Milling of Cereals Questions and Answers

Answer: C

Correct answer: Only three

Three pairs are right. Rittinger's law makes energy proportional to the new surface created, Kick's law to the logarithm of the size ratio, and Bond's law relates energy to the square root of particle size through the work index.

The fourth is wrong. Rittinger's law is best suited to fine grinding, not coarse crushing, because it is the creation of new surface that dominates at small sizes. Coarse crushing is Kick's territory, where the same proportional reduction costs the same energy whatever the starting size.

Answer: D

Correct answer: Only three

Three pairs are right. Flour ash indicates bran contamination, since the bran and aleurone are far richer in minerals than the endosperm; head rice yield is the proportion of whole kernels; and extraction rate is the flour obtained per unit of wheat.

The fourth is wrong. Degree of milling is the proportion of bran removed during whitening, not moisture. Whitening removes bran layers and germ by abrasion; the moisture change across it is incidental and is not what the term measures.

Answer: A

Correct answer: Purification of semolina

Purification belongs to wheat milling. A purifier passes semolina - coarse endosperm particles from the break system - over an oscillating sieve with air drawn upward through it, separating bran flakes from endosperm granules of the same size by their difference in density and terminal velocity.

The other three are rice milling operations: hulling removes the husk from paddy, whitening abrades away the bran and germ from brown rice, and polishing gives the last gloss. Rice milling removes layers by abrasion; wheat milling breaks the kernel and sorts the fragments.

Answer: B

Correct answer: Starchy endosperm

The starchy endosperm becomes the flour. It is about 83 per cent of the wheat kernel, consisting of starch granules in a protein matrix, and the whole object of milling is to release it cleanly from the layers around it.

The other three leave with the bran. The pericarp and seed coat are the outer coats, and the aleurone layer, though nutritionally the richest single layer in the grain, adheres so firmly to them that conventional milling cannot separate it - which is why white flour loses much of the grain's minerals and B vitamins.

Answer: C

Correct answer: Effective purification of semolina

Ash is the mineral residue left when flour is burned, and it is concentrated in the bran and aleurone rather than in the starchy endosperm - so any ash figure is really a measure of bran contamination. A purifier separates light bran flakes from denser semolina of the same size by air current, so good purification removes bran and lowers the ash. The other three all bring more bran into the flour.

Answer: D

Correct answer: Sieve analysis

Rittinger, Kick and Bond are the three classical laws relating energy consumption to the size reduction achieved, each with a different exponent and each suiting a different size range. Sieve analysis is not a law at all - it is the laboratory method by which the particle size distribution is measured, and hence the means of testing whether any of the laws holds.

Answer: A

Correct answer: Slow and even drying of the paddy

Head rice yield is the proportion of whole kernels, and most breakage originates before the mill: rapid drying sets up moisture gradients within the kernel that crack it internally, and rewetting a dried kernel does the same in reverse. Slow, even drying avoids those fissures, and a kernel without them survives whitening. Excessive whitening pressure then breaks whatever the drying has already weakened.

Answer: B

Correct answer: The purifiers

Extraction is on target, so the mill is recovering the right quantity of flour; the problem is what is in it. Ash means bran, and the machine whose job is to keep bran out of the semolina stream is the purifier, which separates flakes from granules by air current where a sieve cannot. A conditioning fault would usually show as poor extraction as well.

Answer: C

Correct answer: The incoming paddy is already fissured

Nothing in the mill has changed, so the change must be in what is entering it. Kernels fissured during drying or by rewetting break as soon as any pressure is applied, and no adjustment of the whitening machines can put them back together. The remedy lies at the drying yard, which is why head rice yield is largely determined before the paddy reaches the mill.

Answer: D

Tempering toughens the bran so it comes off in flakes the purifiers can remove; without it the bran breaks up and follows the endosperm. The ash figure will have risen alongside the colour, and both point to the same cause.