The temperature at which a food begins to freeze is a known parameter, but as the freezing proceeds, the freezing temperature is decreasing because the free water left in the food is becoming more and more concentrated in salts with a consequent decrease in its freezing temperature. The first portion of water freezes at the foods initial freezing temperature, but the last portions will do it at a much lower temperature. Due to this it becomes difficult to define the end of the freezing step. To cope with this problem, Plank considered that all the freezing period happened at constant temperature (¼initial freezing temperature).
It is considered that the freezing of the food is ended when its centre temperature reaches )10 C.