Factors Affecting Pungency in Sweet Onions
Posted: December 9, 2010
Mike Orzolek, Penn State Horticulture
The concentration of pyruvic acid in an onion bulb is used to measure the pungency of an onion and is affected by several environmental factors.
1. Climate and soils also affect pyruvic acid concentrations.
2. Sweet onions should ideally be grown on soils that have a naturally low sulfur level.
3. Follow onion cropping recommendations. Sulfur based fertilizers, if necessary, should be applied only during the early stages of onion growth.
4. Sweet onions should not be subjected to water stress. Water stress causes the pungency level to increase.
5. High summer temperatures during the growth period are known to increase pyruvic acid concentrations. Plant and harvest your onions early to avoid the hottest periods in individual growing regions.
6. Planting density also affects onion pungency. Plant at low densities to avoid overly pungent onions.
7. Sweet onions are at their sweetest when they are harvested. Onion pungency increases during storage while total bulb sugar content decreases.
From: the Sweet Onion Source
Although there is no official industry standard, it is generally accepted that an onion should contain at least 6 percent sugar to be in the "sweet" category. Some sweet onions, like the 'OsoSweet', have recorded sugar levels of up to 15 percent. Storage onions usually range from 3 to 5 percent 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 percent; storage onions usually run 10 to 13 percent. 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.
How a Great Sweet Onion Should Taste
The best sweet onions deliver a burst of sweetness when bitten into, are incredibly mild, with very little, if any, sharpness, and have a subtle, fruity flavor. They should still taste like an onion, but be much sweeter and milder.