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Couple of reasons for this. First, versus a traditional mash tun, there is a lot more drainage area from the grains. Next, the compression of grains caused by lifting the bag releases some of the free water which is contained between the grain particles. Third, many people squeeze the bag and release more of the free water from the grains within.
I use three different brewing set ups: two of them BIAB and the third a cooler mash tun. They each have a different grain absorption rating based upon the amount of grain and my subsequent handling practices for each. It would be better, IMHO, to tie in the grain absorption rate to the equipment profile instead of having it as a globally set variable (hint, hint Brad).
...the compression of grains caused by lifting the bag releases some of the free water which is contained between the grain particles. Third, many people squeeze the bag and release more of the free water from the grains within.
This might answer a question I'd posted in another forum...is squeezing the bag bad chemical-wise? I can see getting extra water that way, and that could make for a gravity that's too low; if so, maybe start with a lower ratio of water to grain? If using less starting water and squeezing would work, I'm wondering if I'd get too many of any undesirable chemicals, e.g. tannins?
There was a lot of comments early in the BIAB days about squeezing the bag releasing tannins. There really is no rationale for it. Chemically, there is not enough pressure being applied to the bag to make a significant impact on chemically altering the contents inside. Tannins are released under conditions of high pH and high heat. You need both conditions to release the polyphenols (tannins). The Germans decocted their mashes (boiled the grains and some wort) for many years (some breweries and brewers still do) and did not release tannins because the pH of the grains and wort was under 6.0.
Also, squeezing the bag does not affect the gravity of the wort already collected. The water still retained in the grains has about the same concentration of sugars as the wort which freely drains out. This I've tested many times in trying to determine if it was worth my time squeezing or not. The difference comes in with the volume being added to the kettle. It really boils down (no pun intended) to your volume balance in your process.