The Sense Of Taste Refers To The Ability Of The

TASTE The sense of taste refers to the ability of the taste organs to perceive and recognize the five basic taste sweet, sour, bitter, salty and unami. The components of food responsible for taste are non-volatile compounds. They must be in solution or dissolve in saliva to produce the taste response. Taste is sensed by the buds which lie mostly in grooves around little projections on the upper surface of the tongue. It refers to the ability to detect the flavor of substances such as food, certain minerals, and poisons. TYPES OF TASTE: 1. BITTERNESS

Bitterness is the taste which detects bases. Bitterness is the most sensitive of the tastes, and is perceived by many to be unpleasant, sharp, or disagreeable. Common bitter foods and beverages include coffee, unsweetened chocolate, bitter melon, beer, bitters, olives, citrus peel. The most bitter substance known is the synthetic chemical denatonium, which has an index of 1,000. Research has shown that TAS2Rs (taste receptors, type 2, also known as T2Rs) such as TAS2R38 coupled to the G protein gustducin are responsible for the human ability to taste bitter substances. Researchers use two synthetic substances, phenylthiocarbamide (PTC) and 6-npropylthiouracil (PROP) to study the genetics of bitter perception. Quinine, the anti-malarial prophylactic, is also known for its bitter taste and is found in tonic water. Bitter taste is sensed at the back side of the tongue. 2. SALTINESS

Saltiness is a taste produced primarily by the presence of sodium ions. Other ions of the alkali metals group also taste salty. Salty taste is sensed at the edges of the tongue

3. SOURNESS

Sourness is the taste that detects acidity. The sourness of substances is rated relative to dilute hydrochloric acid, which has a sourness index of 1. By comparison, tartaric acid has a sourness index of 0.7, citric acid an index of 0.46, and carbonic acid an index of 0.06. The mechanism for detecting sour taste is similar to that which detects salt taste. Hydrogen ion channels detect the concentration of hydronium ions (H3O+ ions) that are formed from acids and water. The most common food group that contains naturally sour foods is the fruit, with examples such as the lemon, grape, orange, and sometimes the melon. Wine also usually has a sour tinge to its flavor. If not kept correctly, milk can spoil and contain a sour taste. Sour taste is sensed at the edges of the tongue 4.

SWEETNESS

Sweetness, usually regarded as a pleasurable sensation, is produced by the presence of sugars, some proteins and a few other substances. Sweetness is often connected to aldehydes and ketones, which contain a carbonyl group. Sweetness is detected by a variety of G protein coupled receptors coupled to the G protein gustducin found on the taste buds. At least two different variants of the "sweetness receptors" need to be activated for the brain to register sweetness. Taste detection thresholds for sweet substances are rated relative to sucrose, which has an index of 1.The average human detection threshold for sucrose is 10 millimoles per litre. For lactose it is 30 millimoles per litre, with a sweetness index of 0.3. Sweet is sensed at the tip of the tongue. 5. UMAMI

Umami is the name for the taste sensation produced by compounds such as glutamate, and are commonly found in fermented and aged foods. In English, it is also described as "meatiness", "relish", or "savoriness". The Japanese word comes from umai ( 旨 い ?) for delicious, keen, or nice. Umami is now the term commonly used by taste scientists.

Humans have taste receptors specifically for the detection of the amino acids, e.g., glutamic acid. Amino acids are the building blocks of proteins and are found in meats, cheese, fish, and other protein-heavy foods.

Examples of food containing glutamate (and thus strong in umami) are beef, lamb, parmesan, and roquefort cheese as well as soy sauce and fish sauce. The additive monosodium glutamate (MSG) produces a strong umami. Umami is also provided by the nucleotides 5’-inosine monophosphate (IMP) and 5’-guanosine monophosphate (GMP). These are naturally present in many protein-rich foods. Some umami taste buds respond specifically to glutamate in the same way that "sweet" ones respond to sugar. Glutamate binds to a variant of G protein coupled glutamate receptors. EXAMPLES OF TASTANT MOLECULES

Sucrose

C12H22O11

Fructose

C6H12O6

Glucose

C6H12O6

Lactose

C12H22O11

Table Salt

NaCl

SUGARS

SALT

Denatonium

C21H29N2O·C7H5O2

Caffeine

C8H10N4O2

BITTER TASTANT Quinine S

C20H24N2O2

PTC (Phenylthiourea C7H8N2S )

UMAMI

Humulone (beer)

C21H30O5

Aspartate

C4H7NO4

GOOD TO KNOW: • Sweet - usually indicates energy rich nutrients • Umami - the taste of amino acids (e.g. meat broth or aged cheese) • Salty - allows modulating diet for electrolyte balance • Sour - typically the taste of acids • Bitter - allows sensing of diverse natural toxins – We have almost 10,000 taste buds inside our mouths; even on the roofs of our mouths. – Insects have the most highly developed sense of taste. They have taste organs on their feet, antennae, and mouthparts. – Fish can taste with their fins and tail as well as their mouth. – In general, girls have more taste buds than boys. – Taste is the weakest of the five senses. Taste

Substance

Threshold for tasting

Salty

NaCl

0.01 M

Sour

HCl

0.0009 M

Sweet

Sucrose

0.01 M

Bitter

Quinine

0.000008 M

Umami Glutamate 0.0007 M

FACTORS AFFECTING TASTE: TEMPERATURE;

Sugared drink appears sweeter when hot than when cold. Lemon drink taste more sour when it is hot than when cold. Coffee and tea appears to be more bitter when cold then hot. TIME:

Salt on the tongue is sensed in a fraction of seconds, whereas a bitter substance may require a full second after it contacts the tongue before it is sensed by the taste buds. Other factors include:

• Aging • Color/vision impairments • Hormonal influences • Genetic variations; see Phenylthiocarbamide • Oral temperature • Drugs and chemicals • Natural Substances (such as Miracle fruit, which temporarily makes sour foods taste sweeter) Lemonade, for example, is made by combining lemon juice (sour), sugar (sweet), and water. Without the sugar, the lemon juice—water mixture tastes very sour. The more sugar is added, the less sour the result tastes. Another example is tonic water, made by combining quinine (extremely bitter), sugar (sweet), and water. The bitterness causes many people to not perceive tonic water as sweet, even though it contains as much sugar as an ordinary soft drink.

TASTE AND THE TONGUE

Taste cells throughout the mouth and throat cluster together to form taste buds. Isolated taste buds are scattered on the surface of the palate and throat, but on the tongue they are grouped in specialized structures called papillae: mushroom-shaped fungiform papillae in the front of the tongue, leafshaped foliate papillae protruding from the sides, and circumvallate papillae arrayed in a chevron at the back of the tongue. The tips of the taste cells reach toward a tiny opening on the tongue - the gustatory pore through which food chemicals can fall. Nerve endings for the taste system and for the touch/temperature/pain system surround the base of the taste bud cells.

FROM TONGUE TO BRAIN

Three nerves bring taste messages to the brainstem: the facial nerve, shown in the picture above, which brings messages from the anterior two-thirds of the tongue; the hypoglossal nerve, which brings messages from the posterior tongue; and the glossopharyngeal nerve, which brings messages from the throat area and the palate. The nerve carrying messages from the touch/temperature/pain system is called the trigeminal nerve. All of these nerves bring their messages to the brainstem, where they combine their signals in areas of the brainstem that are involved with arousal (for example from sleep), As taste messages move further through the brain, they join up with smell messages to give the sensation of flavor.