Onions
Onion (Allium cepa L.)
The onion is a biennial herbaceous species cultivated as an annual vegetable crop and is widely grown in temperate regions, where it represents one of the most important horticultural species. The plant forms a basal rosette of hollow, tubular leaves, typically green in colour, and develops an underground bulb composed of concentric, fleshy scales surrounded by papery outer tunics that protect the internal tissues. Bulb shape and size can vary considerably (spherical, flattened, or elongated), as can the colour of the outer tunic, which may be white, yellow, bronze or red, depending on the cultivar.
From an agronomic perspective, onion prefers well-drained soils of medium texture, with good fertility and adequate structure. Optimal growth is obtained under moderate temperature regimes, with excessive humidity and waterlogging being unfavourable, as they promote fungal diseases and reduce storability. The crop cycle depends on cultivar and planting date (direct seeding or transplanting), with harvest generally concentrated in summer to early autumn. Final quality is strongly influenced by water management, fertilization practices, plant density, and seasonal climatic conditions.
The onion bulb consists predominantly of water, with a fraction of carbohydrates mainly represented by glucose, fructose and sucrose. It also contains dietary fibre, potassium (with lower levels of calcium and magnesium), and a range of sulphur-containing compounds responsible for the characteristic pungent aroma and lachrymatory effect. In addition, the bulb provides flavonoids (such as quercetin), organic acids, and small amounts of water-soluble vitamins. The concentration of these components varies according to cultivar, growing conditions, agronomic management, and maturity at harvest.
From a technological and culinary standpoint, onion is an extremely versatile vegetable ingredient. It is consumed raw, cooked, caramelised, dehydrated, or processed into preserves, dried products, stocks and flavour bases. In many formulations it plays a key aromatic and flavour-building role, contributing to the sensory profile of dishes based on meat, fish, vegetables, sauces and soups. Commercial quality is assessed on the basis of bulb size, firmness of tissues, uniform colour, integrity of the outer tunics, storage potential, and absence of defects such as rots, early sprouting or abnormal desiccation. Different onion types and cultivars (round, flat, elongated; white, yellow, red) are suited to specific markets and uses, including fresh consumption, long-term storage, or industrial processing, with marked differences in pungency, aroma intensity, and sulphur compound content.
Botanical classification (APG IV system)
Common name: Onion
Botanical name: Allium cepa L.
Kingdom: Plantae
Clade: Angiosperms → Monocots
Order: Asparagales
Family: Amaryllidaceae
Genus: Allium
Species: Allium cepa L.
Indicative nutritional values per 100 g (raw, edible bulb)
| Component | Approx. value per 100 g |
|---|---|
| Energy | ~ 40 kcal |
| Total carbohydrates | ~ 9.0–9.5 g |
| — of which sugars | ~ 4.2–4.7 g |
| Dietary fibre | ~ 1.5–2.3 g |
| Proteins | ~ 1.0–1.2 g |
| Total fats | ~ 0.1 g |
| — of which saturated fatty acids (SFA) | trace |
| — monounsaturated (MUFA) | trace |
| — polyunsaturated (PUFA) | trace |
| Sodium | very low |
| Potassium | ~ 140–160 mg |
| Calcium | ~ 20–25 mg |
| Magnesium | ~ 9–12 mg |
| Phosphorus | ~ 25–30 mg |
| Iron | ~ 0.2–0.3 mg |
| Vitamin C | ~ 6–7 mg |
| B-group vitamins (e.g., B6, folates) | small amounts |
Values may vary depending on cultivar, agronomic conditions and maturity stage.
Plant Characteristics
- Bulb: Underground storage organ composed of fleshy, concentric layers. It varies in color (white, yellow, red, purple) and size depending on the variety.
- Stem: Short and hidden within the bulb.
- Leaves: Hollow, tubular, and green, performing photosynthesis.
- Flowers: Small, white or greenish, arranged in spherical umbels on long flower stalks.
- Roots: Fibrous and shallow, staying close to the soil surface.
- Habitat: Cultivated worldwide, preferring temperate climates and well-drained soils.
Chemical Composition
Onions are rich in bioactive compounds and essential nutrients:
- Vitamins: High in vitamin C, B-complex vitamins (B6, folic acid), and small amounts of vitamin A.
- Minerals: Contain potassium, calcium, iron, magnesium, and zinc.
- Sulfur Compounds: Such as sulfides and thiosulfonates, responsible for flavor and health benefits.
- Flavonoids: Quercetin and other antioxidants with protective effects.
- Fiber: Supports digestion and gut health.
- Polyphenols: Contribute to anti-inflammatory and antioxidant properties.
How to Cultivate Allium cepa
Onions are relatively easy to cultivate:
- Climate: Grow best in temperate climates with temperatures between 13°C and 24°C.
- Soil Requirements: Prefer well-drained, loose, organic-rich soil.
- Planting: Can be grown from seeds, sets (small bulbs), or transplants, depending on the variety and season.
- Irrigation: Require moderate watering, avoiding excess moisture that can cause root rot.
- Weed Control: Keeping the field weed-free is essential for good growth.
- Harvesting: Ready when leaves turn yellow and begin to bend over naturally.
- Storage: After harvest, onions are dried in a well-ventilated area before storage.
Uses and Benefits
Culinary Uses:
- A staple ingredient in various cuisines, adding flavor to numerous dishes.
- Can be eaten raw, cooked, fried, baked, or pickled.
- Used in soups, stews, sauces, salads, and meat or vegetable dishes.
Health Benefits:
- Antioxidant Properties: High quercetin levels help combat free radicals.
- Anti-inflammatory: Sulfur compounds contribute to reducing inflammation.
- Cardiovascular Health: Helps regulate blood pressure and cholesterol levels.
- Antibacterial & Antiviral: Traditionally used to support immune function.
- Blood Sugar Regulation: Studies suggest onions may help maintain stable glucose levels.
- Digestive Aid: Stimulates gastric juices and supports gut health due to prebiotic fiber.
Agricultural and Environmental Uses:
- Natural Pest Repellent: Its strong odor deters certain insects in horticulture.
- Organic Fertilizer: Onion scraps can be composted to enrich soil nutrients.
Cosmetic Uses:
- Onion extract is found in skincare products for scar reduction and skin brightening.
- Some traditional remedies use onion juice to promote hair growth.
Applications
Food Industry:
- Dried and powdered for seasonings and flavor enhancers.
- Used in pickles and preserves.
Traditional Medicine:
- Used in home remedies for colds, coughs, and respiratory infections.
- Found in some supplements for cardiovascular and digestive health.
Agriculture:
- Planted in crop rotation systems to improve soil health.
- Being studied for potential applications in organic pest control.
Environmental and Safety Considerations
Environmental Benefits:
- Onion cultivation contributes to agricultural biodiversity.
- Some local varieties are drought-resistant, making them suitable for sustainable farming.
Safety:
- Excessive onion consumption may cause gastric irritation in sensitive individuals.
- Onion juice can irritate the eyes and mucous membranes during preparation.
Research and Future Potential
Onions are being extensively studied for their bioactive compounds and potential health benefits. Recent research explores their role in preventing chronic diseases such as diabetes, cardiovascular conditions, and certain cancers. The food industry is developing new onion-based products to harness their nutritional and functional properties. In agriculture, researchers are working on climate-resilient onion varieties and improving post-harvest storage techniques to reduce food waste.
Studies
Onions have antibacterial properties and can help reduce the risk of certain chronic diseases. Quercetin, found in onions, has been linked to preventing age-related diseases such as diabetes, cancer, and neurodegenerative diseases.
Animal studies have shown the effectiveness of onion as an anticoagulant.
Human studies have found the usefulness of onion combined with other vegetables to prevent cardiovascular problems.
As an anti-inflammatory, a component of the onion, Onionin A found in the bulb, has been shown to perform an action to counteract inflammatory blood activity (1).
There would also appear to be an action to improve the balance of blood sugar.
It is recommended, along with garlic, shallots and chives to prevent prostate cancer (2).
The best known red onions in Italy are the red head of Tropea and Certaldo.
