Storage Engineering Questions and Answers

Answer: A

Moulds need an equilibrium relative humidity above about 65 to 70 per cent, and the sorption isotherm converts that into a moisture content for each grain. That is why safe moisture differs between wheat, paddy and oilseeds even in the same store.

Answer: B

Living grain and the organisms in it burn dry matter, and all three products make matters worse: the heat and the water accelerate further respiration. A hot spot in a store is essentially this process running away with itself.

Answer: C

Air warmed at the centre rises, cools against the cold roof and deposits its moisture on the top layer, where crusting and mould then begin. Aeration to keep the bulk at an even temperature is the standard preventive measure.

Answer: D

Correct answer: Cool and equalise the bulk

Aeration moves a small flow of ambient air through a stored bulk to cool it and even out temperature and moisture differences. Cooling suppresses insect activity and mould growth and slows respiration, while equalising the bulk removes the gradients that drive moisture migration and hot spots.

It is not drying: aeration flows are of the order of \( 0.1 \) cubic metres per minute per tonne, perhaps a hundredth of what a dryer uses, so the grain must already be at a safe moisture before it is stored. Nor does aeration clean or fumigate.

Answer: A

Correct answer: 0.1 cubic metres per minute per tonne

Aeration flows are of the order of \( 0.1 \) cubic metres per minute per tonne - enough to move a cooling front slowly through the bulk over many hours, but far too little to remove significant moisture.

The contrast with drying is the point: a dryer uses one to two orders of magnitude more air, which is why aeration cannot be used to correct grain stored too wet. The low flow is also what makes aeration cheap, since fan power scales steeply with air flow through a packed bed.

Answer: B

Correct answer: 15 degrees Celsius

Most storage insects breed rapidly between about 25 and 33 degrees and their activity falls away sharply below about 15 degrees, with development effectively arrested near that point. Cooling the bulk to 15 degrees or below is therefore a control measure in its own right, and one that leaves no residue.

Below 5 degrees insects are dormant, but holding grain that cold is rarely economic in most climates. At 30 degrees they are at their most active, and 40 degrees is above the optimum for most species but not a practical storage condition.

Answer: C

The gas penetrates the whole bulk and reaches insects in places no contact insecticide could, but it requires a well-sealed structure to hold a lethal concentration. Poor sealing is the commonest cause of failure and of resistance developing.

Answer: D

Correct answer: Raising carbon dioxide

Controlled atmosphere storage suppresses insects by altering the gas composition - raising the carbon dioxide concentration, lowering the oxygen, or both. Insects at every life stage depend on aerobic respiration, so an atmosphere of high carbon dioxide or below about 2 per cent oxygen kills them over days to weeks with no chemical residue.

The method requires a well-sealed structure to hold the atmosphere. It does not depend on temperature or on drying the air, and it uses no insecticide, which is precisely its attraction where residues or resistance are a concern.

Answer: A

Correct answer: Costlier handling, clearer stock

Bag storage costs more per tonne to handle, because every bag must be filled, sewn, stacked and moved individually, whereas bulk grain flows by gravity and conveyor. Its advantage is traceability: each lot stays identifiable, so different varieties, qualities or owners can be kept apart and inspected separately.

Bags give worse protection from rodents, since the sacks themselves are easily gnawed and the stack offers harbourage. And ventilation is still needed - indeed, stack arrangement must leave air paths for it.

Answer: B

Thin steel follows the outside temperature closely, which promotes condensation on the inner surface and moisture migration within the bulk. Concrete's thermal mass evens the swings but costs considerably more to build.