Lesson 9 - Brewing with Hops SAQ

** Your recipe for a beer with an IBU of 19, dictates that 10 kg of aromatic hops (with 5% α-acid) are added 15 mins before the end of boil in a 1000 hL brew. The hop utilisation towards the end of the boil is 8%; at the start of the boil it was 30%.

a) Calculate the quantity of 15% α-acid hops that should be added at the start of the boil.

b) As an experiment, the beer was dry hopped. At the end, it was found to have 13 mg/L of humulinones and 10 mg/L of α-acid in addition to the iso-α-acid. What is the calculated bitterness of this beer?

Response:

a) The beer has an IBU of 19 = 19 mg/L of iso-α-acid. The brew length is 1000 hL, which is 100,000 L, so there is thus a requirement for 1,900,000 mg of iso-a-acid, or 1.9 kg of iso-α-acid to be in the beer.

10 kg of aromatic hops were added towards the end of the boil with 5% α-acid, which means that 0.5 kg of α-acid are added, but there is only 8% isomerisation, which results in 0.04 kg of iso-α-acid.

Therefore, the amount of iso-α-acid from the hops added at the start of boil needs to be 1.9 – 0.04 = 1.86 kg of iso-α-acid.

The utilisation at the start of boil is 30%, so the hops would need 1.86 x (100/30) = 6.2 kg of a-acid. The hops have 15% α-acid, so the amount of hops required is 6.2 x (100/15) = 41.3 kg

b) The projected sensory bitterness is 19 + (0.66 x 13) + (0.10 x 10) = 19 + 8.58 + 1 = 28.58 mg/L or IBU