Lesson 2 - Wort Boiling Theory

Factor

Effect

pH

Hop utilisation is higher at higher pHs. A pH of 8-10 can provide 90% utilisation. At standard wort pH of 5.2, it is only around 35%. For many reasons boiling at pH 8-10 is not sensible. However, some breweries boil hops in a small vessel at a higher pH to isomerise them before they add them to the main boil.

Boil time

The longer the wort is boiled with hops added, the greater the utilisation, up to a maximum point of around 4 h.

Wort gravity

Hop utilisation is greater in lower gravity worts. This makes sense because it is usually harder to dissolve material in more concentrated solutions.

Trub

Hop acids are not completely soluble in wort so can be adsorbed on trub. The more trub formed, the greater the loss, so avoiding excessive trub formation prevents low hop utilisation.

Temperature

The higher the temperature of the wort, the more effective the isomerisation (isomerisation drops significantly below 80^°C). This point links with the next.

Pressure

Wort at a higher pressure, boils at a higher temperature. So boiling at a higher pressure increases the rate of hop utilisation. For example, it can take 4 h to fully utilise hops at 100^°C, but only 20 min at 120^°C.

Hop age

α-Acid levels decrease as hops are oxidised over time. There’s not a lot we can do in wort boiling to affect this apart from not using old or oxidised hops.

Hop product

The α-acids in hop cones are trapped in the hops’ vegetable material so we need to boil hop cones more vigorously to extract them. Even then their utilisation is not as good as that of hop pellets. In pre-isomerised hops the isomerisation into the more soluble iso-α-acids has already been done so they have a higher utilisation than non-pre-isomerised hops.

Quantity of hops added

Just like with the effect of wort gravity, the more material that is dissolved in water the harder it becomes to dissolve additional material. The reduction in utilisation with increase bitterness units is largely linear up to the point where the wort is saturated. The nature of the wort (gravity, pH etc.) exerts an influence as well, so we can’t quote a standard utilisation per quantity of hops added.

Wort boiling system

The vigour of the boil (wort movement and steam bubble formation) affects the rate of hop utilisation. Systems designed to maximise boil vigour provide superior hop utilisation.

Hop α-acid levels

Hop varieties with higher α-acids levels have higher utilisation than varieties with lower α-acids levels. Lower utilisation from hops with lower α-acids, could be due to losses on increased levels of trub, on more hop material or just the difficulty of extracting α-acids from the extra vegetable matter.

Wort composition

This point relates to two earlier points. Wort pH, the concentration of dissolved material (gravity) and level of trub formation all affect hop utilisation.

Foaming

As with their loss on trub, the tendency of α-acids and iso-α-acids to come out of solution means that they are also lost on any foam formed in the boil. The foam sticks to the vessel surface and the iso-α-acids are lost.