A cranberry, honey or a bar – which tastes the sweetest? These foods contain sugars that people perceive differently. A cranberry seems tart, while a bar can be way too sweet and honey is somewhere in the middle. Now, researchers have shown that the perception of sweetness depends on molecular interactions between specific sugars and water in the saliva.
The sugars mannose, glucose and fructose have almost identical chemical structures. Still, fructose (found in many candy bars) is about twice as sweet as glucose (found in honey), while mannose (found in cranberries) is considered bland. Sugars stimulate specific protein receptors on the taste buds of the tongue, which send a signal to the brain that food tastes sweet. But scientists don't know why we view some sugars as sweeter than others. Because these interactions take place in saliva, which is primarily water, the researchers wondered whether water could play a role.
The researchers used a special technique to study the structures of mannose, glucose and fructose in water. They found that none of the sugars substantially disrupted the interaction of water molecules with each other. However, the three sugars interacted with water molecules in different ways. Mannose, the least sweet of the sugars, formed longer and weaker hydrogen bonds with water than glucose or fructose. Fructose, the sweetest of the sugars, formed the shortest and strongest hydrogen bonds with water. The researchers suspect that shorter hydrogen bonds with water could allow the sugar molecule to bind more closely to the protein receptor, causing greater stimulation and perception of sweetness.
