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.
It develops in the mouth after the onion has been chewed, and is not inherent in unbroken cells. The action of this enzyme results in the formation of pyruvic acid, ammonia, and sulfinic acids. The sulfinic acids are further changed to produce additional flavor compounds. The amount of pungency in an onion is measured as the amount of pyruvic acid produced in this process, although it is not the compound that imparts the hot feeling in the mouth. There may be additional compounds that affect hotness that have not been adequately tested at this time.