Low SG for no apparent reason HELP!

I am fairly new to all-grain brewing having done about 5 now. I use a large cooler plumbed with a copper valve for my mash and it holds temp pretty well. I generally have been making 3 gallon batches because my brew kettle has a 5 gallon capacity. Now to my problem, today I brewed an Imperial stout with the intention of the SG being 1.091 and the FG 1.023 and an ABV of ~8.9% a week from now. I mashed 11.75lbs of grain for 60 minutes in 3.75 gallons of water at 154 F. My strike water was 168 bringing the temp of the mash to ~155, reason being I generally lose 2-3 degrees throughout the mash. When I stopped the mash the temp was 152 after 60 min. I sparged once with 3 gallons, then collected the wort for the boil. I took an initial SG reading after cooling the wort to 60 degrees and it came out at just over 1.052 far below the targeted 1.091 I was hoping for. Should I have mashed for 75min? Anyone have any thoughts on what went wrong.....

I normally Mash for 90 minutes and test with iodine which you could not do very well on a Stout. What was your final volume?

The final volume was 3.5 gallons.

People say that most of the starch conversion happens in the first 30 minutes. Who crushed your grain?

Crush is the first thing to look at

What was your pre-boil volume? If you over sparged and your pre-boil gravity was off then you would have needed to adjust your boil time

Did you stir wort before taking your reading as some stratification occurs and you may have gotten a lite reading

The mash temp really has little effect on OG more about FG

oakbarn said:

I normally Mash for 90 minutes and test with iodine which you could not do very well on a Stout. What was your final volume?

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Just dilute the sample with water to thin it out before you add the iodine.

The grain was crushed at the local home brew shop. The guy that crushed it for me seemed to be very knowledgeable about grain. Am I looking for fine or coarse ground? I have never even really paid attention to the way it looked. As for the mash volume and sparge volume, I used the brew365.com mash and sparge volume calculator. Mash volume was approx. 3.9 gallons and the sparge volume was 3.25 gallons. The runnings were not mixed but the valve I have on the cooler dumps them into the brew pot pretty hard. I took the sample off the top, so there is a possibility that the first runnings settled to the bottom??? The pre-boil volume was 4.75 gallons.

At a bare minimum, have him run the grain through the mill twice. You want the outer husk of each grain to be mainly intact, with every single grain crushed.

Are you saying you took a SG reading pre-boil or post-boil? SG should be determined post-boil.

If between mash and sparge you put in over seven gallons of water how did you only get 4.75 gallons back out? I understand grain absorbs a small amount but not two gallons (maybe one gallon). When you dump the grains are you pouring out some liquid with it? Sounds like you might have some dead space in your mash tun that isn't getting drained and you're losing some sugars in that wort. If sugars are getting rinsed bu allowed to settle below where your system is pulling out runnings you might be losing a good portion there. Still doesn't seem to account for a forty point loss.

Looking at that calculator I really disagree with it. First, you should not have a gallon of deadspace in your system that it is accounting for. That's throwing out extra liquid into your kettle unless you really do have a serious deadspace issue like I discussed above. It's also suggesting way too much sparge water which results in more beer but at a lower gravity. At three gallons you should have had a SG over 1.100 and at the 3.5 gallons you should have been around 1.091 so it still seems like there's some losses somewhere in the system. If you took your SG reading pre-boil that probably accounts for the rest.

I wasnt exactly sure what equipment loss meant in the calculator but because it was prefilled I assumed it was a standard loss. That c the amount of sparge water by almost 3 qts. My mash tun definitely does not have very much dead space if any. In the future I will change that entry to maybe .1 gal of equipment loss.That reduces the amount of sparge water by almost 3 qts. I did take the SG reading pre boil so that is probably my other issue. What is the difference between the pre boil SG and post boil SG? Just curious

I really appreciate you helping me out, thanks!

If there's no wort left in the mash tun after draining you might as well set deadspace to zero. A few tablespoons probably gets left behind but not enough to worry about calculating into your water needs.

Pre-boil and post-boil is going to be different due to boil off. The wort pre-boil is less dense than post-boil so pre-boil gravity is always lower than post-boil. By boiling out 1.25 gallons of the 4.75 pre-boil volume you started with you're cutting out a quarter of the volume but keeping all the sugars so it's going to be a much more dense wort with a higher gravity.

I would make the correction on water volumes going forward and assume for this batch your post-boil volume was probably right around where it should be given the extra half gallon of wort you ended up with post-boil.