What is Specific Gravity (SG)?
The quick answer is “concentration of sugar in water”. Pure water has an SG value of 1.000, although this may be sometimes expressed as 1000 (dropping the decimal point). The more sugar dissolved in the water, the more viscous (or “syrupy”) the liquid becomes. This in turn gives a higher SG reading.
Around 3lb of sugar in 1 gallon (8 pints) of water will give about 1.100 SG (commonly expressed as 1100).
The SG that is achieved after the addition of sugar, but before the fermentation begins, is known as the Original Gravity – this phrase is common in the brewing of beer.
How much sugar to add?
With cider making, the sugar naturally occurs in the fruit. So none in needed when you first press the apples. However, if you require a conditioned – sparkling cider – then you will need to disolve sugar into the cider you siphon from a fermentation vessel into bottles.
Cider happily fermenting in the shed!
Alcohol Potential and Estimating % ABV
The general idea is that the fermentation process will turn all of the sugar into alcohol. This represents a reduction in the SG (remember SG measures concentration of sugar). The amount of reduction in SG therefore represents the amount of conversion to alcohol that has taken place – and can be therefore be used to determine the % ABV.
The alcohol content can be estimated, at it’s most simple, by taking 2 SG readings – The first is the Original Gravity (i.e. just after the addition of sugar), and the second is at bottling. The difference in these 2 readings represents the total drop in SG, and therefore the total amount of sugar converted to alcohol. For example an Original Gravity of 1100, and an SG at bottling of 1000 (implying that all sugar has gone) yields an % ABV of 13.6%
Typically, however, the final SG can be either side of 1000, if the fermentation has ended at 1005, this would represent a sweeter cider that one which ends at an SG of 1000, or even 995. The lower the final SG, the less residual sugars are present, and therefore the dryer the cider. SG readings of below 1000 are common, and this is due to a technicality – alcohol being less dense than water, which affects the reading that a hydrometer will take.
The mathematics involved in the simple calculation are: Take the difference in Original Gravity and final SG, and divide this by the magic number of 7.36
In Summary
- Sugar is converted to alcohol during the fermentation process
- The more sugar converted, the higher the final % abv
- SG is the concentration of sugar in water
- Sugar can be added to the must to raise the SG
- Your recipe will tell you how much to add
- You may have to take into account the natural fruit sugars, to prevent over sugaring
- The SG after the sugar is added, and just before fermentation is known as the Original Gravity
- Fermentation reduces the SG
- The final SG on bottling can be compared against the Original Gravity to provide a % ABV estimate
- The magic number is 7.36
Further resources
- Cider Makers FAQ at UK Cider