Poor attenuation, what's the likely culprit?

[Simonh82]

My first AG BIAB brew has been fermenting for two weeks how and shows signs of much lower attenuation than I was expecting. The brew day went well and I got 80% efficiency in to the fermenter. Due to a mess up with acid additions for my hard water, the mash pH was much lower than planned, possibly about 4.6.

The OG was 1.044 and after 10 days it appeared to have stuck at 1.015. I'm using US-05, rehydrated before pitching. Fermentation was started at 17-18oC and was brought up to 20.5oC after 4 days. I checked the gravity at 10 days, expecting to be near the anticipated FG of 1.008 but when checked on two consecutive days I was stuck on 1.015. I roused the yeast and bumped the temp to 21.5oC. Few days later it has drifted down to 1.012 but isn't looking like it is going any further.

The recipe used about 8% oats. I used porridge oats and as I've seen mentioned elsewhere, I boiled them for 10 minutes in 3x their volume in water before adding them to the mash along with the grains at 67oC.

Do the oats get converted as part of the mash? I know they are primarily aimed at developing mouth feel but do they also contribute sugars? If so, could the sugars created be relatively unfermentable?

I'm trying to work out why the fermentation isn't attenuating as far as expected. Could the oats be the factor (i've never used them before)? Is the low mash pH a more likely culprit?

The wort was well aerated with 5 minutes of vigorous shaking in the FV with plenty of air space in it. I also added half a tsp of yeast nutrient. US-05 is normally really reliable so I'm surprised it hasn't gone down to the expected final gravity.

The other outside factor I though might influence things is that I used a whole protofloc tablet for a 6 gallon brew. I now understand I could have used 1/4 or 1/2. Could this have helped pulled the yeast out of suspension too quickly before they had finished their job. I think protofloc is mainly for pulling the break material out of suspension and I don't know if there is enough left in the wort to affect the yeast.

Any insights welcome.

 

[Hello]

I don't think the oats are the factor. Nor do I blame the protofloc tablet.
First, how did you measure your OG? Was the wort hot?

What is the recipe?
How did you figure your efficiency? What was your mash efficiency (different from overall brewhouse efficiency)?
Did you end up with the volume you targeted pre-boil and post boil?
What was your pre-boil OG?

Something is definitely wrong if you used US-05 and moved from 1.044 to 1.015 at those temps. I pitch US-05 into beers up to 1.050 without re-hydrating and hit my anticipated FG each time, in fact lower than what Beer Smith claims it would be. I am surprised by your results.

 

[Simonh82]

I worked out the efficiency using the Brewer's Friend recipe calculator. I set my suggested efficiency at 75% but hit 80% instead. I worked this out by inputting my known parameters (ingredients, volumes, mash temp) and playing with the efficiency figure until it matched my OG. I took the gravity reading at 20 degrees which is what my hydrometer is calibrated to. I didn't take a pre-boil gravity reading so can't tell you my mash efficiency but will do so with future brews.

My post boil volume was half a litre under but I decided not to water it down and just go for a slightly stronger beer.

The recipe is my stab at a clone of Wild Beer Co. Bibble:
2.65KG Marris Otter
1KG Vienna
300g Crushed Oats

Hops -
60 min: 7g Mosaic
15 min: 20g Mosaic + 20g Amarillo
3 min: 30g Mosaic + 30g Amarillo
Dry hop: 50g Amarillo for 7 days

Yeast: Safale US05

22.5 litres in the fermentor
OG: 1044
Anticipated FG: 1008
4.6% Vol
~40 IBU

I was feeling really confident with this one as I thought I had all the bases covered. I was gutted to see it sticking at 1015, although I'm glad it has got a little lower. Do you think the low mash pH could have lead to more unfermentable sugars? The mash temp went from 67oC to 66oC over the course of the hour.

 

[billym99]

What was your mash schedule (temp(s), time at each temp if stepped, etc.)?

 

[Simonh82]

It was a simple infusion mash at 67oC, drifting down by 1 degree over an hour. I then raised the temp to 75oC but heating and stiring, lifted and drained the bag and ran about 6 litres of water through the grains as a sparge.

 

[billym99]

Here's a good read for you. I would look at the pH and temps section.
http://braukaiser.com/wiki/index.ph...in_single_infusion_mashing#Temperature_series


Seems like you did everything right. I punched in your stuff to Beersmith, it told me 1.009 for FG. In my experience with single infusion mashing I've been inconsistent with hitting Beersmith's expected FG's. Over the last two years I've went to a Hochkurz mash and can now consistently hit my FG's (or go lower). Of course, this is my experience as I'm sure many single infusion mashers have no issue.

Biermuncher has a post on here about controlling attenuation though mash times, and then Kai has a ton of stuff on his wiki about mash profiles (where I got my info for Hochkurz style mashing).

 

[ScrewyBrewer]

Man, I would love to get 80% efficiency from my eBIAB system! Even with double crushing the grains and a 90 minute mash I'm happy to get 70-75% efficiency. What temperature was your wort when you used a digital pH meter to take the reading and lastly what did you use to lower your pH?

 

[dmtaylor]

Strange. It must be a mash pH problem (4.6). It's not the oats and you did everything else right. Must be pH. Not much you can do to "fix" it now unless you want to add some simple sugar to jack up the alcohol level -- that's optional.

 

[Simonh82]

Man, I would love to get 80% efficiency from my eBIAB system! Even with double crushing the grains and a 90 minute mash I'm happy to get 70-75% efficiency. What temperature was your wort when you used a digital pH meter to take the reading and lastly what did you use to lower your pH?

I was really surprised and pleased to get 80% efficiency which is why it is a bit of a kick to get such low attenuation. I followed the advice on the BIAB forum here. I crushed fine and squeezed the life out of the bag. I went for a small sparge too.

I took the pH at both mash temp and ~30oC. I was using narrow range pH test strips which I know are not really reliable but should give an indication. They didn't budge from the lowest colour value which corresponded to pH4.6. When I realised the error I'd made in Bru'N Water I was able to recheck and this confirmed the mash pH was likely to be about 4.55.

 

[Simonh82]

Strange. It must be a mash pH problem (4.6). It's not the oats and you did everything else right. Must be pH. Not much you can do to "fix" it now unless you want to add some simple sugar to jack up the alcohol level -- that's optional.

I'm going to bottle this brew so I may go for fairly high carbonation and see if I can get an extra 0.2% out of it. I wasn't going for a strong brew as I have a cupboard full of 6% plus stuff and I want something a little gentler of the liver/head.

 

[Simonh82]

Here's a good read for you. I would look at the pH and temps section.
http://braukaiser.com/wiki/index.ph...in_single_infusion_mashing#Temperature_series

Thanks, that is interesting reading. It looks like the mash pH may have had an affect on attenuation then. Certainly the limited data points at the low end of the mash pH range they have there seem to suggest there could be a real hit in attenuation. I certainly won't be repeating the mistakes I made with my water calculations again.

It is also interesting to see the affect mash pH has on colour, noted in the article. I was aiming for a light golden ale but this is extremely pale. I guess I will bottle it and see how it turns out.

Thanks for the help.

 

[whovous]

Man, I would love to get 80% efficiency from my eBIAB system! Even with double crushing the grains and a 90 minute mash I'm happy to get 70-75% efficiency. What temperature was your wort when you used a digital pH meter to take the reading and lastly what did you use to lower your pH?

Who is doing your double crushing? You, or your LHBS? I always had the store do it and had trouble getting to 70% as well. I then dug out the Corona knock-off that I'd left sitting in the box for many months and set the grind as fine as I could make it and increased my mash from 60 to 90 minutes. I got a compacted grain bed, pump cavitation, and 88%!

That inspired to buy a Barkey Crusher mill (the Corona is an enormous pain for many reasons) and to cut back to 60 minutes again. 80%.

Buying my own mill was one of the few smart steps in my brewing career.