Missed OG by 21 points on first partial mash??

How is this even possible? Just finished a 4 hour brew session and took my gravity, and I'm stunned. I totally expected bad efficiency on my first partial mash using the brew in the bag technique -- but not this bad. I was hoping for 70% efficiency, but according to BeerSmith I got 35% efficiency!

As far as I know, I did everything right for the process. I've done 8 extract batches so I don't think this was an issue with measuring the gravity wrong. I took the gravity after adding my top off water to get to 5.1 gallons and thoroughly mixing the wort by rolling and shaking the carboy. It was at 75 F.

So my imperial stout which was supposed to be 8.3% ABV, OG 1.085 is now going to be a normal stout at 6%, OG 1.064 (but at 50 IBUs it'll probably be rather bitter for that malt profile).

Here are the numbers:

Target OG: 1.085
Actual OG: 1.064

Mashed 8.5 lb of grains at 152 F for 70 minutes in 3.25 gallons of water (water to grain ratio of about 1.5 qt/lb). I covered the pot during the 70 minutes, and the temp held well - at 60 minutes it was only down to 149. I didn't do a mash out or sparge. I checked temp and stirred the grains every 15 minutes to make sure they weren't clumping (as far as I could tell, they weren't). I let the bag drain over the pot until the output slowed after about 5 minutes, and then I bounced and squeezed it a little. It yielded ~2.6 gallons of wort (from the 3.25g strike water).

The grains were:
7 lb US 2-row
0.94 lb chocolate malt
0.44 lb roasted barley
0.19 lb black patent

Then I had 6 lbs of pilsner liquid extract. BeerSmith says that 6 lb extract alone would give an OG of 1.043, so basically my 8 lbs of grain only gave me 21 gravity points. That's about 2.9 lbs of extract. There's no point in doing BIAB again if 8 lbs grain is only going to save me 2.9 lbs in extract.

From the dozens of webpages and forums I've read on BIAB, everyone has said they were getting in the 70-80% efficiency range and that a sparge isn't necessary. I know not doing a mash-out can cost 5% or so in efficiency, but that still should have put me at 65-70%, not 35%.

Any have any clue what could explain this?

Was the grain crushed? The BIAB folks do not stick with typical 1.5qt/lb ratios. Basically you derived too little wort from too much grain. Even if you didn't want to sparge, you would have had to add another gallon of so prior to draining the bag. Ideally, you would have batch sparged by having another pot ready with like 2 gallons to dunk the grain into. How big of a kettle do you have? I'm assuming that is your limiting factor? If so, you should back off on how much grain you use so that you can get more out of it. For example, if your kettle is only 4 gallons, mash 5 pounds and derive 2.5 gallons of wort at much higher efficiency.

I do think there's something else going on though because that 2.6 gallons of wort should have been at about 1.088 and I doubt it was.

Thanks for the reply.
Yes, the grain was crushed. The 2-row was a medium crush and the specialty grains were a fine crush. Right, I probably should have used a higher water to grain ratio but I had a hard time finding the ratio in the writeups I found of BIAB. Also I wouldn't have been able to fit more in my 5 gallon pot (so I guess 8 lbs grain is too much for a 5 gallon pot if I want something like a 2.0 qt/lb ratio).
I'll try sparging next time (if I even attempt BIAB again ), but again from what I read I thought that wasn't really needed. Ex, this guy says he got 80% efficiency without sparging:
https://www.homebrewtalk.com/f36/more-brew-bag-biab-success-88486/

The guy in the thread you linked batch sparged. Post #4,

...and sparged with another 3.25 gal @ 175

Click to expand...

8lbs of grain mixed with 4 gallons of water (that's a 2qt/lb ratio) would be 4.8 gallons in your pot. I agree, it's a little tight.

What you can easily do is stick with your 1.5qt/lb ratio in the main pot for the mash, then heat up 1.5 gallons in another small pot to 175F and dunk your grain bag in that after your mash is done. Then discard the grain and combine the two worts.

Actually, post #4 in that thread is from a different user than the original poster. The user in post #4 said he did sparging, but poster #1 did not say he did it.
Part of the difficulty with BIAB I think is that there's so little clarity on what the BIAB process actually is. There are only a handful of descriptions of it online, and none in the books that I know of. And many of the descriptions disagree about what to do.

One consistent theme of everything I've read though has been that BIAB is surprisingly easy and gets surprisingly good efficiencies. So that's why I'm so disappointed that I got 35% efficiency. I haven't heard of an efficiency that low in any post I've read. Maybe I need to read up more on all grain, but I thought that most of it was a chemical process of the enzymes converting the starches in the grains to sugars and with chemical processes like that normally it's not possible to get drastic differences from the norm.

I'm pretty sure the BIAB movement started in Australia/NZ or at least that's what the majority of brewers there are going. For the most part they mash very thin to the point where the entire batch's preboil wort volume is achieved when the grain is removed. Of course, that takes a very large kettle. What I was asking you was how big your kettle is because if you can accommodate 2.5 qt/lb, try it.

There's some basic arithmetic you can do to figure out what your efficiency CAN be. Let's just go through your scenario.

Each pound of grain is potentially worth 36 gravity units per pound per gallon. You used 8.5 lbs so the max sugar you can derive is 306 units. Since you used 3.25 gallons of water, the max gravity of that mash is 306/3.25 = 1.094. So, you ran off 2.6 gallons of wort (potentially at 1.094) so your total sugar extracted was 2.6 x 94 = 244 and in a 5 gallon batch that contribution would be 244/5 = 1.048 without adding any extract. By the way, if you got 100% mash conversion, your lauter efficiency would be 244/306 = ~80%. The other 20% of that sugar is stuck in the grain from absorption. Sparging or going with a more dilute mash would get some of it.

I have no idea what happened in your situation though. Maybe the grind was coarser than you realize. Maybe your thermometer is off by 5-10 degrees. Are you sure you had 8 pounds of 2-row and you didn't get shorted a few pounds?

The first time I really heard about BIAB in-depth was on Basic Brewing Radio (that's a link to the mp3 of the show). I'm fairly shocked at the efficiency claims, but they say that it's mostly for people who don't have AG setups yet or who want to try something else, so I haven't tried it myself.

One question that comes to my mind is on the grain in the bag. Was the grain very loose in the bag? If it's tightly packed, so you can jam more grain into it, that will interfere with the mash. You want the grain thoroughly wetted, and you want the liquid to easily flow through the mash.

Without a sparge, I'd expect an efficiency in the 50% range, just as a guess. I know the BIAB brewers do better, but again, they are using more liquid than a 1.5 quart/pound. Shoot, even with a double batch sparge, with my water chemistry, I'm in the 72% range. I'm happy there- but I'm just saying that if I didn't do any sparge at all, I'd drop significantly.

Here are some pictures of my brew day: Patrick Davin's Photos - Brewing Day - Stout | Facebook

I didn't get any pictures of the interior of the mash when I was stirring it, but it was fairly dense. Not clumping, but definitely like a thick porridge. All the grain was thoroughly wet and in contact with water though.
I think all the descriptions of BIAB that I read made it sound easier than it is. I listened to the two Basic Brewing Radio podcasts on it too, and that guy said he got like 85% efficiency on his first attempt. Also most of the write-ups I've read said that the only new equipment you really need is the bag, which was part of the attraction for trying this. Sounds now like I'll need a second pot to heat up some sparge water, or a pot larger than 5 gallons so I can use a higher water ratio.

I definitely will check pre-boil gravity next time too. The wiki page (Brew in a Bag - Home Brewing Wiki) kind of glosses over how to do that. It'd actually be really helpful if someone would update that wiki page - I'm willing to do some updates. For starters we should add English-system units so it's not only in Metric. Also that page only has like 2 paragraphs on the Mash, and the rest of the page is mostly standard brewing stuff (how to boil, how to chill, add yeast, etc) that people should already know from other sources. It's still useful but we could certainly add some more depth to the mash instructions.

I'm keeping a list of things I need to do next time:

- finer grain crush.
- use higher water/grain ratio - 2.0 qt/lb?
- Sparge in ~1.5g of 175 F water for 10 min in a second brew pot. Stir frequently. Combine with first runnings.
- don't use the heat guard / false bottom in my mash pot. I'll have more room in the pot without it, and I don't need it since I won't be doing a mash-out in the brew pot if I have a second pot to do a 175 F dunk in.
- take pre-boil gravity! (cool in ice bath). SG * boil volume/final volume = OG

To follow up on this, my second attempt at BIAB partial mash got much better results. 70 to 75% efficiency rather than 35% efficiency!

The things I changed were:
- smaller grain bill (5 lbs - 4 lb US 2-row + 1 lb Crystal 80).
- higher water/grain ratio. 2.0 qt/lb
- "dunk sparged" in 1.5g of 175 F water for 10 min.

Here are my mash notes pasted from BeerSmith:
Mash: 152 F for 75 minutes in 2.5g, stirred every 15-20 min, then steep sparged in 1.5g 175 F for 10 min. Combined for a 4g boil.
Pre-boil gravity = 1.038 @ 80 F. 1.036 = 28.8 OG for 5gal. 77% efficiency? But if we assume my OG was accurate and PBG was inaccurate, then eff = 70%. If I adjust pre-boil volume for sparge temp (170) then actual PBG was 36 * 4g * 0.97 / 5.1g = 27.4, which is about 74% eff.
Pitched at 80 F. OG 1.068? (sorry, kinda drunk) @ 75-80 F

Yeah that sounds a lot better.