I think this is the correct forum area, I apologize if not.
After doing reasonable research on BIAB bag squeezing I think the consensus is:
• Base malts and low color and caramel malts give up their extract easily, so squeezing the bag gives you retained wort instead of retained extract. Squeeze as much and often as possible.
• Roast malts and high color caramel malts, like Special B, don’t give up their extract easily, and aggressively squeezing the grain bag will release more of the roast compounds into the wort. These compounds can taste quite harsh.
To avoid the harsh compounds my notion is that roast malts and high color malts should be crushed separately and put into another bag inside the pot with the base malts. Remove that bag and drain only. DO NOT squeeze.
Regardless what grains are in the bag, I squeeze until I hit my pre-determined post mash volume. Then I stop squeezing. I have never noticed any extra harshness from squeezing roast malts.
That said, the 2 bag theory is kind of interesting. I would love to hear from those that may have tried it.
Squeezing bags will get more insoluble substances into the final wort than dripping.
You can visualize this well in cheese making. Liquid is quite clear when dripping, gets white when you squeeze.
Whether the insoluble substances contribute positively, negatively or both is up to those who taste the beer.
Most but not all these insoluble substances will precipitate after the boil. Some may contribute flavors during the boil.
Summary. It’s not the same but whether it matters or not is up to you.
Another data point: I use a Brew Bag in the MLT. When I used to lift the bag I would get cloudy wort in the BK. But now that I recirculate and pump from the MLT leaving the bag in place, I get crystal clear wort in the BK.
Whether that matters or not is up for debate …but logic dictates there is something in the turbid wort that’s not in the clear wort.
Prior to switching to the Foundry, I did no-sparge in a cooler lined with a BIAB bag. I always squeezed my bag like I was dying of thirst and it was filled with Bear Grylls pee. (If you don’t get the reference, be thankful) I can’t say that I’ve noticed a difference in flavor using darker grains with the Foundry (no bag/no squeeze) vs bag-squeezing. I haven’t brewed a porter or stout with the Foundry yet, but I’ve used plenty of darker crystal malts like C-120 and Simpson’s DRC.
Even if these malts potentially do release some harsher compounds if squeezed, keep in mind that the bulk of the grain bag is going to be base malt and that could potentially end up filtering out much of these insoluble compounds. The two bag method sounds interesting, and I’d love to hear how the experiment goes, but in my experience I don’t know if it’s necessary.
I’ve only done about five batches, but I squeeze like the dickens. By the time I pour into a glass, everything’s precipitated out, and no one’s ever mentioned any off-flavors to me.
