Lesson 5 - Milling Principles SAQ

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a) Explain the relationship between malt modification and successful milling and mashing.

b) Recommend the best mill for a mash tun with a well-modified malt and a mash filter with under-modified malt and explain why this is your recommendation

Response:

a) Malt modification is the sum of the changes that occur during germination in the malting process. Enzymes break down the cell walls and protein matrix of the barley endosperm. This changes a hard barley grain into a friable malt grain. Success in milling is achieving the objectives of milling. We can discuss the effect of malt modification on each of these.

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

Poorly modified malt requires more force to break down its structure, so we need to use a more intensive milling regime to produce the correct particle size. For success in mashing, we need to expose the malt’s starch to water and enzymes. In under-modified malt, the starch granules are encased in protein and cell wall material, so we need to comminute under-modified malt to a greater extent. So success in milling and mashing requires more sophisticated and intensive equipment. If using a lauter tun, as well as reducing the endosperm particle size profile we need to maintain larger husk particles for filtration. This means that we need to be able to separate the husk material from the endosperm before it is too finely divided. Well-modified malt, on the other hand, has very little protein and cell wall material remaining so it is easier to mill and mash, requiring simpler and less intensive equipment. Over-modified malt is very brittle and so is very difficult to mill without producing lots of flour and dust.

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

We have already touched on this in the last point. For mash tuns, we use well-modified malt and need a coarse grist. We therefore need to use a mill that is gentle on the grain but can separate the endosperm, grits from husk particles. For under-modified malt to be used in a mash filter, we are not as concerned about husk integrity so we can mill the grain very intensively to maximise the exposure of the starch to water and enzymes. This task is relatively easy with over-modified malt. In fact, we need to be careful not to mill the grain too much because the endosperm is so brittle that it does not support the husk as the grain is milled.

      3. Maximise extract efficiency during wort production

To maximise extract we need to mill the grain as finely as possible. That said we need to ensure the husk remains intact to the degree necessary for the wort separation system being used. As we said above, under-modified malt needs to be as finely milled as possible to ensure we can produce as much extract as possible. We could do the same for well-modified malt and over-modified malt, but the grist would perform badly in wort separation and ultimately we could achieve poorer extract efficiency.

      4. Operate safely

This is essential for all malt types but becomes harder to achieve with over-modified malt, which turns to dust very readily.

      5. Achieve programmed brewhouse cycle time

Under-modified malt requires more processing, so the mill must be designed to do more in the same amount of time. This aspect also goes back to the first two objectives. If the particle size is incorrect and the husk material is not sufficiently large for the wort separation equipment, wort separation will be slow and difficult and the cycle time will be extended. This is an increased risk with over-modified malt, and we need to be careful with under-modified grain to achieve the balance between fine comminution for extract efficiency and protecting husk integrity.

b) The best mill for a mash tun with a well modified malt is a four-roller mill. This can produce a coarse grist but with an improved extract compared to a two-roller mill. The best mill for a mash filter and under-modified malt is a hammer mill. A six-roller mill can also be used, but the extract available from the very fine comminution achieved by a hammer mill is greater.