Common Chemical Sense in Food Flavor Chemesthesis - People

Chemesthesis
Common Chemical Sense in Food Flavor
At first glance, most people think of the volatile or aromatic compounds perceived by the olfactory organs as the major
contributors to flavor. Others would include the gustatory sense, that is, things that are sweet, sour, salty, bitter, and perhaps
umami, among the more important food flavors. In addition to the classical senses of taste and smell, anatomically separate
systems in the mouth and nose provide additional sensory input from chemical stimulation. Various tasteless and odorless
compounds, such as carbon dioxide and capsaicin, are potent chemical stimuli and important flavor compounds in foods and
beverages. Such stimuli are often classified as trigeminal because they are capable of stimulating nerve endings in the nose,
mouth, and eyes that are subserved by branches of the fifth cranial nerves (the trigeminals). These nerves carry signals for
pain, touch, and temperature stimulation, and are generally associated with painful or irritating chemical stimuli, hot pepper
being a good example.
However, other nerves, such as the glossopharyngeal nerve on the back of the tongue, also are capable of responding to
irritative chemical stimulation. In fact, many skin surfaces and mucous membranes can respond to chemical irritation if the
compound is sufficiently potent. An example is the anal mucosa - an old Hungarian saying states that paprika burns twice,
and Indians say that you feel chili on the way out. For this reason, the old designation of such stimuli as trigeminal is losing
popularity, either to the even older designation of the “common chemical sense” or the newer term chemesthesis.
Chemesthesis was derived for chemical stimuli in an analogy to somesthesis, the sensory system of the skin that responds to
touch (i.e., mechanical stimuli).
Table 1 shows a breakdown of sensory systems subserving the complex of stimuli involved in flavor. Taste, or oral
chemical sensations, can be broken down into the classical gustatory qualities and the oral chemesthetic sensations (e.g., hot
pepper burn and menthol cooling). Similarly, the nasal sensations from chemical stimulation can be divided into true
olfactory impressions and those that arise from stimulation of trigeminal nerve endings in the nose. Small reactive
molecules such as carbon dioxide or isothiocyanates (from mustard) are good examples of chemesthetic flavors in the nose.
However, many other traditionally olfactory stimuli may have trigeminal impact, especially at higher concentrations. In
electrophysiological recordings from animals, responses were observed from the trigeminal nerve to stimuli as benign as
coffee aroma.
Table 1: A scheme for the components of flavor based on anatomical considerations.
Flavor
Oral Chemoreception
Nasal Chemoreception
“Taste”
“Smell”
Oral Chemical
Irritation
Gustation Olfaction
Trigeminal
sensations, e.g.
pepper heat
Sweet, sour, salty, bitter, umami
Sensations from volatiles, sniffed
(“aroma”) or from the mouth –
retronasally (“flavor”)
Nasal Chemical
Irritation
Trigeminal
sensations, e.g.
from carbon
dioxide
Volatile aromatic compounds, even when placed in the mouth, do not cause taste sensations but rather cause flavor
sensations by retronasal passage through the posterior nasopharynx and up to the olfactory epithelium in the reverse
direction from sniffing. People who are mistakenly accustomed to speaking of “taste thresholds” for volatile compounds
such as citral can readily be convinced of their nearly complete olfactory impact by pinching the nose closed while a
solution is introduced into the mouth. This simple maneuver easily blocks retronasal passage by producing a dead air space
in the nose and cuts off the alleged taste sensation. Releasing the pinched nostrils will cause a sudden impression of the
lemon character of citral to develop.
Somewhat more problematic in such a classification is the attribute of astringency. Astringent substances such as tannins are
clearly chemical stimuli, but their attendant sensations seem more tactile than flavorous. Astringency is defined as the
complex of sensations due to shrinking, drawing, or puckering of the epithelium as a result of exposure to substances such
as alum or tannins. Subjects seem to detect three component sensations within the astringent complex: a drying sensation
(lack of salivary lubrication); a roughing sensation (usually perceived when the tongue comes in contact with other oral
tissues such as the palate); and a drawing or puckering sensation, which results from muscular tightening of the cheeks.

Because these sensations are tactile sensations of a chemical origin and are mediated by anatomical systems outside the
classical gustatory and olfactory nerves, they would seem to fall within the boundary of the chemesthetic sensations.
Chemesthetic Flavor Compounds
Several distinct flavor impressions are mediated by chemesthetic sensory systems. These may be broadly grouped into the
following categories:
1. chemical irritation and pungency
2. astringent materials
3. cooling compounds
Pungent Materials
A great deal is known about the chemical basis for pungent sensations because of their importance in various spices and
flavorings. The structures of many pungent materials are known, particularly the components of spices, such as the families
of capsaicinoids from red pepper and the piperines and gingerols from black pepper and ginger, respectively. These
compounds are sometimes referred to as producing oral chemical heat or more generically, irritation.
By far the most thoroughly studied pungent compounds are the capsaicinoids. These are the active principles in so-called
chilies, red pepper, paprika, and the like, contained in the fruits of many pepper plants from the species Capsicum annuum
(e.g., jalapeno peppers) and Capsicum frutescens (e.g., tabasco peppers).
H 3 CO
HO
N
H
O
capsaicin
Black pepper is the dried crushed seed of the southern Asian climbing vine Piper nigrum. White pepper comes from the
decorticated kernel of the same plant. Both have the active compound piperine.
The active pungent compounds in ginger are gingerol and its derivatives.
H 3 CO
HO
O
O
O
O OH
N
piperine
gingerol
Other pungent substances include reactive sulfur-containing compounds, notably those in horseradish, mustard, onion, and
garlic. Because of their volatility, it is unclear whether they should be classified as oral or nasal irritants. Probably, they can
most reasonably be considered as both because the pungent sensations may be sensed throughout the mucosal tissues of the
mouth as well as in the upper respiratory tract. Horseradish and mustard share the common family of isothiocyanates as
pungent principles. These substances are bound as precursors in whole vegetables and are released through enzymatic action
when the plant materials are ground. Allyl isothiocyanate, which occurs in both mustard and horseradish, is the most
common member of this family. Brown mustards contain 3-butenyl isothiocyanate.
N
C
S
allyl isothiocyanate C
N
S
3-butenyl isothiocyanate
The lacrimatory factor in onions appears to be different from the pungent principles. The reactive compound, propanethial
S-oxide is produced from trans-(+)-S-(1-propenyl)-L-cysteine sulfoxide.
HO
O
S
NH2 O
trans-(+)-S-(1-propenyl)-L-cysteine sulfoxide
S O
propanethial S-oxide

Astringent Factors
Astringency has long been recognized as an important sensory attribute of many foods and beverages. In some historical
classifications, astringency has been considered a primary or basic taste category, that is, equal in importance to the classical
four taste qualities of sweet, sour, salty, and bitter. Others have classified astringency as a chemically induced tactile
sensation, a position that is supported by the tendency of astringent sensations to grow in intensity with repeated
stimulation, rather than showing the adaptation that is characteristic of taste receptors.
Astringents have been classified into four main groups (25):
1. salts of multivalent metallic cations (e.g., aluminum sulfate)
2. dehydrating agents (e.g., ethanol and acetone)
3. mineral acids (including halogenated acetic acid)
4. vegetable tannins (e.g., grape seed tannin and tannic acid)
The most common astringent compounds are the vegetable tannins. Tannins are water-soluble polyphenolic compounds that
have the ability to precipitate alkaloids, gelatin, and other proteins. The ecological function of these secondary plant
metabolites is probably that of a deterrent to an animal who would consume the plant, either through unpleasant sensory
characteristics or through undesirable postingestional effects. Regarding the undesirable postingestional effect, high levels
of tannins in the diet can have deleterious nutritional effects as a result of binding nutrients in the intestinal mucosa, which
has a negative impact on the ability to absorb nutrients. Common foods and beverages with high amounts of tannins include
coffee, tea, beer, wine, apples and ciders, many berry crops, and nuts.
Although moderate degrees of astringent sensations contribute to the palatability of some foods in which they are expected
(e.g., red wines), in other foods they are considered flavor defects, such as in fluid milk and other dairy products.
Cooling Compounds
Sensations of coolness are important in consumer products such as cigarettes, chewing gum, candy, toothpaste, mouthwash,
liniments, and pharmaceuticals. The substance most often associated with this cooling sensation is menthol. It is the main
component in a number of peppermint oils obtained from Mentha piperita and Mentha arasis.
OH
(-)-Menthol