50
In order to calculate the alcohol content of a brew, you need
to take two samples:
1. The Specific Gravity (SG) of the initial wort/must on Day 0
when it is made. This measurement is also known as the
Original Gravity (OG).
2. The Specific Gravity (SG) of the final beverage after
fermentation and when cold (preferably after clarification).
This measurement is also known as the Final Gravity (FG).
From these two measurements we can calculate the alcohol %.
The higher the initial SG, the higher the alcohol % will be and
the lower the final SG, the higher the alcohol % will be.
TAKING AN INITIAL SG SAMPLE
When the wort/must is full in the vessel and before the yeast
is added, give it a good stir so that all the contents are mixed
well. Take a 100ml sample out of the top surface using a clean
glass and add this to the 100ml measuring cylinder. Then add
the hydrometer to the cylinder and give the hydrometer a spin
as you let go. The spinning helps remove air bubbles from
the hydrometer. Let the hydrometer come to a stop and then
measure the SG. An example is shown in Figure 62.
You may need to blow off any bubbles so you can read the
hydrometer better. Then read off the scale at the top point
of the liquid where it rises and touches the hydrometer.
Note: some hydrometers require a slightly different reading
point, so read the instructions of the hydrometer you have.
For a WilliamsWarn Standard Kit consisting of a 1.7kg (3.75
lbs.) can of liquid malt extract and a 1.36kg (3lb) pack of dry
malt extract made to 23 Litres (5 UK Gallons/6 US Gallons), the
SG will be 1.0454.
Specific gravity is a measure of density. Water has a density of
1.000 SG. So 1.040 means the wort/must is denser than water
due to the dissolved matter from the barley grain or other raw
materials that are now in your fermenter along with the water.
This includes various sugars, proteins, amino acids, minerals
and vitamins and other compounds.
To get the most accurate reading, the temperature of the sample
should be 20°C (68°F). A few degrees on either side doesn’t
matter too much, but the closer you are to this target the more
accurate the reading. Note: Some hydrometers have different
calibration temperatures than this, so read the instructions of
the hydrometer you have.
SAMPLING DURING FERMENTATION
Once the pressure has built up in the unit, you can take a sample
out of the draft tap at any time. If you’d like to measure the SG
each day you may do that. However unlike the wort/must sample,
there are now carbon dioxide bubbles in the fermenting beverage
so you need to de-gas it first. Take 50ml of the beverage out of
the draft tap and discard this down the drain, this will clear the
line so that you may sample the fresh beverage. Then take a
150ml sample from the tap and pour it between two glasses for
a minute to create turbulence and remove the CO
2
. The more
turbulent the pouring between the glasses, the more the removal
of CO
2
. Then let it settle and ensure it's close to 20°C (68°F) in
temperature and then add it to the measuring cylinder. Add the
hydrometer with a little spin and read the SG.
Different yeast will ferment at different speeds and different
fermentation temperatures also affect the rate. Some graphs
APPENDIX THREE:
TAKING S.G. SAMPLES
AND CALCULATING
ALCOHOL %'S
Fig. 62
