Cereal Grains: Physical and Thermal Properties

Understanding the physical properties of grain are important for equipment design for handling, storage aeration and processing. Even the simple technique of drying and storage requires knowledge on heat and moisture transport phenomenon. Important properties of cereal grains are grain size, shape, volume, density, porosity, angle of repose, heat capacity, thermal diffusivity and many others. However, these properties of cereal grains largely depends on moisture content, temperature and density of grain itself.

Physical Properties:

1. Thousand Grain weight: The weight of 1000 grain indicates grain size. This data gives information about grain conditions such as growing condition and maturity which greatly influence size of grain. When performed at same moisture level condition, this test also provides useful information on determining effective diameter of kernel for handling. Generally this test is performed by taking weight of 1000 grain kernels. It is found that 1000 grain weight increases linearly with increasing moisture content from 10.9 % to 28.4 % [1].

2. Sphericity and roundness: This test is important for determination of terminal velocity, drag coefficient and Reynolds number. Analysis of heat and moisture transport also requires information about shape of the grain. To what extent the solid grain exhibit spherical characteristics is given by shericity of the grain. If di is the diameter of largest inscribed circle and dc is diameter of smallest circumscribed circle, Sphericity = di/dc.

Roundness is the measure of sharpness of its corners. Higher value of sphericity and roundness shows that object shape is closer to sphere.  If Ac represent area of smallest circumscribing circle and Ap is projected area of grain, Roundness = Ap/Ac.

3. Bulk density: bulk density of grain is measured by dividing weight of sample by volume of the container in which it is weighed. This test gives good idea of storage space needed for known quantity of particular grain. Conductivity and other transport properties of grain is also influenced by bulk density of grain. Bulk density of some grain increases with increasing moisture content while for some other grains it decreases [2].

4. Kernel density: Kernel density is also called true density. It is the ratio of mass of grain sample to the solid volume occupied by the sample. This test can be performed by two methods on the basis of Archimedes’ principle of fluid displacement. One methods uses displacement of gas while other method uses displacement of liquid.

Fig: Arrangement for determining coefficient of friction [9]
5. Coefficient of Friction: Coefficient of friction can be static or dynamic. This test parameter is useful in determining pressure of grain against bin wall and silos during grain storage and grain handling (filling and emptying). The static coefficient of various cereal grain is determined relative to different structural material of storage (tank/ silos/bins) such as plywood, plastic, galvanized iron. This test is performed by the use box (140 x 160 x 35 mm) and two plates; fixed and adjustable. The box is filled with sample and then adjustable plate is inclined gradually until the box slides down [3].

6. Porosity: This is the useful test parameter that greatly influences kernel hardness, breakage susceptibility degree of milling, drying rate and resistance to fungal development [4]. Porosity of grain depends on its bulk and kernel densities. The grain porosity is measured by the help of  pycnometer. Porosity is defined as the fraction of the space in the bulk grain that is not occupied by the grain [5].  If ρb and ρt be the bulk density of grain and kernel density of grain, the percentage fractional porosity (Pf) is calculated as below

 

Porosity of some grain such as canola, lentil and wheat increases with increase in moisture content. In other hand, for some grain such as soybean, porosity decreases with increase in moisture content between 8 – 20 % [2].

7. Angle of repose: It is the steepest angle at which sloping surface formed of loose material is stable. To determine angle of repose of grain, grain is allowed to flow vertically from a specially constructed box to its natural slope. The slope of grain is taken as measure of angle of repose. It is found that emptying angle of repose is greater coarse grain such as paddy, wheat, beans and lesser for oil bean seeds. This is because smooth surface of oil bean seed slides on each other [6].
8. Bulk volume shrinkage: To find mathematical solution of drying process (moisture desorption) knowledge on bulk volume shrinkage is required. This test is performed by placing grain sample in cylindrical container and exposing it to dry air. Recording continuous changes in mass and depth of grain bed gives value of bulk volume shrinkage [7].

Thermal properties:

Thermal properties of grain is used for simulating drying and storage condition of different grains. This property is also useful in modeling of heat treatment of cereal grains and legumes. Different types of grain exhibit different thermal properties, depending on its origin, concentration and previous history.

1. Specific heat: It is the amount of heat required to change the temperature of material of unit mass by 1°. Mathematically it is written as

 

 

Where, Q is amount of heat, m is mass of material and ∆T is change in temperature. Different methods have been proposed for determining specific heat of food products such as by using calorimeter, differential scanning calorimeter (DSC) method. However care should be taken that there is no heat exchange with exterior surface of the system. Generally, increase in moisture content increases specific heat capacity of grain. Lighter seeds tend to have high specific heat capacities while heavier seeds have low specific heat capacities.

2. Thermal conductivity: Thermal conductivity of any material is measure of its capacity to conduct heat. Determining thermal conductivity of food material is difficult and more challenging. Thermal conductivity of material depends on its chemical composition as well as structure of sample. Thermal conductivity of grain increases with temperature and moisture content. Thermal conductivity can be measured by using steady state method and transient method. Steady state method is based on Fourier’s Law of heat conduction given by equation,

 

 

Where, q is heat quantity, K is thermal conductivity, A is surface area (m2), dT is change in temperature and L is surface thickness (m)   . Negative sign indicates that heat always flow in direction of decreasing temperature. For transient temperature method, line heat source or a probe at the same point is used.

3. Thermal diffusivity: Thermal diffusivity method indicates how fast heat can propagate through material under transient condition of heat transfer condition. It indicates the ability of material to store heat while conducting heat. If α be the thermal diffusivity (m2/s) of material, K is the thermal conductivity (W/mK),Cp is specific heat (J/KgK) and ρ is density (Kg/m3),

 

 

Thermal diffusivity of grain increases with increase in temperature [8].

References:

[1] Dutta, S. K., Nema, V. K., & Bhardwaj, R. K. (1988a). Physical properties of gram. Journal of Agricultural Engineering Research, 39(4), 259–268.
[2] Sablani, S. S., & Ramaswamy, H. S. (2003). Physical and thermal properties of cereal grains. Hanbook of Postharvest Technology, Eds GSV Raghavan, A. Chakraverty, AS Mujumdar and HS Ramaswamy, 17–40.
[3] Karimi, M., Kheiralipour, K., Tabatabaeefar, A., Khoubakht, G. M., Naderi, M., & Heidarbeigi, K. (2009). The effect of moisture content on physical properties of wheat. Pakistan Journal of Nutrition, 8(1), 90–95.
[4] Chang, C. S. (1988). Measuring density and porosity of grain kernels using a gas pycnometer. Cereal Chem, 65(1), 13–15.
[5] Thompson, R. A., & Isaacs, G. W. (1967). Porosity determinations of grains and seeds with an air-comparison pycnometer. Transactions of the ASAE, 10(5), 693–696.
[6] Oje, K., & Ugbor, E. C. (1991). Some physical properties of oilbean seed. Journal of Agricultural Engineering Research, 50, 305–313.
[7] Lang, W., & Sokhansanj, S. (1993). Bulk volume shrinkage during drying of wheat and canola. Journal of Food Process Engineering, 16(4), 305–314.
[8] Dutta, S. K., Nema, V. K., & Bhardwaj, R. K. (1988b). Thermal properties of gram. Journal of Agricultural Engineering Research, 39(4), 269–275.
[9] Saracoglu, T., & Ozarslan, C. (2012). Moisture-Dependent Geometric, Frictional and Mechanical Properties of Cabbage (Brassica oleraceae L. var. capitata) Seeds. Philippine Agricultural Scientist, 95(1), 53–63.