LEARNING OBJECTIVE

After studying this topic, students will be able to:

• Describe the primary taste sensations, their basic causes, taste thresholds, and taste blindness.

INTRODUCTION

• Taste is mainly a function of the taste buds, with smell, although small, texture,temperature, and stimulation of pain endings also contributing to the overall perception of taste.
• Taste helps a person select food according to personal desires and the metabolic needs of body tissues.
• The sense of smell also contributes strongly to taste perception.
• The texture of food is detected by the tactual senses of the mouth and also affects the taste experience.
• Some substances in food stimulate pain endings and change the taste experience.
  • Example: pepper.
• Taste is important because it helps a person:
  • Select food according to desires.
  • Select food according to the body tissues’ metabolic need for specific substances.

PRIMARY TASTE SENSATIONS

• The specific chemicals that stimulate all the different taste receptors are not completely known.
• For practical analysis, taste sensations are grouped into five main categories:
  • Sour
  • Salty
  • Sweet
  • Bitter
  • Umami
• There is emerging evidence that fat taste may be another major taste modality.
• A person can perceive hundreds of different tastes.
• These different tastes are thought to result from combinations of the elementary taste sensations.
• This is similar to how all visible colors are combinations of the three primary colors, as described in Chapter 51.

Sour Taste

• Sour taste is caused by acids.
• It mainly depends on the hydrogen ion concentration.
• The intensity of sour taste is approximately proportional to the logarithm of hydrogen ion concentration.
• Therefore:
  • More acidic food → stronger sour sensation.

Salty Taste

• Salty taste is produced by ionized salts.
• It mainly depends on the sodium ion concentration.
• The quality of salty taste may vary from one salt to another.
• This occurs because some salts can produce other taste sensations in addition to saltiness.
• The cations, especially sodium cations, are mainly responsible for salty taste.
• The anions also contribute, but to a lesser extent.

Sweet Taste

• Sweet taste is not produced by one single class of chemicals.
• Chemicals that may produce sweet taste include:
  • Sugars
  • Glycols
  • Alcohols
  • Aldehydes
  • Ketones
  • Amides
  • Esters
  • Some amino acids
  • Some small proteins
  • Sulfonic acids
  • Halogenated acids
  • Inorganic salts of lead and beryllium
• Most substances producing sweet taste are organic chemicals.
• Small changes in chemical structure can change the taste of a substance.
• For example:
  • Addition of a simple radical may change a substance from sweet → bitter.

Bitter Taste

• Bitter taste, like sweet taste, is not caused by one single type of chemical.
• Most substances producing bitter taste are organic substances.
• Two groups are especially likely to produce bitter taste:
  • Long-chain organic substances containing nitrogen.
  • Alkaloids.
• Examples of alkaloids include:
  • Quinine
  • Caffeine
  • Strychnine
  • Nicotine
• Some substances may first taste sweet and later produce a bitter aftertaste.
  • Example: saccharin.
• This bitter aftertaste makes saccharin objectionable to some people.
• High concentrations of salts may also produce a bitter taste.
• A strong bitter taste usually causes a person or animal to reject the food.
• This is important because many deadly toxins in poisonous plants are alkaloids.
• These alkaloids usually produce an intensely bitter taste.
• This bitter taste is usually followed by rejection of the food.

Umami Taste

• Umami is a Japanese word meaning “delicious.”
• It is a pleasant taste sensation.
• It is different from:
  • Sour
  • Salty
  • Sweet
  • Bitter
• Umami is the dominant taste of foods containing L-glutamate.
• Examples include:
  • Meat extracts
  • Aging cheese
• The pleasurable sensation of umami is thought to be important for nutrition.
• It promotes the ingestion of proteins.

CONCEPTUAL EXAMPLES

• More acidic food → more hydrogen ions → stronger sour taste.
• Sodium-containing ionized salts → salty taste.
• Sugars and many other organic chemicals → sweet taste.
• Alkaloids → bitter taste → possible rejection of food.
• L-glutamate in meat extracts and aging cheese → umami taste → promotes protein intake.

Threshold for taste

• The molar threshold for different taste sensations varies.
• Average threshold values are:
  • Sour taste
    • Hydrochloric acid = 0.0009 M
  • Salty taste
    • Sodium chloride = 0.01 M
  • Sweet taste
    • Sucrose = 0.01 M
  • Bitter taste
    • Quinine = 0.000008 M
• The bitter taste sensation is much more sensitive than the other taste sensations.
• This high sensitivity provides an important protective function against many dangerous toxins in food.
• Table 54.1 lists the relative taste indices of different substances.
• Relative taste indices are the reciprocals of the taste thresholds.
• In the table, the intensities of four primary taste sensations are compared with:
  • Hydrochloric acid
  • Quinine
  • Sucrose
  • Sodium chloride
• Each of these substances is arbitrarily given a taste index of 1.

Taste Blindness

• Some people are taste blind to certain substances.
• Taste blindness occurs especially with different types of thiourea compounds.
• Phenylthiocarbamide is commonly used by psychologists to demonstrate taste blindness.
• About 15% to 30% of people show taste blindness to phenylthiocarbamide.
• The exact percentage depends on:
  • The method of testing.
  • The concentration of the substance.

KEY CONCEPT

Taste perception = taste buds + smell + food texture + stimulation of pain endings.

• The five main taste sensations are:
  • Sour
  • Salty
  • Sweet
  • Bitter
  • Umami
• Different tastes are produced by combinations of these elementary taste sensations.
• Taste helps the body select food according to desire and metabolic needs.

• Different taste sensations have different molar thresholds.
• Lower threshold → greater taste sensitivity.
• Bitter taste has a very low threshold and is therefore highly sensitive.
• This sensitivity helps protect against dangerous toxins in food.
• Some people may be unable to taste certain substances, which is called taste blindness.

CONCEPTUAL EXAMPLES

• Hydrochloric acid 0.0009 M → sour taste can be detected.
• Sodium chloride 0.01 M → salty taste can be detected.
• Sucrose 0.01 M → sweet taste can be detected.
• Quinine 0.000008 M → bitter taste can be detected at a much lower concentration.
• Phenylthiocarbamide → some people cannot taste it → taste blindness.

CLINICAL IMPORTANCE

• Taste helps in food selection according to personal desire and the metabolic needs of body tissues.
• Smell, food texture, and stimulation of pain endings can strongly modify the overall taste experience.
• Bitter taste has an important protective role because it is much more sensitive than other tastes and helps detect many dangerous toxins in food.
• Many poisonous plant toxins are alkaloids, which usually produce an intensely bitter taste and lead to rejection of the food.
• Umami taste is important for nutrition because it promotes the ingestion of proteins.
• Taste blindness occurs in some people for certain substances, especially thiourea compounds.
• About 15%–30% of people show taste blindness to phenylthiocarbamide, depending on the testing method and concentration used.

HIGH-YIELD POINTS

• Taste is mainly produced by taste buds in the mouth.
• Smell contributes strongly to taste perception.
• Food texture also changes the taste experience.
• Substances that stimulate pain endings, such as pepper, can alter taste.
• Taste helps a person select food according to desire and metabolic needs.
• The five main taste sensations are:
  • Sour
  • Salty
  • Sweet
  • Bitter
  • Umami
• Fat taste may be another major taste modality.
• Hundreds of different tastes are produced by combinations of the basic taste sensations.
• Sour taste is produced by acids.
• Sour taste mainly depends on hydrogen ion concentration.
• More acidic food → stronger sour taste.
• Salty taste is produced by ionized salts.
• Salty taste mainly depends on sodium ion concentration.
• Sodium cations are mainly responsible for salty taste.
• Anions also contribute to salty taste, but to a lesser extent.
• Sweet taste is not produced by one single class of chemicals.
• Most substances producing sweet taste are organic chemicals.
• Sugars, glycols, alcohols, aldehydes, ketones, amides, esters, some amino acids, and some small proteins can produce sweet taste.
• Small changes in chemical structure can change a substance from sweet to bitter.
• Bitter taste is also not produced by one single type of chemical.
• Most substances producing bitter taste are organic substances.
• Two important groups producing bitter taste are:
  • Long-chain nitrogen-containing organic substances
  • Alkaloids
• Important alkaloids mentioned are:
  • Quinine
  • Caffeine
  • Strychnine
  • Nicotine
• Some substances may taste sweet first and bitter later.
• Saccharin can produce a bitter aftertaste.
• High concentrations of salts may also produce a bitter taste.
• Strong bitter taste usually causes rejection of food.
• Bitter taste has an important protective function.
• Many deadly toxins in poisonous plants are alkaloids.
• These alkaloids usually produce an intensely bitter taste.
• Umami means “delicious.”
• Umami is a pleasant taste that is different from sour, salty, sweet, and bitter.
• Umami is mainly associated with foods containing L-glutamate.
• Examples of umami-containing foods include:
  • Meat extracts
  • Aging cheese
• Umami taste is important for nutrition because it promotes protein ingestion.
• Different tastes have different molar thresholds.
• Sour taste threshold for hydrochloric acid = 0.0009 M.
• Salty taste threshold for sodium chloride = 0.01 M.
• Sweet taste threshold for sucrose = 0.01 M.
• Bitter taste threshold for quinine = 0.000008 M.
• Bitter taste has the lowest threshold among the tastes listed.
• Therefore, bitter taste is much more sensitive than the other taste sensations.
• This high sensitivity helps protect against dangerous toxins in food.
• Table 54.1 shows the relative taste indices of different substances.
• Relative taste indices are the reciprocals of taste thresholds.
• Hydrochloric acid, quinine, sucrose, and sodium chloride are each assigned a taste index of 1 in the table.
• Taste blindness means that a person cannot taste certain substances.
• Taste blindness is especially seen with some thiourea compounds.
• Phenylthiocarbamide is commonly used to demonstrate taste blindness.
• About 15%–30% of people show taste blindness to phenylthiocarbamide.
• The percentage of taste blindness depends on:
  • Method of testing
  • Concentration of the substance

QUICK REVISION

Taste mainly depends on taste buds, but smell, food texture, and pain stimulation also influence the taste experience.The five main taste sensations are:

• Sour
• Salty
• Sweet
• Bitter
• Umami

Fat taste may also be an important taste modality.Different tastes are formed by combinations of basic taste sensations.

• Sour taste
• Caused by acids.
• Depends mainly on hydrogen ion concentration.
• More acidity → stronger sour taste.

Salty taste

• Caused by ionized salts.
• Depends mainly on sodium ions.
• Sodium cations contribute more than anions.

Sweet taste

• Is not caused by one single chemical group.
• Most sweet substances are organic chemicals.
• Small chemical changes may convert a substance from sweet to bitter.

Bitter taste

• Is mainly produced by organic substances.
• Important bitter substances include alkaloids.
• Examples: quinine, caffeine, strychnine, and nicotine.
• Strong bitter taste usually causes rejection of food.
• This provides protection against many poisonous substances.

Saccharin may taste sweet first but leave a bitter aftertaste.High concentrations of salts may also produce a bitter taste.Umami

• Means “delicious.”
• Is mainly associated with L-glutamate.
• Found in foods such as meat extracts and aging cheese.
• Promotes protein ingestion.

Taste thresholds:

• Hydrochloric acid for sour = 0.0009 M
• Sodium chloride for salty = 0.01 M
• Sucrose for sweet = 0.01 M
• Quinine for bitter = 0.000008 M

Bitter taste has the lowest threshold and is therefore the most sensitive among these taste sensations.High sensitivity to bitter taste provides an important protective function against dangerous toxins.Relative taste index = reciprocal of taste threshold.Table 54.1 shows the relative taste indices of different substances.Taste blindness

• Means inability to taste certain substances.
• Commonly occurs with some thiourea compounds.
• Phenylthiocarbamide is commonly used to test taste blindness.
• About 15%–30% of people may show taste blindness to it.
• The percentage depends on the testing method and concentration.

Reference: Guyton and Hall Textbook of Medical Physiology, 15th Edition