Why Are Red Onions Purple? The Science Behind Their Surprising Colour
Introduction
Have you ever wondered why red onions look more purple than red? You’re not alone. These vibrant bulbs are a kitchen staple across the globe, but their colour often sparks confusion.
Despite the name, red onions range from deep magenta to vivid violet — and they’re one of the few vegetables where the name doesn’t seem to match the appearance.
In this article, we’ll explore the science behind why red onions are purple, uncover the role of pigments and pH levels, and explain what this colour means for both health and cooking.
Quick Answer: Why Are Red Onions Purple?
Red onions get their purple colour from anthocyanins, a type of water-soluble pigment found in many red, blue, and purple fruits and vegetables. The actual shade depends on the pH level in the onion’s cells, which shifts their hue from red to blue to purple.
So while we call them “red onions,” their pigments naturally express as purple under neutral to slightly acidic conditions — which is why they appear violet.
The Science of Colour: Anthocyanins Explained
Anthocyanins are flavonoid compounds found in:
- Red cabbage
- Blueberries
- Blackberries
- Purple carrots
- And of course, red onions
These pigments:
- Reflect red, blue, or purple depending on the pH of the plant tissues
- Are stored in vacuoles within onion cells
- Are antioxidants — giving red onions their superfood reputation
| pH Level | Anthocyanin Colour |
|---|---|
| Acidic (< 4) | Bright red |
| Neutral (~7) | Purple/magenta |
| Alkaline (> 8) | Blue/green |
Red onions usually fall in the neutral range, hence their characteristic purple hue.
Then Why Are They Called “Red” Onions?
The name “red onion” is traditional, not scientific. Historical naming often prioritised visual contrast or cultural interpretation rather than exact shade.
- In older times, “red” was a general term used for both red and purple vegetables (like red cabbage, red grapes)
- “Red” may also refer to the outer skin, which sometimes appears deep red or maroon
- Language influences: In some cultures, red = any colour from pink to dark violet
It’s a great reminder that food names don’t always match their biology!
Red Onions vs Other Onions: Pigment Comparison
| Onion Type | Colour | Pigment Type | Flavour Notes |
|---|---|---|---|
| Red onion | Purple/magenta | Anthocyanins | Mild, sweet, great raw |
| Yellow onion | Pale yellow | Carotenoids | Rich, strong, versatile |
| White onion | White | Few/no pigments | Sharp, clean flavour |
| Shallots | Pink/rose skin | Low anthocyanin/carotene | Sweet, delicate |
So red onions stand out for their colour and antioxidant load — a double win in the kitchen.
Does Cooking Change the Colour of Red Onions?
Yes. Because anthocyanins are water-soluble and pH-sensitive, cooking can affect both their colour and potency.
| Cooking Method | Colour Change | Why It Happens |
|---|---|---|
| Boiling | Turns pale or bluish | Anthocyanins dissolve and react with water |
| Pickling (acidic) | Turns hot pink or red | Acid intensifies red tones |
| Roasting/Sautéing | Darkens to deep brown/purple | Pigments caramelise with sugars |
| Microwaving | Minimal colour loss if quick | Less heat exposure |
💡 Tip: Add a splash of vinegar when cooking to help retain or enhance the red tones.
Fun Fact: Red Onions as Natural pH Indicators
Because of their pH-sensitive pigments, red onions (like red cabbage) can be used as a natural pH indicator in science experiments.
- In acidic solutions: they turn reddish-pink
- In basic (alkaline) solutions: they shift to greenish-blue
You can try this at home by soaking onion skins in water and adding lemon juice or baking soda — perfect for an edible science project!
What the Colour Means for Nutrition
Red onions’ purple colour isn’t just for show — it reflects higher antioxidant content compared to white or yellow onions.
Key compounds in red onions:
| Compound | Benefit |
|---|---|
| Anthocyanins | Anti-inflammatory, protect cells from damage |
| Quercetin | Reduces blood pressure and inflammation |
| Sulphur compounds | Support immune and liver function |
| Flavonoids | Improve heart and brain health |
The richer the colour, the more potent the health benefits.
Are Red Onions Healthier Than White Onions?
Yes — mainly because of their anthocyanin content.
| Nutrient/Property | Red Onion ✅ | White Onion ❌ |
|---|---|---|
| Anthocyanins | ✅ High | ❌ None |
| Quercetin levels | ✅ Higher | ✅ Moderate |
| Antioxidant content | ✅ More | ✅ Less |
| Flavour (raw use) | ✅ Milder | ❌ Sharper |
That said, all onions are healthy, and each has its place in your diet.
How to Preserve the Colour in Recipes
If you want to maintain or enhance the purple colour in your cooking:
✅ Pickle them – acid from vinegar makes them turn bright pink
✅ Add late in cooking – preserve colour by reducing cook time
✅ Use lemon or lime juice – the acidity boosts red tones
✅ Avoid baking soda or alkaline ingredients – turns them blue or grey
Try quick-pickled red onions on tacos, burgers, or wraps for maximum visual (and flavour) impact.
Global Names and Colour Confusion
In different cultures, red onions are called:
- Purple onions (Asia, Australia)
- Red onions (UK, US, Europe)
- Violet onions (France, Italy)
This reflects the subjectivity of colour naming — and shows that “red” doesn’t always mean bright red.
Historical Use of Onion Pigments
Before synthetic dyes, red onion skins were sometimes used to dye fabrics, yarn, or Easter eggs, creating beautiful shades of:
- Pink
- Violet
- Brownish red
This traditional use is still practiced in folk art and home dyeing today — a natural way to colour without chemicals.
Conclusion
So, why are red onions purple? It’s all down to anthocyanins, the powerful pigments that reflect shades of purple and red depending on pH. Their stunning hue isn’t just aesthetic — it’s a sign of their high antioxidant content, health benefits, and culinary versatility.
Whether you’re adding them raw to salads, pickling them for tacos, or caramelising them for a burger, red onions deliver colour, flavour, and nutrition — all in one humble bulb.
Next time someone calls them red, you’ll know they’re scientifically purple — and proudly so.
Top 10 FAQs About Why Red Onions Are Purple
1. Why are red onions called red if they’re purple?
It’s a historical naming convention — “red” was often used to describe deep-coloured produce, including purple items.
2. What gives red onions their purple colour?
Anthocyanins, natural plant pigments that reflect red to blue tones based on pH.
3. Can red onions change colour when cooked?
Yes — heat, acidity, or alkalinity can shift their colour due to pigment sensitivity.
4. Are red onions healthier than other onions?
Yes — they have more antioxidants, especially anthocyanins and quercetin.
5. Why do pickled red onions turn pink?
Acid (like vinegar or lemon juice) enhances red pigments, making them brighter.
6. Do red onions change colour in alkaline foods?
Yes — they can turn blue or green in basic (alkaline) environments.
7. What does the purple colour mean nutritionally?
It signals high antioxidant power and anti-inflammatory benefits.
8. Do red onions stain?
Yes — their pigments can stain cutting boards or clothing.
9. Are red onions the same as purple onions?
Yes — the terms are used interchangeably in different countries.
10. Can I grow red onions at home?
Absolutely — they grow well in the UK and other temperate climates.