Specific Gravity (SG) and % Alcohol By Volume
This section looks at how you can measure the alcoholic content of your wine, and even target a specific strength during the brewing process. Integral to this process is the measurement of Specific Gravity or SG.
What is 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 of water will give about 1.100 SG (commonly expressed as 1100). This amount of sugar represents a fairly high SG from which to start fermentation, and has a potential to give a wine of about 13.6% alcohol.
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?
After the infusion stage your fruit will have released some of it's sugar, giving a measurable SG. In true grape wine the only sugar that is fermented into alcohol is that which is naturally present in the fruit. However in a good old fashioned English country wine (non grape) it's guaranteed that sugar will need to be added, to raise the SG upto the levels described above.
Depending on the recipe, it may well just say "add 21/2 lb per gallon of water", and this may well produce a quality wine, however every batch will vary in terms of it's final sweetness or dryness on account of a varying amount of natural sugar present in the fruit. A more reliable way to produce a consistent wine, is to measure the SG of the fruit sugars alone, and then determine how much additional sugar to add.
Some recipes will contain a phrase such as "make the sugar up to 3lb per gallon", and that means that the natural sugars must be taken into account. For example, a decent crop of Damsons may well bring the SG up to about 1020 after infusion (this implies that there is 9 oz of sugar per gallon of water that has been provided by the fruit alone). So, in this example, the recipe needs 2lb 7oz of sugar adding per gallon, rather than an additional 3 lb.
In some other recipes, there may be an explicit statement such as "add sugar to raise the SG to 1097", implying that sugar should be continually added until the liquid attains an Original Gravity of 1097.
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 wine 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 wine. 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
- Sugar is converted to alcohol during the fermentation process
- The more sugar converted, the higher the final % abv
- Sugars are present naturally in your fruit, but generally not enough for a decent country wine
- 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