The refrigerating liquid is a eutectic solution made of water and mineral salts. The quality and concentration of these mineral salts can vary in order to modify the water freezing point.
Water gets solid by freezing at or below 0°C. A eutectic solution with a freezing point at -21°C gets solid at or below -21°C.
Freezing point and released temperature are closely related : a eutectic device at -21°C releases a negative cold at -18°C when frozen pure water releases cold at +2°C.
The refrigerating liquid is contained in eutectic plates, more commonly called refrigerating ‘packs’.