Onions pick up Salmonella primarily from contaminated irrigation water, soil fertilized with improperly composted manure, and animal feces in the field. The bacteria don’t come from inside the onion itself. They arrive from the growing environment and can spread further during packing and storage. A 2020 outbreak linked to red onions from California sickened more than 1,600 people in the U.S. and Canada, making this one of the largest produce-related Salmonella events in recent years.
Contamination Starts in the Field
The single biggest source of Salmonella on onions is the water used to irrigate them. Rivers, canals, and reservoirs can carry the bacteria from upstream sources: sewage overflow, animal waste runoff, or contaminated soil. When that water is sprayed onto a field of growing onions, the bacteria land on the bulbs and surrounding soil. The FDA’s investigation into the 2020 red onion outbreak identified contaminated irrigation water from a growing field in Holtville, California as the leading hypothesis for how the onions became contaminated.
Soil itself acts as a reservoir. When farms apply manure, biosolids, or compost that hasn’t been fully broken down, Salmonella can persist in the ground for months. Field studies have detected the bacteria in manure-amended soil 90 to 120 days after application, and some research shows survival ranging from 30 days to seven months depending on conditions.
Animals are the third major route. The FDA investigation found sheep grazing on land adjacent to the onion fields, signs of animal intrusion including fecal droppings on the farm, and large flocks of birds that could spread contamination across growing areas. Even a single animal wandering through a field can deposit enough bacteria to contaminate a section of crop.
How the Bacteria Behave on an Onion
Here’s what makes onion contamination counterintuitive: the dry, papery outer skin of an onion is actually hostile to Salmonella. In lab studies, bacteria placed on the outer skin of red, white, and yellow onions dropped to undetectable levels within three days. Red onions were especially effective, killing off the bacteria within a single day. The outer layers are too dry and contain enough antimicrobial compounds to prevent survival.
The inner flesh is a completely different story. When Salmonella gets past the outer skin, through a cut, bruise, or puncture wound, it thrives. In the same studies, bacteria injected into the inner layers of onion bulbs multiplied significantly over 18 days, growing by roughly 250 to 600 times their starting population in white and yellow varieties. The moist, nutrient-rich interior provides exactly the conditions Salmonella needs. Red onions were the exception, consistently suppressing bacterial growth even in their inner layers.
This means the real danger isn’t an intact onion sitting on your counter. It’s an onion that has been nicked, bruised, or cut during harvest or handling, allowing bacteria to reach the interior where they can multiply.
Cross-Contamination in Packing Houses
Even if only a small number of onions arrive from the field carrying Salmonella, the packing process can spread the bacteria to clean onions. Onions are sorted, graded, and packed using conveyor belts, bins, and shared equipment. Research on cross-contamination in dry produce packing houses found that when onions were placed in a contaminated plastic storage bin with no sanitation step, 49 to 71% of the onions picked up the bacteria from the bin surfaces alone.
This is a critical amplification point. Conveyor belts made of polyester-nylon, high-density plastic bins, and even plywood surfaces all transferred bacteria to onions on contact. Because onions aren’t washed the way lettuce or tomatoes are (they’re a dry-packed product), there’s no rinse step to remove bacteria picked up during packing. The FDA’s outbreak investigation noted that food contact surfaces at the implicated facility had not been inspected, maintained, or cleaned frequently enough to prevent contamination.
Why Onions Are Particularly Vulnerable
Several features of onion farming and handling create a perfect setup for Salmonella contamination. Onions grow in direct contact with soil, often in hot, arid regions that depend on canal irrigation, exactly the kind of water supply most likely to carry pathogens. They’re harvested mechanically, which can cause small nicks and bruises that breach the protective outer skin. After harvest, they’re cured (dried in the field or in storage), but curing only protects the outer surface. Any damage that lets bacteria inside before or during curing creates a pocket where Salmonella can quietly multiply.
Unlike many fresh fruits and vegetables, whole onions are stored and shipped at room temperature for weeks or even months. They aren’t refrigerated, and they aren’t washed or treated with sanitizer before reaching the consumer. This long, unrefrigerated supply chain gives bacteria that have reached the inner tissue plenty of time to grow.
What This Means for Your Kitchen
Cooking kills Salmonella at temperatures above 150°F (65°C), so sautéed, roasted, or grilled onions pose virtually no risk. The concern is with raw onions, the kind you’d put on a sandwich, salad, or burger.
When handling onions at home, discard any that have visible damage, soft spots, or mold. Wash your hands before and after handling raw onions, and clean your cutting board afterward, especially if you’re about to prepare other foods. Store cut onions in the refrigerator, because while refrigeration won’t eliminate Salmonella, it slows bacterial growth dramatically compared to leaving them on the counter. If a recall is announced for onions, check where yours came from: the packaging, the store’s signage, or the sticker on the onion itself can help you trace it back.
Red onions have shown the strongest natural resistance to Salmonella in lab studies, suppressing the bacteria even in their inner layers, while white and yellow varieties are more hospitable to bacterial growth once the skin is breached. That doesn’t make red onions safe from contamination, but it’s a notable biological difference.