BIAB larger batch = lower efficiency

[mtb]

Afternoon all

I hit a fairly consistent BH efficiency of 61% when doing a single BIAB batch in my 70L pot, which isn't too great especially given I sparge. That eff drops to around 55% when doing a double batch (also sparged); that irks me a little more. I put down a mini-BIAB (11L into the fermenter) yesterday and hit 70% (not sparged).

Seems to me that the reduction of BH eff vs batch volume is fairly linear, which makes it easy to predict and account for in my recipes, but I'm keen to know what the contributing factor is and whether there's anything I can do.

With a much larger grain bill in larger batches, can I assume that there is relatively less BIAB bag surface area exposed to liquor/water, and a lower BH eff is inevitable?
Is it worthwhile recirculating with a pump / stirring more frequently? Current I stir once or twice during the mash (stirring the water around the bag, but I leave the bag tied off and the grain undisturbed).

 

[verysupple]

It's hard to pinpoint exactly where the losses are without having the data in front of me, but I'll take an educated stab. If you're using the same equipment and simply using more water and grain then I'd say it's simply because there is more grain absorption. The deadspaces etc. would be the same but there's more grain and so more water absorbed / trapped in the grain. As the batch size increases the ratio of grain absorption to other sources of lost efficiency increases, thus increasing total losses.

I suspect you're getting just as good conversion but more sugary goodness is getting trapped in the grain. This of course assumes you use similar liquor-to-grist ratios.

Happy brewing.

 

[mtb]

Beersmith does adjust my loss ratios as my batch size increases - I use the same equipment profile for all sizes, allowing BS to adjust the volumes and ratios within. That being said, I've never second-guessed them, maybe I ought to weigh the grain bag after I've mashed out to see if I'm truly using as much as I think I am.. good point indeed.

 

[Lyrebird_Cycles]

I'm going to disagree with the bendy man slightly and say that the problem isn't so much the amount of wort in the grain bed as that it isn't coming out due to lower permeability.

It's a known problem in working with grain bed permeability that you can't use the standard D'Arcy model because the bed itself is compressible. As the bed compresses the permeability drops and this can feed back on itself, causing a "stuck mash"

Where I think this becomes relevant to your problem is that a larger grain load means a greater bed depth and at the same time the mechanics of the bag will cause more compression so the permeability of the grain bed will be reduced.

I suspect that this means that the inside part of the grain bed is poorly drained and furthermore will be unavailable to your "sparge".

 

[Bribie G]

How are you calculating efficiency?
I rely on BrewMate to indicate whether I'm hitting target.

For example here's a fairly typical recipe, about as simple as they get.



I've set the expected efficiency to 75%
At the end of the brew if my refractometer reports that I have an OG of 1.052 then I've hit the numbers.
If I've hit a different OG I just use the spin box and adjust eff up or down till I get that OG and it shows what the eff was. I generally just hit 75 on my equipment (urn and voile sheet, no sparge).

I have no idea what calculations BM does in the background but they are probably quite accurate as I have had similar results using Brewers Friend and also Beer Smith - both of them fully paid versions I have owned in the past.

edit: and I've been doing the same process since about 2008 apart from a couple of forays into lauter tuns, malt pipe thingos whatever. BIABrewer might be able to offer some thoughts.

 

[Bribie G]

If you wanted to send me a recipe and the OG you got, I'll see what Brewmate reckons.

 

[mtb]

Cheers Bribie. I'll post it when the BS Cloud refreshes and I can link you direct to the recipe, but the efficiencies I'm seeing are common across any recipe I brew.
I'm calculating efficiency using BeerSmith. I design my recipe using a pre-defined eff (ie 61%), BS will estimate OG based on that efficiency and the recipe. After the brew I enter in the actuals (batch volume, gravity) and it'll tell me my BH eff.

LC - would I be able to offset that compression by using two or three separate grain bags, maybe separating them in the mash so liquor flows between them?

 

[mtb]

http://beersmithrecipes.com/viewrecipe/1380596/pants-party-3

This one, for example, is a double batch and I dialled the recipe in at 61% efficiency in BS. My actual BH eff was 59% (1.047 OG @ 49L, I suspect part of that was a substandard boil)

 

[husky]

I have noticed the same when I brew IPA's on my 1V system(no sparge). I now have different equipment profiles for my IPA's that allows for the lower mash efficiency and extra losses in the kettle due to using approx. 600g hops in a single batch and the wort losses associated with that(BH efficiency).
Below are my recorded numbers over the last few batches, a clear correlation between more grain = lower mash efficiency that I will look to address in the future(more stirring, agitation, better wort return etc).
Careful with beersmiths efficiency, I believe what you're interested is mash efficiency where as the brew house efficiency you are referring to is into fermenter. You can review beersmiths estimated mash efficiency looking at the 'session' tab.

 

[Lyrebird_Cycles]

LC - would I be able to offset that compression by using two or three separate grain bags, maybe separating them in the mash so liquor flows between them?

Possibly. I don't have any experience with BIAB, maybe someone else has tried this approach?

 

[mtb]

Another option could be to stretch the grain bag around the rim of the mash tun, so the grain bed sits across the top. The thickest part of the grain bed would assumedly be smaller. I could also recirculate wort onto the top of said grain bed..

 

[razz]

It would be ideal to get hold of a larger pot, say 100 litres and mash the double batch. That should prove your problem as L_C described.
Edit. I just reread the OP.
So you are saying you have the bag tied off and sitting in the mash water? You mentioned stirring more often, you need to keep the bag opened and secured around the rim of your pot and that way when you are stirring you are getting at all the grain.

 

[mtb]

It would be ideal to get hold of a larger pot, say 100 litres and mash the double batch. That should prove your problem as L_C described.
Edit. I just reread the OP.
So you are saying you have the bag tied off and sitting in the mash water? You mentioned stirring more often, you need to keep the bag opened and secured around the rim of your pot and that way when you are stirring you are getting at all the grain.

I'm beginning to think you're right - I tied off the bag to submerge more of it in water, thus retaining temp, but I think that's caused me other issues. I'm going to try securing around the rim and see what that changes.

Much appreciated gentlemen

 

[razz]

If that is the only issue then you should see a good jump in efficiency on both batch sizes. I wondered why you would have issues of grain compression in such a large pot.

 

[Bribie G]

Aha

I always do this, then cover with a circle of bubble wrap, put lid on and lag the urn with a doonah for an hour or so.



As it's a thin mash, best to keep it thin with good interaction between the liquor and the husk contents: there's no point in turning the grain bag into a stuffed haggis sleeping in the middle of the pot while the water on the outside isn't doing anything helpful.

 

[mtb]

In hindsight, I don't know how I didn't realise this earlier. I used to stretch the bag over the pot but changed method to try maintaining temp more effectively, and didn't notice or correlate loss of eff -_-
Again, much appreciated all, I think this one is in the "bag".. I'll show myself out

 

[Bribie G]

Mate, please report ... I used to tie my bag off as well, until Ross said "Why TF are you doing that?".

 

[Mr B]

Sounds like you have it sussed.

But, of course make sure your temp is ok, and also could consider potential pH issues.

 

[contrarian]

Sounds like you have found the answer. I always find a drop in efficiency when aiming for a higher OG but have never found an issue when increasing batch size.

 

[fdsaasdf]

I had exactly the same issue the first time I BIAB'd in my big w pot. I was stunned to hit about 65% mash efficiency. It wasn't until after much more reading on AHB that I realised I'd missed the point of stirring and keeping the grain as spread out as possible in the pot.

Good thing we have access to such a knowledgeable and helpful community here