Lesson 4 - Wort and Beer Analysis

If you look through any of the analytical guides on brewing the section on beer is almost as large as the other sections put together. Every step of the brewing process increases the complexity and, more importantly, the value of what we are making. 

The information we gain from tests on beer is vital for managing the preceding processes. It also allows us to release the beer to the next stage of production with confidence. We begin with the analysis of ethanol.

The amount of alcohol in our beer is a business-critical and safety-critical parameter. Most countries tax beer on the basis of alcohol percentage and the enforcement authority often has the power to close down a brewery for underpayment of tax. In low-alcohol or alcohol-free beers staying below the threshold for each classification is also essential to avoid potential prosecution. This is especially significant in countries where the consumption of alcohol is prohibited for religious reasons. So how do we measure alcohol by volume (abv)?  

ABV by distillation

Distillation is the definitive method for measuring abv. We add a 100 g sample of beer to a flask, top it up with around 50 mL of water, and boil it. The vapour from the distillation is collected in a condenser until 90 mL of distillate has been collected. We top up the collected distillate with water to 100 ml and cool it to 20 °C. We then measure the SG of the sample using a pycnometer or a densitometer, and use this SG to look up the alcohol by volume from an alcohol table.

If we take the flask from which the alcohol was distilled, top up the residue to 100 g, and measure the SG we can find the real extract (E_R) of the beer. We can use this and the alcohol % by mass (A) to calculate the original extract using this equation:

The real degree of fermentation (RDF) is the amount of extract in the wort before fermentation which has been converted into alcohol. RDF can be calculated from alcohol by mass (A) and real extract (E_R) by this equation: 

ABV by calculation

The simplest method for estimating abv is by a calculation based on the gravity drop and converting the SG into ‘brewers points’. The method is simple but imprecise. We measure the OG and the PG of the sample. We then calculate the abv using the following equation: 

The F factors for a range of gravity drops are shown below.

The specific gravity is often shortened for simplicity by brewers. This shortened version is also known as ‘brewers points’ and is calculated as follows: brewers points = 1,000 (SG – 1). So, for example, an SG of 1.040 becomes shortened to 1,000 (1.040 – 1) = 40.

Table: F factors for various gravity drops