Lesson 4 - Milling Principles

Before we explain the process of milling, it would be a good idea to identify what we need to achieve. So, here are our objectives of milling:

1. Produce the required particle size distribution for the mashing and wort separation systems used

2. Maintain the required level of husk integrity for the wort separation system

3. Maximise extract efficiency during wort production

4. Operate safely

5. Achieve the programmed brewhouse cycle time

Let’s discuss each in some more detail.

Produce the required particle size distribution for the mashing and wort separation systems used

This is objective number one for a reason. The main objective of milling, and indeed selecting a mill, is to ensure that it can produce a grist that will process easily during wort separation. Each wort separation system uses the particles in the grist differently, so we need different proportions of the various particle sizes. The job of the mill is to provide these every time. We will cover how each wort separation system works in detail in the Wort Separation unit. We will also explore the grist profile required for each system later in this unit, but the table below provides a simple summary.

Table: Simplified grist profile for each wort separation system

From the table, we can see that the task of milling is quite different for each system. In other words, each of the mills listed has a different job to do and thus goes about it in a slightly, or sometimes very, different way.

Maintain the required level of husk integrity for the wort separation system

Also, from the table, we can see that the husk integrity requirements of each system are different. By integrity we mean how undamaged the structure of the husk fragments is. The mash tun and the lauter tun both use the husk material to make up the filter bed, so the amount and the nature of the husk fragments are both important. With the lauter tun, because the overall grist profile is finer, the integrity of the husk particles becomes more important to allow wort to flow through the filter bed at the required rate.

Maximise extract efficiency during wort production

This objective connects to the first. The finer the particle size, the greater the extract. This is because with smaller particles it is easier to solubilise, gelatinise, liquefy, and saccharify the malt’s starch. Although each wort separation system requires a different level of grist fineness, the mill still needs to provide particle sizes that can provide the best extract efficiency for the system in question.

Operate safely

This should go without saying for every element of the brewing process, but with milling, it is particularly pertinent. As we discovered in the last lesson, in milling we are producing fuel for explosions, therefore milling is probably the most hazardous step. While the achievement of some objectives could be flexed to improve the others, safety cannot. 

Achieve programmed brewhouse cycle time

The production programme is developed to meet customer demand for the beer. To achieve the production programme, all brews must be completed within the brewhouse planned cycle time. Milling must not cause brews to take longer than the planned cycle time, otherwise the brewery will fail its customers. Milling can delay brews directly if it delays mashing in, or indirectly if the grist produced extends the time required for saccharification or wort separation. Milling must therefore achieve the first three objectives above within the programmed timeframe.

With our new understanding of these milling objectives, we can now explore how mills are designed and the basis of their operation. We will do this starting with roller mills, next.