Externally Reviewed Key Facts
- Onions (Allium cepa) are a widely consumed bulb vegetable of the Liliaceae family represented by several market types: yellow, red, white, or sweet.
- The bulb is cultivated after the first year of growth and is consumed either raw or cooked.
- Pungency is due to the presence of sulfur-containing thiol compounds, e.g., S-alk(en)yl-l-cysteine sulfoxides.
- Onions are hardy with approximately three months of shelf life.
- Field curing causes physiological changes in the outer leaflets of the bulb where the “skin” or crust forms.
- Onions are available year-round, with production reaching approximately 6.75 billion pounds per year in the United States.
- Between 2000 and 2020, at least 74 onion-associated outbreaks were reported to CDC’s National Outbreak Reporting System (NORS), causing 3,572 illnesses, 302 hospitalizations, and 1 death.
- One of the most recent onion-related recalls occurred in 2021 due to a multistate outbreak of Salmonella Oranienburg. This recall has ended.
Externally Reviewed Content
Onions (Allium cepa) are a widely consumed bulb vegetable of the Liliaceae family. This crop is cultivated all around the world, with various centers of domestication reported in the Middle East, Southeast Asia, and Africa. Currently, China and South Korea are the biggest producers of onions (FAO 2019). Although onions flower and complete their life cycle in two years (biennial), the bulb that is consumed is usually cultivated after the first year of growth. The bulb is a compact and fleshy underground stem that acts as a nutrient reserve for its second-year growth.
Dry bulb onions are consumed both raw and cooked and market classes are red, white, yellow, and sweet. Many varieties exist with varying color and phytonutrient composition, imparting different flavors and culinary properties. Some [B2] examples include red, yellow, white, Maui, and Vidalia cultivars. Spring or green onions (shoots) are also frequently consumed. In addition to genetic differences between cultivars, growing conditions play a role in onion development. Onions are broadly classified into short-medium-, and long-day types, which are combined with geographical considerations for optimal growing (Shock 2011).
Onions are consumed in a variety of preparations, including raw, pickled, fried, sauteed, or cooked and incorporated into other dishes. Different culinary outcomes warrant different onion types, however, they can all be generally described as pungent vegetables. This pungency is due to the presence of sulfur-containing thiol compounds, e.g., S-alk(en)yl-l-cysteine sulfoxides (Rose 2019). Organosulfur glycosides, phenols, and flavonoids have bacteriocidal and bacteriostatic effects. It has been postulated that these compounds protect the developing bulb from the soil microbiota. The content and composition of these organosulfur compounds also influence flavor and aroma.
Many of these organosulfur compounds also have demonstrated health benefits, associated with anti-cancer, anti-diabetic, anti-inflammatory, and cytoprotective outcomes (Jacob 2006). In addition to organosulfur, onions contain health-promoting polyphenolics, such as anthocyanins (purple/red onions) and flavonoids (Ren 2017). The all-to-familiar crying that may come while chopping onions can be attributed to a plant cellular defense mechanism. In response to cell damage, inactive sulfur compounds are enzymatically cleaved by alliinases to produce volatile aerosol compounds (Block 2010). Aside from phytonutrients that are important in plant defense and human health, onions provide carbohydrates and minerals. The high carbohydrate content confers a sweetness to onions that is accentuated when cooked (e.g., caramelized onions, French onion soup).
Onions are hardy with an approximate shelf life of three months if stored in cool, dry conditions. Field curing causes physiological changes in the outer leaflets of the bulb where the “skin” or crust forms. The phytonutrients in the skin are typically highly concentrated in comparison to the inner leaflets of the bulbs.
It should also be noted that onions are still susceptible to many plant pathogens. The skin is an excellent barrier in keeping out pathogens, but there are pre-and post-harvest rots and other qualitative issues primarily caused by farming practice, atypical weather events and post-harvest activities like shipping and sorting. Common diseases include fungal botrytis, downey mildew, and fusarium rot (PSU PlantVillage).
Foodborne Outbreaks and Recalls
Between 2000 and 2020, at least 74 onion-associated outbreaks were reported to CDC’s National Outbreak Reporting System (NORS), causing 3,572 illnesses, 302 hospitalizations, and 1 death. The ingestion of contaminated onions can cause foodborne illness, especially in young children, the elderly, and immunocompromised individuals. Onions can become contaminated with bacteria, particularly Salmonella spp., through contact with manure/fertilizer, groundwater, soil, or manufacturing/processing equipment (e.g. sorting for grade and size) containing the bacteria. In June 2020, an outbreak of Salmonella Newport was linked to white, yellow, red, and sweet yellow onions from Thomson International, Inc. that were distributed to grocery stores around the country. Onions relating to this outbreak caused illness in 1,127 patients across 48 states, with 167 patients being hospitalized and 0 reported deaths [B3] [DCC4]. (CDC, 2020a).
A mass recall of onions from Thomson International, Inc. and foods containing contaminated onions was ordered on August 1, 2020. Foods containing recalled onions included cheese dips and spreads, salsa, stir fry kits, salads, frozen pizza, grilled vegetable mixes, premade kabobs, macaroni and pasta salads, chicken salad, breakfast bowls, and other food products (CDC, 2020b). This outbreak was officially declared over by the FDA on October 8, 2020. The FDA provided guidance that some recalled food products that were not yet discarded may still be present in premade foods or homemade freezer products and should not be consumed, emphasizing the importance of monitoring food recalls (CDC, 2020a).
In 2021, a multistate Salmonella Oranienburg outbreak was linked to onions and a recall was initiated. In total, there were 1,040 illnesses, 260 hospitalizations, and no deaths. Thirty-nine states were involved including Puerto Rico and the District of Columbia. The recalled onions were supplied by ProSource Produce LLC and Keeler Family Farms. All onions were imported from the State of Chihuahua, Mexico from July-September of 2021, and whole raw red, white, and yellow onions were recalled.
Onions are available year-round, with production reaching approximately 6.75 billion pounds per year in the United States. Depending on the time of the year, onions and their traits will vary. For example, onions produced in spring and summer usually have a higher moisture content, which shortens their shelf life. These onions are typically sweet, as compared to onions produced in the fall and winter which are more pungent, dense thick, and lower in water content (U.S. Production and Availability, 2019).
Since they are widely adapted vegetable crops, onions can be grown from the tropics to subarctic regions. This adaption is primarily due to differing responses to day length (Ribeiro da Silva et al., 2020). Day length influences bulbing in onions rather than flowering. Onions are grouped into three groups based on their response to hours of daylength:
- Short-day variety: 10-13 hours
- Intermediate variety: 13-14 hours
- Long-day variety: >14 hours
Furthermore, onions can generally withstand light to heavy frosts. However, hard freezes can result in both qualitative and quantitative onion damage (Ribeiro da Silva et al., 2020).
Soils that are fertile and well-drained are a haven for sweet onion growth. Soils containing heavy clay content should be avoided due to high sulfur content, which can lead to overly pungent onions (Ribeiro da Silva et al., 2020).
Onions should be washed and dried immediately after harvesting, to remove soil and other surface contaminants. Typically, most growers undercut and field cure. Curing can be done in the field or in barns, but once dried, onions have a comparatively long shelf life. To maintain the safety of the onion in storage, they should be stored in a cool, dry place, away from walls in order to facilitate proper air movement and prevent bruising. Avoidance of storing onions near other products that might introduce moisture is important as this can encourage the growth of mold on the onion. For most onion varieties, the bulbs are considered safe to eat up to 180 days (6 months) in storage, as long as the skin is intact (NOA, 2010). The actual time depends on pre- and post-harvest conditions and the curing process used.
Black mold on onions is caused by Aspergillus niger, a common soil fungus. The USDA recommends removing the affected layers and anyone that is allergic to Aspergillus niger should not use onions with black mold [B5]. The long shelf life of onions can be further extended by pickling or canning. Onions can be safely canned using a standard boiling water bath technique, however, as with all canned produce, there is a possibility of botulinum toxin being produced in the product if the microbe Clostridium botulinum is present/germinates and proper pickling/canning procedures are not followed. To avoid this danger, carefully inspect the container of canned or pickled onions for bulging, leaking, or loose lids at room temperature. Most cases of foodborne illness can be prevented by proper handling of the onions as stated above. Additionally, proper cleaning of cooking surfaces and implements such as knives and food processors can prevent contamination.
There are several species and even more cultivars of onions, but most are grouped into four types based on color: yellow onions, red onions, sweet onions, or white onions. Yellow onions are the most common in the US with a mild flavor that can be made sweet by caramelizing (NOA, 2021). Red and white onions are much less common in the US, although red onions are becoming more popular because of their distinct color and white onions are commonly used in Mexican and Southwest cuisine (NOA, 2021). Onions are a versatile food that is often eaten raw, cooked in many ways, or pickled for extended storage (NOA, 2021).
Over the past 20 years, onion consumption has steadily increased. In 2018, per capita consumption in the USA was 20 pounds a year and the global per capita consumption was 13.7 pounds a year (NOA, 2021). Globally, onions are very common meal additions to meat-based recipes.
Between 80% to 90% of a raw onion’s mass is water (Sidhu et al., 2019). The average, medium-sized onion has 45 calories, the bulk of which come from the sugars fructose and glucose, as well as one gram of protein and three grams of dietary fiber (FDA, 2020; Sidhu et al., 2019). The sugar, calorie and metabolite content of an onion varies only slightly with cultivar and species (Sidhu et al., 2019). Onions are also a good source of Vitamin C, with one onion containing 20% of the FDA’s recommended daily value and they do also provide some dietary iron and calcium (FDA, 2020).
Metabolomics allows a reliable differentiation among onion varieties and highlights the potential of fingerprinting for food authentication purposes. Comparison of the chemical profiles and metabolomes of the “San Pietro” white onion (WP), the red onion var. Tropea (RT) and the yellow onion var. Montoro (CM) has been reported (Saviano, et al., 2019). There was clear discrimination of A. cepa L. varieties for the fresh and stored batches. The statistical model highlighted higher levels of fructooligosaccharides (FOS) in the fresh WP; RT showed a high content of glucose, citrate and amino acids, while CM had many sulfur components. In the stored samples (CMS, RTS), carbohydrates and sulfur components decreased, while in WPS the free monosaccharides concentration increased. Linoleic acid was overexpressed in the apolar extracts of CMF and WPF cultivars.
Onions are also an excellent source (at least 20 percent) of antioxidants such as flavonoids (Sidhu et al., 2019). Antioxidants quench free radicals, which protect DNA and other cellular machinery from damage (Slimestad et al., 2007). Researchers in Kuwait reported red and white onions had the highest concentration of antioxidants, and the outermost layers of the onions had the greatest concentration with antioxidants declining toward the center of the onion (Sidhu et al., 2019). The nutritional value of the onions is largely unchanged by frying or microwaving (Ioku et al., 2001). Boiled onions lose 30% of their antioxidants to the cooking water, although this isn’t an issue in dishes like French Onion soup (Ioku et al., 2001).
In addition to antioxidants, onions are a good source of organosulfur compounds which are correlated with a reduced risk of lung and bowel cancer (Sparnins et al., 1988). Research indicates that frying, steaming, and microwaving increased the concentration of organosulfur compounds in onions while boiling reduced them (Kim et al., 2016).
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