Abstract
Abstract: Energy feedstuffs, the most important and indispensable components in the pelleted complete feeds, provide energy in the form of soluble carbohydrate, oil and fat. Energy feedstuffs mainly includes cereals, cereal brans, molasses, oil, tuber and tuberous root, among which corn, wheat, barley, sorghum, wheat/rice bran, pelleted cassava residue and beet pulp are widely utilized in diets for livestock and poultry. Knowledge of the thermal properties of different feedstuffs is required for designing and parameter optimization of thermal process such as conditioning and pelleting processing, especially when energy costs, pellet quality and uniformity are the main consideration. The specific heat, thermal conductivity and thermal diffusivity are the most important thermal properties. In this paper, eight energy feed ingredients (4 cereal grains: corn, wheat, barley, sorghum; 4 processed by-products: wheat bran, pelleted cassava residue, pelleted beet pulp, rice bran) were obtained from a feed mill in the suburb of Beijing, and ground in a mini-mill equipped with screens having a aperture size of 1.5, 2.0 or 2.5 mm to obtain feed meals with 3 particle sizes (except rice bran, which could pass through the 1.5 mm screen completely without being ground). The moisture content of all feed meal samples was conditioned to 12% wet basis in order to eliminate the effect of moisture content difference on thermal properties. The specific heat, thermal conductivity of feed meals at the temperature range of 25-100 ℃ were determined by DSC (differential scanning calorimetry) and KD2 Pro (thermal properties analyzer), respectively. Thermal diffusivity of feed meals was calculated using measured thermal conductivity, specific heat, and bulk density. Based on the experimental data, prediction models of three thermal properties as a function of temperature were established. The effects of particle size on thermal properties of feedstuffs were analyzed. The particle size of 8 feedstuffs grinding through screens with aperture size of 1.5, 2.0 and 2.5 mm ranged from 257.61 to 511.79 μm, 325.65 to 594.83 μm and 335.28 to 671.05 μm, respectively, which increased significantly with the increase in mesh size. There were significant differences between the values of particle size of 8 feedstuffs through the same mesh size. The values of specific heat of 4 grain meals (passing a screen with aperture size of 1.5 mm) increased with temperature (25-100 ℃) and ranged from 1.678 to 2.421, 1.644 to 2.402, 1.642 to 2.385, and 1.649 to 2.441 kJ/(kg·K), respectively. For wheat bran, cassava residue and beet pulp, the values were found to range from 1.621 to 2.540, 1.580 to 2.195, 1.746 to 2.351 kJ/(kg·K). The specific heat of rice bran raised from 1.966 kJ/(kg·K) at 25 ℃ to 1.985 kJ/(kg·K) at peak temperature (28.0 ℃), then declined to 1.868 kJ/(kg·K) at end-set temperature (33.5 ℃), and finally increased to 2.671 kJ/(kg·K) at 100 ℃. There was no significant effect of particle size on the specific heat of all feedstuffs (P>0.05). The specific heat followed linear relationships with temperature at the range of 25-100 ℃ for 4 grains, and cubic polynomial relationships for wheat bran, cassava residue and rice bran, and a second order polynomial relationship for beet pulp. The values of thermal conductivity of 4 grain meals (passing a 1.5 mm screen) increased nonlinearly with temperature (25-100 ℃) and varied from 0.089 to 0.299, 0.078 to 0.336, 0.088 to 0.276 and 0.082 to 0.288 W/(m·K), respectively. For 4 processed by-products, the thermal conductivity was found to raise from 0.054 to 0.190, 0.092 to 0.362, 0.085 to 0.263, and 0.064 to 0.202 W/(m·K), respectively. Significant differences were observed between the thermal conductivity of 8 feed mills (passing a 1.5 mm screen) at a higher temperature (85 or 100 ℃), and the thermal conductivity of cassava residue was the highest, followed by wheat, wheat bran with the lowest value. For the same feedstuffs, the thermal conductivity of feed mills, at the same temperature, decreased generally with the increase in particle size, and the higher temperature, the stronger decrease. Cubic polynomial relationships of thermal conductivity with temperature (25-100 ℃) were established for 4 grains and beet pulp with different particle sizes, and quadratic polynomial relationships were obtained for wheat bran, cassava residue and rice bran. The values of thermal diffusivity of 4 grain meals (passing a 1.5 mm screen) varied from 7.459×10-8 to 17.320×10-8, 6.694×10-8 to 19.195×10-8, 7.878×10-8 to 17.045×10-8, and 6.857×10-8 to 16.109 ×10-8 m2/s at temperature range of 25-100 ℃, respectively. The values was observed to ascend from 9.543×10-8 to 20.892×10-8, 8.217×10-8 to 23.254×10-8, 7.258×10-8 to 16.646×10-8 and 8.092×10-8 to 18.718 ×10-8 m2/s for 4 processed by-products, respectively. In general, the thermal diffusivity of the 8 energy feedstuffs had a slow increase with temperature at the range of 25-55 ℃, and a sharply raise with temperature at the range of 55-100 ℃. The thermal diffusivity of all 8 feedstuffs tended to decline with increasing particle size. The thermal diffusivity displayed second order polynomial relationship with temperature at the range of 25-100 ℃ for wheat bran and rice bran, and cubic polynomial relationships for the other 6 feedstuffs.
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