Tasty Chemistry

Taste refers to the ability to detect the flavour of substances. We receive tastes through sensory organs called taste buds which are concentrated on the upper surface of the tongue.
Among the 50 or so cells in each taste bud there are cells responding to each of the five tastes:

• sweetness
• bitterness
• sourness
• saltiness
• umami-ness or savoriness
  

Sweetness is often associated with foods rich in simple carbohydrates such as glucose and sucrose, but many compounds taste sweet. Examples include the amino acids alanine, glycine and serine as well as the glycosides glycyrrhizin (found in licorice root) and stevioside (from the Stevia rebaudiana shrub). Even some inorganic compounds, such as beryllium chloride and lead acetate, taste sweet.

Bitterness is perceived by many people to be unpleasant. It helps prevent us ingesting toxic substances. A bitterant is the chemical that makes a substance taste bitter. Examples of bitterants are sucrose octaacetate which is used as an inert ingredient in pesticides and herbicides, and, brucine which is a bitter alkaloid closely related to strychnine that is found naturally in a number of plant species.

Sourness is the sensation evoked by substances that are acidic such as lemons and pickles. The acids we ingest release protons which enter the cell and cause a direct, detectable, electronic response.

Saltiness is the taste produced by the presence of alkali metal cations such as Na⁺ and K⁺. The less sodium-like the ion is, the less salty the sensation will be, eg, Rb⁺ and Cs⁺ ions are larger than Na⁺ ions so they do not taste as salty.

The umami taste is due to the detection of the carboxylate anion of glutamic acid, a naturally occurring amino acid found in meat, cheese, and other protein-rich foods. Glutamates, the salts of glutamic acid, easily ionize resulting in the same carboxylate anions and therefore producing the same umami taste. As a consequence, glutamates are often used as flavour enhancers, the most common of which is monosodium glutamate (MSG).

Further Reading
Carbohydrates (sugars)
Amino Acids
Properties of Acids and Bases

Study Questions

1. Name the 3 elements common to all carbohydrates.
2. What is the structural difference between molecules classified as monosaccharides and those that are classed as disaccharides or polysaccharides?
   
3. Is glucose an example of a monosaccharide, a disaccharide or a polysaccharide?
4. Is sucrose an example of a monosaccharide, a disaccharide or a polysaccharide?
5. What elements are common to all amino acids?
6. What functional group or groups must a molecule contain in order for it be classified as an amino acid?
7. Draw the structures for glycine, alanine and serine. Identify the functional groups present in each molecule.
8. A number of amino acids are said to taste sweet, but acids generally are said to taste sour. Can you explain these apparently contradictory statements?