Trouble with mash efficiency calculation.

Brewing Processes
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I do BIAB no sparge, with recipes I modify and calculate in Excel  using the formulas from Designing Great Beers by Ray Daniels.

I had been brewing 5-gal batches in a 10-gal pot, but recently began calculating a recipe for 6-gals.  This of course, leads to a larger grain bill.  My calculations were as follows: 13 lbs. of grain adjusted for the extract potential of each type of malt at a stated mash efficiency of 75 % predicting a Target OG of 1.055 using 8.36 gals of treated water for mashing, draining and squeezing the bag to extract all the wort I can, then boiling for one hour.

At the end of the mash, I had 7.1 gals of wort with a hydrometer reading of 1.0364 (adjusted for temperature).  Dividing the total gravity of the wort (7.1 times 36.4) or 258.44 by Potential GUs at 100% efficiency (440) yields a calculated mash efficiency of 58.78 %.

Lousy yield!  But after the boil when I take a hydrometer reading adjusting for temperature,  my Measured OG is 1.0551 – exactly what the original formula predicted using a mash efficiency of 75%!

So how can I get the predicted OG, if my efficiency is that far off.

The only thing I can think of is that somehow the sample of wort (which has hop debris and occasionally some grains that escaped the grain bag) that I am using to calculate the mash efficiency is not representative of the gravity of the entire volume of the wort.

The next time I brew, I may try straining the wort sample before I take a hydrometer reading to  calculate my mash efficiency.

Thanks is advance for any helpful comments and advice.

2

Pull a sample after the boil begins, cool down as close as possible to the ideal temp for your hydrometer, measure.

3

I don’t BIAB so I don’t know if 59% is typical mash efficiency or not.
Since you don’t sparge, the gravity of the wort in your boil kettle will be uniform no matter when you take the sample.
The amount of sugar in the 7.1 gals that you started with doesn’t change.
So, the more water you boil off, the higher the sugar content of the remaining water.
Apparently, you boiled off more water than your original calculation anticipated.

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My typical efficiency is 72% to 75%, and I anticipated the amount of water lost in absorption and evaporation.

But thanks for your comment.

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[quote=“, post:2, topic:25752”]

Pull a sample after the boil begins, cool down as close as possible to the ideal temp for your hydrometer, measure.
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That sounds like a great idea.  Thanks

6

Did you calculate for the concentration of the boil.  You also can get a bad reading if the wort isn’t stirred up after the biab is drained

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