Any all-grain brewer, traditional or BIAB, should view crushing the grain as aiming to break the grain open not pulverise it. Regard the grain as a container that holds sugar. You simply want to open that container.
The recommendation you often read, that, with BIAB, you can crush as fine as you like, is poor advice. If you pulverise your grain or turn it into flour, you are causing two problems. The first is that the resulting beer can develop tannin flavours. These flavours come from the grain 'container', the husks. If you pulverise the husk, these tannins can immediately leach out into your mash water.
The second problem is that too fine a crush can result in poor Efficiency into Kettle (EIK). You will often read that a finer crush gives higher efficiency. This is true, to an extent, however a fine crush, does not mean flour. If you crush your grain to flour, the wort will not drain freely when you pull the bag. In other words, your flour will retain a lot of wort that would otherwise have drained into the kettle.
A good crush will show a majority of the grains being broken in two or three places. You will notice some flour but this should be minimal. Also note that many grain mills have a roller design that is poor. If the grain falls across the roller in a certain manner, it can pass through uncrushed. If you find a lot of uncracked grain in your crush, try passing the grist through the mill a second time. If this does not work, you will need to reduce your roller gap.
If you have questions regarding this topic, feel free to ask them on forum.
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