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. It is not the amount of sugars that are found in onions that allows them to be designated as a 'sweet onion', but rather the relative amount of pungency compounds. Therefore, the amount of pyruvic acid measured in onions is related to the actual pungency, and is used as a measure of 'sweetness'. General Classification A general classification of onions is as shown in the figure.
Pungency is affected by several factors.
As onions age in the field, pungency generally increases until the tops fall over. If onions are grown under higher temperatures, they will be more pungent. The application of sulfur as fertilizer may increase the level of pungency, while irrigation may reduce it. The question of what is a 'sweet onion' is a relative one. The amount of sugars appears to have little to do with the actual 'feeling' of sweetness in the mouth, but is rather the presence or absence of pungency compounds.