Why do onions get sweet when you cook them?

As a culinary expert with a deep understanding of the science behind food chemistry, I can explain why onions become sweeter when cooked. The process involves a series of chemical reactions that transform the complex carbohydrates in onions into simpler forms that our taste buds perceive as sweeter.
When onions are heated, the high temperature triggers a process known as the Maillard reaction. This is a chemical reaction that occurs between amino acids and reducing sugars when heated. The Maillard reaction is responsible for the browning of the onions and the development of a rich, sweet flavor. It's the same reaction that makes seared meats, roasted coffee, and toasted bread so flavorful.
However, the sweetness in onions is not solely due to the Maillard reaction. The primary reason onions become sweeter when cooked is due to the breakdown of complex sugars into simpler forms. Onions contain a type of complex sugar known as fructose, which is already naturally sweet. But it's not the only sugar present; there are also other types of complex carbohydrates, such as starch and inulin, which are not as sweet as fructose.
When onions are cooked, the heat causes the complex carbohydrates to break down into simpler sugars. This process is known as pyrolysis. As the complex sugars are broken down, they release monosaccharides, which are the simplest form of sugar. Monosaccharides like glucose and fructose are sweeter than the original complex sugars, which is why cooked onions taste sweeter.
The breakdown of these complex sugars is also facilitated by enzymes present in onions. When onions are cut or damaged, these enzymes are activated and start to break down the complex carbohydrates. This is why onions can also become sweeter when they are sautéed or caramelized, even before they start to brown from the Maillard reaction.
In addition to the breakdown of complex sugars, the heat also causes the water content in onions to evaporate. As the onions lose water, the concentration of the sugars increases, which also contributes to the sweeter taste.
Furthermore, the heat can also affect the flavor compounds in onions. Some of the compounds that give onions their characteristic sharp and pungent taste, such as sulfur compounds, can be reduced or altered when cooked. This softens the flavor profile and allows the sweetness to come to the forefront.
In summary, the sweetness in cooked onions is a result of several factors:
1. The Maillard reaction, which browns the onions and enhances their flavor.
2. The breakdown of complex carbohydrates into simpler, sweeter monosaccharides through pyrolysis.
3. The activation of enzymes that further break down complex sugars.
4. The evaporation of water, concentrating the sugars and enhancing their sweetness.
5. The reduction or alteration of sharp and pungent flavor compounds, allowing the sweetness to be more pronounced.

Now, let's transition into the Chinese translation of the explanation.

This reaction of breaking down larger sugars into single sugar molecules is what causes saut--ed or caramelized onions to brown and develop a sweeter flavor. Heat causes the sugar molecules that make up polysaccharides (at top) to separate into monosaccharides (at bottom) in a process called pyrolysis.Nov 7, 2011