Thermal Properties of Minor Millet Grains and Flours

Abstract The various thermal properties, viz ., specific heat, thermal conductivity and thermal diffusivity were determined for both the grains and flours of foxtail millet ( Sestaria italia ), little millet ( Panicum miliare ), poroso millet ( Panicum miliaceum ), kodo millet ( Paspalum sorobiculatum ), finger millet ( Eleusine coracana ) and barnyard millet ( Echinochola colona ) in the moisture content range of 10–30%. Specific heat was determined using the method of mixtures and the transient heat flow method, and a thermal diffusivity probe was used for the determination of thermal conductivity and thermal diffusivity of the millets and their flours. An increase in specific heat was observed in the range of 1·33– 2·40 kJ kg −1 K −1 for both grains and flours, with an increase in moisture content. The thermal conductivity of the millet grains increased from 0·119– 0·223 W m −1 K −1 and the flours increased from 0·026– 0·128 W m −1 K −1 in the moisture range of 10–30%. The thermal conductivity of flour was considerably less than that of grains. As the moisture content increased from 10 to 30% (wb), thermal diffusivity of millet grains decreased from 0·731×10 −3 to 0·55×10 −3 m 2 h −1 and that of their flours ranged from 0·820×10 −3 to 0·592×10 −3 m 2 h −1 . Notation A constant D thermal diffusivity, m 2 h −1 H c heat capacity of the calorimeter, kJ K −1 H ca heat capacity of the capsule, kJ K −1 Q heat input, W R inner radius of the cylinder, m S specific heat of sample, kJ kg −1 K −1 S w specific heat of water, kJ kg −1 K −1 T 1 temperature at time θ 1 , K T 2 temperature at time θ 2 , K T cp temperature at the centre of the thermal diffusivity probe, K T e equilibrium temperature of the mixture in the calorimeter, K T s temperature of the sample, K T sp temperature at the outer surface of the thermal diffusivity probe, K T w initial temperature of water in the calorimeter, K W s mass of sample, kg W w mass of water in the calorimeter, kg θ 0 time correction factor, s θ 1 time corresponding to temperature T 1 , s θ 2 time corresponding to temperature T 2 , s