Bitterness

Bitterness is measured in IBU (International Bitterness Units). This is the amount of bitterness that you extract from your hop additions. Bitterness is added to beer to balance the alcohol and sweetness. The style of the beer you are aiming for will give you some guidance on the appropriate bitterness, then the hop additions will determine how this is achieved. What type of hops you use, especially in the late hop additions, is also an important factor.

The IBU scale was introduced in the early 20th century, as a way to put a number to, or quantify, the perception of bitterness, and assess just how bitter a beer turned out to be when it was ready to drink. Almost all the beer you’ll ever drink will have a measured IBU between 5 (which is a very low measured bitterness) up to 120 (which is a very high measured bitterness). Most beer falls in a narrower range within these parameters (between 15-80ish).

Hops are added at various stages of the boil:

Hops added at the beginning of the boil will impart the most bitterness – these are known as ‘bittering hops’.
There are some hop varieties that are added part way through the boil which are known to deliver a particularly bold flavour – these are known as ‘flavour hops’.
Hops added at the end of the boil (at ‘flame out’) are known as ‘aroma hops’ – they add very little to the bitterness, but have a major influence on the flavour and aromas added to the beer.
Hops added during a ‘whirlpool’ at the end of the boil (and at a medium whirlpool temperature of 70°C) have a utilisation figure of about 10%.
‘Dry hops’ (hops that are added to the secondary fermenter) generally add no bitterness, but they also have a major influence on the flavour and aromas added to the beer.

Hop flowers contain alpha and beta resins and essential oils within the lupulin glands of the flower. Alpha resins are not very soluble in water, and therefore hop pellets and whole hops need to be boiled to extract the proper bittering from the alpha acid (at least 60 Minutes). Hop oils are very soluble in water and will boil off quickly with the steam of the boil. For flavour and aroma, you should be adding your pellet hops or whole hops in the last 15 minutes, or during a whirlpool after the boil completes.

The IBU value for each hop addition may be calculated using Glenn Tinseth’s equations, which are considered to be very accurate:

Firstly, the Alpha Acid Units (AAU) value is calculated for each hop:

AAU = the weight of the hop (gms) x Alpha acid percentage

Then the IBU value is calculated for each hop:

IBU = AAU x Utilisation x 10 / the target volume of the brew

The boil gravity is calculated as:

Boil gravity = target OG x post-boil volume / pre-boil volume

where the boil gravity and original gravity are expressed as the gravity value minus 1 ie. 1.045 is expressed as 0.045. For the purposes of our calculation, it is safe to assume a 10% loss of volume during the boil eg. from 30 litres down to 27 litres.

The Utilisation figure is the difficult one to calculate.

Utilisation = Bigness x Boil factor

where:

Bigness = 1.65 x 0.000125 ^ (boil gravity – 1). (where ^ means ‘to the power of’)

Boil factor = (1 – exp(-0.04 x minutes)) / 4.15. (where exp means the exponent of)

where ‘minutes’ is the number of minutes that the hop is boiled for (for instance 60 if the hop is added at the start of a 60 minutes boil).

The use of ‘whirlpool’ hops requires a further calculation. As the name implies, whirlpool hops are added to the hot wort after it has been stirred vigorously for a few minutes. The whirlpool hops are then left in the wort for up to 20 minutes. The calculation of the IBU contribution is calculated in the same way as described above, but a further utilisation factor needs to be applied due to the lower temperature. The required formula for utilisation percentage is:

2.39 * 10^11 * exp(-9773/T) where T is in Kelvin

Here’s some utilisation percentages for whirlpool hops, assuming 100% would be an equivalent boil hop:

Boiling: 100 C – Utilization is 100%
At 90 C – Utilization is 49%
At 80 C – Utilization is 23%
At 70 C – Utilization is 10%
At 60 C – Utilization is 4.3%
At 50 C – Utilization is 1.75%

The overall IBU of your beer is the sum of the individual IBU values calculated for each hop.

These are quite involved calculations, which are incorporated into the designer page.