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water amount and hop utilization


New Forum Member
Dec 26, 2016
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I have just started using BeerSmith. For my extract brews I like to boil all five gallons that I will be using (actually I start with 5.75-6 gallons to account for boil off). I entered my MegaPot 8 gallon into BS with the info below. When I switch the extract recipes from the standard 2 or 3 gallon boils to my kettle the IBU's go through the roof. I have to cut the hops in half or more to get the IBU's back near where they started. Does the hop utilization really increase that much by doing a full boil? Did I enter something wrong? Any help would be great.



Brewhouse efficency: 72%
Large Batch Hop util: 100%
Top Up water for kettle: 0.00
Calculate boil vol auto: checked
Boil Vol 5.72 (grayed out)
Boil time: 60min
Boil Off: .5 gallon
Evaporation rate: 8.7%/hr
Post boil vol: 5.22 gal
cooling shrinkage:4%
cooling shrinkage: .021 gal
Mash turn volume: 5 gal
mash turn weight: 10lb
mash turn specific heat:0.00
mash turn addition: 0.00
lauter tun losses: 0.00
adjust for mash vol losses: checked
loss to trub and chiller: 0.00 gal
top up water: 0.00
Batch volume 5.0 Gal
Fermenter loss: 0.00
Bottlign volume (w/o Starter): 5 Gal
whirlpool time above 85c: 0 min
estimate boil hop util in whirlpool: unchecked



Grandmaster Brewer
Mar 16, 2013
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New Hampshire, US
You are most likely using the Tinseth model for IBU calculation that is the default for BeerSmith.  Glenn Tinseth's data showed an inverse correlation between the specific gravity and the isomerization of hop alpha acids (i.e. the lower the gravity the higher the utilization = more IBU).  In one aspect, it would sort of make sense: for a material with a highly limited solubility in water, the greater the amount of water, the greater the amount of oils which may be available to isomerize.

As with any IBU model, it is both a nice tool and a questionable predictor.  The models developed give the calculated IBU based upon a single person's process and for unfermented wort.  Each process is somewhat different and can lead to differing results.  So take the numbers with a huge grain of salt.

On the other hand, they give us something to try and a standard for which to aim our hop additions.  The phenomenon you encounter may hold for your process and the only way to really prove it out is to make the adjustment, brew and test with your senses (taste, smell) if it really made a change or not.