Development and use of time-intensity methodology ... - Science Direct

Food Research International 26 (1993) 375-385

REVIEW PAPER

Development and use of time-intensity methodology for sensory evaluation: A review Margaret Cliff & Hildegarde Heymann* University of Missouri-Columbia, College of Agriculture, Food & Natural Resources, Dept. Food Science and Human Nutrition, 122 Eckles Hall, Columbia, Missouri 65211, USA Contribution from the Missouri Agriculture Experiment Station Journal Series No. 11,779

Time-intensity (TI) evaluation has been a developing sensory methodology for some 40 years. During this time, the technology has evolved from the use of a simple paper/pen to computerized data collection. Concomitant with this development has been the refinement of average curve calculation and interpretation. This paper summarizes these developments, cross-references alternate nomenclature, and compiles and reviews the research applications.

Keywords: time-intensity,

temporal.

view provided here is an attempt to provide an understanding of the development of this technology. Sjiistriim (1954) and Jellinek (1964) were among the first to quantify the temporal response. They had judges record the perceived bitterness of beer at 1 s intervals on a scorecard, using a clock to indicate time. TI curves were then constructed by plotting the x-y coordinates on graph paper. They found that experienced judges were able to rate two different attributes simultaneously. To facilitate graphical illustration, Neilson (1957) had judges record perceived bitterness directly on stationary graph paper, at 2 s timed intervals. Judges were given caffeine, quinine sulfate, a barbiturate and sucrose octaacetate solutions and asked to record the perceived intensity (O-3) on the Y axis, while moving their hand along the X axis (time). Meiselman (1968), studying taste adaptation of NaCl/water solutions, and later McNulty and Moskowitz (1974) evaluating oil-in-water emulsions, improved the TI methodology by eliminating the potential distraction of the clock. They used audible cues to indicate to the judges when

DEVELOPMENT OF TIME-INTENSITY METHODOLOGY Perception of aroma, taste, and texture in foods is a dynamic, not a static, phenomenon. Typically classical sensory evaluation quantifies the sensory response using a unipoint measurement. Judges must time-average or integrate their responses to provide single intensity values (Lee & Pangborn, 1986). Time-intensity (TI) sensory evaluation is an extension of the classical scaling method providing temporal information about perceived sensations. By having judges continuously monitor their perceived sensations, from onset through extinction, one is able to quantify the continuous perceptual changes that occur in the specified attribute. For some 40 years, TI quantification has undergone an evolution as food scientists and psychophysicists alike have attempted to record the human response. The chronological literature re*To whom all correspondence

should be addressed.

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to record perceived intensities. TI curves were constructed upon completion of the evaluation. However, the greatest improvement occurred when Larson-Powers & Pangborn (1978) utilized a moving chart recorder equipped with a footpedal, for TI evaluation. Judges recorded their response to sweetness in beverages and gelatins, sweetened with sucrose or synthetic sweeteners, by moving a pen long the cutter bar, labelled with an unstructured line scale, from none to extreme. Judges initiated the chart recorder with the footpedal and moved the pen according to the perceived intensity. Cardboard was placed over the moving chart paper, to prevent the judges from biasing their responses by watching the evolving curves. In addition to providing the first continuous data collection method, it freed the judges from distractions caused by a clock or auditory signal. Despite these advantages, the technique required considerable judge training for manipulation of the foot-pedal, sample ingestion and pen coordination. In addition, interpretation of the curves was particularly labor intensive since curves had to be digitized manually. Lawless & Skinner (1979) developed a dial potentiometer connected to a strip chart recorder for TI evaluation of sweetness. The dial was fitted with a scale from 0 to 10. Taste intensity was recorded by turning the dial clockwise for increasing intensity, and anti-clockwise for decreasing intensity. This method avoided the distraction to the evolving plot, but like strip chart recordings, curves had to be digitized manually before evaluation. Shortly thereafter, Birch & Munton (1981) developed ‘SMURF’ (Sensory Measuring Unit for Recording Flux), another dial potentiometer system. They based the name ‘SMURF’ on the rationale that the TI response reflected the physiological flux of the stimuli to the receptor. They found that the TI measurements by ‘SMURF’ did not differ statistically significantly from TI scaling and clock measurements. Schmitt et al. (1984) and later Mufioz et al. (1986) used a strip chart recorder, but improved data manipulation by using a mechanical digitizer to evaluate TI traces. In the former study, a computer program was utilized to obtain the rate of increase and decrease of bitterness, while in the latter study TI was used to evaluate the textural properties of gels. With the ready availability and easy programmability of personal computers in the 1980s

several researchers (Takagaki & Asakura, 1984; Guinard et al., 1985; Lee, 1985; Yoshida, 1986; Cliff, 1987) independently developed computerized TI systems with a variety of hardware and software products. Takagaki & Asakura (1984) were the first to publish the development of a computerized TI system. They interfaced a computer with one of two input devices, either a sliding bar potentiometer or a push-down lever potentio-meter. These input devices, when manipulated along a magnitude estimation scale, resulted in a digitized voltage which was proportional to perceived intensity. TI curves were displayed on the video screen or printed. Shortly thereafter, Guinard et al. (1985) published their development of a computerized TI system using a Digital LSI 1l/2 computer, equipped with a slice bar potentiometer. Fortran programs controlled data acquisition and promoted the judges to perform the required tasks. The judge signalled initiation and termination of data collection by pressing a button on the potentiometer. Data were later transferred to a larger computer for further processing. Also in 1985, Lee (1985) developed a TI system with an Apple II microcomputer, using a game paddle. The game paddle consisted of a dial, with a clicker on the side. A fixed line scale appeared on the video screen and judges used the game paddle to manipulate an ‘X’ along the scale according to the perceived intensity. In this system, unlike the other computer systems previously developed, judges must have some familiarity with the handeye coordination associated with game paddle manipulation. Data were transferred to a larger computer for data analysis. Yoshida (1986) developed a TI system using a Fujitsu PC 9801 computer with a mouse as an input device. The scale was calibrated by magnitude estimation and references were provided. Judges were signalled to ingest the sample with their left hand while their right hand manipulated the mouse on the scale. An auditory signal prompted the judges to expectorate. Cliff (1987) also developed a TI data acquisition unit by interfacing a digitizing pad to a Rainbow 100 computer. The digitizing pad was modified to permit motion of a pointer in the ‘X’ dimension only, and data was recorded at 4 times per s. Programs written in ‘C’ (DEC, 1983) controlled data acquisition and experimental design. Rine (1987) developed a computerized TI sys-

Development and use of time-intensity methodology: A review

tern with a slide bar potentiometer interfaced to a Zenith ZF-158-42 computer. Judges were seated with their backs to the terminal, and were instructed to listen for the auditory signal generated by the computer which prompted ingestion and expectoration. This system was used by Taylor & Pangborn (1990) to evaluate the temporal hedonic responses, by labelling the potentiometer slot from ‘like extremely’ through ‘neither like or dislike’ to ‘dislike extremely’. For hedonic evaluation, judges began and terminated their rating process from the neutral position on the scale. Barylko-Pikielna et al. (1990) used a touchsensitive line satellite and monitor interfaced with a IBM/PC XT computer. Voltage signals were recorded every 0.1 s, according to the position of the finger along the line scale on the satellite. The judge controlled the initiation and termination of data collection by pressing the appropriate button. Janusz et al. (1991) used Apple II Plus computers for TI evaluation of synthetic sweeteners using cross-modality matching of sweetness with sound intensity. Judges were seated at a computer, provided with earphones, and guided through a crossmodality matching/magnitude estimation training session. During the test sessions, judges controlled a 1000 Hz sound between 0 and 90 dB, by using an uncalibrated thumbwheel spanning the entire range of sound intensity. Data were transferred to a mainframe computer of further analysis. More recently, computerized TI systems have been commercially available (Compusense, 199 1; OP&P, 1991) greatly enhancing the ease and availability of TI data collection and data processing. With the Van Buuren (1992) and the OP&P (1991) computerized sensory system each booth was equipped with a computer, monitor, and mouse. The judge indicated his/her response by manipulating a marker on a verticle line scale, using a mouse. Booths were networked to the mother computer. Data were collected at 5 readings per s for a 90 s period. Despite the availability of computerized systems, research is still published using the manual pad and paper method (Lee & Lawless, 1991), and the semi-manual strip-chart recorder method (Lim et al., 1989; Robichaud & Noble, 1990; Ott & Palmer, 1990). Although refinement of computer hardware and software has occurred, further development is inevitable. Although many computer advances have occurred, much controversy still remains about the method of interpretation and analysis of the TI curves.

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INTERPRETATION AND ANALYSIS OF TI CURVES

Before computerization, TI evaluation resulted in more information than could be realistically or conveniently handled. As a result, interpretation was limited to quantification of key-points or parameters on the curve. The parameters selected were relatively easy to access. Most frequently these parameters included maximum intensity, time-to-maximum intensity and total time. Less frequently, parameters such as plateau time (Birch et al., 1980; Schmitt et al., 1984; Cliff, 1987; Rine, 1987), lag time (Birch et al., 1980; Cliff, 1987; Liu & MacFie, 1990; Janusz et al., 1991) highest intensity before expectoration/ingestion (Ott & Palmer, 1990), time of half maximum (Lawless & Skinner, 1979; Janusz et al., 1991; Pecore, 1992), decline time (Liu & MacFie, 1990), and time for taste to linger (Kemp & Birch, 1992), have been reported (Table 1). Some researchers, with considerably more effort, reported area under the curve (Larson-Powers & Pangborn, 1978; Harrison & Bernhard, 1984; Lim et al., 1989; Ott & Palmer, 1990) by integration of the curve with the aid of a planimeter or by weight measurements. However, with computerization, parameters requiring computation were more easily obtained and parameters such as area under the curve (Yoshida, 1986; Cliff, 1987; Barylko-Pikielna et al., 1990; Janusz et al., 1991; Kemp & Birch, 1992; Pecore, 1992), area under the curve before and after maximum intensity (Yoshida, 1986; Cliff, 1987; Pecore, 1992), as well as, rate of onset (Birch & Munton, 1981; Schmitt et al., 1984; Leach & Noble, 1986; Cliff, 1987; Nasrawi & Pangborn, 1990; Robichaud & Noble, 1990; Janusz et al., 1991; Kemp & Birch, 1991; Pecore, 1992) and rate of decay (Gent & McBurney, 1978; Lawless, 1984; Schmitt et al., 1984; Leach & Noble, 1986; Lee & Pangborn, 1986; Cliff, 1987; Nasrawi & Pangborn, 1990; Robichaud & Noble, 1990; Janusz et al., 199 1; Pecore, 1992) are more frequently mentioned. However, not all researchers have used the same terminology and abbreviations when referring to these TI parameters. Therefore, Table 1 was compiled to cross-reference equivalent nomenclature and facilitate interpretation of the literature. As listed in Table 1, maximum intensity (I,,,) has been referred to as initial intensity, height to maximum intensity and maximum perceived intensity. Lag time (Tag), the time required before initial

M. CII~ H. Heymann

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Table 1. Summary of parameters, aliases, and abbreviations used to characterize TI curves Parameter

Alias

Maximum intensity Initial intensity Height to max. intensity Max. perceived intensity Maximum intensity Time-to-maximum

intensity Time to max Onset time Appearance time

Abbrev.

Reference

Zmax

Overbosch et al. (1986) Harrison & Bernhard (1984) Ott & Palmer (1990) Dubois & Lee (1983) Robichaud & Noble (1990) Cliff (1987) Nasrawi & Pangbom (1990) Birch et al. (1980) Janusz et al. (1991) Dubois & Lee (1983) Robichaud & Noble (1990) Cliff (1987) Birch et al. (1980) Kemp & Birch (1992) Portmann et al. (1992) Liu & MacFie (1990) DuBois & Lee (1983) Robichaud & Noble (1990) Cliff (1987) Birch et al. (1980) Cliff (1987) Liu & MacFie (1990) Birch et al. (1980)

4 HTMAX (I),, MAX T l-%4 it+

TIME to MAX Total time Persistence time Time Persistence Finish time Extinction time Total duration Plateau time Protraction

of max. int.

Lag time

? P Tend

ET DUR $lat ti,,

Start time Reaction time

T Start

Highest intensity before expectoration Highest intensity before ingestion

HIBE HIBI RT T,

Expectoration

Recording time Total recorded time Total elapsed time Time of % maximum

T,

Thdec T hmax Tdec

Ott & Palmer (1990) Ott & Palmer (1990) Ott & Palmer (1990) Birch et al. (1980) Janusz et al. (1991) Lawless & Skinner (1979) Janusz et al. (1991) Pecore (1992) Pecore (1992) Liu & MacFie (1990)

fiT

Kemp & Birch (1992) Kemp & Birch (1992)

TCR STIP AUC

Yoshida (1986) Kemp & Birch (1992) Nasrawi & Pangbom (1990) Ott & Palmer (1990)

t, Time of % max (decay) Time of % max (onset)

Decline time Time after maximum Time of taste to linger Maximum intensity-time Area Total Total Total Area

amplitude gustatory resp. intensity under curve

Rate of increase Max. rate of adsorption Maximum intensity rate Rate of onset

MIR RATE MAX ONSET

Slope rising Max. rate of onset

M O”W

Rate of decrease Max. rate of desorption Rate of decay Slope tailing Max. rate of decay Area-before-maximum

Mads

intensity

Mdes

DECAY

M decay A H area

Area-after-maximum Aftertaste

intensity

B OH,,,,

Cliff (1987) Kemp & Birch (1991) Ott & Palmer (1990) Robichaud & Noble (1990) Leach & Noble (1986) Pecore ( 1992) Cliff ( 1992) Cliff (1987) Robichaud & Noble (1990) Leach & Noble (1986) Pecore ( 1992) Cliff (1992) Yoshida (1986) Cliff (1987) Matuszewska (1992) Yoshida (1986) Cliff (1987) Matuszewska ( 1992)

Area after max./area before max. B/A Ratio AT

Yoshida (1986) Cliff (1987) Ott & Palmer (1990)


perception occurs, has been referred to as start time, reaction time and initial time. Time-to-maximum (T,,,,,), the time to maximum intensity, has been designated as onset time and appearance time. Plateau time (T,,,,), the duration of maximum intensity, has been termed protraction of maximum intensity. The total time (r,,,) or total duration of perception is sometimes called persistence, finish time or extinction time. Sometimes the product to maximum intensity and time has been reported, as an approximation for total gustatory response or area under curve. Area under the curve (Area) has sometimes been called total amplitude or total gustatory response. The ratio of the area-after-max to the area-before-max has been termed aftertaste and ratio. Slopes or rates of the increasing (Monset) and decreasing (Mdecay) portions of the curve have also been calculated. While most researchers have fitted linear regression for evaluation of rates of onset, some researchers (Swartz, 1980; Ott & Palmer, 1990; Kemp & Birch, 1992) have approximated the rate of onset by dividing the maximum intensity by the time-to-maximum intensity, thereby assuming a perfectly linear onset function. Individual judge differences are very unique and reproducible within individuals (Swartz, 1980; Pangborn et al., 1983; Schmitt et al., 1984; Guinard et al., 1985) due to differences in anatomy, oral manipulation, and scaling (Noble et al., 1991). However, this information is often sacrificed when interpretation of the TI parameters occurs using analysis of variance. The variance due to judges is partitioned out, and it is then possible to evaluate sample differences for each of the TI parameters. By summarizing the data in this way, a typical or average TI curve can be reconstructed for the mean TI parameters. In an attempt to more fully characterize the TI traces, average curves are usually calculated by averaging the intensity values at given times, and connecting the mean values. However, with this method there is no provision for an atypical response. All judges’ scores are used to calculate the mean response. To avoid irregular curve shapes (Overbosch et al., 1986) it may be necessary or desirable to group judges with similar responses (Cliff, 1987) before calculation. Overbosch et al. (1986) noted that when a curve is fitted through the arithmetic means calculated at given times, the resulting curve may show multiple peaks, peak broadening and tailing due to the declining number of judges contributing to the response with

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time. To partially alleviate this problem, average curves could be considered for the portion of the curve represented by the majority of the judges (Cliff, 1987). However, Overbosch et al. (1986) also pointed out that the average curve will not contain the mean values from the individual curves. To alleviate these problems, Overbosch et al. (1986) proposed another method of average curve calculation. They suggested normalizing the data in the intensity direction and calculating of the averages in the time direction for the ascending and descending portions of the curve. Using this methods, each curve is divided into the ascending and descending parts, separated by the T,,, value. The intensity values for each curve are normalized so that each judge’s maximum intensity equals the mean I,,,. To average in the T direction, time values are calculated for the selected percentages of I max (say 2%) (I = 0,2%1,,,,...,98%1,,,, I,,,, 98%Z,,,, . . .,2%1,,,,,, 0), for both the ascending and descending portions of each curve. A plot of the mean T and corresponding I values, generates the average curve. Using this method, the resultant average curve contains the averages for I,,,, T,,, and Tot, and is believed to be more indicative of the typical TI response. However, Liu & MacFie (1990) indicated that the Overbosch et al. (1986) method does not make provision for plateaus or stable sections because each intensity must correspond to only one time. To account for a plateau of maximum intensity, they suggest the Overbosch et al. (1986) method would require modification so that the starting point of the second would be Tdec, rather than I max, and the stable phase (plateau) assigned the I,,, value. In addition, with inadvertent multiple peaks preliminary curve smoothing should be preformed, to avoid filling the valley in the increasing region of the curve and eliminating the peaks in the decreasing region of the curve (Overbosch et al., 1986). Liu & MacFie (1990) proposed a modification to the Overbosch et al. (1986) method. They recommend normalization in the I and T directions, so that all curves have the same I,,,, Tstart, T,,,, Tdec, and Tend. Averaging of the intensity values are then performed for the ascending and descending phases of the curves at fixed T intervals. This is in contrast to the Overbosch et al. (1986) method which averages T at fixed I intervals. Using this method, the main parameters for the resulting curve are the averages of the corresponding

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parameter on the individual curves. The difference between this method and the Overbosch et al. (1986) method is relatively small and only observable where there is a plateau section or when missing values are present. When curves are strictly monotonically increasing or decreasing in the ascending and descending portions of the curve, the two methods result in almost identical average curves. Van Buuren (1992) suggested using principal component analysis (PCA) for the generation of summary curves rather than the usual average curves. This allowed information common to all judges to be extracted and weighted more heavily than information from deviating responses. This method is a tool for accounting for inter-judge agreement and produces more representative summary curves.

TIME-INTENSITY

RESEARCH

Over the last 40 years, TI research has been used most frequently for product research. However, it has also served as a tool for fundamental research on sweetness (Birch et al., 1980, 1982) bitterness (Leach & Noble, 1986) and oral irritation (Cliff, 1992). Birch et al. (1980) used TI evaluation to provide evidence for an orderly queue model for sweetness perception. Leach dz Noble (1986) used TI to speculate on bitterness perception. They found that for equal maximum intensities, caffeine had a faster rate of onset and a slower rate of decay than quinine. Since the time-to-maximum values were identical for the two compounds, they suggested that the differences in onset rates suggested different response mechanisms. They hypothesized that if caffeine was bound more strongly than quinine, then a faster adsorption and slower desorption would be expected, and the decay in perception might be controlled by desorption of the species from the receptor. They noted, however, that this faster adsorption and slower desorption may have been an artifact of the higher molar concentrations required to obtain a similar maximum intensity, and could possibly be due to different ratios of tastants to receptors for the two species. Cliff (1992) used TI methodology to develop an adsorptiondesorption model for oral irritation. Maximum intensities were used to evaluate the appropriateness of the Beidler taste equation and calculated the degree of affinity of the stimuli for

the receptor (KJ. Correlation coefficients for the proposed Beidler taste model were 0.996-0.999. The large association constants (&) for cinnamaldehyde (25 M-l) and capsaicin (5.2 X lo4 M-l) were consistent with their steep psychophysical functions and persistent aftertastes. The concentration dependencies for time-to-maximum (T,,,), plateau time ( Tplat), total time (T&, maximum rate of onset (M,,,,J and maximum rate of decay (Mdecay)quantified the adsorption-desorption processes and were congruous with the proposed model. TI research has also been used in methodological studies to assess the effect of multiple sample ingestions for bitterness (Guinard et al., 1986a), astringency (Guinard et al., 1986b), and irritation (Nasrawi & Pangborn, 1990) as might occur in food consumption. It has also been used to assess the effect of sample residence time in the mouth for saltiness (Matuszewska, 1992). Discrimination was greatest for samples held in the mouth for 10 s, compared to 5 s, or the sip-and-spit technique. Most recently, time-intensity has also been used to assess judge performance (Issanchou & Porcherot, 1992) for evaluation of bitterness in water and beer. The authors found that large individual judge differences occurred, especially for maximum intensity and total duration. In other methodological TI research, Ott & Palmer (1990) compared expectoration and ingestion of sweet, sour, salty and bitter solutions. Aftertastes for ingested NaCl solution were statistically significantly greater than expectorated solutions. Overall TI profiles looked very similar prior to expectoration or ingestion, but judge inconsistencies made interpretation after expectoration or ingestion difficult. In other words, the judges were more consistent when they were evaluating the solutions in their mouths than they were when evaluating the aftertaste of the solutions. In other TI research (Lawless & Skinner, 1979; Lewis et al., 1980; Moore & Shoemaker, 1981), temporal evaluations have been shown to be consistent with traditional scaling evaluations. Lundahl (1992) compared time-intensity scaling to categorical scaling for sweetness, sourness, bitterness and astringency of strawberry juice. He concluded that TI was superior to category scaling in the ability to resolve and explain sample differences. Lee (1989) compared the unipoint sensory methods with TI methods using informational entropy theory. His theoretical analysis showed that TI evaluation generated more useful information than


unipoint analyses, and that it could be obtained efficiently and with less susceptibility to errors. TI evaluation has also been used for hedonic response of chocolate milk (Taylor & Pangborn, 1990), basic tastes (Yoshida et al., 1992) and umami substances (Yoshida et al., 1992). The hedonic response for chocolate milk with O-36% milk fat (Taylor & Pangborn, 1990) increased with ingestion, achieved maximum liking sometime after swallowing or expectoration and returned to a neutral (neither like or dislike) upon termination of the taste. In addition, Pecore (1992) used TI methodology to predict consumer perception of sweeteners. However, TI evaluation has most frequently been used for intensity evaluations on a variety of products and compounds (Table 2). The majority of the TI research has focused on quantification of sweetness and bitterness, with relatively little research on saltiness, sourness, astringency, irritation, flavour and textural attributes. Although some research is available on saltiness (Meiselman, 1968; Meiselman, 1975; Takagi & Asakura, 1984; Barylko-Pikielna et al., 1990; Ott & Palmer, 1990; Matuszewska, 1992) and sourness (Larson-Powers & Pangborn, 1978; Norris et al., 1984; Takagi & Asakura, 1984; Mufioz et al., 1986; Barylko-Pikielna et al., 1990), an extremely limited amount is available on astringency (Guinard et al., 1986b; Robichaud & Noble, 1990; Lee & Lawless, 1991) and irritation (Lawless, 1984; Cliff, 1992). Despite the apparent appropriateness of TI methodology to quantify changes during ingestion, mastication and swallowing for flavor and texture assessment (Lee & Pangborn, 1986), TI has yet to be widely used as a tool in this area. To date, TI research on flavor attributes is only available for beer (Sjiistriim, 1954; Neilson, 1957), fruit flavored beverages/model systems (LarsonPowers & Pangborn, 1978; Cliff, 1987; Matysiak & Noble, 1991), chocolate puddingcreme (Pangborn & Koyosako, 1981) palm oils (Lee, 1986), 2pentanone in vegetable oil (Overbosch et al., 1986) anethole in oil/water emulsions (McNulty & Moskowitz, 1974), and sour rye breads (BarylkoPikielna et al., 1990). TI research on textural attributes is only available for the melting and perceived viscosity of ice cream (Moore & Shoemaker, 1981), viscosity of chocolate pudding/creme (Pangborn & Koyosako, 1981), firmness of gels (Larson-Powers & Pangborn, 1978; Mufioz et al., 1986) and adhesiveness and cohesiveness of peanut butter (Rine, 1987).

381

Since several excellent reviews (Lee & Pangborn, 1986; Cliff, 1987; Rine, 1987) have been published summarizing TI applications through 1987, the remainder of this review will serve to update these works. Lim et al. (1989) obtained TI profiles for sweetness and bitterness for six sweetener combinations and sucrose in shortbread cookies. Temporal sweetness profiles for the six sweeteners were similar to that for sucrose. Temporal bitterness refor all non-sucrose were similar sponses sweeteners, but higher than that for sucrose. Barylko-Pikielna et al. (1990) used TI methodology to evaluate the effect of salt on sour rye and wheat breads. Maximum intensity, total duration and area under the curve increased with salt concentration, whereas overall flavor reached a peak intensity 1.35% NaCl. Robichaud & Noble (1990) used both scalar and TI methodology to evaluate bitterness and astringency of gallic acid, catechin, grape-seed tannin and tannic acid in wine. They found that both maximum intensity and duration of astringency and bitterness increased with concentration; little or no changes were observed for time-to-maximum. However, a slightly longer time-to-maximum was observed for bitterness as compared to astringency. Lee & Lawless (1991) evaluated the time-course of astringent sensations. They had tasters record the decay of overall astringency, drying, puckery feeling, roughing, bitterness and sourness for alum, tannic acid, and tartaric acid. TI profiles were both compound and concentration dependent. All attributes showed an exponential decay of intensity with time. Noble et al. (1991) summarized the factors affecting the TI parameters for sweetness including sweetener identity, concentration, viscosity, temperature, complexity of the system, and method of evaluation. Matysiak & Noble (1991) evaluated sweetness and fruitiness (orange) of equisweet solutions of aspartame (APM), aspartame and acesulfame-K (APM + AK), and sucrose using TI methodology. They found solutions of APM had longer persistence times for both fruitiness and sweetness than either solutions of sucrose or APM + AK, which had similar persistences. Nasrawi and Pangborn (1990) used TI methodology to evaluate repeated oral stimulation from 2 ppm, capsaicin solutions. They found that repeated stimulation resulted in increased time-to-

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Table 2. Summary of TI applications for bitterness, sweetness, sourness, saltiness, astringency, irritation, flavor, and texture Reference Sweetness Meiselman (1968) Larson-Powers & Pangbom (1978) Lawless & Skinner (1979) Swartz (1980) \ ’ Birch et al. (1980) Birch & Ogunmoyela (1980) Birch & Munton (1981) Pangbom & Koyasako (1981) Dubois & Lee (1983) Takagi & Asakura (1984) Harrison & Bernhard (1984) Lee (1985) Yoshida (1986) Cliff (1987) Lim et al. (1989) Ott & Palmer (1990) Liu & MacFie (1990) Matysiak & Noble (1991) Kemp & Birch (1992) Portmann et al. (1992) Bitterness Sjbstrom (1954) Neilson (1974) Jellinek ( 1964) Meiselman (1968) Lee (1985) Larson-Powers & Pangbom (1978) Lewis et al. (1980) Schmitt et al. (1984) Takagi & Asakura (1984) Guinard et al. (1985) Guinard et al. (1986a) Leach & Noble (1986) Lim et al. (1989) Robichaud & Noble (1990) Ott & Palmer (1990) Van Buuren (1992) Kemp & Birch (1992) Issanchou & Porcherot (1992) Sourness Larson-Powers & Pangborn (1978) Norris et al. (1984) Takagi & Asakura (1984) Mut’ioz et al. (1986) Ott & Palmer (1990) Barylko-Pikielna et al. (1990) Saltiness Meiselman (1968) Meiselman (1975) Takagi & Asakura (1984) Ott & Palmer (1990) Barylko-Pikielna et al. (1990) Matuszewska (1992) Astringency Guinard et al. (1986b) Robichaud & Noble (1990) Lee & Lawless (199 1) Irritation Lawless (1984) Nasrawi & Pangbom (1990) Cliff (1992) Flavor . Siiistrijm (1954) Neilson (1957) ’ McNulty & Moskowitz (1974) Larson-Powers & Pangbom (1978) Pangborn & Koyasako (198 1) Birch & Ogunmoyela (1980) Lee (1986) Cliff (1987) Overbosch et al. (1986) Barylko-Pikielna et al. (1990) Matysiak & Noble (1991) Texture Larson-Powers & Pangborn (1978) Moore & Shoemaker (198 1) Pangborn & Koyosako (198 1) Munoz et al. (1986) Rine (1987)

Product/compound Sucrose solutions Orange gelatin Sucrose solution Sweeteners Sucrose, thaumatin Chocolate drink Sugar solutions Chocolate pudding and creme SweetenersSweet compound Sugar solutions Chocolate Sweeteners Glucose solutions Sweeteners in shortbread cookies Sucrose Sucrose solutions Sweeteners Amino acid solutions Sugar solutions Beer Bitter compounds Beer quinine sulfate Chocolate Flavored beverages, gelatin Iso-cr acids, water and beer Beer Bitter compound Iso-cwacids in water Iso-cy acids in beer Caffeine, quinine Sweeteners in shortbread cookies ;;4iiFt compounds Beer Amino acids Water and beer Flavored beverages, gelatin Binary acid solutions s;“;; compound Citric acid Sour rye and wheat breads Taste adaptation NaCl Taste adaptation NaCl Salty compound Sodium chloride Sour rye and wheat breads Salt solutions Tannic acid in wine Astringent compounds Astringent compounds Capsaicin, piperine, ginger oleo Capsaicin Capsaicin, cinnamaldehyde, piperine Beer Beer, bubble gum Anethole in oil/water emulsion Flavored beverages Chocolate pudding and creme Chocolate drink Butter flavour, palm oils Peach flavored solutions 2-Pentanone in vegetable oil Sour rye and wheat breads Orange flavored solutions Hardness gelatins Viscositv ice cream Viscosity pudding Firmness aels AdhesiveiEohesive peanut butter


maximum with successive stimulation, and either a progressive increase (18 subjects) or decrease (14 subjects) in perceived intensity. Janusz ef al. (1991) evaluated a number of aspartic acid amide sweeteners using TI methodology. Correlations of sensory results with measured and calculated hydrophobicities suggested the hydrophobicity may be important in characterizing the temporal characteristics of sweeteners. Kemp & Birch (1992) used TI methodology to investigate sweetness and bitterness of amino acids. They fit linear and power law functions to describe the psychophysical functions and calculated K, and R,,,. They found limited relationships between chemical structure of the amino acids and their temporal properties. Portmann et al. (1992) used TI to evaluate glucose, fructose and sucrose solutions at 22°C and 35°C. For all sugars, persistence increased linearly with concentration. Equi-sweet solutions of sucrose, D-glucose, and D-fructose were prepared with apparent viscosities of 5, 15, 25, and 35 mPa using maltodextrin. Increases in solution viscosity increased sweetness intensity and persistence. They concluded that intensity and persistence of sweetness was related to the physico-chemical properties affecting the accessibility of the sweet molecule to the receptor, and the effect of temperature on water structure. Cliff (1992) used TI methodology to quantify the temporal response for cinnamaldehyde, piperine and capsaicin. For equipungent solutions, the pungency of cinnamaldehyde was perceived and eliminated quickly, whereas the pungency of piperine and capsaicin was perceived and eliminated slowly. Also, eaters and non-eaters of chili peppers differed statistically significantly in their TI profiles. Eaters of chili peppers had lower maximum intensities, shorter total times, and slower maximum rates of onset and decay for perceived pungency. Judges with substantial desensitization also showed shorter time-to-maximum intensities.

SUMMARY AND PERSPECTIVES Clearly over the last 40 years, time-intensity has had an increasingly important function in sensory evaluation research. With the commercial availability of computerized TI systems, notable improvements in calculation and interpretation of summary curves (Overbosch et al., 1986; Liu &

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MacFie, 1990; Van Buuren, 1992) and better understanding of the underlying processes of temporal perception (Cliff, 1992), it is anticipated that TI will become more widely utilized in both academia and industry. Although Rine (1987) and Lee & Lawless (1991) innovatively used TI methodology to evaluate descriptive textural (adhesiveness, cohesiveness) and astringency (drying, pucker-y feeling, roughing) attributes, respectively, time-intensity has yet to be used in conjunction with descriptive analysis for product development. It is believed that TI methodology is highly underutilized in the evaluation of textural and flavor characteristics. It has had limited application in the evaluation of persistent flavor and aftertastes relating to food quality. Research has yet to be conducted relating the textural breakdown and flavor release characteristics of food with phase transformations during food mastication. In addition, TI methodology has yet to be used for the evaluation of the retronasal component of food flavor, by having judges wear nose-clips during temporal evaluation. Furthermore, TI methodology has not been applied for the assessment of visual characteristics such as degree of carbonation of sparking beverages or rate of browning of apples. It is believed that TI methodology could also be used to assess the degree of nasal pungency or flavour release of onions during chopping or heating, respectively. TI has not been applied for the assessment of consumer and pharmaceutical products. TI methodology could be used for the evaluation of nonthermal heating and cooling of products such as toothpaste, mouthwashes, throat lozenges and muscle pain-relievers. It could also serve as a tool for the further development of theoretical and mechanistic models for taste perception. As noted by Cliff (1992) TI is particularly well suited for the evaluation of bitterness, astringency, burning and cooling; responses which are not limited by the reaction time of the judges. Finally, it is anticipated that the theoretical and practical applications of TI will lead to innovative advances in the field of sensory science. REFERENCES Barylko-Pikielna, N.. Matuszewska, I. & Hellemann, (1990). Effect of salt on time-intensity characteristics bread. Lebensm- Win. u.-Technol.. 23, 422-6.

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(Received 23 September 1992; accepted 30 November 1992)