THRESHOLDS AND SUPRATHRESHOLD INTENSITY FUNCTIONS FOR CAPSAICIN IN OIL AND AQUEOUS

BASED CARRIERS

HARRY T. LAWLESS. CAROLINE HARTONO and SUSANA HERNANDEZ

Department of Food Science Cornell Wniversiq Ithaca. Ny I4853

Received for Publication April 1, 1999

ABSTRACT

Thresholds and responses to capsaicin in oil- and water-based model systems were compared. Thresholds were measured among 23 individuals using an ascending forced choice method of limits. Detection thresholds were I I . 75 (& 1.31) mg/L in oil and 0.31 (& 0.03) mg/L in water. A horizontal labeled magnitude scale was used to obtain suprathreshold scaling functions in corn and soybean oils and in water systems using an emulsifier @olysorbate) or alcohol to solvate the capsaicin. In the water-based systems, capsaicin ranged from weak to very strong ratings over a range of 0.3 to 10 mg/L while the same perceptual range was achieved in oil over a range of I0 to 316 mg/L. Nonconsumers of hot/spicy foods generally had higher thresholds and higher suprathreshold responses in oil systems. Differences between user groups were less pronounced in water-based stimuli.

INTRODUCTION

Capsaicin and its homologues are the active sensory components in chili peppers that provide oral sensations of heat or irritation. The primary approach of the psychophysical literature in studies of capsaicin's effects has been to present capsaicin in simple aqueous-based solutions or emulsions in order to assess responses in isolation from the complicating effects of other stimuli and sensations. However, the fat content of products varies in more complex food systems. Capsaicin and other trigeminal irritants are lipophilic and thus more readily soluble in lipids. Prediction of capsaicin's sensory impact based on aqueous systems, then, might considerably overestimate the perceived intensity in a fat containing food. Conversely, reduction in the fat content of food

Contact author Harry T. Lawless, Department of Food Science, Stocking Hall, Cornell University, Ithaca. N Y 14853. TEL: 607-255-7363; FAX: 607-254-4868; E-mail: htll@cornell.edu

Journal of Sensory Studies 15 (2000) 437-447. All Rights Reserved. "Copyrighr 2000 by Food & Nutrition Press, Inc.. Trumbull, Connecticut 437

438 H.T. LAWLESS, C . HARTONO and S. HERNANDEZ

products would conceivably increase the delivery of capsaicin to oral tissues due to changes in partitioning between the food system and oral perireceptor environment.

Several studies have examined the response to capsaicin in more realistic food systems of varying fat content. Baron (1995; Baron and Penfield 1996) examined responses to capsaicin in water, and in cheese sauces and starch pastes that differed in fat content. Baron observed decreasing responses with increasing fat content. This is consistent with the idea that less of the hydrophobic capsaicin molecule would partition onto oral tissues and thus to trigeminal receptors in an oily carrier. Carden et af. (1999) extended this work by looking at cheese sauces with different levels of fat and fat mimetics. Reducing the fat level increased maximum heat level, the total area under the time-intensity curve and the heat level at 180 s in the decaying portion of the time function. Hutchinson et al. (1990) examined the ability of various foods to influence the oral bum associated with capsaicin pretreatments. They found that butter was slightly more effective than other foods in reducing the capsaicin burn after samples were expectorated. Naswari and Pangborn (1990) examined the time course of capsaicin bum with various rinses following capsaicin expectoration. Milk rinses were generally more effective than water in reducing the capsaicin bum and the rebound in bum experienced after expectoration of the rinse. However, fat content of the milk was not observed to have any differential effect. The general pattern of results is consistent with the concern that fat reduction in foods will be accompanied by increased release to oral surfaces upon ingestion and gain better access to chemoreceptors (Rosin and Tuorila 1992; Shamil el al. 1992; Plug and Haring 1993).

Our main goal in the present study was to examine the degree of difference in the sensory properties of capsaicin in extreme model systems, pitting water-based stimuli against 100% oil carriers. We also classified subjects into user and nonuser groups to see if any differences between groups as a function of the carriers might be found. Individuals who regularly consume hot spicy foods often show lowered responsiveness to oral irritants (Lawless et al. 1985; Prescott and Stevenson 1995, 1996). A secondary issue concerned the use of different agents to solubilize the capsaicin in water based systems. Due to the lipophilic nature of these compounds and other trigeminal irritants, psychophysi- cal studies of oral chemical irritation have generally used one of two additional compounds to solvate the irritant molecules. When making stock or parent solutions before dilution, either ethanol (Stevens and Lawless 1987; Green 1989) or an emulsifying agent such as polysorbate (Lawless 1984; Cliff and Heymann 1992) has been used to help dissolve the capsaicin. It is generally presumed that the small amounts of ethanol or polysorbate remaining after dilution have little or no sensory impact and that these agents act in a similar manner.

CAPSAICIN IN OIL AND WATER 439

MATERIALS AND METHODS

Subjects

In the threshold study, 23 adults were recruited from the Cornell campus. Subjects filled out a questionnaire on chili liking and frequency of eating hot/spicy foods (Lawless et al. 1985). Those with a score of 18 or higher were classified as high-frequency users of chili (“users”, N = 9, mean score = 25.7). Those with a score lower than 18 were classified as “nonusers” (N = 14, mean score = 10.0). In the scaling study, 31 adults from the Cornell campus served as subjects, with 20 “users” (mean score = 24.6) and 11 “nonusers” (mean score = 9.6).

Stimuli

Aqueous-based carriers for capsaicin (8-methyl-N-vanillyl -6- nonenarnide, Sigma Chemical Co.) solutions were prepared by adding 10 g of polysorbate or 10 g ethanol per 500 mL of stock solution. Stock solutions contained 1 mg/L capsaicin in the threshold study and 360 mg/L in the scaling study. Spring water (Chemung Water Co.) was used as the solvent and diluent for the aqueous stimuli. Threshold concentrations in aqueous systems were produced by serial diluation to yield 0.03125, 0.0625, 0.125, 0.25 and 0.5 mg/L capsaicin. Suprathreshold concentrations for scaling were 0.316, 1.0, 3.16 and 10 mg/L. In the oil-based carriers, capsaicin was first dissolved to make a 32 mg/L stock solution for the threshold work and a 720 mg/L stock solution for the supra- threshold scaling. Concentrations for threshold testing were 1, 2, 4, 8 and 16 mg/L in vegetable oil for thresholds and 10, 32, 100 and 316 mg/L in vegetable oil (100% soybean) and corn oil for the scaling study. Concentrations were chosen based upon pilot work to provide a reasonable bracketing of threshold and approximately equal ranges of perceived capsaicin burn (oral heat) for the scaling concentrations.

Threshold Procedure

A modification of ASTM procedure E679-79, an ascending forced-choice method of limits, was used. Three samples (two water or oil only and one capsaicin) of 10 mL each were presented at each concentration level at room temperature. Subjects were asked to choose the one item from the three that they thought contained the capsaicin, and to give a certainty judgment (guessing or not guessing). Certainty or sureness judgments have often been added to forced choice tests to improve discriminative power (Rousseau et al. 1998). The individual best estimate threshold was taken as the geometric mean of the last concentration with an incorrect response and the first concentration with a

440 H.T. LAWLESS, C . HARTONO and S. HERNANDEZ

correct response except for the following sequence: if the subject indicated a “guessing” response for the correct choice, that concentration was increased by a factor of 1.41 to bring it up to the geometric mean between the two concentra- tion steps, to adjust for the possibility of chance correct responding. Group thresholds were then taken as the geometric mean of the individual best estimate thresholds.

Scaling Procedure

Subjects tasted the four concentrations in a different carrier (vegetable oil, corn oil, polysorbate emulsion or ethanol solution on each of four days. Order of the carriers was counterbalanced. Stimuli were 15 mL capsaicin samples at room temperature (24.5C). Ratings were made on a 15 cm horizontal labeled magnitude scale (Green et al. 1996), recorded using a computerized sensory analysis system (Compusense five, Release 3.0, Compusense Inc., Guelph, ON, Canada) and scored from 0 to 100 based on the position of a line mark from the left (zero) end of the scale. Stimuli were “swirled” in the mouth for 5 s and expectorated. An interstimulus interval of 5 min was allowed to permit the capsaicin bum to subside. Three-way analyses of variance (ANOVAs) with repeated measures (mixed model, subjects as a random effect) were conducted with carrier and concentration as within subjects variables and user group (high or low chili consumption) as a between-groups effect. Separate ANOVAs were conducted on the oil-based and the water-based concentration series since they only had one concentration in common.

RESULTS

Thresholds in oil were higher than those in water by a factor of about 30. Figure 1 shows the individual thresholds for the user and nonuser groups in oil and water-based systems. There was no overlap between the two carriers with a mean threshold of 11.75 (i- 1.31, S.E.) mg/L inoil and 0.31 (i- 0.03) mg/L in water. These were slightly higher but in the general range of previous findings (Krajewska and Powers 1988; Govindarajan et al. 1987). Considerable overlap was evident between the two user groups, but a few regular users of chili had lower thresholds than the nonuser group in the oil system (means of 9.82 and 14.74 mg/L in oil, t (19.8) = 2.074, p = 0.051). Consistent with this difference, a small negative correlation was observed between the chili questionnaire score and thresholds (r = -0.41, p = 0.055), indicating that higher chili use was associated with lower thresholds. No difference was seen between user group thresholds in the water system (mean thresholds = 0.30 and 0.34 mg/L for users and nonusers, respectively).

CAPSAICIN IN OIL AND WATER 44 1

loo ,, I I 1 11 water "Users " "Non users" I I

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Panelist Number (arbitrary)

FIG. 1. THRESHOLDS (mglL, ON A LOG SCALE) IN OIL-BASED AND WATER-BASED CARRIERS FOR CAPSAICIN, AMONG FREQUENT AND INFREQUENT CONSUMERS OF

HOT AND SPICY FOODS

Suprathreshold scaling functions showed a similar difference between oil and water-based systems. The psychophysical functions for capsaicin bum in oil were displaced on the concentration axis, with isointense concentrations increased by a factor of about 50 compared to the water-based stimuli (Fig. 2). Figure 3 shows that nonusers of chili had higher responses than users in the oil system (F( 1,28) = 10.7, p = 0.003). With the exception of the 3 mg/L ethanol stimulus, this was not the case (concentration by user group interaction, F(3,18) = 4.58, p = 0.015) (Fig. 4).

DISCUSSION

Responses to oral capsaicin are severely attenuated in lipid carriers as opposed to water-based stimuli. Our results are consistent with the observations of Baron (1995), that increasing the fat level in a food system decreases responses to capsaicin. In his studies with cheese sauce and starch pastes, isointense responses were found at levels of approximately 10 to 1, comparing his high fat conditions to water. Our ratios were even greater, but these experiments present more extreme conditions of comparison. Within each carrier type, no differences were observed, and one can conclude from this that any

442 H.T. LAWLESS, C. HARTONO and S . HERNANDEZ

-t- Polysorbate

-0- Ethanol

-0- Soybeanoil

-A- Corn Oil

(User Groups Combined)

7 0 1

I 0.1 1 10 100 1000

Concentration (mg/L)

FIG. 2. MEAN SUPRATHRESHOLD DOSE-RESPONSE CURVES FOR CAPSAICIN IN OIL AND WATER-BASED CARRIERS. BOTH USER GROUPS COMBINED

60

50

% 40

.- I3 2 30

.- F n

P) C .- - c 2o = 10

0

Oil Based Carriers

--+--- Soybean 011

--+-- &moil

llsers + Soybean Oil

4 Corn 011

I I

10 100

Concentration, mgL

FIG. 3 . MEAN SUPRATHRESHOLD DOSE-RESPONSE CURVES FOR CAPSAICIN (LOG SCALE) IN OIL BASED CARRIERS, AMONG FREQUENT (“USERS”) AND INFREQUENT

CONSUMERS (“NONUSERS”) OF HOT AND SPICY FOODS

CAPSAICIN IN OIL AND WATER 443

70

60

50 .- c) m a: .- c 40 E 2 30 C -

“Aqueous “-based carriers

-

1 10

Polysorbate ---A- - - Ethanol --+--

llsers Polysorbate *

Ethanol

Concentration, m g L

FIG. 4. MEAN SUPRATHRESHOLD DOSE-RESPONSE CURVES FOR CAPSAICIN (LOG SCALE) IN WATER-BASED CARRIERS, AMONG FREQUENT (“USERS”) AND

INFREQUENT CONSUMERS (“NONUSERS”) OF HOT AND SPICY FOODS

differences among oil types or between the solvating agents (ethanol or polysorbate) would be much smaller in magnitude than the differences between oil and water seen in these data. This may aid somewhat in interpreting the earlier literature, as ethanol has irritative properties, and there has been speculation that it might not be a good choice as a solubilizing agent. Krajewska and Powers (1988) reported that subjects were unable to differentiate blanks containing ethanol at their threshold level of 0.03 % . Polysorbate is a powerful emulsifier and could potentiate soapy or bitter tastes (for example, see the emulsifier control conditions, Fig. 3, in Lawless and Stevens (1984)). However, comparisons of ethanol and polysorbate at these levels show no clear effects.

Several mechanisms could give rise to this phenomenon of increased flavor impact with water-based carriers. Simple partitioning of the capsaicin molecule from oil into the aqueous medium of saliva might occur in similar ratios to our isointensity matches. Such a finding would provide a simple explanation for the observed differences. Examination of partitioning from oil into artificial saliva suggests that simple partitioning will overpredict the observed oil/water ratio (Hartono 1999). That is, the amount of capsaicin partitioned to saliva is even less than the isointensity ratio. Based on this observation, a model for this partitioning might also take into account the association of capsaicin with

444 H.T. LAWLESS, C. HARTONO and S . HERNANDEZ

epithelial tissues. Not only is the equilibrium between oil and saliva important, but also the binding, association or sequestering of capsaicin by the oral tissues that affects the dispersion of capsaicin in the mouth. Kinetic factors could also play a part, as the rate of capsaicin entry into saliva and then onto oral tissues could affect sensory impact. Simple equilibrium partitioning considerations may oversimplify the modeling.

As in the previous literature, we noted that people who consume hot and spicy foods on a regular basis are partially desensitized to the sensory effects of oral capsaicin. That is, they are much less responsive above threshold. Threshold effects are less clear, with early work by Rozin ef al. (198 1) showing small effects of desensitization among frequent chili consumers. Our current study showed a paradoxically higher sensitivity (lower thresholds) among frequent consumers, but once again it was a small effect. Our data showing reduced responses above threshold among chili consumers is consistent with previous findings (Lawless et al. 1985; Cowart 1987; Cliff and Heymann 1993; Stevenson and Yeomans 1993). The question remains as to why differences between user groups were more apparent in oil than water. Given the small group sizes in these studies, it is possible that lack of statistical power obscured group differences in the aqueous media. However, since the group sizes were the same for the oil conditions, statistical power alone cannot explain the effect size differences noted here.

Our results did not address issues regarding the time course of capsaicin irritation in the oral cavity and potential differences in carrier effects. Previous work with discrete-point TI scaling showed little information in the time-related parameters regarding fat content differences (Baron 1995) although rebound effects following expectoration were found by Naswari and Pangborn (1990). Carden et al. (1999) examined rate of growth (time to maximum) for their reduced fat cheese sauces, but effects were nonmonotonic, with the fastest onset occurring in low fat, as opposed to full fat or nonfat formulations. Further research should examine parameters of the time intensity (TI) curves for differences due to fat content. One time intensity study of capsaicin in salad dressings showed similar effects to those observed here, with much lower sensory impact from oil-based dressings, higher impact from water-based dressings and intermediate impact from emulsions (Hartono 1999). Time to maximum of the water-based carriers was delayed, but this is confounded with overall intensity level. Other studies should examine time-related parameters of the TI curve when maximum intensity is equated.

Sensory studies with irritants are further complicated by the sequential effects of sensitization (Stevens and Lawless 1987), desensitization (Green 1989) and stimulus induced recovery (Green 1996). The five minute interstimulus interval was chosen based on its previous use in the sensory literature (Krajewska and Powers 1988) and the fact that using any shorter time intervals

CAPSAICIN IN OIL AND WATER 445

would have entailed frequently adding a new sensation to an existing back- ground, an undesired effect in most sensory studies. A five minute interstimulus interval would normally induce some level of desensitization. This would serve to depress the psychophysical function and lower the overall level of response. However, if the degree of desensitization is dependent upon the delivery and perceptual impact of the capsaicin stimulation, it should have been the same for all of our scaling conditions since the perceptual ranges (rather than concentra- tions) were set to be approximately equal. Had we held concentration constant and compared intensities across media, sequential effects would have potentially played a larger role. In the threshold studies, the presence of desensitization effects is generally unlikely due to the low levels of stimulation. The matter is further complicated by the finding that under repeated stimulations, the desensitization is to some degree undone by repeated stimulation with capsaicin, a phenomenon called stimulus induced recovery (Green 1996).

ACKNOWLEDGMENTS

The authors thank John Home for technical support and the panelists for their participation.

REFERENCES

ASTM 1991. Standard practice for determination of odor and taste threshold by a forced-choice method of limits. E-679-79. In Annual Book of Standards. Vol. 15.07, pp. 34-39, American Society for Testing and Materials, Philadelphia.

BARON, R.F. 1995. Carrier type, fat content, capsaicin concentration and sample temperature: Effects of perceived pungency. Ph.D. Dissertation, Univ. Tenn. Knoxville.

BARON, R.F. and PENFIELD, M.P. 1996. Capsaicin heat intensity - Concentration, carrier, fat level and serving temperature effects. J . Sensory Studies I I, 295-3 15.

CARDEN, L.A., PENFIELD, M.P. and SAXTON, A.M. 1999. Perception of heat in cheese sauces as affected by capsaicin concentration, fat level, fat mimetic and time. J . Food Sci. 64, 175-179.

CLIFF, M. and HEYMANN, H. 1992. Descriptive analysis of oral pungency. J. Sensory Studies 7, 279-290.

CLIFF, M. and HEYMANN, H. 1993. Time-intensity evaluation of oral bum. J. Sensory Studies 8, 201-211.

446 H.T. LAWLESS, C. HARTONO and S. HERNANDEZ

COWART, B.J. 1987. Oral chemical irritation: Does it reduce perceived taste intensity? Chem. Senses 12, 467-479.

GOVINDARAJAN, V.S., RAJALAKSHMI, D. and CHAND, N. 1987. Capsicum - production, technology, chemistry and quality. Part IV. Evaluation of quality. CRC Crit. Rev. Food Sci. Technol. 25, 185-282.

GREEN, B.G. 1989. Capsaicin sensitization and desensitization on the tongue produced by brief exposures to a low concentration. Neurosci. Let. 107,

GREEN, B.G. 1996. Rapid recovery from capsaicin desensitization during recurrent stimulation. Pain 68, 245-253.

GREEN, B.G., DALTON, P., COWART, B., SHAFFER, G., RANKIN, K. and HIGGINS, J. 1996. Evaluating the labeled magnitude scale for measuring sensations of taste and smell. Chem. Senses 21, 323-334.

HARTONO, C.L. 1999. The effects of water and oil carriers on the perception of capsaicin pungency by users and nonusers of chili peppers. M.Sc. Thesis, Cornell University.

HUTCHINSON, S.E., TRANTOW, L.A. and VICKERS, Z.M. 1990. The effectiveness of common foods for reduction of capsaicin bum. J. Sensory Studies 4, 157-164.

KRAJEWSKA, A.M. and POWERS J.J. 1988. Sensory properties of naturally occurring capsaicinoids. J. Food Sci. 53, 902-905.

LAWLESS, H.T. 1984. Oral chemical irritation: Psychophysical properties. Chem. Senses 9, 143-155.

LAWLESS, H.T., ROZIN, P. and SHENKER, J . 1985. Effects of oral capsaicin on gustatory, olfactory and irritant sensations and flavor identification in humans who regularly or rarely consume chili pepper. Chem. Senses 10, 579-589.

LAWLESS, H.T. and STEVENS, D.A. 1984. Effects of oral chemical irritation on taste. Physiology and Behavior 32, 995-998.

NASWARI, C.W. and PANGBORN, R.M. 1990. Temporal effectiveness of mouth-rinsing on capsaicin mouth-bum. Phys. Behav. 47, 617-623.

PLUG, H. and HARING, P. 1993. The role of ingredient-flavor interactions in the development of fat-free foods. Trends Food Sci . Technol . 4, 150- 152.

PRESCOTT, J. and STEVENSON, R.J. 1995. Effects of oral chemical irritation on tastes and flavors in frequent and infrequent users of chili. Physiol. Behav. 58, 1 1 17-1 127.

PRESCOTT, J. and STEVENSON, R.J. 1996. Psychophysical responses to single and multiple presentations of the oral irritant zingerone: Relationship to frequency of chili consumption. Physiol. Behav. 60, 617-624.

ROSIN, S. and TUORILA, H. 1992. Flavor potency of garlic, pepper and their combination in different dispersion media. Lebensm.-Wiss. u.-Technol. 25,

173- 178.

139- 142.

CAPSAICIN IN OIL AND WATER 441

ROUSSEAU, B., MEYER, A. and O’MAHONY, M. 1998. Power and sensitivity of the same-different test: comparison with triangle and duo-trio methods. J . Sensory Studies 13, 149-173.

ROZIN, P., MARK, M. and SCHILLER, D. 1981. The role of desensitization to capsaicin in chili pepper ingestion and preference. Chem. Senses 6, 23-3 1.

SHAMIL, S . , WYETH, L.J. and KILCAST, D. 1992. Flavour release and perception in reduced-fat foods. Food Qual. Pref. 3, 35 1-360.

STEVENS, D.A. and LAWLESS, H.T. 1987. Enhancement of responses to sequential presentation of oral chemical irritants. Physiol. Behav. 39, 63-65.

STEVENSON, R.J. and YEOMANS, M.R. 1993. Differences in ratings of intensity and pleasantness for the capsaicin burn between chili likers and non-likers: Implications for liking development. Chem. Senses 18, 471-482.