The strength of an onion comes down to how much sulfur it contains and how actively its enzymes convert that sulfur into pungent, eye-stinging compounds. Every onion carries the same basic chemical machinery, but genetics, soil, water, storage, and even temperature during the growing season all dial that machinery up or down. Two onions sitting side by side in the grocery store can taste wildly different because of where and how they were grown.
The Chemistry Behind Onion Bite
An intact onion is mild. The sharp flavor only appears when you cut into it, breaking open cells and mixing two ingredients that are normally kept apart: sulfur-containing amino acids (called flavor precursors) and an enzyme called alliinase. When they meet, alliinase rapidly breaks those precursors down into volatile sulfur compounds, along with pyruvic acid and ammonia. Those volatile compounds are what you taste and smell as “onion.”
A second enzyme, lachrymatory factor synthase, grabs one of the sulfur byproducts and converts it into a tear-inducing gas. This is the compound that makes your eyes water. A stronger onion produces more of both the flavor volatiles and this tear gas, because it started with a higher concentration of sulfur precursors, more enzyme activity, or both.
Sulfur in the Soil Is the Biggest Driver
Onion pungency tracks closely with how much sulfur the plant absorbs while growing. In controlled studies, onions grown in sulfur-fertilized soil had 25% more sulfur in their bulbs than onions grown without added sulfur. That extra sulfur translated directly into a 35% increase in pyruvic acid, the standard lab measurement for pungency. The compound responsible for making you cry increased tenfold in sulfur-fertilized onions compared to controls. The precursor behind fresh onion odor was 6.5 times higher.
This is why geography matters so much. Vidalia onions from Georgia are famous for being sweet, and a big part of that reputation comes from the region’s naturally low-sulfur, sandy soil. The same variety planted in sulfur-rich soil would taste noticeably sharper. Sweet onion–growing regions around the world share this trait: soil that starves the plant of the raw material it needs to build pungent compounds.
Water and Temperature Play a Role Too
Onions that experience water stress during growth tend to become more pungent. When a bulb loses moisture, its dry matter concentrates, and the flavor precursors concentrate along with it. Think of it like reducing a sauce on the stove: less water means a more intense result. Heat stress during the growing season can have a similar concentrating effect. Onions grown in hot, dry conditions generally pack more bite than those grown with consistent irrigation in mild weather.
Genetics Set the Baseline
Even in identical soil, different onion varieties produce dramatically different levels of pungency. Plant breeders measure this using pyruvic acid content: onions scoring below about 5.5 micromoles per gram qualify as “sweet” under certification standards like those used for Vidalia onions. Hot varieties can score well above that threshold.
Breeding programs have made real progress in creating genuinely low-pungency onions. Researchers working with short-day onion varieties started with parent populations averaging 3.5 to 4.1 on the pyruvic acid scale. After four rounds of selecting and replanting only the mildest bulbs, they produced families averaging just 1.5, a reduction of more than half. The commercial result of similar work is “Sunions,” a branded onion developed through traditional crossbreeding that gets milder during storage rather than stronger.
Scientists have also identified the specific gene (lachrymatory factor synthase) that controls tear gas production. Experimental onions with that gene silenced or mutated produce almost no tear-inducing compound and taste significantly milder. One Japanese team achieved this by bombarding seeds with ion beams to inactivate the alliinase enzyme entirely, eliminating both the tears and most of the pungency in a single step.
Why Red, Yellow, and White Onions Taste Different
Color is a rough guide to flavor, though it’s not absolute. Yellow onions tend to have the highest sulfur content of the three common types, averaging about 3.5 grams of sulfur per kilogram of dry matter. Red onions average around 2.7 g/kg, which is one reason they often taste milder when raw. White onions fall in between at roughly 3.3 g/kg.
Sugar content also shapes perception. Red onions contain the most total sugar of the three types, which masks whatever pungency they do have. Yellow onions carry the most reducing sugars, the type that caramelize readily during cooking, which is why they’re the go-to choice for sautéing and French onion soup. White onions tend to have a cleaner, sharper bite because their sugar-to-sulfur balance leans slightly toward sulfur without the caramelization potential of yellow varieties.
How Storage Changes Pungency
An onion’s flavor isn’t static after harvest. Some pungent compounds break down over weeks of storage while new ones form, and the balance between creation and loss determines whether the onion gets milder or stronger over time. The compound behind fresh onion odor decreases steadily during storage, which is why an onion that’s been sitting in your pantry for weeks can smell less “oniony” when you first pick it up but still taste sharp when cut.
Sulfur-fertilized onions hold onto their pungency longer and more intensely during storage. The tear-inducing precursor actually increased during 16 weeks of storage in those onions. Temperature matters too: whole onions stored at room temperature retain more total sulfur compounds than those kept cold, and cutting an onion accelerates these chemical changes significantly.
Why Cooking Tames the Burn
Heat disarms the entire pungency system. Alliinase, the enzyme that kicks off the chain reaction, is completely inactivated at relatively low temperatures. Once the enzyme is destroyed, no new pungent compounds can form, and the volatile sulfur molecules already present evaporate off or break down into sweeter, mellower compounds. This is why a raw onion can make you cry while the same onion, slowly caramelized, tastes almost like candy.
The practical takeaway: if you want maximum sharpness, keep your onion raw and chop it right before serving. If you want to soften the bite without cooking, slice the onion thin and soak it in cold water for 10 to 15 minutes. The pungent compounds are water-soluble and will leach out, leaving the crunch without the sting.
Picking a Milder (or Stronger) Onion
If you want a mild onion, look for sweet varieties like Vidalias, Walla Wallas, or Maui onions, all of which are grown in low-sulfur soils and bred for gentleness. Sunions are specifically marketed as a no-tears option. Red onions are generally your next mildest choice for raw eating.
If you want an onion with real punch for cooking, standard yellow storage onions are your best bet. They’re grown for durability and flavor concentration, and they’ve typically been stored long enough to develop a robust pungency profile. Small onions and shallots also tend to pack more bite per volume, since their lower water content concentrates the sulfur compounds further.