Answer: 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.
Sweet onions have a thinner, lighter color skin than storage onions and tend to be more fragile. Signs in produce sections usually differentiate between sweet onions and storage onions. Most producers also put stickers on each individual onion, such as "Vidalia", "Texas 1015 SuperSweet," "Sweet Imperials," "Walla-Walla", etc. Another indication is price - sweet onions are a premium product and generally cost more than storage onions.
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