Answer: Heat in onions comes from the sulfur content. Most storage onions are pungent because sulfur is present as a natural fungicide to prevent rotting and decay. Shep Ogden, market gardener and owner of The Cook's Garden in Londonderry, Vermont, explains that as they grow, onions take up sulfur from the soil. Any kind of stress, particularly water stress, can concentrate the sulfur, causing the onion to be more pungent, he says. Give the onions at least an inch of water a week for best growth. Addinglime to raise the pH won't affect the pungency. You can't get rid of sulfur in soil, but you can grow varieties of onions that don't scavenge sulfur so easily, adds Ogden. Ringmaker, Ailsa Craig and Sweet Sandwich storage onions are known for their mildness and are recommended for the Northeast, Ogden notes. Sweet Sandwich starts out pungent, but the sulfur breaks down after three months in storage and the onion becomes milder.
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