Foam accumulates on the surface of dal

Dear Student,
Please Find Your answer -

When we cook lentils i.e; Dal , chemicals are released from their Tissue. These chemicals are often , Proteins. More dissolved gases are held by air particles , than those by Hot water. So these air particles leech out forming Foam on the surface .
Proteins gets denatured (change their shape) and become non functional , if kept for a long period of time. 

Hope this will clear your doubt.
Regards!