How to use hops is the biggest question new brewers have when making the transition from pre-hopped beer kits to all-grain or extract brewing. An important skill to have is predicting how the hop additions will contribute to the bitterness of the beer which is described by the International Bittering Unit or IBU.
Although there are plenty of computer programs that will do this for you, by understanding the process you will gain insight in to what is happening in your kettle.
Making beer bitter? - Humulone, isohumulone and boiling.
If you tear open a hop flower you will notice patches of yellow powder running along the length of the cone, these are grains of lupulin, which is the main resinous component of hops.
The chemicals that make up the grains of lupulin are called humulones which come in three main flavors; humulone, cohumulone and adhumluone. These are collectively referred to as the alpha (α) acids of the hop. The alpha number printed on the bag of hops in your hand is the fraction of the hops weight comprised of alpha acids. This has been measured in a lab by the company which processed your hops after harvest.
Aside from the role they play in beer-bittering α-acids are of interest to chemists because they have anti-microbial and anti-viral properties.
At temperatures above 80°C (i.e. the rolling boil in your brew-kettle) something called isomerization takes place, which is a process whereby the structure or shape of a molecule changes. In the case of humulone (pictured below courtesy of wikipedia) a double bond in the central ring structure of the molecule breaks. This allows part of the molecule to rotate producing trans- and cis- isohumulone, these new configurations are far more soluble in water.
There are other compounds such as beta (β) acids which contribute to the bitter component of beer flavor though they are of less concern to home brewers when formulating a beer recipe.
International Bittering Units.
The number of IBUs in a brew unsurprisingly describes how bitter a beer is, a typical lager will be 5 IBU, a stout or pale ale around 40 IBU while the holy-grail of IPAs is 100 IBU or greater.
The number of IBUs in a particular beer is usually measured by consensus by a panel of skilled tasters, however 1 IBU is defined as a concentration of 1 ppm isohumulone.
Alpha acid units and utilization.
The first step in predicting how many IBU’s a particular hop addition will contribute is to describe the quantity of α-acids contained in the addition, this is known as the alpha acid units (AAU).
To do this we simply multiply the weight of hops in the addition by their α-acid content expressed as a decimal.
AAU (g) = Weight of hops (g) x Fraction of alpha acids (%)
e.g. 10 grams of Green Bullet with an α-rating of 15% would be:
10 x 0.15 = 1.5 AAU
To convert the α-acid rating to a decimal value; take the percentage-value and divide it by 100.
e.g. Green Bullet with an α-rating of 15% would be:
15 / 100 = 0.15
How well the AAUs are isomerized by boiling is described by the fraction hop utilisation, U. This depends on the duration of the boil and the gravity of the wort. A longer boiling time means a greater conversion while a lower gravity will help to dissolve the isohumulone.
There are many different methods to predict the hop utilisation during the boil but the Tinseth method is the most popular. The mathematics in his method starts to get a little complicated so in practice a utilisation value selected from the following table is used:
<span style="font-size: x-small;"> </span> |
<span style="font-size: x-small;">Gravity of the boil.</span> | |||||||||
<span style="font-size: x-small;">t </span><span style="font-size: x-small;">(min)</span> |
<span style="font-size: x-small;">1.030</span> |
<span style="font-size: x-small;">1.040</span> |
<span style="font-size: x-small;">1.050</span> |
<span style="font-size: x-small;">1.060</span> |
<span style="font-size: x-small;">1.070</span> |
<span style="font-size: x-small;">1.080</span> |
<span style="font-size: x-small;">1.090</span> |
<span style="font-size: x-small;">1.100</span> |
<span style="font-size: x-small;">1.110</span> |
<span style="font-size: x-small;">1.120</span> |
<span style="font-size: x-small;">0</span> |
<span style="font-size: x-small;">0.000</span> |
<span style="font-size: x-small;">0.000</span> |
<span style="font-size: x-small;">0.000</span> |
<span style="font-size: x-small;">0.000</span> |
<span style="font-size: x-small;">0.000</span> |
<span style="font-size: x-small;">0.000</span> |
<span style="font-size: x-small;">0.000</span> |
<span style="font-size: x-small;">0.000</span> |
<span style="font-size: x-small;">0.000</span> |
<span style="font-size: x-small;">0.000</span> |
<span style="font-size: x-small;">5</span> |
<span style="font-size: x-small;">0.055</span> |
<span style="font-size: x-small;">0.050</span> |
<span style="font-size: x-small;">0.046</span> |
<span style="font-size: x-small;">0.042</span> |
<span style="font-size: x-small;">0.038</span> |
<span style="font-size: x-small;">0.035</span> |
<span style="font-size: x-small;">0.032</span> |
<span style="font-size: x-small;">0.029</span> |
<span style="font-size: x-small;">0.027</span> |
<span style="font-size: x-small;">0.025</span> |
<span style="font-size: x-small;">10</span> |
<span style="font-size: x-small;">0.100</span> |
<span style="font-size: x-small;">0.091</span> |
<span style="font-size: x-small;">0.084</span> |
<span style="font-size: x-small;">0.076</span> |
<span style="font-size: x-small;">0.070</span> |
<span style="font-size: x-small;">0.064</span> |
<span style="font-size: x-small;">0.058</span> |
<span style="font-size: x-small;">0.053</span> |
<span style="font-size: x-small;">0.049</span> |
<span style="font-size: x-small;">0.045</span> |
<span style="font-size: x-small;">15</span> |
<span style="font-size: x-small;">0.137</span> |
<span style="font-size: x-small;">0.125</span> |
<span style="font-size: x-small;">0.114</span> |
<span style="font-size: x-small;">0.105</span> |
<span style="font-size: x-small;">0.096</span> |
<span style="font-size: x-small;">0.087</span> |
<span style="font-size: x-small;">0.080</span> |
<span style="font-size: x-small;">0.073</span> |
<span style="font-size: x-small;">0.067</span> |
<span style="font-size: x-small;">0.061</span> |
<span style="font-size: x-small;">20</span> |
<span style="font-size: x-small;">0.167</span> |
<span style="font-size: x-small;">0.153</span> |
<span style="font-size: x-small;">0.140</span> |
<span style="font-size: x-small;">0.128</span> |
<span style="font-size: x-small;">0.117</span> |
<span style="font-size: x-small;">0.107</span> |
<span style="font-size: x-small;">0.098</span> |
<span style="font-size: x-small;">0.089</span> |
<span style="font-size: x-small;">0.081</span> |
<span style="font-size: x-small;">0.074</span> |
<span style="font-size: x-small;">25</span> |
<span style="font-size: x-small;">0.192</span> |
<span style="font-size: x-small;">0.175</span> |
<span style="font-size: x-small;">0.160</span> |
<span style="font-size: x-small;">0.147</span> |
<span style="font-size: x-small;">0.134</span> |
<span style="font-size: x-small;">0.122</span> |
<span style="font-size: x-small;">0.112</span> |
<span style="font-size: x-small;">0.102</span> |
<span style="font-size: x-small;">0.094</span> |
<span style="font-size: x-small;">0.085</span> |
<span style="font-size: x-small;">30</span> |
<span style="font-size: x-small;">0.212</span> |
<span style="font-size: x-small;">0.194</span> |
<span style="font-size: x-small;">0.177</span> |
<span style="font-size: x-small;">0.162</span> |
<span style="font-size: x-small;">0.148</span> |
<span style="font-size: x-small;">0.135</span> |
<span style="font-size: x-small;">0.124</span> |
<span style="font-size: x-small;">0.113</span> |
<span style="font-size: x-small;">0.103</span> |
<span style="font-size: x-small;">0.094</span> |
<span style="font-size: x-small;">35</span> |
<span style="font-size: x-small;">0.229</span> |
<span style="font-size: x-small;">0.209</span> |
<span style="font-size: x-small;">0.191</span> |
<span style="font-size: x-small;">0.175</span> |
<span style="font-size: x-small;">0.160</span> |
<span style="font-size: x-small;">0.146</span> |
<span style="font-size: x-small;">0.133</span> |
<span style="font-size: x-small;">0.122</span> |
<span style="font-size: x-small;">0.111</span> |
<span style="font-size: x-small;">0.102</span> |
<span style="font-size: x-small;">40</span> |
<span style="font-size: x-small;">0.242</span> |
<span style="font-size: x-small;">0.221</span> |
<span style="font-size: x-small;">0.202</span> |
<span style="font-size: x-small;">0.185</span> |
<span style="font-size: x-small;">0.169</span> |
<span style="font-size: x-small;">0.155</span> |
<span style="font-size: x-small;">0.141</span> |
<span style="font-size: x-small;">0.129</span> |
<span style="font-size: x-small;">0.118</span> |
<span style="font-size: x-small;">0.108</span> |
<span style="font-size: x-small;">45</span> |
<span style="font-size: x-small;">0.253</span> |
<span style="font-size: x-small;">0.232</span> |
<span style="font-size: x-small;">0.212</span> |
<span style="font-size: x-small;">0.194</span> |
<span style="font-size: x-small;">0.177</span> |
<span style="font-size: x-small;">0.162</span> |
<span style="font-size: x-small;">0.148</span> |
<span style="font-size: x-small;">0.135</span> |
<span style="font-size: x-small;">0.123</span> |
<span style="font-size: x-small;">0.113</span> |
<span style="font-size: x-small;">50</span> |
<span style="font-size: x-small;">0.263</span> |
<span style="font-size: x-small;">0.240</span> |
<span style="font-size: x-small;">0.219</span> |
<span style="font-size: x-small;">0.200</span> |
<span style="font-size: x-small;">0.183</span> |
<span style="font-size: x-small;">0.168</span> |
<span style="font-size: x-small;">0.153</span> |
<span style="font-size: x-small;">0.140</span> |
<span style="font-size: x-small;">0.128</span> |
<span style="font-size: x-small;">0.117</span> |
<span style="font-size: x-small;">55</span> |
<span style="font-size: x-small;">0.270</span> |
<span style="font-size: x-small;">0.247</span> |
<span style="font-size: x-small;">0.226</span> |
<span style="font-size: x-small;">0.206</span> |
<span style="font-size: x-small;">0.188</span> |
<span style="font-size: x-small;">0.172</span> |
<span style="font-size: x-small;">0.157</span> |
<span style="font-size: x-small;">0.144</span> |
<span style="font-size: x-small;">0.132</span> |
<span style="font-size: x-small;">0.120</span> |
<span style="font-size: x-small;">60</span> |
<span style="font-size: x-small;">0.276</span> |
<span style="font-size: x-small;">0.252</span> |
<span style="font-size: x-small;">0.231</span> |
<span style="font-size: x-small;">0.211</span> |
<span style="font-size: x-small;">0.193</span> |
<span style="font-size: x-small;">0.176</span> |
<span style="font-size: x-small;">0.161</span> |
<span style="font-size: x-small;">0.147</span> |
<span style="font-size: x-small;">0.135</span> |
<span style="font-size: x-small;">0.123</span> |
<span style="font-size: x-small;">70</span> |
<span style="font-size: x-small;">0.285</span> |
<span style="font-size: x-small;">0.261</span> |
<span style="font-size: x-small;">0.238</span> |
<span style="font-size: x-small;">0.218</span> |
<span style="font-size: x-small;">0.199</span> |
<span style="font-size: x-small;">0.182</span> |
<span style="font-size: x-small;">0.166</span> |
<span style="font-size: x-small;">0.152</span> |
<span style="font-size: x-small;">0.139</span> |
<span style="font-size: x-small;">0.127</span> |
<span style="font-size: x-small;">80</span> |
<span style="font-size: x-small;">0.291</span> |
<span style="font-size: x-small;">0.266</span> |
<span style="font-size: x-small;">0.243</span> |
<span style="font-size: x-small;">0.222</span> |
<span style="font-size: x-small;">0.203</span> |
<span style="font-size: x-small;">0.186</span> |
<span style="font-size: x-small;">0.170</span> |
<span style="font-size: x-small;">0.155</span> |
<span style="font-size: x-small;">0.142</span> |
<span style="font-size: x-small;">0.130</span> |
<span style="font-size: x-small;">90</span> |
<span style="font-size: x-small;">0.295</span> |
<span style="font-size: x-small;">0.270</span> |
<span style="font-size: x-small;">0.247</span> |
<span style="font-size: x-small;">0.226</span> |
<span style="font-size: x-small;">0.206</span> |
<span style="font-size: x-small;">0.188</span> |
<span style="font-size: x-small;">0.172</span> |
<span style="font-size: x-small;">0.157</span> |
<span style="font-size: x-small;">0.144</span> |
<span style="font-size: x-small;">0.132</span> |
<span style="font-size: x-small;">100</span> |
<span style="font-size: x-small;">0.298</span> |
<span style="font-size: x-small;">0.272</span> |
<span style="font-size: x-small;">0.249</span> |
<span style="font-size: x-small;">0.228</span> |
<span style="font-size: x-small;">0.208</span> |
<span style="font-size: x-small;">0.190</span> |
<span style="font-size: x-small;">0.174</span> |
<span style="font-size: x-small;">0.159</span> |
<span style="font-size: x-small;">0.145</span> |
<span style="font-size: x-small;">0.133</span> |
<span style="font-size: x-small;">110</span> |
<span style="font-size: x-small;">0.300</span> |
<span style="font-size: x-small;">0.274</span> |
<span style="font-size: x-small;">0.251</span> |
<span style="font-size: x-small;">0.229</span> |
<span style="font-size: x-small;">0.209</span> |
<span style="font-size: x-small;">0.191</span> |
<span style="font-size: x-small;">0.175</span> |
<span style="font-size: x-small;">0.160</span> |
<span style="font-size: x-small;">0.146</span> |
<span style="font-size: x-small;">0.134</span> |
<span style="font-size: x-small;">120</span> |
<span style="font-size: x-small;">0.301</span> |
<span style="font-size: x-small;">0.275</span> |
<span style="font-size: x-small;">0.252</span> |
<span style="font-size: x-small;">0.230</span> |
<span style="font-size: x-small;">0.210</span> |
<span style="font-size: x-small;">0.192</span> |
<span style="font-size: x-small;">0.176</span> |
<span style="font-size: x-small;">0.161</span> |
<span style="font-size: x-small;">0.147</span> |
<span style="font-size: x-small;">0.134</span> |
To select the appropriate utilisation value, first find the column corresponding to the approximate gravity of your boil – a value between the pre-boil and desired final gravity can be used for additions after the boil begins (or use your hydrometer). Once you have the appropriate column the utilisation is found by selecting the row containing the time of the addition to the kettle.
e.g. A sixty minute addition in a wort with a gravity of 1050 would have a utilization of 0.231, or 23.1%
Even with a two hour boil the conversion of alpha acids will at best approach 30%, this low efficiency can be attibuted to evaporation of the humulones during boiling.
Calculating the IBU of an addition.
Now that you have a value for utilisation the final calculation is easy, the IBU contributed by an addition is equal to 1000 times the AAU multiplied by the Utilisation, U as a percentage and divided by the batch size:
IBU (mg/L) = 1000 x AAU (g) x U / Volume (L)
e.g. A 1.5 AAU addition with a utilisation of 23.1% in a 23L batch would produce:
(1000 x 1.5 x 0.231) / 23 = 15 IBU.
The final IBU for your brew is the sum of all the additions.
Why we do, while we brew.
As you can see from the Tinseth table at best only about 30% of the α-acids are converted to isohumulones during the boil, the majority of the losses can be attributed to evaporation from the kettle. Understanding the relationship between gravity and time in the boil will let you use your bittering hops more efficiently. This will reduce problems down the line and reduce ingredient costs.