• Cross-Modal Integration
• Trigeminal Sensitivity
• Trigeminal Interactions
• An Outline of Trigeminal Chemoreception
• Chemical Structures of some Irritant Compounds

Cross-Modal Integration

Figure 1 shows the anatomical subsystems mediating chemical sensations in the nose and mouth. One striking characteristic of flavor perception is that information from all modalities may be synthesized into a unitary experience with a single hedonic response, appetitive or aversive. Consumers taste food and decide immediately whether it is likable or something to reject. As gatekeepers of the alimentary tract, the nose and mouth function together, and do so with speed and seemingly little cognitive effort. This synthesis of sensory inputs is not limited to the chemical senses. Foods are also appreciated for their appearance, visual texture and color, as well as tactile sensations during active manipulation and destruction in the oral cavity. Cross-modal information is integrated. Influences of color on taste and smell and of texture on taste are common in everydat eating. Food perception is an active process - it cannot be effectively understood when stimuli are imposed on a passive organism. Humans are by nature foragers and sensation seekers. This provides an important place for studies of flavor perception in intact and behaving organisms.

FIGURE 1: Schematic of the anatomical systems mediating perception of flavors.

In contrast to this synthetic sort of perception, it is also possible to focus attention on the individual systems producing chemical sensations. Trained tasters on an industrial descriptive analysis panel are taught to analyze taste sensations from the mouth as distinct from volatile perceptions mediated by the olfactory organs. Sapid substances in the oral cavity give rise to classical taste sensations such as sweet, sour, salty, bitter and other impressions such as umami, an important taste category in Japan ans other countries, roughly equivalent to a savory or meaty flavor arising from stimuli such as monosodium glutamate. In contrast to this relatively limited number of distinct categories for taste experience, there are a much larger number of distinct experiences from volatile chemicals stimulating smell. Sensations as diverse as the smells of citrus, cut grass, mint, various flowers, woods, fruits, herbs, spices, burnt aromas, sulfidic aromas, ehtereal, fatty and sweet (like vanilla) smells all seem so distinct as categories as to be unrelated. Thus the qualitative range of olfaction seems quite wide compared to taste. It could be argued that smells provide the majority of diversity in out flavor experiences.

Trigeminal Sensitivity

In addition to these two classical systems for chemical sensitivity, there is a more generalized chemical sensitivity in the nose and mouth, and over the whole body. Mucus membranes such as the anus are also sensitive - an old Hungarian saying has it that the good paprika burns twice, and similar sentiments are known in other countries with hot, spicy cuisines. In the nose and mouth, this more general chemical irritability is primarily mediated by the trigeminal nerves (Fig. 1). These systems have recently been described by the term "chemesthesis" in an anology to somesthesis. A variety of everyday flavor experience arise from trigeminal stimulation: the fizzy tingle of CO₂ in soda, the burn from hot peppers, black pepper, and spices such as ginger and cumin, the nasal pungency of mustard, horseradish, the bite from raw onions and garlic, not to mention their lachymatory effects, to name a few. This important chemical sense is easily overlooked in considerations of taste and smell, because it has received less experimental study than the classical taste and smell modalities. However, a simple demonstration, such as comparing the sensations form warm, flat (decarbonated) soda to warm, fizzy soda should easily convince one of the importance of trigeminal sensations.

Of course, this set of nerves also mediates tactile, thermal and pain sensations, so the distinction between a chemical sense and a tactile sense becomes blurred somewhat. This blurring is perhaps worst in the sensations of astringency. Tannins in foods are chemical stimuli, and yet the astringent sensations they produce seem largely tactile. They make the mouth feel rough and dry, and cause a drawing, puckery or tightening sensation in the cheeks and muscles of the face. Although scientific analysis would categorize astringency as a group of chemically-induced oral tactile sensations, most wine tasters would say that astringency is an important component of wine "flavor." This highlights, once again, the integrative nature of flavor in combining inputs from multiple modalities.

The mechanisms giving rise to these sensations are poorly understood, but one long-standing popular theory has it that tannins bind to salivary proteins and mucopolysaccharides (the slippery components of saliva), causing them to aggregate or precipitate, thus robbing saliva of its ability to coat and lubricate oral tissues. One feels this result as rough and dry sensation on oral tissues, even when there is fluid in the mouth. Note that "roughness" and "dryness" are difficult to perceive unless a person moves the tongue against other oral tissues (which we do all the time when eating). An active perceiver is required for astringent perception. Astringency is well-suited to study with active human observers and less well-suited to study with immobilized animals in electrophysiological preparations.

The importance of chemesthesis can be defended on two grounds - one anatomical and the other economic. The sheer numbers of trigeminal fibers, relative to the other sense organs are impressive. One study found three times as many trigeminal fibers in the fungiform papillae of the rat as facial nerve fibers innervating taste buds. Students of taste anatomy normally consider a fungiform papilla as an anatomical structure that holds taste buds and provides a taste organ, or more accurately, it is thought of as the organ for the perception of chili pepper burn! Even the taste bud itself seems organized to provide trigeminal access to the oral milieu. Trigeminal fibers ascend around the taste bud itself, forming a chalicelike structure. The trigeminal endings seem to use the specialized structure of the tastebud to find a chennel to the external environment. This speculation is consistent with the observation of high responsiveness to pepper chemicals in areas such as the top of the tongue that are rich in fungiform papillae.

The economic impact of trigeminal stimulation on the food and flavor industry is easy to underestimate. If CO₂ is considered a trigeminal flavor, the carbonated beverage industry - soda, beer, sparkling wines, etc. - amounts to several billion dollars in sales of a trigeminal flavor in this country alone. Putting aisde CO₂. we can ask about the economic impact of individual spices or their use in various products. The pepper business amounts to several hundred million dollars anually. Furthermore, so-called ethnic foods are experiencing a period of rapid growth due to a continuing influx of immigrants from cultures with hot, spicy cuisines, and a growing trend toward less neophobic and more adventurous dining on the part of many Americans. As evidence of this, the sales of salsa surpassed the sales of ketchup for the first time in 1992.

Trigeminal Interactions

Several psychophysical studies have examined interactions of trigeminal irritation from chemicals with taste and with odor perception. As in most laboratory psychophysics, these studies have focused on simple intensity changes in single chemicals in simple mixtures. The first workers to examine effects of chemical irritation on olfaction found mutual inhibition of smell by CO₂ in the nose (Cain & Murphy, 1980). This seems to occur even though the onset of the sting from CO₂ is delayed somewhat compared to the onset of smell sensations. Because many smells also have an irritative component (Tucker, 1971), it is probable that some of this inhibition is a common event in everyday flavor perception. If a person had decreased sensitivity to nasal irritation, the balance of aromatic flavor perception might be shifted in favor of the olfactory components. If irritation impact is reduced, then the inhibitory effects of nasal irritation would also be reduced. This might explain, in part, why smokers who are less responsive to irritation have little or no apparent olfactory deficiency when tested under controlled conditions.

Short-term effects of multiple capsaicin stimuli within an experimental session. Following a strong sample, the stimulus is perceived to be less intense than when it follows a weaker sample or is the first in a series.

Application of the red pepper compound, capsaicin, to the skin or oral epithelium has profound desensitizing effects. This is also known to occur with systematic administration of capsaicin - animals injected with capsaicin become inured to chemical irritants to a remarkable degree. This is believed to reflect a depletion in substance-P, a peptide neurotransmitter for pain. Because effects of substance-P have also been linked to functioning of endorphins, there is at least an indirect explanation for the apparent addiction that occurs to spicy foods among osme people. Hign dietary levels of capsaicin also result in a chronic desensitization, as shown in psychophysical tests (Lawless, et al 1985). The extent and influence of this desensitization should be investigated further.

Reduced response to capsaicin among regular consumers of hot, spicy foods. Conditions are at two points in the tasting sequence.

When confronted with the seemingly shocking level of hot pepper use by the regular spicy food consumer, a common comment by people who do not eat hot spices is, "how can you taste your food?" There are several possible replies, one of which is that the burn or irritation is not unpleasant, and therefore does not command the attention the way other painful stimuli do, and that the other flavors are still present in addition to the pain (you merely have to shift your attention to them - if you want to). In spite of a vast literature on capsaicin in animal models, Nagy once commented that "no detailed quantitation has been conducted on the influence of capsaicin on non-nociceptive sensory stimuli" (Nagy, 1982). This raises the fundamental question of whether chili burn can mask tastes in the mouth, the way that CO₂ sting masks smell in the nose. Partial inhibition of taste responses has been found following pretreatment of oral tissues with capsaicin, particularly of sour and bitter tastes. In contrast, Cowart (1987) observed little or no effect of capsacin on tastes when capsacin was mixed with taste stimuli, even though such direct mixing produced equal or higher levels of overall irritation than capsacin pretreatments. A potential resolution of this paradox is suggested by the finding that capsacin desensitization takes several minutes to develop (i.e., it depends upon a delay between treatment and test stimuli) (Green, 1989). Such a temporal gap would have occurred to varying degrees in pretreatment experiments with tastants. Conversely, when capsacin stimuli were given in a more continuous sequence (as in the mixture studies) irritation grew over intervals.

The potential time dependence of capsaicin inhibition of taste as well as the fact that capsaicin inhibition is most reliably observed for acid and quinine, substances sometimes reported as partially irritative, suggests that the inhibitory effect seen in pretreatment studies may have been due to desensitization to an irritative component of the presumed "tastants," rather than a direct effect on gustatory intensity per se. If so, two simple predictions can be made regarding time and concentration variables:

1. Inhibition of taste should parallel the time course of capsacin self-desensitization, including the need for a hiatus between capsaicin treamtent and testing as observed by Green.
2. More intense taste stimuli (strong acid, strong salt) should show proportionally larger decrements after capsaicin treatment, because they have more of an irritative component than weaker, more dilute stimuli.

The reciprocal issue of whether tastes can modulate or ameliorate chili burn is a subject of some speculation. There are folk remedies in various cultures, such as starchy corn (Peru), ghee (India), pineapple (Phillipines), sugar (various Latin countries), and beer (Ithaca, New York, among other places). Systematic studies of trying to wash out chili burn with different tasting rinses has shown some effect for swwet (most pronounced), sour, and perhaps salt. Cold stimuli provide a temporary but potent inhibition of pepper burn, as known to many habitues of ethnic restaurants. Since capsacin is highly lipid soluble, the Indian remedy of ghee (clarified butter) would seem to have some merit. Sour things stimulate salivary flow, which may provide some relief to abused oral tissues. The combination of fatty, sour, cold and sweet suggests the author's favorite antidote, forzen yogurt. Certainly the Indian culinary practice of alternating cool, sweet chutneys with hot curries would seem to have some merit from this perspective.

• Important component of food flavor:
  • There is vast trigeminal innervation of the nose and mouth, including the fungiform papillae!
  • many food components have irritative aspects, including:
    • stings of horseradish and mustards,
    • burn of chili peppers,
    • tingle from CO2,
    • astringency of tannins.
  • Economically important: salsa outsold ketchup in the U.S. in 1992
• Pepper compounds are used throughout the world.
  • Columbus sailed to the "West Indies" to find a shorter route for the spice trade, and when Alarik sacked Rome, he asked for peppers as part of his duty.
  • Manycuisines utilize peppers!
  • There are vast differences between potencies: capsaicin is much more hot than piperine;
  • There is disagreement about the methods used to measure these potencies (e.g., Scoville Units, or ASTM).
  • Peppers have long-lasting time properties, making them ideal for time-intensity scaling applications.
  • They cause defensive reactions (e.g., salivation)
  • The have desensitizing properties -- both chronically and within a session
  • The qualitative variation between peppers and concentrations remains largely unknown.
• Astringency -- At the intersection of chemical and tactile
  • Tannins bind to salivary proteins, delubricating the mouth
  • Astringency may have subqualities (i.e., drying, roughing, puckering)
  • Potential definitions of astringency

    "The complex of sensations due to shrinking, drawing pr puckering of the epithelium as a result of exposure to substances such as alum or tannins."

    "A complex sensation combining three different aspects: drying of the mouth, roughing of the oral tissues, and a puckery or drawing sensation felt in the cheeks and muscles of the face."

  • Wine judges identify astringency as a "taste" even though it may result from tactile phenomena.
• An unanswered mystery of the trigeminal nerve:
  • Do different qualities exist for different oral chemical irritants? Is it possible to tell the difference between capsaicin and piperine after controlling for all other sensory properties (i.e., intensity, side tastes and smells)?
  • Evidence for qualitative differences:
    • Synergies and cross-potentiation suggest separate physiological mechanisms -- substrate for different qualities is present (but not sufficient).
    • Different areas of the mouth are stimulated -- suggests different receptor density (but could be a result of access factors).
    • M. Cliff & H. Heymann (1992). Descriptive analysis of oral pungency. Journal of Sensory Studies vol 7, pp. 279-90:
      Differences were found in lag time (short vs. long), burning vs. tingling and longitudinal extent between capsaicin, piperine, cinnamaldehyde, eugenol, ginger oleoresin, cuminaldehyde and ethanol.