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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.

Why Milk Curdles

Milk curdles when its pH drops, allowing protein molecules in milk to clump together. (Tess Watson)

Milk curdles when its pH drops, allowing protein molecules in milk to clump together. (Tess Watson)

Curdled milk is what you get when lumps form in smooth milk. Although the clumps form in spoiled milk, the chemical reaction that causes curdling also occurs in fresh milk, under the right conditions. Intentional curdling of milk is used to produce foods, such as yogurt, cheese, and buttermilk. Here’s a closer look at how curdling happens:

Curdling Chemical Reaction

Fresh milk is an example of a colloid, consisting of fat and protein particles floating in a water-based solution. The colloidal suspension scatters light, causing milk to appear white. The protein molecules, mainly casein, repel each other so they naturally distribute evenly through the liquid. Milk is slightly acidic. When the pH is lowered even more by the addition of another acidic ingredient, the protein molecules stop repelling each other. This allows them to stick together or coagulate into the clumps known as curds. The watery liquid that remains is called whey.

How Sour Milk Curdles

When milk goes “off” or turns sour, it is because acids produced by bacteria lower the pH of milk so the proteins can clump together. The increased acidity of the milk also causes it to taste sour. The bacteria living in milk naturally produce lactic acid as they digest lactose so they can grow and reproduce. This occurs whether milk is fresh or pasteurized. You won’t notice the effect on flavor until enough acid has been produced. Refrigerating milk slows the growth of bacteria. Similarly, warm milk helps bacteria thrive and also increases the rate of the clumping reaction.

Curdling Milk in Coffee and Tea

If you enjoy milk in your coffee or tea, you may have noticed sometimes milk immediately curdles when added to the hot beverage. Except for the chunkiness, the drink may taste perfectly fine. This is because coffee and tea contain just enough acidity to tip the pH of milk to the point of curdling. The effect is most often seen in milk that is close to going sour or when adding milk to very hot coffee or tea, since the high temperature can coagulate casein.

Intentional Curdling of Milk

No one wants to drink chunky milk straight from the fridge, but the chemical reaction that causes curdling isn’t always bad. The same reaction produces buttermilk, cheese, and yogurt.

Adding lemon juice or vinegar to fresh milk is an easy way to make homemade buttermilk. Why isn’t buttermilk clumpy? It would be, if you added the acidic ingredient to hot milk. However, adding acid to cold milk allows casein to coagulate more slowly. Rather than forming clumps, the chemical reaction simply thickens the liquid. The ingredient also affects the flavor of the buttermilk, adding a tangy note.

Yogurt and cheese are slightly more complicated because you usually control the type of bacteria (the bacterial culture) used to make a product with a pleasing flavor and texture. However, fresh cheeses, such as ricotta, is very simply made by heating milk, adding an acidic ingredient, and straining the curd.