Properties of Agricultural Materials Questions and Answers

Answer: A

Correct answer: A is false but R is true

The reason states the definition correctly:

\[ \Phi = \frac{A_{\text{sphere, equal volume}}}{A_{\text{particle}}} = \frac{(LWT)^{1/3}}{L} \]

The assertion, however, is the opposite of what that definition gives. A flat seed has one dimension much smaller than the others, so the geometric mean \( (LWT)^{1/3} \) is well below \( L \) and sphericity is low - often 0.3 to 0.5. Sphericity approaches unity only for a nearly spherical particle such as a pea or a mustard seed. A true reason with a false assertion again.

Answer: B

Correct answer: 1 and 2 only

Statements 1 and 2 are the standard definitions: bulk density uses the total occupied volume with the voids included, true density only the solid material.

Statement 3 inverts the relation. Porosity is the void fraction, so

\[ \varepsilon = 1 - \frac{\rho_b}{\rho_t} \]

The ratio \( \rho_b/\rho_t \) is the solid fraction, its complement. For wheat at 790 and 1360 kg/m\(^3\) the ratio is 0.58 and the porosity 0.42, so the two are plainly not the same quantity.

Answer: C

Correct answer: 1 and 2 only

Statements 1 and 2 follow from the denominators. The wet basis divides by the whole sample, which necessarily contains the water, so the ratio cannot exceed one. The dry basis divides by the dry matter alone, so a sample with more water than solids gives a value above 100 per cent - a tomato at 95 per cent wet basis is 1900 per cent dry basis.

Statement 3 is wrong. Trade standards and grading quote the wet basis, because the delivered mass is what is weighed and paid for. The dry basis belongs to drying calculations, where the constant denominator is the convenience.

Answer: D

Correct answer: 1 and 2 only

Statements 1 and 2 hold. The angle of repose rises with moisture, from about 26 degrees at 10 per cent to 35 degrees at 22 per cent for wheat, because liquid bridges increase cohesion; and it is the parameter that sets chute and hopper slopes, which must be steeper than it for the material to discharge.

Statement 3 is false. The angle of repose is a bulk property, measured on a poured heap - a single kernel has no slope to measure. Distinguishing bulk properties from single-kernel ones such as sphericity and rupture force decides how a measurement is made at all.

Answer: A

The first two are the standard definitions. Because part of the deformation is time-dependent, how long a load is held matters as much as how large it is, which is why stacking height and transit time both cause bruising.

Answer: B

Correct answer: A-2, B-1, C-4, D-3

Each property belongs to the category defined by the physics behind it. Sphericity is purely geometric, comparing the particle's form to a sphere, so it is a shape descriptor.

Terminal velocity arises from drag balancing weight in a moving air stream, making it aerodynamic. Specific heat is the heat needed per unit mass per degree, hence thermal. The angle of repose describes resistance to sliding within the mass, so it is frictional. The grouping is useful because it points to the equipment each governs: shape to screening, aerodynamic to aspirators, thermal to driers, frictional to hoppers and bins.

Answer: C

Correct answer: A-2, B-1, C-4, D-3

Each measurement has one standard method. Moisture content is determined by oven drying and weighing - the reference procedure against which every meter is calibrated.

Terminal velocity is found in a vertical air column with a variable fan, by raising the air speed until the particle is just suspended. Colour is read on a Hunter or CIE colorimeter as the coordinates \( L \), \( a \) and \( b \). Viscoelastic behaviour is obtained on a universal testing machine in relaxation mode, holding a fixed deformation and recording the force as it decays.

Answer: D

Correct answer: A-2, B-1, C-4, D-3

Terminal velocity sets aspirator air settings, since the air speed must sit between the terminal velocities of the fraction to be lifted and the fraction to be kept. The angle of repose sets hopper and chute slopes, which must exceed it for discharge.

Rupture force sets sheller and thresher clearances, which must break the hull without cracking the kernel. Porosity governs airflow resistance in a drying bed, because the air travels through the void space and a lower void fraction means a larger pressure drop for the same flow. Each pairing is the property that actually enters the relevant design calculation.

Answer: A

Correct answer: A-2, B-1, C-4, D-3

Water activity measures the availability of water for microbial activity, being the ratio of vapour pressure over the product to that over pure water. Equilibrium moisture content is the value at which the grain neither gains nor loses water to its surrounding air.

Hysteresis is the gap between the adsorption and desorption isotherms, so that the same relative humidity corresponds to two different moisture contents depending on the sample's history. Rehydration ratio is rehydrated mass divided by dried mass, a quality index for dried products - the only one of the four that is a simple mass ratio rather than a thermodynamic quantity.

Answer: B

Correct answer: Angle of repose

The angle of repose is a bulk property. It is the slope at which a poured heap stands, and a single kernel forms no heap and has no repose angle.

True density, sphericity and geometric mean diameter are all measured on the kernel itself - the first by displacement of the solid material, the other two from the three principal dimensions. Bulk density and porosity join the angle of repose on the bulk side. The distinction decides the measurement: kernel properties are averaged over a sample of individuals, bulk properties over a filled container.