Priming Bottled Beer for Consistency and Reproducibility

The Prime Directive

by Dave Draper and Mark Hibberd (Brewing Techniques - Vol. 4, No.4)

Carbonation level is one of the most readily identifiable features of any beer. A simple method enables home brewers to bottle with the same control over carbonation as those who keg.

Put a glass of beer in front of just about any drinker, from the least to the most experienced, and one of the first things that will be noticed after taking a sip is the level of carbonation. The image of bubbles climbing through the beer to form a head is in fact what virtually everyone pictures at the mention of the word “beer.”

But carbonation is more than just a matter of image. A beer’s carbonation also influences a person’s sensory perception of it, from aroma to mouthfeel to flavor. Some beer styles are conventionally carbonated to specific levels, and these carbonation levels actually help define those styles. Most English pub bitters, for example, are carbonated to fairly low levels; in contrast, European lagers and wheat beers are characterized by much higher carbonation levels.

As craft brewers, we generally seek to achieve certain stylistic targets and to maximize the favorable impression that proper carbonation can give. To succeed in this finishing touch, we need a reliable way to produce specific carbonation levels in our beers. On an even more basic level, good priming techniques are important if only to be sure that our beers are neither so poorly carbonated as to be flat, nor so overly carbonated that bottles might explode!

Brewers who dispense from kegs have the luxury of virtually complete control over the carbonation levels of their beers. For those of us who bottle, however, the results are typically less precise. This article describes a simple method for bottling homebrew, a method that can control carbonation levels much more accurately than most think to be possible. First we present one simple change that can help you gain considerable control over bottle conditioning results — measuring priming sugar by weight rather than by volume. Next we cover some of the basic chemistry by which carbon dioxide is dissolved in beer during fermentation and produced during bottle conditioning. This basic background can then be applied to calculating the priming rates needed to produce desired amounts of carbon dioxide in the finished beer. Finally, these relationships can be combined into a set of simple, easy-to-use charts and equations for rapid determination of the amounts of priming sugar needed.

Measuring by Weight, not Volume

Most brewing textbooks instruct brewers to prime their beers with an amount of priming sugar figured on a volume-per-volume basis. The level most commonly cited is ¾ cup per 5 gal. This approach can create several problems. Not all sugars are manufactured the same way; for example, ¾ cup of one type of sugar may yield a very different amount than ¾ cup of another. In addition, should the sugar be packed in the cup or kept loose? Dick Dunn reported via e-mail that corn sugar (equivalent chemical names are dextrose and glucose) can compress up to 30% with just a gentle tap. Clearly, measuring by weight is a more accurate way of getting predictable, reproducible results.

If you measure sugar on a weight-by-volume basis, however, you might ask whether sugar’s adsorption of water from the atmosphere could bias the results; that is, wouldn’t much of the weight be accounted for by adsorbed water? This theory was tested, and the results strongly suggest that water adsorption produces negligible effects (see box, top left).

It may be true that if you brew a style of beer the same way every time, and tweak the recipe enough times, you will eventually arrive at a set of ingredient amounts — and priming rates — that work. Thousands of home brewers have started out by using the venerable ¾ cup per 5 gal rate with no complaints. When entering uncharted territory and exploring new styles, however, you can reduce your trial and error by using methods based on first principles. If you are a brewer who derives satisfaction from hitting stylistic targets squarely, or are looking for an added degree of authenticity to excel in competitions, then this method is for you.

Carbonation in the Bottle

Carbonation is most usefully described and measured in terms of volumes of carbon dioxide. A beer carbonated to 2 volumes would have 2 L of carbon dioxide in every liter of beer. (For a more complete discussion of volumes of carbon dioxide, read Dave Miller’s Brewing the World’s Great Beers [1].) Before examining the chemistry of carbonation, let’s first look at the carbonation levels of various beer styles (see Table I). Most authors of beer texts agree on the guidelines to carbonation levels shown in the table.

Carbon dioxide content before bottling: Beer that has just finished fermentation already contains a significant amount of dissolved carbon dioxide. To achieve specific carbonation levels, you first need to know how much carbon dioxide is already present in your beer at that point. Then when you prime the beer, you add only enough carbon dioxide to achieve the desired level for the particular style you are bottling. Thus, our first task is to find out how much carbon dioxide is in the beer before we add any sugar at all.

Beer that is ready for bottling has had carbon dioxide bubbling through it more or less continuously throughout primary and secondary fermentation. As a result, it is very close to being saturated with carbon dioxide. If the beer sits for weeks after the final fermentation, however, its carbon dioxide content will drop as the dissolved gas slowly exsolves out (essentially the opposite of dissolving in). In the reasonably well-sealed fermentors used by most home brewers, this drop is of minimal concern. For our purposes, we assume that the beer is being bottled soon after fermentation is complete and that most of the dissolved carbon dioxide is still present.

The maximum amount of carbon dioxide that can be dissolved into beer (which chemists refer to as the solubility of carbon dioxide) is strongly dependent on the beer’s temperature (at a fixed pressure, namely atmospheric). The lower the temperature, the more carbon dioxide can be dissolved. Because we are interested in the amount of carbon dioxide in the beer at the time of bottling, we want to know the temperature of the beer during fermentation. If the beer has been sitting for more than a few days at a different temperature, however, then that temperature is the one to note. Table II lists the number of volumes of carbon dioxide present in beer at particular temperatures, before any priming sugar has been added. Interestingly, the solubility of carbon dioxide is independent of the beer’s gravity or chemical composition.

Bottle conditioning and carbonation: A reaction that produces carbon dioxide during carbonation can be written as follows:

C₆H₁₂O₆ + yeast = 2CH₃CH₂OH + 2CO₂

This equation says that yeast will convert one unit of glucose (C₆H₁₂O₆)* into two units of ethanol (CH₃CH₂OH) and two units of carbon dioxide.^† The “units” in this case are units of measurement that chemists refer to as moles. When we look up the numbers and do the background math, we find that we add 1 volume of carbon dioxide for every 0.49 oz per gal (3.7 g/L) glucose added to the beer. As an example, if we have 5 gal (19 L) of beer and we want to add 1 volume of carbon dioxide to it, we need 5 X 0.49 = 2.45 oz (19 X 3.7 = 70 g) of glucose.

A Simple, Precise Method for Predicting Carbonation

Armed with the temperature dependence data in Table II, which shows how much carbon dioxide is present before bottling, and the equation for determining the amounts of sugar needed per volume of added carbon dioxide, we can make general predictions for use in our brewery. The information can be combined into a set of diagrams and simple equations for use at bottling time.

One potential complication of this method, which should be mentioned for the sake of completeness, is that we use fermentation rather than serving temperature to calculate the amount of carbon dioxide to add. This complication, however, is more theoretical and should be of negligible significance in practice. Serving temperatures of cold-conditioned beers such as lagers are typically close to cold-conditioning temperatures, so the actual carbonation levels should be very close to the target levels. For ales, where serving temperature might be 10 or 15 °F (5–10 °C) cooler than fermentation temperature, you might expect the actual carbonation level to be less than anticipated when you open the bottle. At the lower serving temperatures, the beer has greater capacity for holding carbon dioxide. If, however, the ales are conditioned at temperatures close to those at which they were fermented and then chilled just before serving, the carbon dioxide has hardly any time to redissolve into the beer. In fact, it would probably take days for this extra carbon dioxide to redissolve. In any case, it is likely that any temperature fluctuations would be compensated by slight changes in the pressure that had built up in the headspace of the bottle. The carbonation levels for the styles listed in the table may already take some of these potential effects into account. In our experience, these concerns have been quite trivial, and we routinely ignore them with perfectly good results.

Getting graphic: We said at the beginning of this article that this method was easy, and it is, especially with the help of a simple graph. Figure 1 shows how many volumes of carbon dioxide will be produced in finished beer when it is primed at the level found on the x- (horizontal) axis. Each line is labeled for the temperature of the beer at the time of priming and incorporates the amount of carbon dioxide present before priming. To use the graph, choose a desired carbonation level from the y- (vertical) axis, find the line that corresponds to the beer’s temperature, and read off the appropriate priming rate beneath.

Example 1, carbonating a lager. We pull a lager from cold conditioning at 39 °F (4 °C) and want it carbonated at 2.75 volumes. We find 2.75 on the y-axis, move right until we hit the 39 °F (4 °C) line, and then move down to the x-axis for a reading of about 0.6 oz/gal (4.5 g/L). To get the same level of carbonation in a beer at 68 °F (20 °C), the graph tells us to prime it at about 0.93 oz/gal (7 g/L) because of the lower amount of carbon dioxide present in the beer before priming at that higher temperature.

Example 2, carbonating a bitter. A pub bitter has just finished fermentation at 61 °F (16 °C), and we want it carbonated at 2 volumes. Reading the graph, we find that we’ll need to use about 0.49 oz/gal (3.7 g/L).

Playing by the numbers: The lines on Figure 1 can be expressed as equations as well. To calculate the priming rate, first find the saturation level for the temperature of your beer at the end of fermentation (Table II). Let’s call it V₀. Then choose the volume of carbon dioxide that corresponds to the desired carbonation level — let’s call that V. That’s all we need to complete the following equations:

priming rate (oz/gal) = (V – V₀) x 0.49

or

priming rate (g/L) = (V–V₀) X 3.7

You can confirm these equations with the lager and bitter examples above. In example 1, V₀ = 1.5 at 39 °F (4 °C), and V= 2.75. Applying the equation tells us that (2.75 – 1.5) X 0.49 = 0.61 oz/gal (4.6 g/L), which matches the result from the graph. In example 2, V₀ = 0.99 at 68 °F (20 °C) and V = 2.00; (2.00 – 0.99) X 0.49 = 0.49 oz/gal (3.7 g/L).

Priming with sucrose: Sucrose (table or brown sugar) produces more carbon dioxide for every ounce per gallon (or gram per liter) of priming sugar compared with corn sugar. The results for sucrose are directly analogous to the volume and temperature relationships described above — the numbers are slightly different, and the lines on the graph are displaced to different values, but the same relationships are maintained between them. To add 1 volume of carbon dioxide with sucrose, we need only 0.46 oz/gal (3.5 g/L), less than the amounts needed for glucose. The equations to use if priming with sucrose are:

priming rate (oz/gal) = (V – V₀) X 0.46

or

priming rate (g/L) = (V – V₀) X 3.5

Hitting the Target

Carbonation levels not only help to define styles but are also matters of personal preference. Our method enables brewers to achieve desired carbonation levels without resorting to guesswork. Because the effect of carbonation is so important to the overall impression of a beer, it makes little sense to take a chance on having the carbonation levels come out differently than desired. It is easy to predetermine how carbonated a beer will be when you measure your priming sugar by weight and add the precise amount for the style. Once we know what a given number of volumes of carbon dioxide “feels like,” we can aim for — and achieve — those target levels of carbonation in our beers.

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