Why Do Some Foods Change Color When Cooked?

When certain foods are cooked, their color can change due to chemical reactions that alter the pigments present in their tissues. The most common changes are caused by protein denaturation, the degradation or transformation of colored compounds, and the formation of new compounds through reactions such as the Maillard reaction or caramelization.

Main causes of color change

• Protein denaturation: When proteins are heated, they lose their three-dimensional structure and may expose groups that were previously hidden, changing the way they absorb light. A clear example is meat, which shifts from a pink or red tone to a grayish brown as myoglobin coagulates.
• Pigment degradation: Some pigments are heat-sensitive and break down. Chlorophyll, responsible for the green color of vegetables, turns into pheophytin (a more olive color) when it loses its magnesium ion during boiling.
• Maillard reaction: This is a reaction between amino acids and reducing sugars that generally occurs above 140 °C. It produces brown compounds that give the characteristic toasted tone to bread, roasted coffee, grilled meat and fried potatoes.
• Caramelization: When sugars are heated enough (around 160 °C), they break down and form compounds ranging from amber to dark brown. This can be seen in bread crust, melted sugar and vegetables such as carrots or onions when sautéed.
• Changes in anthocyanins and carotenoids: Flavonoids (anthocyanins) that give red, purple and blue colors can become more red or blue depending on the pH of the medium; during cooking, the medium sometimes becomes more acidic or basic, causing a shift in tone. Carotenoids (responsible for yellow, orange and red tones) are more heat-stable, but can oxidize slightly and become more opaque.

Specific examples

1. Red meat: Myoglobin, an iron-containing protein, shifts from a ferrous state (bright red color) to a ferric state (brown color) as it coagulates and oxidizes during cooking.
2. Broccoli and spinach: Chlorophyll loses its Mg²⁺ ion when boiled, turning into pheophytin, which changes the intense green to an olive or yellowish green.
3. Toasted bread: The sugars and amino acids in the dough react through the Maillard reaction, generating a brown crust and the characteristic aroma of toasted bread.
4. Caramelized onion: The onion's natural sugars break down and react, shifting from a translucent white color to a golden tone and then dark brown.
5. Strawberry or blueberry in an acidic sauce: Anthocyanins can shift toward redder tones when the medium becomes more acidic (for example, when adding vinegar or lemon).

Factors that influence the intensity of the change

• Temperature and time: The higher the temperature and the longer the exposure time, the more pronounced the Maillard reaction or caramelization will be.
• pH of the medium: An alkaline medium favors the formation of darker compounds in the Maillard reaction, while an acidic medium can preserve certain pigments like anthocyanins in redder tones.
• Presence of moisture: In very humid environments (such as boiling), the surface temperature of the food does not exceed 100 °C, limiting the Maillard reaction; in dry methods (roasting, baking, frying), much higher temperatures are reached, favoring browning.
• Type of food: Foods rich in protein and reducing sugars (meat, bread, legumes) show a greater tendency toward the Maillard reaction; those rich in chlorophyll or anthocyanins (vegetables, fruits) show changes related to the degradation of those pigments.

In summary, the color change that occurs during cooking is the result of chemical processes that modify or create compounds that absorb and reflect light differently. Understanding these mechanisms makes it possible to better control the final appearance of dishes, whether to achieve an appetizing golden-brown color or to preserve the bright green color of vegetables.