BIAB 2, an Oatmeal Stout- questions

[Hengoedbrewer]

Hi all, I have just completed my second BIAB brew, an oatmeal stout. Recipe and method I followed is here...
https://www.brewersfriend.com/homebrew/recipe/view/844423/biab-stout

Seemed to go well other than my mash temp dropped a bit lower than I planned (mashed in at 66c not 68 and it was measuring 64.5c after the hour mash was up despite insulating my brew kettle with radiator foil) and it then took well over 2 hrs to get the wort cooled to yeast pitching temp, which I know is not best practice. Things to think about next time, mainly to get wort straight into an ice bath to cool, but I am not too concerned as my first biab took longer to cool and tasted fine.

I do have some questions which hopefully you may be able to help with.

1. The pre boil gravity measured 1.068 at 40c which equates to 1.073 at 20c. The estimate using brewers friend for pre boil was only 1.043!! I would have had to have had efficiency of 85% for it to be 1.068 so I don't think this is right. What could explain this high pre boil gravity, especially as my post boil gravity was then only 1.056 (which is actually exactly what Brewers friend estimated)? Was the wort maybe more concentrated and stronger perhaps as I had my pre boil volume set as 13L when it was actually lower (see below)?

2. I am struggling a bit to get my head around water needed for BIAB. I am going to be aiming mostly for 10L brews. In total I used 7L to mash the oats and Maris Otter, 3L to sparge them, cold steeped dark grains in 2L overnight, and then cold sparged those grains with a further 1L of cold water, so 13L of water used in total. The dark grains liquor was only added in the last 10 mins of the boil. I ended up, according to my FV, with 2 gallons, which is about 1L short of the 10L I was aiming for. I don't mind that too much, so didn't bother topping up, but am I right in thinking that this doesn't sound like much lost to grain absorption or boil off? Brewers Friend estimated I would need 18.7L of mash water to get 10L into the FV which conversely, sounds like a lot. I use a 15L stock pot on a portable induction hob to brew with if this matters at all.

Including tidy up and a break to feed the baby as wort was cooling, brew day was 6 hrs start to finish, so a good 90 mins shaved off my first go at BIAB. 7g of yeast was pitched at 22c and the FV is now sat in the cupboard under my stairs at 20c. Really enjoying the AG process and I hope learning more will need to consistently decent beer, hence the questions! Any advice welcome.

Thanks

 

[steveinUS]

First impression here is that your guess about the wort being more concentrated is accurate. If you added what appears to be a considerable amount of additional liquid to the wort near the end of the boil, the pre-boil gravity reading is not truly "pre-boil" because it did not include the full volume of liquid.

 

[foxy]

Agree with steve, but will also add if you are drawing off the test sample from the bottom of the mash tun then the wort will be more dense with the dissolved sugars
Loss of liquor grain to absorption is generally stated in round figures as being 1 litre / kilo, for myself I drain into a bowl for a considerable time so I gauge my loss as about 900 ml, you will have to work out your evaporation loss on your system, and at your boil rate.

 

[Hengoedbrewer]

Thanks both for your responses, it was the first time I have ever added some of the liquour late in the boil so had no idea how to reflect this in Brewers Friend. When I have a spare hour or so I will try and do a boil to work out boil off losses, should then let me know how much water I need more accurately. Next brew will probably be a more straightfoward citra and galaxy IPA so hopefully no panics about my numbers!

 

[foxy]

You can add it to the fermenter but it is better to add it to the boil just to be safe, and always make sure to know the volume of water from the steeped grains, someone I know forgot to add it to the calculations.

 

[Hengoedbrewer]

I am still really in my infancy with BIAB as I say, so missing the little things is still quite easy so I can imagine doing that! I forgot yesterday to lift out clips to hold the BIAB bag bag in place for eg! I will get slightly less beer out of this attempt than planned, but never mind (though probably more than my first time actually as not so much lost to hop and trub). I got 15 500ml PET bottles last time from the Citra IPA; lost quite a lot to yeast cake and hop mess.