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Chemistry of Caramelization: Why does the onion turn brown when you fry it

Fine dining dish of a caramelized onion created by AI
published January 30, 2023 | 5 min read

Have you ever wondered why onions caramelize when you fry them? It’s all thanks to the Maillard reaction, a chemical reaction between amino acids and sugars that causes browning. Keep reading to learn more about the Maillard reaction and how it affects onions!

Macro photography of onion caramelization process created by AI
Macro photography of a caramelized onion

The Maillard Reaction

The Maillard Reaction is a chemical reaction that occurs when food is heated, resulting in the browning of the food. This reaction is responsible for the browning of onions when they are fried. The Maillard Reaction is a complex series of reactions between the amino acids in the food and the sugars in the food. These reactions result in the formation of new molecules, including melanoidins, which give the food its brown color. The Maillard Reaction is a key component of the flavor of many foods, including caramelized onions.

AI created microscopic molecules with maillard reaction of caramelized onion
Microscopic photography of Maillard reaction

What Happens During the Reaction?

When you fry an onion, the individual cells of the onion start to break down. This process is called caramelization, and it results in the formation of new compounds that give the onion its characteristic brown color. Caramelization also affects the flavor of the onion, making it sweeter and more complex.

microscopic molecules with maillard reaction of caramelized onion creadet by AI
Individual cells of the onion breaking down

Caramelization is the browning of sugar that occurs when it is heated. Caramelization is a complex process that involves the breaking down of the sugar molecule into smaller molecules. The sugar molecule is made up of carbon, hydrogen, and oxygen atoms. When the sugar is heated, the bonds between the atoms break and the atoms rearrange themselves into new molecules. This process is called oxidation. The new molecules that are formed during caramelization are brown in color.

Microscopic molecules of maillard reaction of caramelized onion created by AI
breaking down of the sugar molecule into smaller molecules

Why Does It Cause Caramelization?

Amino acids play a vital role in the chemistry of caramelization. When you fry an onion, the amino acids in the onion react with the heat to create new compounds that give the onion its brown color. This process is known as the Maillard reaction, and it’s responsible for the flavor and color of many cooked foods. Caramelization is a similar process that occurs when sugar is heated, and it’s what gives caramel its characteristic flavor and color.

Microscopic photography of amino acids created by AI
microscopic photography of amino acids

The Chemistry of Browning

There are several types of reactions that occur when food is cooked. One of the most important reactions is decomposition, also known as breaking down molecules into smaller pieces. This process releases energy that the cells in our body can use to survive. Another type of reaction that occurs when food is cooked is known as condensation. During condensation, many small molecules join together to form a few bigger molecules; this is the opposite of decomposition. One example of a condensation reaction is the formation of glucose (a type of sugar) from numerous type A glucose enzymes. The last important reaction that cooks should be aware of is oxidation. During oxidation, onion loses its wholesome, allicin-rich cellulose coverings, which are responsible for its potent flavor and health benefits.

 

Macro photography created by AI of onion caramelization process
Macro photography of onion caramelization process

When onions are sliced or chopped into foods such as salsa. Within a few minutes, the onions begin to turn brown. Luckily, this doesn’t affect the flavor much, but it does affect the appearance.

Caramelization is responsible for the brown color of many foods, including onions, bread, and coffee. Caramelization occurs when the food is heated to a high temperature. The browning of the food is a result of the Maillard reaction, which is a chemical reaction between amino acids and sugars.

BY AI FOOD CHEMIST PILA MAN

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