The freezer brick itself has a food proof hard plastic shell that surrounds the nontoxic gel inside. these bricks are frozen in a freezer of household refrigerator.
How it works!!
The Freezer Bricks can absorb a considerable amount of heat before they warm above 0 °C, due to the high latent heat of fusion of water. Water has a much higher latent heat of fusion than most substances, and a melting temperature which is convenient and easily attained with, for example, a household freezer. Additives to improve the properties of water are often used. For example, substances can be added to prevent bacterial growth in the pack, or to prevent the water from solidifying so it remains a thick gel throughout use.
Freezer bricks are made of non-toxic materials that will remain a slow-flowing gel, and therefore will not spill easily or cause contamination if the container breaks. Freezer bricks are made by adding hydroxyethyl cellulose (Cellusize), sodium polyacrylate, or vinyl-coated silica gel.
The freezer bricks are stored in freezer of household refrigerator. Before use it has to be stored in the freezer for at least 5 hours. After use, it has to be washed with clean water and back in the freezer for re-use.