Technical Guide to Bulk Priming

The topic of carbonation deals with the dissolution of carbon dioxide, a compound that exists as a gas at regular temperature and pressure, in fermented beer. By definition, carbonation by priming involves the physics of gas behaviour and the chemistry of converting sugar into carbon dioxide. So to consider the topic in any detail, some understanding of certain scientific principles is unavoidable. The two key principles to understand are 1) that the amount of gas that can be dissolved in a liquid is a function of temperature and pressure, and 2) that a given weight of sugar contains a given amount of carbon, and therefore, can produce a particular amount of carbon dioxide. With these points in mind, read on.

Fermentation Temperature:

The first thing that must be considered for more accurate priming is the temperature at which the beer has fermented. Green beer is saturated with carbon dioxide. The beer will have absorbed as much of the CO₂ produced during fermentation as it is capable of holding at that temperature. The amount of a gas that can dissolve into a liquid at a given pressure is temperature-dependent. Since we are fermenting at a constant one atmosphere of pressure, the lower the temperature the more carbon dioxide (CO₂) will be dissolved in the beer and the less priming sugar needed to achieve the desired carbonation. Table 2 shows the approximate level of CO₂ in green beer depending on its fermentation temperature. (This also assumes the beer does not contain excess CO₂ due to super-saturation; this is discussed in more detail later.) Table 3 lists the typical carbonation ranges for the main categories of beer styles.

Table 2. Approximate level of CO₂ in green beer in grams per litre and volumes.

Source: Mark Hibberd’s: A Primer on Priming

Table 3. Carbonation ranges for different beer styles.

How much CO₂ is formed from a given amount of priming sugar?

Inside the yeast cells, glucose follows the glycolytic metabolic pathway when it is being broken down, and under anaerobic conditions the major products are ethanol and carbon dioxide (CO₂). Theoretically one molecule of glucose should yield two molecules of ethanol and two molecules of CO₂, and since the molecular weights of glucose and CO₂ are known, we could easily work out how much glucose we require. In practice, this is not quite true because some glucose goes into producing by-products other than CO₂ and ethanol; mostly these by-products go into building yeast cells.

Balling used empirical measurements to determine the amount of CO₂ formed from glucose:

2.0665 g of glucose —› 1 g of ethanol + 0.9565 g of CO₂ + 0.11 g of losses.

Balling’s formula is more useful for our purposes if we set CO₂ to a unitary value:

2.16 g of glucose —›1.0455 g of ethanol + 1 g of CO₂ + 0.12 g of losses.

This shows that for every gram of CO₂ that we want to add to our beer, we will need 2.16 grams of glucose.

How much priming sugar?

Now that we know how much CO₂ remains in the beer after fermentation (Table 2), and that 2.16 g of glucose per litre of beer will give one gram of CO₂, we can calculate the quantity of sugar required to achieve our desired carbonation level (from Table 3) for the whole batch of beer. If we require a total of 4.7 g/L of CO₂ for a beer that has been fermenting at 20 °C, then we will need an extra 3.0 g/L of CO₂ to add to the 1.7 g/L already in the beer. We need to add 2.16 g x 3.0 = 6.48 g of glucose per litre of beer to get an extra 3 g/L of CO₂ into the beer.

For a 20 L batch of beer this is 6.48 x 20 =130 g.

Now, these calculations are based on molecules of pure glucose. However, glucose is sold as dextrose monohydrate, which means that one water molecule is bound to each glucose molecule, so an extra 15% by weight is required. Using the same example as above, then the weight of dextrose monohydrate required is 149 g.

Sucrose (table sugar) is made of one glucose molecule and one fructose molecule bound together. Fructose follows the same metabolic pathway as glucose and can thus be considered equivalent, so the calculations proceed the same way as for pure glucose, i.e. we would need 130 g of sucrose to prime our 20 L of beer at 20 °C to 4.7 g/L of CO₂.

Table 4 contains a worked list of quantities of dextrose to add per litre of green beer depending on the fermentation temperature and desired carbonation level.

Table 4: Amount (g/L) of dextrose monohydrate (dextrose) needed to achieve varying carbonation levels depending on fermentation temperature.

Note: For table sugar (sucrose) or pure glucose, multiply these numbers by 0.87.

Is priming necessary?

Some higher gravity all-malt beers will fully carbonate over a few months without the addition of any priming sugar at all. This carbonation is the result of the very slow fermentation of the residual dextrins in the beer and is difficult to estimate, but for most beers some priming is required.

English ales, which generally have low levels of carbonation, also may not require priming so long as they have a moderate to high finishing gravity.

During lagering, there may be slow fermentation, especially in high gravity dextrinous beers. It may not be enough to make priming unnecessary, but may alter the required priming rate.

How to get more consistent carbonation levels

Supersaturation

There is a school of thought that suggests that the figures in Table 2 may actually underestimate the amount of CO₂ present in a beer at the end of fermentation. This is because of the so-called supersaturation of CO₂. Although information is scarce, some estimates are that this super-saturation may result in the numbers in Table 2 being exceeded by 20-50%. Supersaturation occurs because fermenters typically have a very smooth surface (especially glass fermenters), which provides very few nucleation sites for bubbles of gas to form on. If a green beer still in a sealed fermenter is swirled to re-suspend yeast (called rousing), the currents in the beer will act as nucleation sites and will simultaneously force the excess CO₂ out of the beer. As long as the fermenter is not opened, there is no risk of oxidation in this process. The CO₂ levels in the beer will also come back to the levels quoted in Table 2. A prominent advocate of this technique within the international homebrewing community is Domenick Venezia, the maker of PrimeTabs (www.primetab.com), a convenient dextrose tablet which, unfortunately, is not readily available in Australia (as far as we know). Domenick maintains that yeast rousing over two or three consecutive days late in fermentation will give you more complete attenuation, whilst simultaneously causing the effusion of excess CO₂. You will then be able to use Table 2 with greater confidence that it represents the correct amount of CO₂ already in your beer.

Variable temperature History

Another possible complication to selecting the correct value from Table 2 could be a complex temperature history of your fermenting/fermented beer. Apart from not allowing for the super-saturation effect, Table 2 assumes 1) a constant fermentation temperature and 2) that bottling proceeds with no other change in temperature. We all know that life is not that simple. Some of us have little control over temperature and are at the mercy of fickle Australian weather patterns, so our beer may experience a five degree Celsius or more change in temperature during the course of fermentation or after fermentation prior to bottling/kegging. Some folk like to cold condition their ales, so should they use the fermentation temperature or the cold conditioning temperature to determine the amount of CO₂ in the beer? Others lager their true lagers at 4 °C or less, where lager yeast still has the ability to slowly ferment. Do they use the fermentation temperature or the lagering temperature? Still others will take their lager through a 18-20 °C diacetyl rest before lagering, introducing three quite different temperature rests. What value should they use?

Considering that less CO₂ can be held in the beer at higher temperature, the short answer is to use the highest temperature the beer has been at since the end of fermentation, since we expect the CO₂ level to come to a new equilibrium if rested at a higher temperature. Dropping the temperature in the absence of active CO₂ production is not going to cause any significant re-dissolution of CO₂ into the beer. There may of course be some fermentation during lagering, the extent of which will depend on the completeness of fermentation prior to lagering, including the amount of dextrins in the original wort. Before and after lagering gravity readings could assist to this extent. If you can detect a point or two drop in gravity, then the lagering temperature may best represent the correct value to determine the CO₂ level in the green beer.

Conclusion

Selecting the correct value for CO₂ in solution is not quite as simple as you may have been lead to believe from previous guides to priming. We hope that we have shed some light on this aspect to assist you achieve more precise and consistent priming. If you do not lager or cold condition your beer, simple yeast rousing should be enough to ensure that you can use the tables and methods to accurately and consistently achieve your desired carbonation level. If you do lager, it may be best to rouse after fermentation or the diacetyl rest and use the final pre-lagering temperature to calculate your dissolved CO₂. It is important that you take careful notes of the temperature stages your beer has been through and adjust your priming levels accordingly.
It should also be evident that there is a fairly broad margin for error when it comes to carbonation, and this is why we have presented both a simpler essential guide and a technical guide to priming. The technical guide will allow you to be as consistent as possible, but the essential guide is probably all that is required, and will give you close enough to the correct level of carbonation. The final step, deciding what the correct level of carbonation is for you, should be relatively straightforward now that you have the information with which to experiment.

References

Mark Hibberd,A Primer on Priming,http://www.brewery.org/library/YPrimerMH.html
George Fix, Principles of Brewing Science, 2^nd Ed, Brewers Publications, Boulder Colorado.
Eric Warner, German Wheat Beer, Brewers Publications, Boulder Colorado.