Finite,
Thanks for the observations and numbers. A couple of things that I think might go a little way to explaining your experience.
BIAB has 3 distinct features that are "unusual" It uses the full volume of liquor during the mash process resulting in a high L:G ratio, it is No-Sparge and it uses only 1 vessel in order to save space, complexity and money. Pretty much all of them would contribute to the results you have seen.
Your comment that you tasted the spent grains and noticed different levels of sweetness seems to be the key here. Its not about the mashing process, thats working fine or there wouldn't be sweetness to taste. Its about the lautering process, which in the case of BIAB means no sparge.
No sparge brewing isn't exactly unusual in its own right. Lots of people do it. Some do it because its believed that the less sparging a mash gets, the maltier and smoother the flavour of the beer. No sparging being at one end of the scale and oversparged harsh watery astringent beers at the other. Other people do it simply because it saves time and effort and just costs a few more $$ for the increased amount of grain/litre of finished beer. The no sparge method is simply to mash at a traditional L:G and at the end of the mash add all the remaining liquor required for your volumes to the mash tun, stir the bejeezuz out of it, recirc till it clears and drain to the kettle. Thats it. BIAB is different in that we dont use a HLT and therefore have nowhere to heat/store the extra liquor. So it all goes in at the start.
Most people who No-sparge get somewhere around 55-65% efficiency. When BIAB idea was first being tossed about in this thread, I suspected and I imagine that most others did as well, that this is the sort of efficiencies that we were going to see. I think that there was almost universal surprise when efficiencies of 75+ were reported from the guys who did the first batches. Using a finer crush will certainly increase efficiency and BIAB means you dont have a problem lautering finely crushed grist. So that would be a major contributing factor. Apart from that, I dont really know why BIAB seems to be more efficient than No-sparge. It shouldn't be, and yet it is. Go figure.
In no-sparge, batch sparge and BAIB, the sugars aren't gently rinsed out by a flow of water. They are bashed out into solution by a vigorous stirring, then just drained off. This is why these methods cant hope to be as efficient as a properly running fly-sparge set up. Any liquid that stays trapped in the grain, should contain the same concentration of sugar as the wort being run off. So some sugar gets left behind no matter what you do. With fly sparging, ultimately you could have it so the liquid trapped by the grain was pure water, with all the sugar rinsed out and run off. Nothing left behind. Of course, by then you would have a kettle full of tannins, but it would be 100% efficient.
Lets think about the numbers here. Take a 20 litre batch of BIAB, with 5kg of grain. Allowing for evap you want around 24 litres in the kettle. So you need to allow around 29 litres of liquor because the grain will absorb about 1litre/kg of your liquid. Near enough. So you dough in, mash for the hour, take out your bag and let it drain for a while and you hit your numbers perfectly. 24litres in the kettle. But the liquid absorbed by your grain has just as much sugar in it as the liquid in the kettle!!! So... that means that 5/24 x 100 = 21.83% of your sugar is still in the bag. Your best case scenario is 79.16% efficiency.
Do the numbers again with a 10kg grist and you get 10/24 x 100 = 41.67 % left in the bag with a maximum possible efficiency of 58.33%
If you give the bag a good squeeze (please do, I dont see ANY reason why it should extract tanins) and adjust your calculations for less absorbtion by the grain; it will improve the numbers, but it still wont stop the incremental decrease in efficiency as the amount of grain goes up.
You mentioned that you thought that the concentration of sugr in the grain bag area might be higher than the concentration of the rest of the wort. If this were so it would certainly make the situation worse. And looking at your grain bag set up, I can see that it might happen to you. Your bag seems to be distinctly away from the edges of your pot. This means that there are inded two seperate zones in your mash. In the bag and out of the bag. In his guide, Pistol Patch reccomends that you make your bag big enough to "line" your pot. Mine is set up that way, and while there is a bit of floating about, the entire volume of liquid is in the In the bag zone, only the liquid undeneath the rack I have to keep the bag away from the flame is in the Out of the bag zone. I stir the absolute buggery (without splashing!) out of the whole thing during the mash, and ESPECIALLY just before removing the bag, precisely to make sure that the concentration of sugar is evenly spread.
So...... The whole increase in grain bill = decrease in efficiency thing is something you are going to have to learn to live with I'm afraid. Even after you make your planned move to a 3 vessle system, if you are going to No-sparge or Batch sparge, it will still happen.
Things that will improve your efficiency for now are:
A looser fit for your grain bag
Vigorous stirring before pulling the bag
Squeeze that bag (I'd calculate for only 0.6litres/kg absorbtion and see how your volumes work out)
Finer Crush (crush till you're scared!)
Your concern about the no-sparging and the idea about dunking the bag has lead me to an epiphany. So I'm going to end this post with the hope that I have maybe helped a bit and ask you to see the next post for my ultimate solution to all the woes and worries of BIAB (well maybe)
I gotta learn to type faster
Thirsty
