Lesson 5 - Milling Principles SAQ
This lesson allows you to test yourself on the material covered in the Milling Principles lesson, and determine the level to which you have met the learning outcomes.
SAQ Suggested Response - Q3
**Why is shear important in roller milling and how is it achieved?
Response:
Shear is when an object is subject to forces from opposite sides. In roller milling, the grains are comminuted by compression and shear. Compression squeezes the grain, causing it to fracture. If only compression is applied to the grain, it results in a less effective comminution. The grain would have to be processed by more sets of rollers to achieve the same degree of comminution and the particle size distribution would be inadequate to provide effective wort separation in a mash or lauter tun. Shear helps to pull the grain apart and to separate endosperm grits from husk fragments. This improves extract efficiency and makes wort separation easier.
Shear is achieved in two ways: roller fluting and differential roller speeds. Fluting increases the coefficient of friction between the rollers and the grain, which helps the rollers grip the grain. It also provides a surface to cut the husk material. Fluting is produced during the manufacture of the rollers by milling out the metal surface of the roller. The fluting has a twisted pattern across the rollers, so as the rollers crush the grain there is a kind of wringing action, helping to shear the grain.
Differential roller speeds mean that the surface of the roller attached to one side of the grain is travelling at a different speed to the surface in contact with the other side. This means that the grain is pulled (sheared) apart while it is in contact with the rollers.