A Primer on Priming

by Mark Hibberd (Bayside Brewers, Melbourne, Australia), mfh@dar.csiro.au
Most homebrewers carbonate their beer by adding priming sugar at bottling time. Usual instructions call for about a teaspoon of sugar per bottle. But exactly how much sugar is needed and what types of sugar are suitable? And what can you do if a beer is over- or under-carbonated?

Carbonation levels

The amount of carbon dioxide in a beer can usefully be described in terms of the volumes of CO2, i.e. how many volumes of CO2 (at atmospheric pressure) are dissolved in one volume of beer. This terminology is familiar to those who keg. Charts for kegging systems show the gas pressure to apply at each temperature to achieve a particular carbonation level. If this pressure is held for several days, the carbonation reaches its equilibrium value, i.e. the beer will absorb all the CO2 it can at that temperature. In bottle conditioning, the CO2 is produced by the fermentation of an accurate dosing of priming sugar.

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 much less carbonated than lagers or wheat beers. The accompanying table shows typical carbonation levels for various styles of beer. Exact values are a matter of personal preference, but a good starting point for a homebrew is 2.4 volumes CO2. 

Beer style                     Volumes CO2
----------                     -----------
British-style ales             1.8 - 2.2
Most lagers, Aussie beers      2.2 - 2.6
American, highly carbonated    2.6 - 3.0
Table 1: Typical CO2 levels in bottled beers

How much priming sugar

To achieve a particular carbonation level, you need to know the initial CO2 content of your 'green' beer as well as the amount of priming sugar that will give the additional CO2.

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 graph. 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 tastes much brighter than a sample from an ale fermenting at 20 degC. For the following example, we will assume an initial 0.9 volumes CO2. 

Temp (degC)    Vol. CO2            Temp (degC)    Vol. CO2
-----------    --------            -----------    --------
        0        1.7                     12        1.12
        2        1.6                     14        1.05
        4        1.5                     16        0.99
        6        1.4                     18        0.93
        8        1.3                     20        0.88
       10        1.2                     22        0.83
Table 2: Chart substituted here for graph in original article

Determining the amount of priming sugar is based on the fact that adding 4 grams of fully fermentable sugar (dextrose, glucose or normal white sugar) per litre will ferment to give 1 volume of CO2: 4 g/l sugar --> 1 vol. 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*4 = 6 g/l 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), the sugar is dissolved and sanitised by boiling in about 500 ml water, then cooled and added to a clean fermenter. The green beer is then racked into this fermenter, carefully mixed and bottled. 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. Balanced against this are the risks of oxidation and infection with the extra racking as well as the additional time involved.

Variations in the priming rate as small as 1 g/l can produce noticeable changes in the final CO2 levels (0.25 vol. CO2) so that fairly accurate measurements are required to obtain consistent results. 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 U.S. gallons called for in many American recipes is equivalent to a priming rate of 6 g/l.

This simple picture 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).

Measuring priming sugar

The most accurate method of measuring priming sugar is by weight but for bottle priming the most convenient method is by volume using a measuring spoon. However, much confusion arises here because the same spoon holds different weights of different sugars.

Measurements show that a standard (5 ml) kitchen teaspoon holds 4.5 g of normal white sugar but only 3.4 g of dextrose or glucose powder - about 25% less dextrose. 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 with time.

Another useful 'spoon' is a homebrew bottling measure, which holds 6 g of white sugar (4.5 g dextrose) on one side and 3 g (2.3 g dextrose) on the other. Thus, either a standard teaspoon of white sugar or a bottling measure of dextrose per 750 ml 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 white sugar), 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!

Types of priming sugar

The above calculations are based on using fully fermentable sugars. These include glucose, dextrose (corn sugar), fructose, and sucrose (white sugar or castor sugar). Icing sugar should be avoided because of the small amounts of cornstarch added to prevent clumping. In fact, a given weight of sucrose produces 5% more CO2 than the other sugars, but the difference can safely be ignored.

When used in small amounts for priming, none of these sugars contributes any flavour to the beer. On the other hand, some people enjoy the note added by using brown sugar or Demerara sugar to prime ales; the same weights should be used as for dextrose. 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 (extra 40% by weight), genuine maple syrup (+50%) or molasses (+80%). Furthermore, the results will be less predictable and carbonation will take longer.

Finally, the all-malt purist may want to prime with malt extract, either dried (+30%) or liquid (+40%). Again the results may be variable. More involved methods include adding unfermented or actively fermenting wort (krausening); details can be found in good brewing books.

Problems

Sometimes things just don't work out and you find a whole batch is over- carbonated. It may have occurred because of bottling too soon, over- priming or possibly because of an infection. In any case, the batch can be saved by releasing some of the pressure. With swing-top Grolsch bottles, just release the pressure momentarily a number of times over several days. For crown-sealed bottles, it's best to cool them as much as possible to avoid gushing. Prise off the caps but leave them sitting loosely in place to minimise possible contamination of the beer. The time to wait before resealing with new caps can only be determined by trial and error so experiment with one bottle at a time, starting with 10 to 30 minutes.

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 if you want to avoid 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, the factors discussed here should enable you to consistently produce beer that is carbonated the way you like it.