Why are some onions considered sweet and why?

Answer 1

From Sweet Onion Source:

"Although there is no official industry standard, it is generally accepted that an onion should contain at least 6% sugar to be in the "sweet" category. Some sweet onions, like the OSO Sweet, have recorded sugar levels of up to 15%. Storage onions usually range from 3%-5% in sugar content. Unlike sweet onions, regular onions have high levels of sulfur compounds. It's the pyruvic acid in the sulfur that causes tears, harshness, and indigestion. That's why great sweet onions are always grown in soil with low amounts of sulfur. Typically, sweet onions have pyruvic acid levels that measure below 5%; storage onions usually run 10%-13%. Because a sweet onion is also a fresh onion it is very high in water content, which further dilutes the effect of the sulfur and increases mildness."

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