We all have experienced the burning sensation of Hispanic foods and the cooling sensations of minty chewing gums, but how do food companies test the degree of “heat” and “cooling” when developing new foods?
Salsas, enchiladas, spicy and minty chewing gums, and mentholated lozenges appear to be very different products. But they all contain irritating ingredients that require sensory scientists to adjust some of the traditional sensory analysis protocols used in descriptive or discrimination tests.
It should be noted that “irritation” and “sensitization” are common terms in the sensory science community and are not associated with any harmful effect. Rather, “irritation” refers to an innocuous chemical feeling and generally can encompass perceptions of prickling, numbing, and burning; some scientists might include cooling and heat in its definition.The terms do not have the same meaning as in the medical field. For example, one study conducted with certain gum flavors prior to launch was conducted to ensure that the formula would not cause actual irritation.
Common chemesthetic irritants, such as capsaicin from chile pepper, piperine from black pepper, and menthol, stimulate multiple receptors within the trigeminal nervous system. The trigeminal nervous system contains exhaustible neurotransmitters, and their depletion will have a significant impact on the performance of panelists and their consistent measurement of chemesthetic attributes, such as heat (warmth), cooling, and burn (irritation). Often, heat and burn are used synonymously.
There are several protocol procedures that sensory panels might need to consider when testing chemesthetic attributes. Because we tend to treat menthol and capsaicin as being in the same class of sensory irritation, some protocols for managing their irritation should be similar. Although the protocols could be varied depending on the intensity of the irritation, they should provide sensory scientists with a general understanding of how to manage panel evaluations of irritating products, thus avoiding panelist “burnout.”
Sensitization and Desensitization
Desensitization is defined as the reduced sensitivity to stimuli, and typically results in the delayed ability to recover full sensitivity. It differs from adaptation, in which a person tends to recover quickly, e.g., within a few minutes. Based on research with capsaicin, desensitization may be caused by the depletion of neuropeptides (Substance P) following repeated activation of pain receptors in the trigeminal system (Silver et al., 1985). An is an example a person’s becoming “used to” higher intensities of peppers with continued exposure. The irritation by menthol has also demonstrated desensitization (Cliff and Green, 1994;Silver et al., 1985; Green, 1989), although the cooling quality showed little desensitization (Cliff and Green, 1994).
In contrast, sensitization is the increased sensitivity to stimuli. Sensitization by capsaicin has been demonstrated with rapid repeated exposures to capsaicin, although its mechanism is not completely understood (Green, 1991; Lawless and Stevens, 1988). Sensitization by menthol has been documented less, although menthol does tend to sensitize the oral cavity to subsequent thermal stimuli. A typical example of menthol sensitization is when a person drinks a glass of cold water while chewing mentholated gum. What might have been considered a refreshingly cool glass of water may turn into an almost painfully chilly experience.
A few protocol procedures could be used to manipulate the onset of either sensitization or desensitization for capsaicin or menthol irritation. The ultimate goal is to prevent either phenomenon from occurring, thus maintaining confidence in the panel’s assessment of irritating stimuli.
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Protocol Procedures Affect Results
Although most research to date does not explore protocol procedures in finished products, there are numerous examples of factors that should be considered when designing the protocol procedures for the sensory testing of “fiery” and “frosty” foods:
• Serving Temperature. Menthol cooling and capsaicin heat/burn are influenced by temperature, either sample temperature, oral cavity temperature, or residual “rinse” temperature.Capsaicin and menthol have both been shown to stimulate thermal receptors (Cliff and Green, 1994, 1996). Thus, it is not surprising that reduced temperature samples tend to mask the burn of capsaicin (Sizer and Harris, 1985; Baron and Penfield, 1996) or that warm samples sensitize the warmth of menthol (Green, 1985).
Rinsing the oral cavity with cold rinses also can reduce the residual burn perception (Nasrawi and Pangborn, 1990). However, as often reported both anecdotally and in research by Green (1991), this residual relief is only temporary, and the burn sensation returns. Thus, there appears to be a significant interaction between thermal and chemical perception, and sample temperature should therefore be considered when conducting sensory analysis of these types of irritants.
• Rinsing. Folklore has suggested that common foods such as beer, milk, sour cream, chocolate, and sugar water can be used to reduce the intensity of spicy foods, and a few researchers have tackled this issue head on (Hutchinson et al., 1990; Nasrawi and Pangborn, 1990; Stevens and Lawless, 1986).
Because capsaicin is fat soluble, a few scientists have hypothesized that rinsing the receptor with a fatty substance might prevent excessive heat buildup. Ethanol and dairy products were ineffective in reducing burn (Nasrawi and Pangborn, 1990; Stevens and Lawless,1986); this may indicate that either the capsaicin molecule is extremely tightly bound or the transduction process is irreversible once it starts.
Although other researchers identified some relief by the use of rinses or tastants, most agree that the effect may be driven by the very presence of food in the mouth, thus decreasing the oral cavity temperature. However, Allison et al. (1999a) demonstrated that the use of crackers and water between tasting salsa samples accelerated the burn decay more than if no rinse had been used; this was most effective with longer time delays between samples. That recommendation may be less appropriate for for burn perceptions of higher magnitudes.
• Tasting Frequency. The timing of sample exposure will influence the onset of sensitization or desensitization (Green, 1989; Lawless, 1984). At lower capsaicin concentrations, rapid and repeated exposure to capsaicin (every minute) resulted in sensitization. Additionally, once sensitization sets in, delaying the next sample by 15 min resulted in desensitization. Indeed, Green (1989) proposed that an between-sample delay of only 2.5 min was all that was necessary to create the onset of mild desensitization (Green, 1989). This implies that multiple bites of a sample could impart sensitization, but that desensitization could occur when the next new sample/prototype is served following a brief break.
In contrast, Allison et al. (1999a) demonstrated that descriptive panelists could accurately evaluate a “medium” salsa with up to 16 min between samples, with only minimal sensitization of residual temporal assessments. However, shorter time intervals (<4 min between samples) demonstrated slight sensitization, which may have implications for discrimination tests where samples are typically served more quickly. In any case, shorter time intervals will most definitely result in sensitization.
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So if complete desensitization could be achieved, how long would it take to for a panelist to fully recover? Karrer and Bartoshuk (1991) suggested that 1–3 days may be required to recover from desensitization of a 10-ppm capsaicin exposure. It is strongly recommended that a panel never achieve complete desensitization. To date, it is unclear whether mild desensitization would require such stringent guidelines.
• Number of Samples. As demonstrated previously, the number of samples that should be evaluated may depend on the amount of time between them. No published research has focused primarily on this topic. However,multiple exposures has been a major research paradigm for a few authors (Green 1989). Allison et al. (1999a)demonstrated the ability of a panel to evaluate 8 salsas in a session if sufficient time and rinsing (crackers/water) were allowed between samples. In that study, a “medium” salsa required at least 16 min between samples to allow the residual burn to decay to a negligible level and prevent desensitization. Samples with higher or lower irritation intensities may require more or less time between samples.
• Concentration of Heat/Cooling Ingredients. Higher levels of menthol and capsaicin will demonstrate higher intensities of heat and cooling (Allison et al., 1999b, 2001; Lawless, 1984) and will demonstrate a greater chance for desensitization. There is not as much research on protocol procedures for high-intensity products, but most scientists prefer to keep the “hot stuff ” to the end of their sessions. More research is needed on procedures for testing multiple heat or cooling levels within a single testing session. This is particularly true for test sessions where experimental designs require that panelists randomly evaluate these products.
• Ingredient Interactions. Capsaicin perception may affect the perception of other taste qualities, as well as be affected by other ingredients such as salt, sugar, and fats. Lawless et al. (1985) demonstrated a modest masking effect on the fundamental tastes-bitter, sweet, salt, and sour-following exposure to a low level of capsaicin (2 ppm). However, Cowart (1987) found no effect of capsaicin on basic tastes of a chicken broth.
Pungency and nasal irritation have also been shown to diminish odors of accompanying stimuli (Cain and Murphy, 1980), and desensitization by high levels of capsaicin (100 ppm) caused slight reduction in the perceived intensity of bitter and sour qualities (Karrer and Bartoshuk, 1995). The interaction of irritants and other fundamental taste qualities may be somewhat confounded, in that some of these basic tastes may be accompanied by irritating qualities, such as the burn from citric acid and sodium chloride.
Some studies have demonstrated that ingredients such as sugars and fats can attenuate the perceptions of capsaicin and menthol in finished products (Baron and Penfield, 1996; Allison et al.,1999b; 2001).
• Evaluation Methods. Many researchers have proposed or investigated methods for sensory evaluation of irritant stimuli. The famous Scoville Heat Unit (Scoville, 1912) is used by the American Spice Trade Association (ASTA, 1968). The test employs a dilution schedule for assigning the SHU value. This method has been criticized for its panelist subjectivity, heat buildup, desensitization, and lack of statistical validity and standardized references (Gillette et al., 1984).They are synonymous.
Because of the lingering characteristic of chemesthetic attributes, temporal analysis is common for spicy products. Lag time, onset, time of maximum intensity, and area under the curve are important characterizing elements of an irritant’s sensory perception. Several authors have used time intensity to demonstrate the variation between irritants and the interaction of ingredients on irritant profile (Baron and Penfield, 1996; Nasrawi and Pangborn, 1990; Lawless, 1984; Cliff and Heymann, 1992; Allison et al., 1999b; 2001).
A method sponsored by ASTM International (ASTM, 1994) uses a 15-point scale with standardized references of diluted capsaicin. Developed by Gillette et al. (1984), this method was intended to allow reproducible and reliable assessment of heat/burn, and its resulting scores are highly correlated with results of high-performance liquid chromatography. This method is essentially a scale typically found in descriptive sensory analysis, which has been used in other published studies to characterize and measure pungency from piperine, cuminaldehyde, cinnamaldehyde, eugenol, ginger, and alcohol (Cliff and Heymann, 1992).
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• Descriptive Language. Irritation is difficult to measure because it is not always the same perception to everyone. Terms such as irritation, prickly, throbbing, warmth, numbing, tingling, burn, heat, cooling, painful, etc., may be used to describe the sensation of irritants. As long as a panel has a good reference of the irritation quality, the attribute can be customized to the preference of the sensory scientist. Burn and cooling are the most common and highly recommended because of their simplicity (Table 1), but the language may greatly depend on the project objective and the irritating character of the product.
• Location of Perception. Different irritants can affect a single area or multiple areas in the oral and nasal cavities. Menthol has a noticeable nasal and oral impact, while capsaicin predominantly elicits an oral sensation. Depending on the irritant, the panel may want to specify where the irritation occurs in these areas. Indeed, some areas of the oral cavity, for example, are more sensitive to irritants than others.
Many authors have reported the multiple location effect of various irritants and also identified the quality of the burn, e.g., diffuse or acute (Lawless, 1984; Lawless and Stevens, 1988; Krajewska and Powers, 1988; Cliff and Heymann, 1992). Thus, it is encouraged that several locations be considered for irritant measurement, including the throat, tongue (apex or posterior),nasal cavity, inside cheek, and lip. This is also true for cooling or other irritant qualities.
• Standardized References. Several standardized intensity references have been used in the past, but only a few may have long-term success. Finished food products, such as salsas, pepper sauce, mint gums, and lozenges, are not recommended for use as references because they contain interfering flavors. Indeed, mint flavor may be mistaken for menthol cooling because they are often synonymous with each other. Most important, these products may be subject to production variation, especially those composed of natural ingredients, such as salsa.
ASTM (1994) promoted the use of diluted pure capsaicin with a tasteless emulsifier and water as a standardized reference. Menthol may also be prepared in the same way, although some descriptive research studies have diluted menthol in ethanol (Allison et al., 2001; Gwartney and Heymann, 1995). However, because ethanol is also an irritant and possesses an unpleasant aroma, it is recommended that improved menthol references be developed and standardized.
The biggest benefit of using standardized references is obvious when we consider panelists’ variable neurological responses to irritation. The ASTM (1994) standardized references allow all panelists to “calibrate” with each other, preventing sensitive panelists from scoring products differently than a less sensitive panelist. Table 1 provides examples of standardized references for heat and cooling perceptions.
There is one major caveat to consider when using heat or cooling references. Because sensitization or desensitization can be elicited by even a brief exposure to a standardized reference, sensory panels need to minimize reference testing during the testing sessions. Indeed, reference exposure should be considered a “sample” rather than a simple flavor stimulus.
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Only a few researchers have truly focused on testing a variety of protocol options. But from the wider range of physiological research, it is evident that there are several factors that must be considered when conducting sensory evaluation on mentholated or spicy products:
• References. A reliable standardized reference is recommended; the ASTM (1994) heat reference is a good example.
• Evaluation Method. Because of the enduring nature of burning, cooling, and warmth, it is recommended that panels employ temporal descriptive sensory evaluation with consensus
on attributes and definitions.
• Rinses. Although a “magical elixir” rinsing agent has not yet been proven, any type of interfering stimulus, such as crackers and water, might be helpful in accelerating decay. But it is not necessarily proven to significantly reduce the time needed between samples.
• Daily Exposure. The frequency and number of samples have a significant impact on evaluation accuracy. Thus, time between samples appears to a major protocol element that is required to manage panelist burnout. But because the time between samples would greatly depend on the stimulus intensity, we cannot provide a single protocol recommendation. Panelists need enough time for the residual irritation to attenuate to a negligible level. But they also need enough time to prevent subsequent samples from exhibiting sensitization or desensitization. Current research has only demonstrated that for a moderately spicy product (salsa), at least 16 min would be needed for an accurate and repeatable evaluation of up to 7 samples per day (Allison et al., 1999a). Intense products would require more time, and possibly fewer samples per day.
• Weekly Exposure. Although milder samples could most likely be tested daily, it is encouraged that panels evaluate highly intense products only a few days a week, because desensitization is an ongoing process that could be exacerbated by daily testing of irritating compounds. Although this is a difficult proposition with the current trend toward fast-paced development cycles, the panel’s long-term reliability could be preserved.
by Ann-Marie Allison and Terry Work
Author Allison is Sensory Scientist , Kellogg Co., 1 Kellogg Square, Battle Creek, MI 49016. Author Work is Manager, Sensory Science, McCormick & Co., Inc., 204 Wight Ave., Hunt Valley, MD 21031 Send reprint requests to author Allison ([email protected]).
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