In most cases finer crush provides better conversion efficiency than a coarser crush. But if you are using a traditional MLT (with false bottom, manifold, or braid), if you crush too fine you will get stuck run-offs. So, you back off the crush to the point where you no longer get stuck run-offs, but this reduces your conversion efficiency. However, if you add a sparge, your lauter efficiency goes up, and you still get pretty good mash efficiency.
When you use a bag, and pull it out of the mash vessel for lautering, finer crushes don't have stuck run-off issues. This allows you to crush finer than for a traditional MLT, and take advantage of the higher conversion efficiency. If you keep doing a sparge you will get a gain in mash efficiency. On the other hand, you could say, "I want to have a simpler brew day, so I'll skip sparging." If you did this with a coarse crush, your mash efficiency would go down, because your lauter efficiency went down. But with a finer crush, the conversion efficiency goes up, while the lauter efficiency goes down, and you may or may not see a reduction in mash efficiency. The finer crush makes no-sparge brewing more acceptable from a mash efficiency standpoint, than no-sparge with a coarse crush. Fine crush is never a
"necessity", but if you are not happy with your mash efficiency, then finer crush is desirable.
Let's do a couple of real world examples. First a traditional MLT with typical coarse crush, where the grain absorption rate is 0.125 gal/lb, you typically get 80% mash efficiency with a 10 lb grain bill, and 6.5 gal pre-boil volume, while employing and optimal single batch sparge. Using my mash/lauter simulator, the results you get are:
Conversion efficiency = 92%
Lauter efficiency = 87%
Mash efficiency = 80%
Pre-boil SG = 1.044
If we leave everything the same, except do a no-sparge process the results are:
Conversion efficiency = 92%
Lauter efficiency = 78%
Mash efficiency = 72%
Pre-boil SG = 1.039
If you wanted to achieve the same pre-boil SG (1.044) as with your sparge process, you would have to increase your grain bill to 11.5 lb, a 1.5 lb increase.
If you go to a bag, and use a very fine crush that allows you to get 98% conversion efficiency (typical of what I get), and lift the bag when lautering (which will drop the grain absorption rate to about 0.11 gal/lb), the results with the original 10 lb grain bill are:
Conversion efficiency = 98%
Lauter efficiency = 79%
Mash efficiency = 77 - 78%
Pre-boil SG = 1.042 - 1.043
And the grain bill required to achieve the 1.044 pre-boil SG goes down to 10.4 lb. The finer crush saved a little more than a lb of grain.
If you add back the sparge step, with the fine crush and bag, the 10 lb grain bill results become:
Conversion efficiency = 98%
Lauter efficiency = 88%
Mash efficiency = 86 - 87%
Pre-boil SG = 1.047
And to hit the 1.044 pre-boil SG target only requires 9.1 lb of grain.
If you decide to squeeze the bag instead of sparging, and squeeze enough to get your grain absorption down to 0.07 gal/lb, the 10 lb grain bill, no-sparge results become:
Conversion efficiency = 98%
Lauter efficiency = 83%
Mash efficiency = 81 - 82%
Pre-boil SG = 1.044 -1.045
Slightly better than the coarse crush, traditional MLT with batch sparge process.
The take away is that there are a lot of ways to achieve similar results, all of which are valid choices.
It turns out that a lot of brewers who are having efficiency issues have conversion efficiencies much less than the 92% used in our coarse crush case. These are the folks that really need to crush finer and/or mash longer to get their efficiency up.
Brew on