Carmelization Chemistry – Why Sugar Browns


The toasted part of a marshmallow has been carmelized. (John Lustig)

The toasted part of a marshmallow has been carmelized. (John Lustig)

Carmelization is one of the food browning processes, used to give foods a desirable color, flavor, and texture. It is also a process responsible for a burnt sugar flavor or blackening of food.

How Carmelization Works

Carmelization, like the Maillard reaction, is a form of non-enzymatic browning. It occurs when foods containing a high concentration of carbohydrates are heated above a certain temperature. The temperature at which carmelization occurs depends on the type of sugar. The rate at which carmelization proceeds depends on the acidity or pH of the food. Carmelization occurs more quickly at neutral pH than under either acidic or alkaline conditions.

Note fructose has a lower carmelization point than other sugars. Baked goods made using fructose brown more readily than those made using other sugars and often end up darker in color.

Carmelization Temperature of Different Sugars
Sugar Temperature
fructose 110°C, 230°F
galactose 160°C, 320°F
glucose 160°C, 320°F
sucrose (table sugar) 160°C, 320°F
maltose 180°C, 356°F

Carmelization is a process and not a single chemical reaction. As it occurs, water is removed from the carbohydrate. Isomerization and polymerization then take place. This is seen as melting, boiling, foaming, and darkening of sugar.

Carmelization is used to make candies, caramel, ghee, carmelized onions, and carmelized potatoes, among other foods. Foods that contain both carbohydrates and proteins brown from a combination of carmelization and the Maillard reaction.

How Carmelization Works in Sugar

Regular table sugar or sucrose is the best-studied carbohydrate for the carmelization process. It proceeds in the following manner:

  1. The disaccharide sucrose breaks down into the monosaccharides glucose and fructose. This is called a sucrose inversion.
  2. Condensation occurs, where the sugars lose water and react with each other, forming difructose-anhydride.
  3. Further dehydration occurs. Aldoses isomerize to ketoses.
  4. Molecules fragment and polymerize, producing the characteristic caramel color and browned sugar flavor associated with the process. The three main products from sucrose carmelization are the dehydration product caramelan (C12H18O9) and two polymers, carmelen (C36H50O25) and caramelin (C125H188O80).

Carmelization Flavors

Carmelization products have different flavors from each other. Here are some common compounds:

Diacetyl: Diacetyl forms during the first stages of caramelization. Diacetyl contributes a buttery or butterscotch flavor.
Hydroxymethylfurfural (HMF): Hydroxyacetylfuran (HAF) has a sweet aroma and flavor. Other furans have a nutty flavor.
Maltol: Maltol is the compound associated with the toasty flavor and aroma of freshly baked bread.
Esters and Lactones: These compounds have a sweet flavor, reminiscent of rum.