The structure of proteins is destroyed by the application of heat or chemical agents and the process is called denaturation. It involves the disruption of both the secondary and tertiary structures of a protein, but the primary structure, which involves a sequence of amino acids does not get disturbed and remains intact. This results in the loss of the biological activity of the proteins. The protein in milk is normally suspended in a colloidal solution, which means that the small protein molecules float around freely and independently. On adding lime juice to milk the pH decreases and the small protein molecules gets coagulated and forms large chain. This is denaturing of protein and causes curdling. Therefore the (A) is false but (R) is true.