Just responding to something Pat said in the BIAB register thread. Thought my answer was more appropriately posted here.
Blackbock: Mate thanks so much for doing the BIAB summary stuff. That was a great effort. Interesting stuff on the high gravity beers. Your experience reflects ThirstyBoy's assumptions though he said a similiar decreas would occurr with batch-sparging. (I don't really understand all that stuff by the way
but I'll get there.) . . . . . . . . . . . . . . .
Hopefully someone else will test out the high gravity beers alongside a batch-sparger. I'd do it but I can't even handle normal strength beers.
Pat, the drop in efficiency is higher gravity beers is simply because of the amount of sweet liquid that is trapped in the left over grains. A high gravity wort will not only have more sugar content per ml of liquid, but there will also be more grams of grain for the liquid to be trapped in.
This isn't the case with fly sparging, because you are rinsing out the sugar with pure water, you could theoratically rinse out basically all the sugar if you just kept on sparging. Yeah, its more complex than that, but its close.BIAB and Batch sparging are much more simple because of the homogenous nature of the sugar solution, whereas there is a gradient accross the grain bed in fly.
Suffice it to say, that in both BIAB and batch, as you increase the gravity of the beer and the mass of the grain, there is an increase in the proportion of sugar left behind in the grist when you pull the bag or drain the tun. The proportion is different between batch and BIAB because of the "batch" aspect. The second and maybe third drainings in a batch sparge are much less concentrated.. so while the amount of sugar lost to the grist also increases with gravity and grain mass in batch, it increases at a slower rate than it does with BIAB
You can crunch the numbers yourself --
extract loss to grist in BIAB = gravity of wort to kettle (plato) x volume of Wort lost to grist in ml
extract loss to grist in Batch = 0.3333 x graity of wort to kettle (plato) x (Wort lost to grist + Wort left in MT dead space)
throw in the numbers for a sliding increase in gravity and grist and make a graph
I've assumed that the lost wort in a batch sparge has a gravity of one third that of the kettle wort because thats roughly what it is in my system. It'll change depending on how you conduct your batch sparge, but the principle will be the same. You also need to remember that most of the time, BIAB loses less volume of wort to grain than does batch sparging, and has no dead space.
When you make these two graphs, you will see that at certain gravities BIAB has less totally unrecoverable sugar than batch sparging but batch overtakes BAIB as the grian bill increases. Where that crossover occurs depends on the size of your Mashtun deadspace and how much liquid you lose to your grain in each method.
If you are 2 lazy to crunch the numbers yourself here is one I prepared earlier. I assumed a pre-boil volume of 30litres. A wort to grain loss ration of 0.5litres per kg for BIAB and 1.1 litres per kg for batch and there are three seperate graphs for three different sizes of mashtun deadspace. The whole thing is expressed as percentages of the final amount of extract in the kettle
data on the first sheet, graph on the second ok
View attachment BIAB_vs_Batch._Sugar_extracton.xls
The upshot of this is that it really isn't that big a deal. I just compensate by using a sliding expected efficiency when I BIAB. I start with an expected efficiency (into the kettle) of 75% @ a gravity of 1.030 and I subtract an efficiency point for each ten points of gravity
that comes from grain
so
1.04 = 74%
1.05 = 73% etc etc
1.1 = 68%
and that works for me.
I dont really bother when I batch sparge, maybe a couple of points between an ordinary bitter and a barleywine if I am feeling keen.
Now there are practicalities and other real things that get in the way of nice clean maths. Your method of dong things may be twice as efficient as mine or it might suck. Doesn't really matter, this effect MUST occur given the nature of the methods. Its just a matter of what the rates and proportions are.
I get perfectly acceptable efficiencies with BIAB even at 1.09 (the biggest beer I have BIAB'd) even if they are generally a little lower than my batch sparege effs. Its just nice to know that you need to take into account the size of the grain bill.
Thirsty
