Taste Receptors

Sweetness is related to a substance's ability to hydrogen bond to a protein-based receptor in taste buds at the tip of the tongue.

In the figure at right, you see that molecules with a hydrogen bond donor (an OH group in a sugar) about 3 x 10-8 cm away from a hydrogen bond acceptor (another OH group in the sugar) are able to strongly interact with the sugar receptor protein in the taste bud. It is also important that there is a relatively non-polar part between the hydrogen bond donor and acceptor. In sugars, the non-polar part consists of the C-H bonds adjacent to the OH groups.

Sweetness varies with structure. In the table below, table sugar (sucrose) is given an arbitrary sweetness value of 1.0 and other sweet substances are rated relative to that. All sugars are sweet as are many other organic and even inorganic molecules.

Lactose 0.16 milk sugar
disaccharide of galactose and glucose
Galactose 0.32 one of the simple sugars in lactose
Maltose 0.33 malt sugar
disaccharide of glucose
Glucose 0.74 main product of photosynthesis
Sucrose 1.00 table sugar
disaccharide of glucose and fructose
Fructose 1.74 fruit sugar
Cyclamate 30 artificial sweetener
Aspartame 100-150 artificial sweetener
Saccharin 450 artificial sweetener

Notice that sugars in the table above are classified as monosaccharides (one sugar unit) or disaccharides (two sugar units). Next time we'll talk about how monosaccharides are converted to disaccharides, trisaccharides, and polysaccharides.

Several artificial sweeteners have been used in foods and drink. These were all discovered accidentally by sloppy chemists who didn't wash the chemicals off their hands before leaving the laboratory.

The use of cyclamate and saccharin has been restricted because they can cause bladder cancer in rats at high dosage levels. These compounds are not metabolized by the body and so provide no calories to food.

Aspartame is made from two amino acids: aspartic acid (tasteless) and phenylalanine (bitter) that give a sweet compound as the dipeptide. It is metabolized like other peptides and proteins in the body but it has far fewer calories than sugars.

Professor Patricia Shapley, University of Illinois, 2012