Lesson 2 - Wort Boiling Theory

Point of reflection – reconvene: Welcome back. So, what were your thoughts?

If the starting gravity and volume are correct, but the final gravity is too low, it may be due to fouling on the wort kettle heating surfaces (in which case you should clean the kettle before the next boil).

It could also be because a steam or thermal fluid control valve has failed to open, because there are steam leaks, or a steam trap has failed. If the kettle is heated by an electrical element, this may have failed. 

If the starting gravity is lower than normal (10.0°P), it may be because too much sparge water or chase water has been used. This would coincide with an increased start of boil volume. Other possibilities are a fault on the malt/adjunct weighing system meaning less malt/adjunct was added earlier in the process. 

Although not asked in this question, if the final gravity is higher than normal, it could be that you have boiled the wort excessively. This could be due to a steam or thermal fluid valve failing to close sufficiently.

If you have a well-run brewhouse, you wouldn’t have nasty surprises like this at the end of the boil. You need to monitor all processes as they occur either automatically or manually and intervene before problems become significant and affect downstream processes.

Vigour

Effective boiling does not only rely on adding heat. In order for the changes we discuss in this lesson to be achieved, we must also ensure that the boil is vigorous. We can think of vigour as the movement of the wort in the boil and also steam (vapour) bubble formation. Wort boiling systems must ensure that the wort moves as much as possible during the boil as this movement helps drive the required changes and promotes evaporation. It is not easy to quantify or measure vigour but wort boiling systems are designed to move the complete volume of wort across the heating surface between six and thirty times during the boil. It is essential that all the wort is able to circulate across the heating surfaces and that there are no areas of the kettle where wort movement is limited.

With so much of the lesson dedicated to explaining how evaporation is measured and achieved it might seem that the more evaporation the better. This is not the case. We pay both financially and environmentally for every hL of water we evaporate and the energy we have used to evaporate it. Modern wort boiling systems are designed to reduce evaporation to the bare minimum required to remove volatiles. Evaporation is not so much an objective of boiling but a consequence of achieving the other objectives. The thing we must achieve however is a consistent rate and amount of evaporation.