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IBU's wrong on Belgian wit

phopkinsiii

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I'll be brewing a Belgian witbier in a couple of weeks. I've created the recipe in BS2, but the IBU estimate is clearly wrong or the recipe is off for this style.
The kit calls for 2.0 ounces Celeia pellet hops (alpha 4.7, beta 3.1) 15 minutes into a 60 minute boil (45 minute boil time).
The predicted IBU is 26.6 which is too bitter for this style which is predicted by the retailer to come to about 16.
If I adjust the desired IBU down to 16, the BS design drops me down to 1.2 ounces of the pellet hops.
I've double-checked that all the hop parameters(%alpha, beta, pellet, purpose) are correct and they are.
Am I doing something wrong? Why is the BS2 recipe calling for a little over 1/2 of the hops quantity for the same IBU as the recipe kit?
I really love the Belgian witbier style and would prefer the bitterness to be in the nominal range. From what I've read, that tops out at about 17. Should I reduce the amount of hops or go with what the kit recommends?
The rest of the recipe:
3.75 lbs pale LME
3.75 lbs wheat LME
1 lb flaked wheat (20' BIAB steep)
1 lb torrified wheat (20' BIAB steep)
1 oz coriander
1 oz dried orange peel
Kit from Adventures in Home Brewing

Thank you.
 
Do you know what IBU model the kit is using versus what your version of BeerSmith is using?  There are three basic predictive models: Tinseth, Garetz, and Rager and they each have subtle differences which can account for some of the difference.  I would ask the supplier which model they used to figure out the IBU number for the kit as a start.

Are you following the water additions according to the recipe?  It looks like from the web site that they are boiling with 2.5 gallons of water and the topping off with additional water to 5 gal.  If you are adding all of the water up front (boiling 5+ gal) then the lower gravity of the wort will increase the isomerization of the AA from the hops and that will be reflected in the IBU calculations.

Further, after looking at the recipe, it says to add the hops, boil for 45 minutes, then add the spices and boil for an additional 15 minutes, which means that the hops would be added at 60 minutes (the beginning of the boil).

Beyond that, What the model in BeerSmith says it is may have little to do with the actual bitterness you will get in your final beer.  I would brew the recipe as per the supplier's kit instructions before judging that the IBU are out of spec for the style.  What really matters is what your taste buds perceive and not what some model which may or may not well represent your process tells you.
 
Unfortunately, I don't know what predictive model they used for the kit.
I made a mistake when I listed the recipe steps. The hops are added 15 minutes after the start of the boil and boil for the remaining 45 minutes. Changing this in BS2 didn't have an appreciable effect.
I will be doing a full boil with the pre-boil volume of 6.5 gallons, so this may make a difference.
Your point about letting my taste buds be the judge is well-taken and I'll probably just make the beer as suggested on the recipe.
I'll try to find out the model used as I'm still curious to know where the difference arose.
Thank you.
 
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