Properties of Agricultural Materials Questions and Answers

Answer: C

Correct answer: It equals bulk density divided by true density

That ratio is the solid fraction, not the void fraction. Porosity is its complement:

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

The other three statements are sound. Porosity near 0.4 is typical of cereal lots; it governs airflow resistance in a drying bed, since the air must pass through the voids; and it is by definition the void fraction of the bulk volume. Wheat at 790 and 1360 kg/m\(^3\) gives a ratio of 0.58 and a porosity of 0.42 - two clearly different numbers, which is the quickest check on the false statement.

Answer: D

Correct answer: An increase in bulk density

Bulk density generally falls as moisture rises. The kernels swell, packing less closely, and the added water does not make up for the lost packing efficiency - wheat drops from about 800 to 758 kg/m\(^3\) between 10 and 22 per cent moisture.

The other three do increase. Liquid bridges between kernels raise both the angle of repose and the wall friction coefficient, and specific heat rises because water's greatly exceeds that of dry matter. So a single cause produces three increases and one decrease, and it is the decrease the question asks for.

Answer: A

Correct answer: It is a percentage of the dry matter

Water activity is not a percentage of anything. It is a dimensionless ratio of vapour pressures, running from 0 to 1, and it measures availability rather than amount.

The other three statements are correct. It does range between zero and one; it does govern microbial growth, with most bacteria halted below 0.90 and moulds below about 0.70; and at equilibrium it equals the relative humidity of the surrounding air expressed as a fraction, which is what makes it measurable with a humidity sensor in a sealed headspace. Percentage of dry matter describes dry basis moisture content, an entirely different quantity.

Answer: B

Correct answer: 20 per cent wet basis is 25 per cent dry basis

The two bases give materially different numbers for the same sample, so "25 per cent" is ambiguous without the basis. The conversion makes it concrete:

\[ M_{db} = \frac{M_{wb}}{1 - M_{wb}} = \frac{0.20}{0.80} = 0.25 \]

So a reading of 25 per cent could mean the grain is at 25 per cent wet basis, or at 20 per cent wet basis quoted on the dry basis - and those differ enough to decide whether the lot is safe to store. The first option has the inequality backwards, the third is false since the gap widens with moisture, and the fourth is wrong because trade uses the wet basis.

Answer: C

Correct answer: It is low, because the voids hold air

Roughly 40 per cent of a bulk grain mass is air, which conducts at only 0.026 W/m K, and the kernel-to-kernel contact area is small. The effective conductivity of the bulk is therefore around 0.1 to 0.2 W/m K.

The first option reverses the moisture trend: conductivity rises with moisture, since water at about 0.6 W/m K conducts far better than the air it displaces. It plainly depends on porosity, which fixes how much of the path is through air, so the second is false; and the fourth mistakes the kernels' own conductivity for that of the bed. The low value is why hot spots persist in stores.

Answer: D

Correct answer: Raising the moisture content

Raising moisture makes flow worse. Surface water forms liquid bridges between kernels, increasing cohesion and the angle of repose, and it is exactly what turns a working hopper into one that bridges.

The other three all help. Steepening the walls puts the slope further above the repose angle. Reducing the wall friction coefficient - a smoother liner - lets the mass slide more readily. Removing chaff and fines helps because those fill the voids between kernels and increase interlocking. Together they are the standard remedies for a bin that discharges sluggishly.

Answer: A

Correct answer: Values above one indicate an elongated particle

Sphericity cannot exceed one. Since a sphere encloses a given volume with the least surface area of any shape, the equal-volume sphere's area is always the smaller of the two in the ratio

\[ \Phi = \frac{A_{\text{sphere, equal volume}}}{A_{\text{particle}}} \le 1 \]

An elongated particle gives a low value, not one above unity. The other three statements are true: a perfect sphere gives exactly one, flat seeds give low values around 0.3 to 0.5, and the working formula \( (LWT)^{1/3}/L \) uses the three principal dimensions.

Answer: B

Correct answer: Desorption lies above adsorption at a given humidity

At the same relative humidity, material that arrived by drying holds more water than material that arrived by wetting, so the desorption curve lies above the adsorption curve.

The usual explanation is capillary condensation together with structural change in the matrix: pores that fill at one humidity do not empty until a lower one is reached. The first option reverses this, the third denies hysteresis exists, and the fourth is wrong because the gap is generally widest in the mid-humidity range rather than vanishing there. The practical consequence is that an equilibrium moisture figure is incomplete without the sample's history.

Answer: C

Correct answer: The material is viscoelastic

A viscoelastic material's response depends on the rate and duration of loading, not on the load alone. Held under a given force, it continues to deform with time - creep - so the same force applied for longer produces more permanent damage.

A purely elastic material would recover completely whatever the duration, so bruising could not depend on handling time at all. Specific heat and sphericity are thermal and geometric properties with no bearing on mechanical damage. It is the time dependence that makes handling speed, and the dwell time in a chute or on a grader, a design consideration in fruit lines.

Answer: D

Correct answer: It generally falls as moisture content rises

Water plasticises the kernel's structure, so a moist kernel deforms more and fails at a lower force than a dry one. The property is therefore strongly moisture-dependent, which rules out the first two options.

It is measured by compressing a single kernel to failure in a universal testing machine, not in a colorimeter, which reads colour. The practical consequence is direct: sheller and thresher settings established at harvest moisture will crack the same crop once it is dried, so clearances and drum speeds have to be reset as the moisture of the incoming material changes.