Most small brewers are using 2 roller mills because of capital cost considerations. Well, now you can easily afford Brewmax 4 and 6 roller mills.
Multi-nip malt mills control astringency by keeping the malt husk more intact, which then leaches less astringent polyphenols into your wort.
I wrote an article about polyphenol extraction and crush sizes back in 1997 and it is still kicking around the Internet http://www.brewery.org/library/Phen_CS0497.html
The classic crush distribution for brewing is tested by standard sieves, and Brewmax will supply those with our 4 roll and 6 roll mills. ( or we will sell them separately if you need)
I have attached an article from some Tasmanian academics who claim they got an approximation of the perfect crush with just two passes through a two roller mill. That equals a 4 roller mill doesn’t it? Small Lauter.pdf
Cold steeping and two-roller wet milling also reduces the leaching of higher-weight, oxidisable, polyphenols into your mash. The wet, intact husk also increases bed volume by 25 to 30% and that contributes to faster runoffs.
Fast hot steeping (at 100C) leaches astringent polyphenols into the discarded steep water before the two roller crush, and it also contributes to keeping the husk intact.
These wet processes happen in a special grist case and after N2 purging, the two-roller crush begins immediately above the grist hydrator. Total astringent compounds can be reduced by over 30%.
In all wet milling, Brewmax advise pre-acidifying strike water to inhibit lipo-oxygenase activity.