2-roll Mill Gap
Does anyone know a typical gap for a two roll mill? We are in the process of opening ours up to try and improve lautering. Currently it is set to 0.05". Knowing how others have theirs set may help.
I would think that the proper mill gap would depend on the quality and homogenity of the malt being used. With that said, we recently reduced our mill gap (2 roll mill) dramatically and it allowed us an increase (~5 to 10%) in extract yield. However, our mill makes it nearly impossible to measure the gap (it is very difficult to get at the actual rollers without disassembling the mill...).
Briggs, Hough, Stevens, and Young in Malting and Brewing Science Vol. 1 (Malt and Sweet Wort) suggest that for a system that utilizes a lauter tun (compared to a mash filter), "...the grist should contain 15% husks, 23% coarse grits, 30% fine grits, and 32% flour..." (p.314). They also point out that there should be no uncrushed kernels, endosperm particles should be of uniform size, and husks should be longitudinally split, but otherwise whole.
Although we did not sift our malt to get an accurate analysis, we did eyeball the size of the grist on multiple occasions and, in conjunction with extract yield and time to wort clarification, we found a mill gap that works well for us.
Hope this helps. I would be curious to know where you end up setting your mill gap.
We have left it at 0.05" at the moment and are still getting around 95% efficiency (we use a lauter tun). I think it is difficult to get the ideal grist sizings with a 2 roll mill as the sizing achieved cannot match that of a 6-roll mill (as discussed in most texts).
We first started adjusting our mill because we did a beer with 22% crystal in the grist and lautering was a complete bastard - the crystal basically shattered during milling, creating a lot of fines resulting in a 5-hr run off - not good for a Friday afternoon shift!!
However, we haven't seen a reduction in extract, so maybe we should open it up a bit more and then see what happens..... (just came back from our Xmas break, so will do some more trials in the weeks to come).
One cannot pick a consistent gap on a mill and expect not to alter it as different malts, or maltster's changing blends are encountered or even different crop years. (Don't expect your supplier to tell you each time there is a change.) The more pulverized your grist, the greater potential extract and yet the longer lauter times-especially on Fridays.
My experience is to tighten the mill until there are few or no whole kernels, and toy with it more or less from there depending on extract and lautering. Screens are a guide if you have them, and your 2-roll mill will rarely be able to give the ideal ratios anyway. Just (safely) take a handful of grist and look at it.
Fluting on rolls will change everthing, too. Fluted rolls are more likely to destroy that critical husk at a wider gap than ideal for extract.
That is really the probem with a two roller mill, verses a six roller. It like the difference between driving a standard and an automatic. If you want the perfect grind, (which is almost impossible) you will constantly have to re adjust the gap from brew to brew. Depending on the size of your batches, you may want to forgo some efficiency for ease and consistancy of laudering, by increasing the gap size. Investigate the true savings of your laudering efficency, and don't get too hung up on always reaching the highest number.