How/can you figure pre boil gravity in BIAB?

[rdbrett]

I'm almost a year in. I've been reading a little today on how to figure pre boil gravity...and adjusting gravity pre boil to hit accurate FG. However, most of what I've read is for AG the standard way. How do you do it BIAB?
I mash in my total water volume, including accounting for grain absorbtion, boil off and trub. Can a person just mash in enough water for the batch size and boil off and hit it? Then, add the other before boiling? I'd love to figure out how to do this and not be a big surprise after the boil. Seems BIAB shouldn't have a lot of gunk in the bottom, but mine does....I usually account for it with .25/.5 gall. Should have asked this question long ago!
Any help is appreciated.

 

[Oginme]

Traditional BIAB starts with all the water you will need, including volumes for grain absorption, trub and boil off. You can modify this if you wish to do mashes at less than the full volume and then top off at some other point in the process (i.e. pre-boil, post boil, into the fermentor).

I do a modified BIAB which includes a batch sparge. I figure the total water and divide that by half, heat half in one pot and mash in. While that pot is mashing, I heat the rest of the water in a second pot. Once mashing is done, I lift the bag from the first pot, allow it to drain with some help from squeezing and then plop the bag into the second pot. I can then start heating the first pot to the boil.

Personally, I check volume and brix on the initial mash and record these values. I stir the grains in the bag in the second pot, allow it to steep for about 10 to 15 minutes, then lift and squeeze whatever I can from the bag. I measure volume and brix values from the batch sparge and then pour it into the first pot. I mix well and measure total volume and pre-boil gravity both with my refractometer and with a hydrometer.

The reason for measuring both mash and sparge values is a check on the final pre-boil figure. They should reconcile.

On the 'gunk' from BIAB, there usually is more grain fines that come through the bag than you would get with a normal mash tun. In the mash tun, the grains will act as a filter bed and only a small amount of the fine grains get through. By vorlaufing, this amount is reduced even further.

 

[beaksnbeer]

Doesn't matter if you BIAB or use a screen, false bottom there will always be hot/cold break material left behind.

I also compact mash lift the bag and sparge the rest of my batch water through the bag.
I have stick marked with batch size, pre-boil and finish, you need to really need to know your boil off rate vs amount vs time

 

[Cider123]

I use total water volume to mash just like you. Similar to you, I felt a little lost when it came to getting the OG where I want it. It seems to many folks that sparging makes more sense, since you can make more adjustments and add more water later based on what is needed. There is also a belief that sparging helps rinse out more sugars than would be left behind in the grains if it was not done.

So I am not an expert but I played one on TV. Here is what I can offer.
You have a lot of good advice on this post. When I was first learning the process, I was led to another site called BIABrewer.info This site includes people who were very involved when the process was first introduced on a large scale. They base their logic on the scientific method and offer solid advice to the beginner. Topics such as, is sparging really necessary? and do you really need a finer crush? are often tested and discussed.

Above all, I would recommend looking at the spreadsheet they developed called the BIABacus. This thing has made all the difference for me. You enter all your data for your recipe and it tells you exactly how much water, grain and hops to use to get the OG, expected FG and IBU's you enter. You can obviously adjust all values based on what you want. Does it work? Since using it, I have hit my expected OG's EVERY time. It takes a while to learn it but they will help. They are all helpful, but in particular there is a gentleman who goes by the name PistolPatch on the site who is one of the moderators and, as far as many are concerned, what he offers is golden. My BeerSmith DVD now sits in my drawer collecting dust. The BIABacus is all I need. There is no cost and I don't work for them But I talk highly of the site because of the profound effect it has had on my brewing.

Good luck and have fun

 

[Porchfddler]

I second the the use of the Biabacus, it works every time for me. And Pistolpatch is the go to guy.

 

[Oginme]

If you take readings of your volumes: starting, mash, sparge (if you do one), pre-boil, post boil and trub loss, you will be able to predict the ending volume pretty accurately on subsequent batches.

From the volumes, calculate the amount of water absorbed in the grains (as gal/lb of grain or l/kg of grain), boil loss in gal/hr or kg/hr, and trub loss (I calculate this as gal/lb grain or l/kg grain also because the amount of grain fines, protein break material is fairly proportional to the amount of grain I start with).

It took me three batches to get these numbers to give me consistent values and now I am usually within a couple of percent of my predicted ending volume without any need to add make-up water.

 

[dzlater]

I don't brew in a bag, but why would it be any different then traditional methods?
I assume one might get slightly less efficiancy since you are not sparging, but wouldn't you use the same math?

 

[Yooper]

I don't brew in a bag, but why would it be any different then traditional methods?
I assume one might get slightly less efficiancy since you are not sparging, but wouldn't you use the same math?

Yes, absolutely. The preboil SG would be the same whether you did no-sparge, BIAB, batch sparging, traditional fly sparge, etc.

You take the SG of the full volume of preboil wort.

 

[laserghost]

Yep, BIABacus will reveal all.