Lesson 2 - Wort Boiling Theory

The change in gravity is directly proportional to the amount of water evaporated so we can calculate the percentage evaporation using this equation:

Let’s work through an example: The start of boil wort gravity is 10.2°P; at the end of boil, the gravity has increased to 10.8°P. What is the percentage evaporation?

It is important to monitor the evaporation rate as the boil progresses. To achieve our target evaporation in the required time we must evaporate water at the correct rate.

The evaporation rate is calculated as follows using volume:

Let’s use this equation to calculate the evaporation rate in our earlier example:

Wort gravity can be measured using a hydrometer. This consists of a glass stem (with a graduated scale on it) and a bulb that floats upright in the liquid being measured. The hydrometer floats at a different height in the liquid depending on the liquid’s gravity. The gravity is read from a scale on the hydrometer stem where it is level with the surface of the wort.  

The most modern method involves using a U tube densitometer. Wort is fed into a U shape tube that is caused to oscillate. The frequency at which the U tube oscillates is measured. In the same way as it is more difficult to wiggle a cannonball than a ping pong ball with your hand (so the cannon ball is wiggled more slowly), a heavier U tube oscillates at a lower frequency. The frequency of oscillation is progressively reduced by an increasing density, so measuring oscillation frequency enables the machine to calculate the SG.

Inline mass flow meters can also estimate wort gravity.