Many people are confused by how an onion is designated as a 'sweet onion', and standards for this designation have not yet been established. In order to understand what a sweet onion is, one must understand what are the components of onion flavor. The perception of flavor is also dependent on the taste buds of the person eating this succulent bulb. Simple Sugars Important Sweetness is primarily attributable to the accumulation of simple sugars. The three sugars found in onions are fructose, glucose, and sucrose.
Fructose contributes the most sweetness per unit, followed by glucose and sucrose.
The amounts of these components vary, depending on the genetics of the cultivar, the environment in which it is grown, and some cultural practices. Polymers of sugar are also found in onion bulbs. These polymers are composed of one glucose molecule attached to two or more fructose molecules. These polymers contribute a small amount of sweetness and a large amount to the total dry matter of the onion. Pungency is a term given to a feeling of hotness in the mouth when onions are eaten.