Storage Engineering Questions and Answers

Answer: A

Lowering the pressure evaporates water from the leaf surface and the latent heat cools the product within minutes. It works only where the surface to volume ratio is large, which is why it suits leafy crops and not potatoes.

Answer: B

Correct answer: Humidity was high enough for moulds

Fifteen per cent moisture in a warm humid climate puts the grain in equilibrium with air well above 70 per cent relative humidity, which is the threshold above which storage moulds grow. Warmth accelerates them further, so spoilage within two months is exactly what would be expected.

The safe moisture for cereals is around 13 to 14 per cent, precisely because that corresponds to an equilibrium relative humidity below the mould threshold. Nothing here suggests the grain was too dry, and aeration would have helped rather than hurt.

Answer: C

Correct answer: The other three load terms

Designing on the product cooling load alone omits the respiration, transmission and infiltration loads. The product load is large but transient - it ends once the produce reaches temperature - whereas the other three run continuously, so a plant sized on the product load alone can pull down but cannot hold.

For 40 tonnes of respiring produce those continuous terms might total \( 1.8 + 1.4 + 2.6 = 5.8 \) kW, plus internal loads from lights and fans. The arrival temperature and specific heat are inputs to the product load itself and would have been used already.

Answer: D

Correct answer: aeration

Aeration is the movement of small quantities of ambient air through a stored bulk, at flows of the order of \( 0.1 \) cubic metres per minute per tonne, to cool it and even out its temperature and moisture.

The cooling suppresses insect activity and mould growth, and the equalisation removes the gradients that drive moisture migration and allow hot spots to form. Precooling is the equivalent operation on harvested perishables, fumigation is a gaseous insect treatment, and curing is a wound-healing process for tubers and roots.

Answer: A

Correct answer: hermetic

Hermetic storage seals the grain in an airtight structure so that the respiration of the grain, insects and moulds consumes the oxygen and raises the carbon dioxide. The resulting atmosphere kills the insects and suppresses the moulds - the infestation destroys its own environment.

It leaves no chemical residue, which is its principal attraction. It differs from controlled atmosphere storage, where the composition is actively established and maintained by external equipment; here the biology does the work, and the only requirement is that the seal holds.

Answer: B

Correct answer: precooling

Precooling is the rapid removal of field heat from harvested produce, by forced-air cooling, hydrocooling, vacuum cooling or icing, as soon as possible after picking. Because respiration roughly doubles for every ten degree rise, produce left at field temperature consumes its storage life quickly.

Speed matters more than the final temperature: hours at field temperature cost days of shelf life, and the loss cannot be recovered by cooling later. Tempering, curing and conditioning are all different operations applied to grain and to tubers.

Answer: C

Correct answer: equilibrium

The equilibrium relative humidity is that of the air with which a stored product neither gains nor loses moisture - the point at which the product's vapour pressure matches the air's.

It is the more useful variable for storage safety than moisture content itself, because moulds respond to the humidity in the interstitial air rather than to the water in the grain. Storage moulds require above about 70 per cent, so safe storage moisture for any commodity is whatever content puts its equilibrium relative humidity below that - 13 to 14 per cent for cereals but only about 7 per cent for groundnut.

Answer: D

Correct answer: fumigation

Fumigation is the treatment of a sealed bulk with a toxic gas - most commonly phosphine, released from aluminium or magnesium phosphide - which penetrates the whole mass and kills insects at every life stage.

Its effectiveness depends entirely on holding a lethal concentration for the full exposure period, which for phosphine means several days, so the quality of the seal matters more than the dose. Aeration is a cooling operation, sanitation is hygiene, and irradiation is a quite different physical treatment.

Answer: A

Correct answer: Its oil content holds far less water

Groundnut kernels are roughly half oil, and oil holds virtually no water. Moisture content is expressed on the whole-seed basis, so the water present is confined to the much smaller non-oil fraction; a groundnut therefore reaches the same equilibrium relative humidity - and hence the same mould risk - at about half the moisture content of a cereal.

The governing variable is the equilibrium relative humidity, not the moisture percentage, and 70 per cent is the mould threshold for both. Respiration rate, insect attractiveness and bulk density do not enter the relation.

Answer: B

Correct answer: Moisture and temperature compound one another

The two variables multiply rather than add. At 13 per cent, cooling from 30 to 10 degrees extends the life from 180 to 1200 days, a factor of nearly seven. At 19 per cent the same cooling gives 8 to 60 days - the same rough factor, but from a base twenty times smaller.

Equally, at 30 degrees, drying from 19 to 13 per cent multiplies the life more than twentyfold. Each variable multiplies the effect of the other, which is why the two must be managed together: drying alone or cooling alone leaves most of the available benefit on the table.