Measuring specific gravity & calculating alchol content
Most homebrewers want to know the alcohol content of their beer, and taking specific gravity (SG) readings is the key to being able to calculate it.
Specific gravity is a measurement of the density of liquid relative to pure water. As yeast converts sugars in the wort to alcohol and carbon dioxide, the wort becomes less dense and the specific gravity drops (alcohol being less dense than sugar). By comparing the specific gravity before fermentation (known as original gravity, or OG) to that after fermentation (final gravity, or FG) and applying a formula it's possible to calculate the alcohol content.
Taking gravity readings is the only sure way to know that a beer has stopped fermenting. The same gravity reading on three consecutive days — or two identical readings three days apart — indicates that fermentation has finished, provided the beer is about the expected final gravity. (A higher-than-expected gravity could indicate that the beer has not fully fermented, perhaps due to the temperature dropping too low for the yeast to work or fermentation becoming "stuck".)
Measuring specific gravity also becomes important if you start doing all-grain brews because it allows you to work out the efficiency of your mash and whether you need to add water or a little malt extract to achieve a particular gravity before pitching the yeast.
On the other hand, many brewers make perfectly good beer without ever taking a specific gravity reading, but for them the ever-present risk is that they will bottle a beer that hasn't finished fermenting and end up with "bottle bombs" caused by the beer continuing to ferment in the bottles and exploding due to the pressure.
The most common method of taking an SG reading is using a hydrometer, which comes with most starter brewing kits. A hydrometer is a glass tube that's floated in a sample of several hundred millilitres of wort or fermented beer so that a reading can be taken from the increments on its side. Pure water at the temperature for which the hydrometer is calibrated (usually 20C) will give a reading of 1.000. Wort before fermentation will have a higher reading because it is more dense due to the sugars dissolved in the liquid. For instance, a "kit and kilo" brew made up to 23 litres should have a gravity before fermentation about 1.045. This reading will drop about 75 per cent during fermentation depending on the ingredients and yeast used, so an expected final gravity would be about 1.011.
When talking about specific gravity the decimal point is dropped, so 1.045 becomes 1045 (or "a thousand and forty-five" or "ten forty-five") and 1.012 becomes 1012.
Specific gravity of the wort before fermentation has begun can also be taken with a piece of equipment called a refractometer, which requires just a few drops of wort for a reading. A refractometer is a particularly useful tool for all-grain brewers because they will often take SG readings at various stages during the mash and boil, and boiling wort can be used the few drops cools to room temperature almost immediately.
Refractometers work by measuring the refraction of light through liquid due to the presence of sugar. Once fermentation has begun they cannot be used to measure gravity accurately because the presence of alcohol affects the reading. However, some brewers use a correction table, which adjusts the refractometer reading for the presence of alcohol.
As a guide, the gravity of a beer should drop about 75 per cent during fermentation, so a wort with a gravity of 1.040 should ferment to a beer of a gravity of about 1.010.
Step by step: Taking gravity readings with a hydrometer
Some brewers measure the gravity by putting the hydrometer directly into the wort. Only do this if you want to run the risk of ending up with an undrinkable beer due to an infection introduced by putting the hydrometer into the wort or fermenting beer.
- Take the first SG reading immediately after you have pitched the yeast and mixed the wort well.
- Slowly turn on the tap and fill the test vessel that comes with the hydrometer to about three centimetres from the top (there needs to be enough liquid for the hydrometer to float but not so much that the hydrometer displaces so much wort that the tube overflows!). If you're using a lid and air lock, the air lock will bubble as air is sucked into the fermenter. Don't worry about this.
- Carefully place the hydrometer into the test tube. Don't drop it in because it may hit the bottom of the test tube and smash.
- Spin the hydrometer. This is important, to remove bubbles that may be on the surface of the hydrometer and cause it to float higher than it should, resulting in an SG reading higher than it should be.
- To take an accurate reading you need to be level with the surface of the liquid, so bend down and look at the reading. Most hydrometers are designed for readings to be taken level with the surface of the liquid, not the meniscus that will be clinging slightly to the side of the hydrometer. If in doubt, check the instructions that came with the hydrometer.
- This reading is the original gravity, or OG. Write it down.
- Don't tip this liquid back into the fermenter. Pour it down the drain or, even better, put it in a drinking glass and let it ferment (see below).
- Leave the wort to ferment.
- When you think fermentation has finished, draw off 100ml or so of beer through the tap. Tip this liquid down down the sink because it will be full of yeast and other sediment that will give a false SG.
- Fill the hydrometer tube and take a reading as above.
- Take a reading on the following two days, or another reading in two days. If reading has remained unchanged over this period and it's about the expected gravity, you're ready to bottle.
- If it has changed wait another day or two before testing again. If it has changed DO NOT BOTTLE because it indicates that fermentation has not finished and you risk bottles exploding.
TIP: After taking a gravity reading, don't pour the wort back into the fermenter because it may have become contaminated. For the sample used to take the original gravity reading, pour it into a glass, cover it with plastic lunch wrap and leave it to ferment. Smell and taste it every couple of days as the sweet liquid turns into beer during the course of fermentation. Likewise, don't be afraid to have a taste of specific gravity readings taken during fermentation.
Step by step: Calculating alcohol content
If you have brewing software it will calculate the alchol content for you when you enter the original and final gravity.
However, you can also work it out using this simple formula:
( OG-FG⁄7.46) + 0.5
- OG= Original gravity (your initial reading)
- FG = Final gravity (the reading at the end of fermentation).
- The 0.5 is an allowance for the alcohol created during secondary fermentation. If you force carbonate in a keg omit the extra 0.5.
When applying the formula, remove the decimal points from OG & FG (so 1.042 becomes 1042).
For example, let's say the original gravity was 1.045 and final gravity was 1.010, the formula would be:
( 1045-1010⁄7.46) + 0.5, which equals ( 35⁄7.46) + 0.5, which is 4.69 + 0.5.
Therefore, the estimated alcohol by volume (rounded) is 5.2 per cent.