Why do beans change colors?

The main factor influencing the change in bean color is the oxidation of phenolic compounds during the cooking process. Here is a more detailed explanation:

1. Phenolic Compounds: Beans are rich in various types of phenolic compounds, which are natural pigments responsible for their characteristic colors. These compounds include anthocyanins, flavonols, and tannins, among others. Anthocyanins, in particular, are responsible for the bright colors found in many beans, such as red, black, and purple hues.

2. Oxidation: When beans are exposed to heat during the cooking process, the phenolic compounds undergo oxidation reactions in the presence of oxygen. This oxidation process converts the colorless precursors of these compounds into colored pigments. As a result, the beans change from their original color to various shades of brown or tan.

3. Acidic Environment: The cooking water's pH can also influence color changes in beans. In acidic conditions (low pH), the anthocyanins in beans tend to be more stable, maintaining their original colors better. On the other hand, in alkaline conditions (high pH), the anthocyanins break down more readily, resulting in a color shift towards brown tones.

4. Metal Ions: Some metals present in the cooking water or cookware can interact with phenolic compounds and affect their stability and color. For instance, iron ions from cast iron pots can lead to darker brown hues, while copper ions can enhance red or purple colors.

It's important to note that the rate and extent of color change during cooking can vary depending on the type of bean, the cooking conditions (temperature, duration, pH), and the initial concentration of phenolic compounds in the beans.

To maintain the original color of beans better during cooking, you can employ certain strategies, such as using a slightly acidic cooking medium (adding a small amount of lemon juice or vinegar) and limiting the cooking time.