Why Are Onions Sweet When Cooked Heres What We Know

Onions are an integral part of the kitchen. They are used in cooking, salads, soups, stews, sauces, and even desserts. Onions are also known as Allium cepa, and their botanical name is Allium sativum.

They are a member of the lily family, and they are native to Europe, Asia, Africa, and North America. The onion bulb has been cultivated since ancient times. It was first domesticated in Mesopotamia around 5,000 years ago.

Today, there are two main types of onions: spring onions and storage onions. Spring onions are harvested before the bulbs start to form. Storage onions are harvested after the bulbs have formed. Spring onions are usually smaller than storage onions. They are often used raw in salads and sandwiches. They are also commonly used in stir-fries, soups, and other dishes.

Onions are an essential ingredient in almost every recipe. They add flavor, texture, and color to dishes. But did you know that onions also have a sweet side? When cooked, onions become sweeter. And while there are many theories behind why this happens, the science behind it is still being researched.

In this article, we’ll explore the science behind why onions turn sweet when cooked.

Why are onions sweet when cooked?

The answer lies within the chemistry of how onions work. There are three major components found inside all onions: water, carbohydrates, and proteins. These compounds react differently during different stages of cooking.

Cooking Onions: Flavor Chemistry Changes - Sweetness

When onions cook at high temperatures for long periods of time, these reactions occur quickly. This causes sugars to break down into simpler molecules called pyruvic acid. Pyruvate is what gives onions their sweetness. When onions cook slowly over low heat or simmer, however, the reaction between sugar and protein takes longer. As a result, more complex chemical bonds form. In addition to forming new chemical bonds with each other, the proteins begin to crosslink together. Crosslinking creates a stronger structure that makes the onions harder and firmer.

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This process explains why onions get softer and sweeter when they’re boiled instead of sautéed. Boiling breaks down the starch faster than frying does. Because boiling doesn’t require much energy, less oxygen gets involved in the reaction. Oxygen speeds up the breakdown of starches, which results in fewer simple sugars.

This means that when onions are boiled, they don’t develop any pyruvic acids. Instead, they produce large amounts of acetic acid. Acetates give onions their pungent taste. Onions contain sugar, but not enough to make them sweet. So if you want your onions to be sweet, boil them!

Do onions turn to sugar when cooked?

No, onions do not actually change from one type of molecule to another. However, they do undergo changes in composition. For example, when onions are heated, some of the water evaporates away. Some of the moisture leaves because it becomes trapped inside the cell walls. At the same time, the amount of free water decreases.

As a result, the concentration of solids increases. Solids include both carbohydrates and protein. Carbohydrates are made up of carbon, hydrogen, and oxygen atoms. Proteins consist of nitrogen, oxygen, phosphorus, sulfur, and sometimes calcium. The increase in solid content can cause the onion cells to swell. Swelling forces out liquid, causing the onion to shrink.

As the onion shrinks, its volume decreases. Since the total number of particles remains constant, the ratio of solids to liquids increases. That’s why onions get sweeter as they cook.

What onions are the sweetest?

There isn’t just one kind of onion that tastes sweet. All varieties of onions will eventually turn sweet after prolonged exposure to heat. It depends on where the onion was grown and harvested.

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Sweetness varies depending on the variety of onions. Red onions tend to be milder tasting than yellow ones. White onions are usually considered sweeter than reds. Yellow onions are often used raw in salads. Their mild flavor allows people to enjoy them without adding extra salt. But white onions also have an intense aroma that comes off strong even when eaten alone. They work well in dishes like soups and stews.

Onion growers use different techniques to control how sweet onions become. One way is by controlling the temperature during harvest. Another method involves growing onions under controlled conditions so that they stay small throughout development. Smaller onions take longer to mature. And since they grow slower, they’ll reach maturity before they start producing too many seeds.

Caramelized vs sauteed onions

When we talk about caramelizing or browning food, we mean cooking something until it turns dark brown.

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This happens when certain types of molecules called polyphenols react with other compounds called phenolic antioxidants. These reactions create new chemical bonds between the two substances. As a result, the original compound loses electrons and gains a positive charge.

The colorless phenolics then bond together into colored pigments. In this case, the pigment is known as melanin. Melanin gives foods such as carrots, potatoes, and apples their characteristic colors.

Sauteed onions are ..well, cooked! Sautee means "to fry" in French. So if you’re frying your own onions for a recipe, you might want to consider using a pan instead of boiling them in a pot. Boiling causes the outer layers of the onion to dry out first. Then those dried-out bits fall apart. If you don’t remove them, they may burn while the rest of the onion cooks through.

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Should you cover the pan when caramelizing onions?

Yes, but not all the time. Covering the pan helps prevent moisture from escaping. Moisture makes the surface of the onion soggy. Sogginess prevents the onion from crisping properly. You should only need to do this once or twice per batch of onions. Otherwise, leave the lid open.

What is the best way to remove the sweetness from onions?

You can try adding some acidity to onions if you want to deal with the sweetness problem. For example, lemon juice works great. Or you could add vinegar. Vinegar has a lower pH level than most acids. The higher the pH level, the more acidic the solution becomes. Acidity cuts back on the amount of sugar present in the onion.

But there’s another option: water. Water lowers the overall concentration of sugars in any substance. By diluting the onion, you reduce the amount of sugar available to interact with the phenolic antioxidants.


If you’ve ever wondered why onions taste sweet after being cooked, here’s what happened. First, the heat breaks down the cell walls of the onion. That lets the liquid inside escape. It also creates steam which evaporates away much of the remaining water content.

Finally, the high temperatures cause the onion cells to rupture. All these things combine to make the onion very soft.I hope this post helped you with all your food curiosities.

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