Why is it that when you boil beans and lentils a layer of scum appears on the water? Should you throw it out?


When you boil beans and lentils they release chemicals from their tissues.  These chemicals, often proteins, leach out as water boils, and the air that is 'dissolved' in the water when cold (cold water holds more dissolved gases than hot water) bubbles out, catches the proteins and floats to the surface, forming a foam.

Beans and lentils are the seeds of plants, and in order that the seeds get the best chance to grow into new plants, they are often packed full of toxins, poisons to prevent being eaten.  This works well with herbivorous animals, but less well when you know how to cook.  Since the poisons are proteins they can be denatured, that is the shape of the protein is damaged so that it is no longer a functional protein, when heated.

This is exactly what happens when you get scum on your bean pan when you are cooking.  The denatured protein is floating to the surface.  It is therefore a good idea to remove the scum, because you may still have some unpleasent chemicals in the foam.