I have recently started BIAB, and my wort is very cloudy after the boil. Much "cloud material" ends up in the fermentor, since it takes really long to settle and is easily stirred. It mostly looks like flour, and is present even without hops (I have tested).

Question: Is it OK to filter the wort after chilling, or will I remove some important stuff? (I could also filter just the last part.)

Will Irish Moss or WhirFloc help with this problem?

  • Filtering after the chill sounds like a pain. I'd do things like adding the Irish Moss, or using a finer bag for mashing, before I attempted to do sanitized filtering. Are you familiar with the concept of a "Hop Back"? This technique uses a vessel filled with hops to strain out material like pellet hops before the wort enters a counter flow chiller. Might be worth looking into as well.
    – GHP
    Commented May 13, 2013 at 12:16
  • Thanks! Will Irish Moss have any effect at all on this cloud material? We are considering filtering only the very last part of the chilled wort: we siphon off the cooled worth, and place a funnel with a filter at the end, when the wort turns cloudy at the bottom of the kettle.
    – Nemis L.
    Commented May 13, 2013 at 20:53
  • Its hard to say. I mentioned the Irish Moss because it can't hurt. Actually a better idea might be to rack the wort into a temporary carboy for about 2-3 hours. Let it settle, then transfer the wort into the primary fermentor and leave behind whatever is at the bottom inch or two. That should cut down on that stuff getting into the primary.
    – GHP
    Commented May 14, 2013 at 12:10
  • Thanks! We will go for WhirFloc and see what happens, and take it from there.
    – Nemis L.
    Commented May 14, 2013 at 16:39
  • I always add whirlfloc. But based on the amount of material in my wort when I used a wider spaced mesh bag it probably is going to be overwhelmed by the particulate matter. On the other hand it couldn't hurt. Commented May 15, 2013 at 21:12

2 Answers 2


I also have had a similar issue. I switched bags from the handmade swiss voile to one I bought at a homebrew shop which had wider mesh. What happened was pretty disturbing. The beer was suddenly full of fine material which was boiling up and creating a very nasty mess on the side of the pot. I cleaned this crap out several times as it built up over the course of the boil. But there was still material in the wort. I just put it into the fermenter and it settled out on its own and I got a very nice clear beer after fermenting.

Needless to say I went back to my old bag because I liked that it filtered that stuff out. But what I assume is going on is that the mesh on the swiss voile is much finer and is doing this filtering for me. This material is definitely coming from crushing the barley and other grains. So there are a few possibilities to fix this:

  1. Crush the grain less or after wetting it to reduce the amount of dust created by crushing.
  2. Get a finer mesh bag, I suggest using swiss voile it seems to be pretty much perfect for this.
  3. But most importantly don't worry about it, most likely it is going to settle out and be cemented into the trub anyway. Just be careful not to disturb it when you syphon off the yeast cake.
  • Thanks! The finer mesh is a good idea; the particles are definitely due to the mesh width (~1 mm). My first BIAB batch turned out fine -- the stuff settled nicely in the end. However, I am concerned about the taste effects of boiling lots of husk.
    – Nemis L.
    Commented May 15, 2013 at 6:54
  • Check out this article about wet or conditioned milling (beersmith.com/blog/2010/07/11/…). That would reduce the amount of husk particles in the boil. Otherwise I don't really know of a great way to reduce the amount of particulate matter added to the boil. You could pre-sift your crushed barley in a pine mesh screen or a collinder to get rid of the fine stuff before adding to the boil. But that might affect your efficiency a bit but probably wouldn't affect it too much. you could always add more grains to account for the loss. Commented May 15, 2013 at 21:10
  • Thanks for the link! Alas, for now, we grind our grains in the brewshop ... I tried the sifting. I actually did an experiment, making two identical mashes with and without the sifting. Sifting produced as much as 10 % fine flour that was discarded, and a corresponding decrease in efficiency. But the amount of cloudy material was the same -- virtually no change. I should perhaps sifted even more strongly, but IMO this is not the way to go. Conditioned milling must be much better.
    – Nemis L.
    Commented May 16, 2013 at 5:01
  • I also mill at the brew store, but they do say you get a lot less fine particles with the conditioned milling. I guess the best option for you would be a finer mesh bag. That works for me. Commented May 16, 2013 at 17:10

I also use BIAB with a bag bought online. The flour from the crushed grain flows pretty freely out of the mash and is present after the boil and chill as a huge cloud settling towards the bottom of my fermenter. Whirlfloc definitely helps clump the stuff together with the cold-crash material, but the goop clogs every filter I have tried.

I've also tried letting the wort settle for 24-36 hours while refrigerating to fermentation temp, then racking to a clean fermenter. That was a huge pain because I did not have an extra gallon to discard, and filtering the remainder at the bottom was impossible. There was also the fear of contaminating my batch by messing around with it in various containers. It seems one could do this effectively with additional equipment like vacuum pumps and Buchner funnels, or maybe a closed system that you could let drip for hours into a giant filter bag...

Long story short, the flour does compact nicely into the trub, and a good cold crash for 2-days at the end of fermentation pretty much takes care of the entire problem for me. On my next batch I am going to try filtering the flour BEFORE the mash by screening it with a #60 screen. Maybe I lose 5-10% of the grain bill that way.

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