Via Bermuda, if I remember right.
The first sweet onions grown in Texas were Bermuda variety but the seeds came from the Canary Islands. The “mother” of most super-sweet onions (Vidalia, Maui, Texas 1015, etc.) however is the Grano 502 which traces back to Valencia, Spain. The Granex has ancestry in both lines.
Ah, but but you ignore that storage onions have substantial quantities of fructans, which when heated become fructose. (Slowly sweating the oninons is the best way to do this: it's moist heat that does the hydrolization.)
Sugars and fructans make up almost all of the dry weight onions. Since sweet onions have more sugar, storage onions should have more fructans.
The hydrolysis products of fructan polymers are fructooligosaccharides and fructose. I believe that at 100% fructan conversion, fructose makes up about 80% of the total converted carbohydrates. Complete conversion isn’t fast unless you are at temperatures that will quickly burn the sugars – as you note, slowly sweating is the best. Any bite (as in al dente) is long gone before you get there – not to say that some conversion and a noticeable increase in sweetness doesn’t happen relatively quickly. With agave, for example, it takes 24+ hours to maximize the conversion of fructans to fermentable sugars for making tequila.
Froctose is perceived ~3-5x sweeter than fructooligosaccharides. Fructose tends to make up a much higher percentage of the total sugar in sweet onions as compared to storage onions, but you’re right, when a large portion of the fructans are converted to fructose, a lot of sweetness is created. Onion chutney cooked for hours is good stuff… The heat of cooking also degrades the sulfur compounds reducing pungency which masks sweetness.
(And your numbers seem low: what's the method used to measure that?)
I’ve never done it. I would think HPLC (High Performance Liquid Chromatography), refractive index, or NIR (Near Infrared Radiation) absorbance. The % figures I noted are of fresh weight. Of dry weight, I think the sugars typically represent 40-55% in storage onions and 55-60%+ in sweet onions.
The lower fructan content and higher moisture level of sweet onions is one of the big reasons they keep poorly.
Hence the name “storage onion.”So what does this all have to do with pizza???
I like sweet onions for pizza because, depending on the pizza, I can roast them until they just begin to turn translucent or a little longer until they just begin to brown and in either case have a beautifully sweet flavor with no sulfur pungency yet still have a nice texture that contrasts slightly with the soft cheese.