarization amplitude and rate were correlated with papilla density and perceived bitterness, and associated with taster status and TAS2R38. Our study provides a robust and generalizable research tool for the quantitative measure of peripheral taste function, which can greatly help to resolve controversial outcomes on the PROP phenotype role in taste perception and food preferences, and be potentially useful for evaluating nutritional status and health.

Dose-Dependent Effects of L-Arginine on PROP Bitterness Intensity and Latency and Characteristics of the Chemical Interaction between PROP and L-Arginine

Genetic variation in the ability to taste the bitterness of 6-n-propylthiouracil (PROP) is a complex trait that has been used to predict food preferences and eating habits. PROP tasting is primarily controlled by polymorphisms in the TAS2R38 gene. However, a variety of factors are known to modify the phenotype. Principle among them is the salivary protein Ps-1 belonging to the basic proline-rich protein family (bPRP). Recently, we showed that oral supplementation with Ps-1 as well as its related free amino acids (L-Arg and L-Lys) enhances PROP bitterness perception, especially for PROP non-tasters who have low salivary levels of Ps-1. Here, we show that salivary L-Arg levels are higher in PROP super-tasters compared to medium tasters and non-tasters, and that oral supplementation with free L-Arg enhances PROP bitterness intensity as well as reduces bitterness latency in a dose-dependent manner, particularly in individuals with low salivary levels of both free L-Arg and Ps-1 protein. Supplementation with L-Arg also enhanced the bitterness of caffeine. We also used 1H-NMR spectroscopy and quantum-mechanical calculations carried out by Density Functional Theory (DFT) to characterize the chemical interaction between free L-Arg and the PROP molecule. Results showed that the -NH2 terminal group of the L-ArgH+ side chain interacts with the carbonyl or thiocarbonyl groups of PROP by forming two hydrogen bonds with the resulting charged adduct. The formation of this PROP•ArgH+ hydrogen-bonded adduct could enhance bitterness intensity by increasing the solubility of PROP in saliva and its availability to receptor sites. Our data suggest that L-Arg could act as a 'carrier' of various bitter molecules in saliva.

Temporal characteristics of gustatory responses in rat parabrachial neurons vary by stimulus and chemosensitive neuron type

It has been demonstrated that temporal features of spike trains can increase the amount of information available for gustatory processing. However, the nature of these temporal characteristics and their relationship to different taste qualities and neuron types are not well-defined. The present study analyzed the time course of taste responses from parabrachial (PBN) neurons elicited by multiple applications of “sweet” (sucrose), “salty” (NaCl), “sour” (citric acid), and “bitter” (quinine and cycloheximide) stimuli in an acute preparation. Time course varied significantly by taste stimulus and best-stimulus classification. Across neurons, the ensemble code for the three electrolytes was similar initially but quinine diverged from NaCl and acid during the second 500ms of stimulation and all four qualities became distinct just after 1s. This temporal evolution was reflected in significantly broader tuning during the initial response. Metric space analyses of quality discrimination by individual neurons showed that increases in information (H) afforded by temporal factors was usually explained by differences in rate envelope, which had a greater impact during the initial 2s (22.5% increase in H) compared to the later response (9.5%). Moreover, timing had a differential impact according to cell type, with between-quality discrimination in neurons activated maximally by NaCl or citric acid most affected. Timing was also found to dramatically improve within-quality discrimination (80% increase in H) in neurons that responded optimally to bitter stimuli (B-best). Spikes from B-best neurons were also more likely to occur in bursts. These findings suggest that among PBN taste neurons, time-dependent increases in mutual information can arise from stimulus- and neuron-specific differences in response envelope during the initial dynamic period. A stable rate code predominates in later epochs.

Relation between parotid saliva flow and composition and the perception of gustatory and trigeminal stimuli in foods

The objective of this study was to investigate whether and how parotid saliva flow and composition correlated with the perception of gustatory and/or trigeminal stimuli in foods. Thirty (15 male and 15 female) subjects tasted seven foods or beverages (lemonade, beer, wine, soup, methyl cellulose, peanut butter, and crackers) with three levels each of a gustatory or trigeminal stimulus and rated the perceived intensity of the corresponding sensation over time using the time-intensity (TI) method while their parotid saliva was being collected. Salivary flow rates of males were significantly higher than those of females for all stimuli (p < 0.001). That did not translate, however, into consistent differences in perception of sensory attributes between males and females. Significant positive correlations were found between saliva flow and (1) TI parameters for adhesiveness of peanut butter and cohesiveness of mass of crackers (p < 0.05 or lower) and (2) time from intake to swallowing of crackers and peanut butter (p < 0.05). No correlations were found between saliva composition (e.g., sodium and total protein) and TI parameters. These results indicate that parotid saliva flow may correlate with the perception of some texture and mouthfeel attributes (presumably through oral work and bolus formation) but not with that of the taste attributes examined in this study (at the concentrations studied).

Chemoreception and perception of the bitterness of isohumulones

Psychophysical experiments were conducted to determine whether isohumulones share a common receptor mechanism with other bitter compounds, and whether parotid saliva flow affects perception of their bitterness. Findings from a study of inter-individual differences in sensitivity to 23 sweet and/or bitter compounds among 25 subjects using the time-intensity (TI) method suggest that isohumulone and tetrahydroisohumulone may share a common receptor mechanism with other bitter compounds except those with the thiourea moiety. Isohumulone and tetrahydroisohumulone displayed a unique dome-shaped TI profile. The bitterness of the two compounds took longer to develop, but it lasted as long as for other bitter stimuli. In a study of the relation between perception of bitterness in beer and parotid saliva flow in 20 young adults, no significant difference was found among the mean saliva flows triggered by 0, 15 and 30 mg/L of isohumulones added to beer, and no significant correlation was found between saliva flow and maximum intensity or total duration of bitterness.

Time-quality tracking of monosodium glutamate, sodium saccharin, and a citric acid-saccharin mixture

The temporal patterns of taste-quality descriptors evoked by 1000-ms duration stimulus liquids flowed through a closed delivery system over the anterodorsal tongue tip region were indicated using touch-typing on a computer keyboard. Single keys corresponded to the taste words of a 23 item code. A computer monitor displayed for subjects the keys pressed and when they were pressed, starting at stimulus delivery. For 2 mM sodium saccharin (NaSac), 75% of the responses were "sweet," 6.5% "sugar"; for NaSac in 10 mM citric acid (ArtLem), 43% "sour," 20% "citrus," and 11% "sugar"; for 214 mM monosodium glutamate (MSG), 28% "salty," 14% "sour," and 10% 1st "soapy," then "no taste," and finally "bitter." Distilled water received "no taste" on all trials. Response durations were 657 ms for ArtLem, 594 ms for NaSac, 577 ms for MSG. MSG yielded multiple quality responses on 25.5% of the trials; ArtLem, 9%; and NaSac, 1%. These results are compared with temporal patterns for taste intensity and with unrestricted verbal descriptions of the solutions.

Constraints imposed on taste physiology by human taste reaction time data

The speed with which an organism responds to stimulus events is reaction time (RT): the minimum time interval between stimulus arrival at a receptor organ, and an overt response by the organism. This time interval specifies maximum duration of all processes necessary for the RT sequence. Responses to any change in taste have RT less than 1 sec for suprathreshold concentrations. Therefore, constituent events at taste receptors, in the central nervous system (CNS), and at the response organ, must have sufficient durations less than 1 sec (Constraint 1). Taste stimulus durations of 50 msec, and therefore taste receptor events of approximately 50 msec, are sufficient for these responses (Constraint 2), as well as for taste quality identification responses (Constraint 3). Taste receptor latencies, neural conduction times, and RT response organ events are even briefer. Thus, 60% to 90% of human taste RT is CNS events. Taste receptor events remain crucial, but CNS processing is important, and apparently time limiting, in all human taste judgments.