IBU Blues

[knightstreet]

Hi,

I'm new to brewing and have bought the book 'How to Brew'by John J Palmer. This is a bit of a long newbie post so you might have to bear with me....

I'm having some trouble with the calculations with regard to the 'Cincinnati Pale Ale' recipe and was wondering if I could get some help finding out how some of the values have been arrived at. The recipe is as follows:

OG=1.045
IBUs=30

Ingredients for a 5 gallon batch:

3.3lbs pale malt extract
2.5lbs amber dry malt extract

6AAUs of bittering hops (0.75oz of 8% AA Northern Brewer)
5AAUs of finishing hops (1oz of 5% AA Cascade)

Dry Ale yeast

I'm having trouble arriving at the 30 IBU measure. I seem to always get a really high IBU for the calculations I have made:

The wort is made with 3 gallons of water, the pounds per point per gallon of dry malt extract is described as being around 40 in the book so...

OG=(3.3x40)/3 = 44 or 1.044 - this differs from the 1.045 quoted. Not sure why that is since the number is exact.

Then the utilization should be calculated so I used the chart provided and used his OG of 1.045 since it's exactly halfway between 1.040 and 1.050 (The chart has these values thus making the calculation easier and probably pretty close to the final IBU)

The hop addtions are done from the hot break and are as follows:

T=60 mins add all the bittering hops.
T=30 mins add half the finishing hops.
T=15 mins add the remaining half of the finishing hops.

So using the utilization equation:

@T=60 the Utilization chart gives me half way between 0.252->0.231 so this gives me a value of 0.021+0.252=0.273
@T=30 the Utilization chart gives me half way between 0.194->0.177 so this gives me a value of 0.017+0.194=0.211
@T=15 the Utilization chart gives me half way between 0.125->0.114 so this gives me a value of 0.011+0.125=0.136

So I can calculate the total IBU as:

IBU60=(6x0.273x75)/5 =24.57
IBU30=(2.5x0.211x75)/5=7.9125
IBU15=(2.5x0.136x75)/5=5.1

So the total IBU=24.57+7.9125+5.1=37.5825

This is quite a bit higher than the stated 30. Can anyone shed light as to why this is? Is it a rounding thing perhaps?

The reason I want to get it right is because the AA% of my hops are different from the value quoted in the recipie and I need to be able to calculate the modified values to adjust. Also I just want to know how he arrived at an IBU of 30 from that recipie.

Any help would be much appreciated!

Cheers,

Chris.

 

[sptaylor70]

What year/which edition, was your copy of Palmer?

I ran your ingredients through Brewer's Friend and got OG 1046. Both of our OG calculations are off of Palmer's by 0.001. That's more than close enough.

Brewer's Friend gave me 41 IBU for your hopping schedule. 0.5 oz. Northern Brewer gives 32 IBU. I don't know exactly how Palmer's utilization equations work, but it looks like there's an approximation that is inappropriate to your situation.

For an APA, with dry yeast, it is traditional to use a neutral yeast, like Nottingham or US-05. If you want something a little fruitier, you might try S-04 or Munton's.

 

[knightstreet]

It's the 3rd edition, how did you come up with 1.046?


Is this equation wrong: OG=(3.3x40)/3 = 44 or 1.044?

The assumption is 40ppp for dry amber malt.

The approximation works using the chart in Table 7 for the Utilization as a function of time vs Boil Gravity. Apparrently the equations were determined by Tinseth....

 

[dkevinb]

What year/which edition, was your copy of Palmer?

I ran your ingredients through Brewer's Friend and got OG 1046. Both of our OG calculations are off of Palmer's by 0.001. That's more than close enough.

Brewer's Friend gave me 41 IBU for your hopping schedule. 0.5 oz. Northern Brewer gives 32 IBU. I don't know exactly how Palmer's utilization equations work, but it looks like there's an approximation that is inappropriate to your situation.

For an APA, with dry yeast, it is traditional to use a neutral yeast, like Nottingham or US-05. If you want something a little fruitier, you might try S-04 or Munton's.

Interesting. When I run it through Brewer's Friend I get OG = 1.044, FG = 1.011 and IBU = 28. TastyBrew, OTOH, gives OG = 1.045, FG = 1.011 and IBU = 44. I've been having a hard time getting the two calculators to even come close for IBU, though everything else seems about the same. Neither one agrees with Palmer's recipes for IBU.

 

[boydster]

The difference between 28 and 30 calculated IBUs is not something you are going to notice, and a .001 gravity difference is also negligible. There are different ways of estimating IBUs (Rager, Tinseth, others too) that all yield different results. Sometimes dramatically different. The important thing is to figure out what the recipe tastes like and then make adjustments as needed from that baseline.

 

[knightstreet]

OK, so it looks like I'm doing the right base calculations, I can tolerate variations I guess, just wanted to know if it was significant or not. I'll just take the plunge and adjust accordingly based on the results. Thanks for the replies, much appreciated!

 

[sptaylor70]

I had the all-grain setting selected, with a 7 gallon boil. When I switched to the extract setting, with a 3 gallon boil, I got exactly what Kevin got.

The assumption that I was thinking of had to do with hop utilization as it relates to boil volume - i.e. because alpha acids are sparingly soluble in water, IBUs are sensitive to boil volume. As you can see, the difference between a 3 gallon and 7 gallon boil is 13 IBU. I can get Chris's result with a 5.5 gallon boil, and the target 30 IBU with 3.5 gallons.

Boydster makes good points - such small differences are not noticeable, and there are several methods for calculating IBU (Brewer's Friend includes Tinseth's and Rager's).

 

[ajf]

Firstly, lets look at the malts:
3.3lbs pale malt extract
2.5lbs amber dry malt extract

The 3.3 lbs pale malt extract is not dry malt extract, it is a syrup (commonly referred to as LME or liquid malt extract). This has about 36 pppg (points per pound per gallon)
In addition to that, there are 2.5 lbs dry malt extract (commonly referred to as DME). This has about 45 - 46 pppg, not as Palmer says 40 pppg. (I don't know where he got the figure of 40, but it's wrong.)
In your calculations, you assumed the LME was DME, and failed to take the DME into account.
The next thing is you are not applying the pppg correctly. If you take 1 lb LME at 36 pppg and make it up to 1 gallon, you should have 36 points, or a gravity of 1.036. This is very different to taking 1 lb LME and adding 1 gallon of water, which will result in a volume of much more than 1 gallon.
Then you made the mistake of assuming that the gravity of the boil should equal the OG of the recipe. This is not true because during the boil you will evaporate a certain amount of water (which will increase the gravity), and after the boil you will top off with water (which will decrease the gravity).
By the time you have mixed the extracts in the water, you will have a volume of about 3.5 gallons, and a gravity of ~1.065. (The only way to check the gravity correctly, is to take a sample, and measure it.)
Your notes on calculating the utilization factor also seem to be wrong
@T=60 the Utilization chart gives me half way between 0.252->0.231 so this gives me a value of 0.021+0.252=0.273
@T=30 the Utilization chart gives me half way between 0.194->0.177 so this gives me a value of 0.017+0.194=0.211
@T=15 the Utilization chart gives me half way between 0.125->0.114 so this gives me a value of 0.011+0.125=0.136
should read
@T=60 the Utilization chart gives me half way between 0.252->0.231 so this gives me a value of (0.252+0.231)/2 =0.2415
@T=30 the Utilization chart gives me half way between 0.194->0.177 so this gives me a value of (0.194+0.177)/2 = 0.1855
@T=15 the Utilization chart gives me half way between 0.125->0.114 so this gives me a value of (0.125+0.114)/2 + 0.1195
I took the average of the two figures, while you took the lower figure divided by 10, and added it to the larger figure which would be approximately correct for a gravity of 1.039
Assuming that your pre boil gravity is 1.065:
@T=60 the Utilization chart gives me half way between 0.211->0.193 = 0.202
@T=30 the Utilization chart gives me half way between 0.162->0.148 = 0.155
@T=15 the Utilization chart gives me half way between 0.105->0.096 = 0.100
Plugging these figures into your IBU calculations gives
IBU60=(6x0.202x75)/5 =18.18
IBU30=(2.5x0.155x75)/5=4.65
IBU15=(2.5x0.100x75)/5=3.0
Total IBUs = 26, and I doubt you can tell the difference between 26 and 30 IBUs.

If your hops have a different AA%, simply multiply his weight by his AA%, and divide by your AA% to get the equivalent weight for your hops.

Hope this helps,

-a.