The enthalpy of a system defined as h = U + pV where U is the internal specific energy, p is the pressure and V is the specific volume of a substance. Like pressure, temperature, and volume, enthalpy is a property of a substance. The enthalpy is normally expressed with respect to some reference value. The specific enthalpy of water or steam, for example, is zero at 0.01°C and 101, 325 Pa.

For the freezing of foods, a value of 0 kJ kg⁻¹ is taken at −40^ºC.