Hit my Pre-Boil gravity, missed my OG by a lot.

[Woodbrews]

Can anyone advise as to where I've gone wrong in my calculations or my method?

I came close to my predicted pre-boil gravity of 1.050 (actual was 1.047), but missed my predicted original gravity of 1.064 (actual was 1.057).

The recipe is It's a Fat Head Headhunter IPA clone:

Brew Method: BIAB
Style Name: American IPA
Boil Time: 60 min
Batch Size: 5.25 gallons (fermentor volume)
Boil Size: 6.75 gallons
Pre-Boil Gravity: 1.047

STATS:
Predicted Original Gravity: 1.064
Predicted Final Gravity: 1.015
ABV (standard): 6.46%
IBU (tinseth): 55.57
SRM (morey): 6.01

FERMENTABLES (12.25 total):
6.75 lb - American - Pale 2-Row (55.1%)
3.25 lb - United Kingdom - Maris Otter Pale (26.5%)
1.25 lb - German - CaraHell (10.2%)
0.5 lb - Torrified Wheat (4.1%)
0.5 lb - American - Carapils (Dextrine Malt) (4.1%)
0.5 lb - Malt dextrin (added during boil)

I hit my predicted pre-boil volume of 6.75 gallons and my post-boil volume of 5.5 gallons, which suggests that my equipment profile is correct.

One possibility is that I mashed for only 30 minutes, instead of 60, but I did hit my pre-boil gravity, so I'm not sure that's it.

Any ideas? Note, I used both Beersmith and Brewer's Friend for the predictions and the lined up pretty much the same.

Thanks!

 

[duboman]

Did you temp correct your readings and did you use the same instrument for both readings.

Did you verify the calibration of your volume markings? Either your readings are off or you actually have a greater volume in your primary than you think and didn't boil off as much as you thought

 

[trentm]

Caveat: I don't know anything about BIAB, I am stuck in my ways and still fly sparge. That said, with fly sparging, the sugars in will be stratified, making pre-boil gravity mistakes easy. To be accurate the pre-boiled wort must be stirred thoroughly. I worry about hot side aeration so I now skip the pre-boil gravity or do it just after a rolling boil has been achieved. If your measurement is accurate you can trust your post-boil gravity.

 

[kyle187]

I think your expected OG (the 1.064) incorrect. preboil gravity of 1.050 at 6.75 gallons calculates out to 1.061 at 5.5 gallons

.050/5.5*6.75=.061

and calculating your numbers it works out perfect

.047/5.5*6.75=.0576

 

[LandoLincoln]

Expected OG: 1.064
Expected fermenting volume: 5.5 gallons
64*5.5 = 352 gravity units

Pre-boil SG: 1.047
Pre-boil volume: 6.75g
47*6.75 = 317 gravity units

317/5.5 = an estimated OG of 1.058 (rounded)

Actual OG: 1.057
Actual post-boil volume: 5.5g
57*5.5 = 313.5 gravity units

You hit 1.057, which was pretty close to the estimate.
If you read the hydro at 70°, you need to add another point to compensate for calibration of the hydro (which is usually at 60°), so you hit the OG where you were supposed to.

So in order to hit 1.064 at 6.75 gallons (pre-boil) you would have needed:
352 GU's / 6.75g = 1.052 pre-boil SG

Does that make sense?

 

[Rockn_M]

The only thing that I can see is that in the recipe it calls for 5.25 gallons in the fermenter but in your description you show 5.5 gallons, is that correct?

If so I bet that is the difference. If you boiled off the last .25 gallons you would probably hit your anticipated OG. You can test this by updating your brewing software to show 5.5 gallons in the fermenter and see what the OG should be.

 

[Near-Beer-Engineer]

Are you confident that the amount of malto dextrin you added during the boil was exact? That can certainly throw the two measurements off.

 

[nutty_gnome]

There will be some error in the measurements due to several things (temp, volume, user's eyeball, junk in the hydro tube, ionized flux capacitor in the neighborhood, etc....) your readings don't sound too far off from what you expect if you consider that you aren't using precision instruments to take your measurements. Your Pre-Boil measured low so it is not surprising that your post boil is low too... at least your recordings match that trend.

 

[Woodbrews]

Thanks for the great input, folks. I used a refractometer for both readings. I cannot rule out measurement error, particularly with respect to volumes, because those are judged by eye using my sight glass.

So in order to hit 1.064 at 6.75 gallons (pre-boil) you would have needed:
352 GU's / 6.75g = 1.052 pre-boil SG

Does that make sense?

This does make sense, although one question is how can I increase my pre-boil gravity? Does it increase with a longer mash? I've been reading multiple threads about shorter mashes, so I thought I'd try a 30-minute mash this time, but maybe that was a mistake?

 

[duboman]

Thanks for the great input, folks. I used a refractometer for both readings. I cannot rule out measurement error, particularly with respect to volumes, because those are judged by eye using my sight glass.



This does make sense, although one question is how can I increase my pre-boil gravity? Does it increase with a longer mash? I've been reading multiple threads about shorter mashes, so I thought I'd try a 30-minute mash this time, but maybe that was a mistake?

This pertains to you mash/lauter efficiency. Crush, mash pH and time for proper conversion are all factors. With today's highly modified malts conversion is pretty easy but if you pH is off and your crush is less than ideal your efficiency will fall off, sometimes dramatically. You were pretty close so it's possible had you mashed for 60 minutes you might have had better conversion meaning better extraction and efficiency.

 

[joyceman]

Ive done that recipe 3-4 times and I believe there is supposed to be a lb of sugar in there as well. If your OG is based upon that addition, that could be your problem.

 

[Woodbrews]

Sorry my original post didn't mention that; I added the sugar (0.5 lbs. of maltodextrin) during the boil.

@joyceman: I've noticed that different versions of the recipe call for a 90-minute boil. Do you do a 90-minute or 60-minute boil?

Regarding pH -- I've not started testing that yet, but I just picked up some test strips and will starting testing. Aside my refining my process, I think the next area of improvement is learning about water adjustments. Baby steps!

 

[joyceman]

ive done 90, at first cause the recipe said so now cause it saves a little columbus. ive also shifted alot of the mid boil hop additions to later.

my recipe called for a lb of corn sugar no maltodextrine, we must be working off variants.

 

[RM-MN]

Sorry my original post didn't mention that; I added the sugar (0.5 lbs. of maltodextrin) during the boil.

@joyceman: I've noticed that different versions of the recipe call for a 90-minute boil. Do you do a 90-minute or 60-minute boil?

Regarding pH -- I've not started testing that yet, but I just picked up some test strips and will starting testing. Aside my refining my process, I think the next area of improvement is learning about water adjustments. Baby steps!

Some of the pH strips are less accurate than we would like to think. If your reading is way off from what you expect, don't start throwing minerals at the mash to make it come out right until you're sure the pH is way off.

 

[LandoLincoln]

This does make sense, although one question is how can I increase my pre-boil gravity? Does it increase with a longer mash? I've been reading multiple threads about shorter mashes, so I thought I'd try a 30-minute mash this time, but maybe that was a mistake?

Well...I'm not so sure about the length of mash giving you a better mash efficiency. I mean, sure...if you only mash for five minutes you're not going to get a lot of conversion from starches into sugars. But with today's highly modified malts it doesn't take long for the mash to convert as much as it's going to convert.

30 minutes is definitely on the low side, though. That might have contributed to the low pre-boil SG.

If you come in low on your pre-boil SG, typically there's really only two things that you can do about it: either boil longer or add some malt extract.

If you had boiled off a half gallon more...
317 GU's / 5g = 1.063 OG

Calculating how much malt extract to add is a bit more complex. Don't have the time right now to go into that.

 

[Woodbrews]

Thanks for the input.

If you come in low on your pre-boil SG, typically there's really only two things that you can do about it: either boil longer or add some malt extract.

If you had boiled off a half gallon more...
317 GU's / 5g = 1.063 OG

Calculating how much malt extract to add is a bit more complex. Don't have the time right now to go into that.

I've been relying on various online calculators for predicting OG without knowing the formulas behind the calculations, so this is really helpful. I very likely will repeat this recipe, so I'll have a chance to tweak the mash and boil times and see where I come out. I'm sure I've made good beer this time around, but there's always room for improvement and for learning.

 

[TexasWine]

I've been reading multiple threads about shorter mashes, so I thought I'd try a 30-minute mash this time, but maybe that was a mistake?

Nope, not a mistake. But make sure your crush was fine enough. Just did my first 30 minute mash this past weekend and hit, or exceeded, all my gravity readings.