The Onion's Outer Layers Have the Most Quercetin

November 24, 2025 •

3 min read

According to research published by the National Institutes of Health, the outer scales and skin of onions can contain a concentration of quercetin up to 77 times higher than the inner parts. This astonishing fact reveals that the most nutrient-dense part of the onion, rich in the flavonoid quercetin, is often discarded.

Quick Summary

The highest concentration of the antioxidant flavonoid quercetin is found in the outer, fleshy layers and the dry, papery skin of onions, with levels decreasing significantly toward the core. Darker-colored onions like red and yellow varieties also tend to have more quercetin overall.

Key Points

Quercetin Concentration is Highest in Outer Layers: Scientific research confirms that the outer, fleshy rings and the dry papery skin of an onion contain the most concentrated levels of quercetin.
Concentration Decreases Toward the Center: A steep gradient of quercetin exists within the onion, with levels diminishing significantly from the outside toward the inner core.
Red and Yellow Onions are Higher in Quercetin: Darker-colored varieties like red and yellow onions typically have a higher total content of quercetin compared to white onions.
Cooking Method Matters: To preserve the most quercetin, avoid boiling, as the flavonoid can leach into the water. Sautéing, microwaving, and eating raw are better methods for retention.
Don't Discard Nutritious Outer Layers: The first few fleshy layers often peeled off along with the dry skin are rich in quercetin and should be kept or used in cooking whenever possible.
Utilize Onion Skins for Broth: For a simple way to extract the benefits of the inedible dry skin, simmer them in water to create a nutrient-rich stock for soups and other dishes.

The Surprising Truth About Onion Peels

Most home cooks meticulously peel away the dry, papery skin and the first few fleshy layers of an onion before cooking, discarding them as waste. However, extensive scientific research confirms that these very parts of the onion contain the highest concentrations of quercetin, a potent flavonoid with numerous health benefits. While the inner bulb is still a good source of nutrients, the outer layers are a hidden powerhouse of antioxidant potential that is often thrown away.

Comparing Quercetin Concentrations by Onion Part

Research using High-Performance Liquid Chromatography (HPLC) has precisely quantified the distribution of quercetin throughout the onion bulb. Studies consistently show a steep gradient, with the concentration of quercetin highest in the outermost parts and lowest in the core. The dry, inedible skin contains the most concentrated form, predominantly as the aglycone, or sugar-free, form of quercetin. The first few fleshy layers directly beneath the skin are also exceptionally rich in quercetin, mainly in the form of glucosides. This nutrient concentration gradually diminishes as you move toward the white, juicy center.

The Impact of Onion Color on Quercetin Levels

While the location of quercetin within the onion remains consistent across varieties, the total quantity of the flavonoid varies significantly by color. Darker-colored onions, specifically red and yellow varieties, generally contain higher levels of quercetin than their white counterparts. This is because flavonoids are often responsible for the pigmentation in plants, with the deep hues indicating a higher concentration of these beneficial compounds. For instance, one study found that red-skinned onions can have significantly higher total quercetin concentrations compared to yellow-skinned ones.

Preserving Quercetin During Cooking

Heat and water can affect the bioavailability and concentration of quercetin in onions. Different cooking methods have varying impacts on how much of this flavonoid you retain.

Boiling: This method is the least ideal for preserving quercetin. A significant portion of the water-soluble flavonoid will leach out into the boiling water, with studies showing an 18–30% decrease in total content. If the cooking liquid is not consumed, this quercetin is completely lost.
Sautéing and Frying: These methods are much better for retention, as they don't involve water. Sautéing can even increase the concentration of quercetin by breaking down the onion's plant matrix, making the flavonoids more available for absorption.
Microwaving and Baking: Research indicates that cooking with minimal or no water, such as microwaving or baking, can help retain or even increase the concentration of quercetin.
Eating Raw: Consuming raw onions is the most direct way to ingest the maximum amount of quercetin, as no processing reduces its levels.

How to Utilize the Most Quercetin-Rich Parts

Instead of discarding the outer layers, consider incorporating them into your cooking to maximize your quercetin intake. While the dry, papery skin is generally considered inedible, it can be used to make a flavorful and nutritious broth or stock.

To make onion skin broth:

Gather the dry, papery skins and the first few fleshy layers from 2-3 onions.
Place them in a saucepan with water, covering completely.
Simmer for at least 30 minutes, or until the liquid has a rich brown or amber color from the flavonoids.
Strain the liquid to remove the skins and use it as a base for soups, gravies, or as a flavorful cooking liquid for grains.

Comparison of Quercetin Content and Uses


Onion Part	Quercetin Concentration	Bioavailability Form	Best Culinary Use	Notes
Dry, Papery Skin	Highest	Mostly aglycone	Infusions for stock or broth	Not meant for direct consumption. Imparts rich color.
Outer Fleshy Layers	Very High	Glycosides	Lightly sautéed, raw in salads, or simmered in stews	Often discarded; should be kept to maximize intake.
Inner Bulb	Low	Glycosides	Cooking and raw uses	Still a good source, but significantly less concentrated than outer parts.

Conclusion

The science is clear: the most quercetin in an onion is concentrated in its outermost layers and skin, a fact that challenges conventional kitchen practices. By being mindful of which parts we discard, we can significantly increase our intake of this powerful antioxidant. Whether using the peels for a flavorful stock or simply retaining the first few fleshy layers when chopping, making these small changes can help you get the maximum nutritional benefit from this versatile vegetable. For those seeking to boost their intake even further, choosing red or yellow varieties and opting for cooking methods with minimal water will also help preserve and even enhance quercetin content.

https://www.sciencedirect.com/science/article/pii/S1756464625002312

Frequently Asked Questions

Red and yellow onions contain higher total concentrations of quercetin compared to white onions. The color is often an indicator of a higher flavonoid content.

The dry, papery skins are generally not eaten, but the fleshy outer layers just beneath them are edible and very high in quercetin. It is advisable to keep these layers rather than discarding them.

Boiling causes the greatest loss of quercetin, as it leaches into the water. However, cooking methods like sautéing, baking, or microwaving can preserve or even enhance the flavonoid's concentration by breaking down the plant's cell walls.

To retain the highest amount of quercetin, eat onions raw, or use cooking methods that involve minimal to no water, such as sautéing, baking, or microwaving.

Quercetin is a powerful antioxidant and anti-inflammatory compound. It may help support heart health, lower blood pressure, and reduce oxidative stress in the body.

Yes, simmering dry onion skins in water is an excellent way to extract their rich quercetin content and other flavonoids into a flavorful broth for soups and stews.

Eating onions raw is the most surefire way to get the maximum amount of quercetin, as it is consumed without any heat processing. However, lightly cooked onions still provide a significant amount.