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Gardening Articles: Edibles :: Vegetables

Sweet Onions

by Charlie Nardozzi

A fresh crop of 'Granex 33' onions drying in the late summer sun.

'Vidalia', 'Texas Supersweet', 'Walla Walla Sweet' ... sweet onions are becoming the rage across the country. "In the last five years, we've seen a big increase in demand for sweet onions -- and it's continuing to grow," notes Bucky Parker, general manager at Piedmont Plant Company, growers of more than 15 million onion transplants in Albany, Georgia. Onions imported from as far away as Chile and New Zealand are called "sweet onions," but since there are no national standards on what constitutes a sweet onion, growing sweet varieties yourself is the best way to go.

In many areas of the country, Fall is the time to start growing these mild-flavored, large-sized globes for spring. There are just three ideas to keep in mind for a bumper sweet onion harvest: Choose the right varieties for your area, plant at the right time and keep the onions as stress-free as possible.

What Makes an Onion Sweet

Sweetness in onions is more accurately termed "lack of pungency or mildness." Sugar levels really don't determine if an onion will be sweet. "Some pungent onions actually have more sugar in them than 'sweet onions,' but the sugar is masked by a high amount of sulfur compounds," explains Bill Randle, onion expert at the University of Georgia in Athens. "The low amount of sulfur compounds in sweet onions allows the sugar flavor to come through."

"Though there's no national standard for what constitutes a sweet onion, researchers at New Mexico State University and University of Georgia have developed a test to compare the sweetness of differ onion varieties. The pyruvic acid test combined with a sensory rating scale gives us an idea of onion pungency," explains Randle. The rating scale is 1 to 18. An onion that scores 1 has very little pungency when eaten raw. "At 4 the onion taster begins to feel some heat, and anything above 6 you can't comfortably eat raw," he says.

"Most sweet onions such as Vidalias and Texas Supersweets score between 2.5 and 4.0. It's primarily the genetics of onions that make them pungent or sweet, but the sulfur content of the soil and poor growing conditions can add to the pungency," adds Randle. The higher the soil sulfur content, the more pungent the onion. That's one reason areas of the country with naturally low sulfur levels, such as Vidalia, Georgia, are known for their sweet onions. It's a delicate balance between having enough sulfur for proper plant growth, but keeping sulfur levels low so the onion stays mild," explains Randle.

Any stress while the onion is growing also tends to make it more pungent. "There is a natural variation of sweetness in onions from year to year, depending on the weather, fertility and any insect and disease damage to the crop," Randle notes.

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