Determination of the thermal diffusivity of foods from temperature measurements during cooling

Abstract

Two techniques are described for estimating the thermal diffusivity of spherical foodstuffs from the temperature/time histories during cooling in a medium at constant temperature. The first method (the ‘Slope Method’) calculates thermal diffusivity from the slope of the cooling curve for the food centre, whilst the second method (the ‘Lag Method’) uses the maximum temperature difference between the centre of the food and a point half way from the centre to the surface. For values of thermal diffusivity between 0.85 × 10 −7 and 3.23 × 10 −7 m 2 s −1, a range which includes almost all foodstuffs, the mean and maximum errors were 1.3% and 4.0% by the slope method and 1.2% and 5.3% by the lag method. Additional errors arising from the imprecision of thermocouple location are discussed. The mean and maximum differences between values of thermal diffusivity calculated by the two methods for a range of fruits and vegetables were 2.43% and 6.7%. The methods could be modified for foods of other simple geometric shapes, for example semi-infinite slabs and cylinders.