Lesson 5 - Milling Principles SAQ

*State whether each of these statements is true or false and explain your reasoning.

a) To increase the throughput of a roller mill, you just need to increase the speed of the rollers.

b) Compression of the grain in a roller mill strips the husk away from the endosperm.

c) Husk integrity is more important in a grist for a mash tun than for a lauter tun.

d) The particle size produced by a hammer mill is determined by the speed of the rotor.

Response:

a) This is true up to a point, in that if rollers are running slower than optimal speeds, throughput will be slower than optimum. However, if a mill is running optimally, increasing the speed will actually reduce throughput. This is because higher speeds cause grains to bounce around, so they are less likely to arrive at the rollers for crushing. This phenomenon is called ‘riding the rollers’.

b) This is false. Compression of the grain causes the grain to fracture, which reduces particle size. Shear helps tear the husk so can help to separate husk fragments from endosperm fragments.

c) This is also false. You could argue that as both vessels rely on the husk to form the filter bed during wort separation, husk integrity is equally important. However, because the filter bed on a lauter tun is thinner and the grist for a lauter tun is finer, the nature of the husk fragments is probably more important in a lauter tun.

d) This is false as well. In a hammer mill, the grain is comminuted through collisions with the beaters. It is true that the faster the rotor turns the greater the impact, but the holes in the screens are most significant in determining the particle sizes of the grist. If the malt was over-modified at a very low moisture level and the rotor speed was very high, a finer grist could be produced due to the grain exploding on impact. However, as it is not sensible to process very dry, over-modified grain in a hammer mill with a high rotor speed, these cannot be considered normal (or sensible) operating conditions.