Measuring the specific gravity
(Source: VigoPress) Hydrometers measure the specific gravity or density of a liquid. With apple juice they give a good indication of the sugar content, which in turn will give a good indication of the potential alcohol of the fermented cider.
To use a hydrometer, place your sample of juice/cider in the hydrometer jar (also called a ‘trial jar’) and lower the hydrometer carefully until it floats freely. Spin the hydrometer to remove clinging bubbles. When it is steady and not touching the jar sides, take the specific gravity reading at the level of the bottom of the liquid surface. Make a note of this reading. The potential alcohol will be produced if all the sugar is fermented. The specific gravity of the juice is likely to be somewhere between 1040 (potential alcohol of 5%) and 1065 (potential alcohol of 9%).
Our apple juice of September 2014 has been measured with 2 different hydrometers. The Boots one showed a sg of 1043, the Geordy one a sg of 1049.
After fermentation, to work out the actual alcohol level, take a new specific gravity reading, read the equivalent ‘potential alcohol’ from the scale, and subtract this reading from the potential alcohol reading taken before fermentation. The result will give you the alcohol level of your cider.
Hydrometers also determine when the cider is ready for racking (syphoning off) the cider from the yeast deposit (lees). The specific gravity drops during fermentation. If you are fermenting the cider to natural dryness, when the specific gravity is below 1005 then it is time to rack the cider from the yeast deposit (lees) – this helps to stabilise and clear the cider. This racking process may need repeating if more sediment forms over the next few weeks or months, before you store your cider.