Just as each style of beer has its own balance of hops and maltiness, so the appropriate level of carbonation varies from beer to beer. British ales should be less carbonated than lagers or wheat beers. The accompanying table shows typical carbonation levels for various styles of bottled beer. Exact values are a matter of personal preference, but you can see that a good starting point for a homebrew is 2.4 volumes CO2.
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Beer style Volumes CO2
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British-style ales 1.5 - 2.0
Porter, stout 1.7 - 2.3
Belgian ales 1.9 - 2.4
European lagers 2.2 - 2.7
American ales & lagers 2.2 - 2.7
Lambic 2.4 - 2.8
Fruit lambic 3.0 - 4.5
German wheat beer 3.3 - 4.5
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Typical CO2 levels in bottled beers
Green beer, i.e. beer that has finished fermenting and is ready for bottling, is saturated with carbon dioxide because it has had CO2 bubbling through it continuously during fermentation. This amount of CO2 can be estimated from the accompanying table. It shows that the CO2 level depends on the temperature (at which fermentation was completed) and explains why a sample taken from a secondary fermenter at 2 degC (36 degF) tastes much brighter than a sample from an ale fermenting at 20 degC (68 degF). For the following example, we will assume an initial 0.9 volumes CO2.
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Temp (degC) Vol. CO2 Temp (degF) Vol. CO2
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0 1.7 32 1.7
2 1.6 35 1.6
4 1.5 40 1.45
6 1.4 45 1.3
8 1.3 50 1.2
10 1.2 55 1.1
12 1.12 60 1.0
14 1.05 65 0.92
16 0.99 70 0.85
18 0.93 75 0.78
20 0.88
22 0.83
Solubility of CO2 in beer at atmospheric pressure
Determining the amount of priming sugar is based on the fact that
adding 4 grams of sucrose (cane/beet/granulated sugar) per litre will
ferment to give 1 volume of CO2:For our sample homebrew with a final 2.4 volumes CO2, we subtract the initial 0.9 vol. CO2 in the green beer to find that we need another 1.5 vol. CO2. This is achieved by adding 1.5 x 4 = 6 g/l (0.8 oz/US gall) priming sugar. It can be added directly to each bottle (4.5 g per 750 ml bottle) or by bulk priming the whole batch.
For bulk priming (in this example, 140 g for a 23 l batch or 4 oz for 5 US gall), the sugar is dissolved and sanitised by boiling in about 500 ml (1 pint) water, then cooled and added to a clean fermenter. The green beer is then racked into this fermenter with the hose outlet resting on the bottom so that the swirling mixes in the priming sugar. It is then bottled in the usual way.Bulk priming has the advantages of sterilising the sugar, consistent carbonation for all bottles and not having to worry about siphoning yeast sediment at the end of bottling. It's also simpler if you want to fill a number of different sized bottles. Balanced against this are the risks of oxidation with the extra racking as well as the additional time involved.
Variations in the priming rate as small as 1 g/l (0.1 oz/US gall) can produce noticeable changes in the final CO2 levels (0.25 vol. CO2) so that reasonably accurate measurements are required to obtain consistent results.
This simple method of calculating the priming rate can be complicated by the CO2 generated by the slow breakdown and fermentation of dextrins, particularly in strong all-malt beers. This is rather difficult to estimate. Although it will be negligible in most beers, it is said to be sufficient to fully carbonate some high gravity beers that are stored many months before drinking (maybe producing up to 1 vol. CO2).
Many homebrewers prefer glucose or dextrose (corn sugar) because of the 'cidery' flavour supposedly imparted by sucrose, although, if present, this 'flavour' can probably only be detected in lightly hopped lagers.
Dextrose is also fully fermentable but its chemical composition means that an extra 15% (by weight) is needed to get the same carbonation level as with sucrose. It's easiest to do the basic calculation for sucrose and then add 15%. In the above example requiring 6 g/l (0.8 oz/US gall) sucrose, you would need 7 g/l (0.9 oz/US gall) dextrose.
Particularly for ales, some people enjoy the note added by using brown sugar or Demerara sugar for priming; the same weights should be used as for white sugar. For the adventurous, syrups can also be used, but the weights need to be increased to account for the water and different types of sugars present: honey (suggested extra 40% by weight), genuine maple syrup (+50%) or molasses (+80%). Furthermore, the results will be less predictable and carbonation will take longer. Some report that molasses produces a very unpleasant overpowering taste that only moderates after many months. You may need to experiment.
Finally, the all-malt purist may want to prime with malt extract, either dried (+30%) or liquid (+40%). Again the results may be variable, depending on the type of malt and the amount of water present. More involved methods include adding unfermented or actively fermenting wort (krausening); details can be found in good brewing books.
If you want to do your own tests on the relative effectiveness of various priming agents, you need to know: i) its strength at increasing specific gravity, and ii) its fermentability. To calculate the increase in specific gravity, take a litre (quart) of fresh water and dissolve the priming agent at the rate of 120 g/l (1 lb/US gall). For sucrose, this should give a solution with a gravity of 1047. Other sugars will tend to give lower values. The fermentability of simple sugars including honey is very close to 100%. Priming agents containing more complex sugars such as malt are not fully fermentable and the exact value will have to be estimated as best you can, but 80% is probably a good starting point.
To see how to use these numbers, take the example of a dried malt extract which gives a gravity of 1042 when dissolved at the above rate and assume its fermentability is 80%. If used for priming at the same rate as sucrose, the carbonation level will be 42 x 80/(47 x 100) = 0.71 times that with sucrose. To get the same carbonation level as with sucrose, it would have to be used at a rate that is 1/0.71 = 1.4 times larger.
Measurements show that a standard (5 ml; 1/6 fl oz) kitchen teaspoon holds 4.5 g (1/6 oz) of sucrose but only 3.4 g (1/8 oz) of dextrose or glucose powder. This difference is sufficient to explain the changes homebrewers report when switching from one priming sugar to another, particularly as many kit recipes suggest rather high priming rates to produce a beer ready for drinking soon after bottling; these beers often become over-carbonated after a few weeks/months.
Another useful 'spoon' is a homebrew bottling measure, which holds 6 g (1/5 oz) of sucrose (4.5 g; 1/6 oz dextrose) on one side and 3 g (2.3g dextrose) on the other. Thus, either a standard teaspoon of sucrose (4.5 g; 1/6 oz) or a generous bottling measure of dextrose (5.2 g; 1/5 oz) per 750 ml (26 fl oz) bottle will give the same final level of carbonation (+1.5 vol. CO2). But if the sugars were reversed (teaspoon of dextrose or bottling measure of sucrose), the final beer would be under- or over-carbonated by 0.4 vol. CO2.
For really reliable results, you need to know exactly how much priming sugar your measuring spoon holds. If you have accurate scales, you can check directly. However, it's best to average by adding, say, 20 scoops to a small container and weighing them all at once. If your scales aren't accurate enough, you could ask your homebrew shop to do the weighing. Or buy some good scales - they're also useful for weighing hops and letters!
For comparison with the priming rate suggested above, it is useful to note that the 3/4 cup corn sugar (4 oz. dry weight) per 5 US gallons called for in many American recipes is equivalent to a priming rate of 6 g/l.
On the other hand the beer may be under-carbonated or even flat. The simplest explanations are that you forgot to prime or that the caps are not sealing properly. But it may also be that they just haven't had time to carbonate properly in which case you'll probably be able to taste the sugar. The bottles should be held at the yeast fermentation temperature for a few days for an ale yeast to several weeks for a cool-fermenting lager yeast. A longer time may be required if the beer sat for a long time before bottling allowing more of the yeast to sediment out. But there should always be enough yeast left to do the job provided they are given sufficient time. If you're really worried, it is possible to add extra yeast - a few grains of dried yeast or drops of liquid yeast. But extra priming sugar should only be added as a last resort after waiting several weeks. Otherwise you may end up producing a batch of grenades.
In conclusion, a teaspoon of sugar per 750 ml bottle is a good rule of thumb and if you're happy with the result then stick with it. But if you're having problems, I hope that the factors discussed here will enable you to consistently produce beer that is carbonated the way you like it. Otherwise you may have to delve into the joys of kegging, which also avoids all that bottle washing!
Finally, thanks to the many homebrewers who have helped me in putting together this article by passing on their own priming experiences. I'd be glad to hear any further comments you may have. Please send by email to mfh@dar.csiro.au or c/- Bayside Brewers Club, PO Box 175, Chelsea 3195, Australia.
1 oz (weight) = 28.4 g 1 US gall = 128 US fl oz = 3.8 litre 1 Imperial gall = 160 Imp fl oz = 4.5 litres 1 US fl oz = 29.6 ml 1 Imp fl oz = 28.4 ml