Lesson 2 - Wort Boiling Theory

During wort boiling, typically four to ten percent of the water is evaporated. Correspondingly, the wort volume decreases, whilst the concentration of material dissolved in the wort increases.

At the end of wort boiling, the gravity is called the Original Gravity (OG) or Original Extract (OE). OG/OE is usually measured in degrees Plato (°P). °P is defined as the percentage of extract by weight. Therefore, in a 10°P wort there are 10 g of extract in every 100 g of wort. It can also be defined as specific gravity (SG). SG is a way of showing the density of a substance in relation to a reference substance, typically water.

Density is the mass of a substance divided by its volume. Density tells us how much mass of a substance fills a specific volume. Water has a density of 1.000 g/mL at 4°C, which means that 1 mL of water weighs one gram at 4°C. 1 mL of a material that is denser than water weighs more than 1 g. Specific gravity is simply a ratio of the density of the substance we are measuring to the density of water. So back to our wort.

If our wort has a density of 1.060 g/mL, and water has a density of 1.000 g/mL, we then know that the specific gravity of our wort is equal to 1.060 g/mL divided by 1.000 g/mL, which is simply 1.060. Notice that the units cancel out in the equation, so specific gravity is a unitless quantity.

Tables and calculators are available online to help you convert between °Plato and SG but be careful to use a trusted source.

Achieving the target OG/OE is essential for making consistent beer. If the OG/OE is too high, we could stop the fermentation early to achieve the required abv but that would mean the level of unfermented extract was higher than required. This is likely to affect the beer’s flavour and potentially its microbiological stability. We could allow the beer to ferment until normal attenuation is achieved and then dilute the beer to achieve the required abv. Again, this is likely to affect flavour with the beer tasting dilute.

To achieve a consistent OG, we must achieve the correct gravity at the start of the boil and then ensure we evaporate the required amount of water. Failing to control the gravity at the start of the boil means we need to boil the wort for longer/harder or for less time/more gently than is required. Ensuring the changes that we are covering in this lesson are achieved consistently, requires a consistent boil in terms of time, vigour and heat input. If we need to vary these factors, we will vary the effectiveness of the boil. Also, boiling more than is necessary, means we are wasting energy, time, money and probably damaging the environment more than is necessary.

There are a few ways to measure the amount of evaporation during the boil, so that you can achieve the level required.

Percentage evaporation based on kettle volume

If we can measure the volume of wort supplied to the kettle and the volume of wort that leaves the kettle, we can calculate the volume of water evaporated. We do this using the following equation:

Let’s work through an example. The kettle contains 400 hL of wort at the start of boil and 378 hL of wort at the end of boil. Total boil time is 60 min. What is the % evaporation?

Point of reflection: Why might this not be a very practical way of measuring evaporation? 

Give yourself 5-10 mins to think about this and then click ‘Next’ to continue with this lesson.