Cooked green beans can be a vivid green color, or they can turn gradually less colorful, sometimes becoming greyish or brownish.
Generally salt is added to the water before boiling vegetables. The reasons given for this include:
- It makes them greener
- It makes them firmer
- It raises the boiling point of water to make them cook faster
- It improves the flavor.
Chemists studied the truth behind these claims and found that first 3 of them are totally false. Adding salt slightly improves the flavor. The increase in the boiling point is insignificant to make any difference in the cooking time.
The color of the beans is dependent on the pH of the cooking water. The green color is due to chlorophyll present in the beans. If the water is acidic, the Magnesium ion bound to the chlorophyll is replaced by hydrogen ions and color is discharged. So depending upon the pH, their will be different degree of color changes.
If you cook the beans in hard water which contain bivalent ions calcium and magnesium, the pectin sugars present in the beans become firmly attached to each other and form a nice three dimensional network and give it a nice firm texture. Soft water on the other hand, dissolves the pectin quickly giving the cooked beans a mushy texture.