A Guide To Bulk Priming

This page provides the essential information needed to carry out bulk-priming. Technical details have deliberately been kept to a minimum in the first section and progresses to the more technical discussion. (Dissolved CO² levels will increase with lower temperature and decrease with higher temperature)

The following procedure assumes we are working with a beer fermented at around 20 °C. A difference of plus or minus a couple of degrees won’t matter much, but more than around 5 °C certainly will. You will have to see the technical section for more details on temperature effects and how to consider them.

Procedure

• Measure the required quantity of priming sugar as determined from Table 1 (we assume here that you are using dextrose).
• Add to saucepan with only enough water to dissolve it (say, 200 mL).
• Bring to boil for a minute or two.
• Add to beer a few minutes prior to bottling and gently stir to distribute evenly. If using a bottling bucket, add the priming sugar as or just before you commence racking – the swirling wort will mix the sugar. You can add it directly to the primary fermenter, but then you also risk stirring up the yeast cake. Be Careful to avoid oxidation
• Don’t forget to sanitise the bottling carboy and any equipment that contacts the beer. Take care not to splash the beer when racking as this will cause oxidation.

How much dextrose to add (grams)

The most important column is the ‘Rate of dextrose to add’; – multiply this by your volume of fermented beer (in liters) to obtain the total quantity of dextrose to add.

** NOTE ABOUT LAGERED BEER (COLD STORED) Before bulk priming:

When a fermented beer has been cubed and cold stored for a lengthy period (more than a few weeks or so) at the time of bulk priming and racking to bottles, it may be necessary to re-seed the batch with a small culture of yeast to achieve effective carbonation in a reasonable time frame.

To do this, we'll need a small amount of a highly floculent yeast strain such as Fermentis S23, Mangrove Jacks M10 "Workhorse" or Fermentis S33.. A reasonable dose for a 25 Litre batch is to rehydrate 5g of yeast in 50ml of water. After 20 minutes when yeast has creamed, stir well and draw off 15ml using a sterile syringe or measuring pipette. Add this to the batch to your bulk priming vessel after the first few litres of beer have been transferred into it . There will be sufficient yeast in the 15ml of solution to provide for a clean bottle fermentation of the bulk primed batch. If preferred, you can add 0.5ml using an insulin syringe to each 750ml bottle and then simply fill each bottle with the primed beer.

The alternative is to simply rouse the yeastin the secondary fermentation vessel each day for a few days, to ensure sufficient yeast is suspended in the beer solution.

Table 1. Guide to Dextrose Addition for Bulk Priming

This table assumes you have a beer fermented at 20°c

• Use "high" to replace standard priming practices. Good for most Australian style lagers (230g), pale ales (160g), and so on. Use “High” for Weizens and some Belgian styles. Use “Low” or "medium" for English style ales. Stouts are best served at a low-medium carbonation as well. Or use whatever you like to suit your taste: it’s your beer!

For sugars other than dextrose:

The most predictable priming results are obtained from simple fully fermentable sugars such as dextrose or sucrose, but for if you like to experiment here is a guide for adjusting the quantity of priming sugar depending on the type you use. Start with the numbers in Table 1 then adjust as follows:

• Table sugar (sucrose) – decrease numbers by 10% Increase the boiling time to 20 minutes (more impurities than dextrose)
• Dry malt extract – increase by 20-25% (this depends on the brand and may take a little trial and error)
• Liquid malt extract – increase by 40% (this depends on the brand and may take a little trial and error)
• Honey – increase by 50%

The topic of carbonation deals with the dissolution of carbon dioxide CO₂ , 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 behavior 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 atoms, 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 at sea level of 1013 hectopascals, 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 ( also see note on super-saturation.)

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.

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 (without oxygen) conditions the major products are ethanol and carbon dioxide (CO₂) (there are several other compounds in tiny amounts as well). 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 so simple because some glucose goes into producing by-products other than CO₂ and ethanol; mostly these by-products go into building yeast cells.

Fermentation is the conversion of a simple sugar (such as Glucose AKA α-Dextro-Glucose) This process takes 1 mole of glucose and yields 2 moles each of ethanol and carbon dioxide.  This is the formula C₆H₁₂O₆→2(C₂H₅OH)+ 2 (CO₂). Note that molecular weight of Glucose is 180 (1 mole glucose =180g) and that of ethanol is 46 (i.e. 46g per mole of ethanol)

So  [(180g/mol of glucose) x (1 mol of glucose)]             180g of glucose

      ——————————————————                =      ——————–                 = 1.9565

      [(46g/mol of ethanol) x (2mol of ethanol)]                   92g of ethanol

Therefore to produce 1 gram of ethanol requires 1.9565 grams of glucose in a perfect world. However we are utilizing yeast to carry out the fermentation and they need some of the glucose for their biological processes. It has been shown to be more like 2.0665g of glucose per gram of ethanol by Carl Jose Napoleon Balling who used empirical measurements to determine the amount of CO₂ formed from glucose his measurements showed that:

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 most commonly sold as dextrose monohydrate, which means that one water molecule is bound to each glucose molecule, (one molecule of water of crystallization) 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 a similar metabolic pathway to 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.

Some benefit can be gained for inverting sugar if used for priming (brewing?). This is achieved by boiling sucrose and water solution, the hydrolysis reaction being catalyzed by the addition of an acid such as Citric or Ascorbic acid at the one gram per kg or lemon juice at 10ml/kg.  This breaks sucrose ( a disaccharide) into its constituent mono-saccharides Glucose and Fructose. Fructose and Glucose are both able to pass directly into yeast. Sucrose is unable to pass directly into yeast and must be inverted by the yeast. It is able to pass the cells outer wall. This means the enzyme Invertase is released into the periplasmic space between the inner and outer wall of the cell to break sucrose down (there being no sucrose permease in brewers yeast similar to the Maltose permease mechanism for Maltose uptake)

Table 4 shows 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) / 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.  Gentle yeast rousing over two or three consecutive days late in fermentation may give you more complete attenuation, whilst simultaneously causing the separation and escape of excess CO2 out of the fermentation vessel . You will then be able to use Table 2 with greater confidence that it accurately represents the amount of CO2 saturation 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.

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 sugar (or other) 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 straight forward now that you have the information with which to experiment.