Perfect Priming & Kettle Souring: Mr. Wizard

Q I can’t seem to find any information that I feel like I can trust on the amount of priming sugar to use if I cold crash my homebrew. I have heard you need less priming sugar, but the calculations I’ve seen haven’t been reliable. Also some say not to worry and it might take a little longer to carbonate. This is probably the most confusing thing I have tried to get info on in almost two years of homebrewing. I don’t keg yet, which really makes it worse because it seems like most people offering knowledge do. I’m just scared of getting bottle bombs or 48 flat beers.

Chris Larivey
Richmond, Virginia

A This topic confused me many years ago for multiple reasons, but mainly because of all of the assumptions and loosey-goosey measurements tossed about in brewing procedures and recipes. The two fundamental questions that must be answered to go about calculating how much priming sugar to add to beer are: What is the target carbon dioxide content in the finished beer, and what is the current carbon dioxide content? Most homebrewers and commercial brewers in the United States express carbon dioxide content in terms of volumes. Another way to express carbon dioxide is by weight, typically in grams of carbon dioxide per liter. The weight method works easier for doing calculations about carbonation.

So let’s create a scenario here to review the fundamentals. Assume we have a carboy of beer containing 20 liters (5.3 gallons) of beer with 3.2 grams of carbon dioxide per liter of beer (1.6 volumes; 2 g/l = 1 volume), and assume the target carbonation level in the finished beer is 5.2 g/l (2.6 volumes). In order for this to happen, 2 grams (the final 5.2 grams minus the initial 3.2 grams) of carbon dioxide need to dissolve into every liter of our 20 liters of beer during bottle conditioning, meaning we need 40 grams of carbon dioxide. This is really an easy problem to solve if we use dry glucose (aka corn sugar or dextrose) or dry sucrose (aka table or cane sugar) as the priming sugar. Liquid sugars muddy things a bit because they don’t all contain the same amount of moisture and wort, liquid malt extract (LME) and dried malt extract (DME) all contain a mixture of fermentable sugars, unfermentable sugars, and some protein; making for less surety about what is really being added.

Glucose (C6H12O6) has a molecular weight of 180 grams per mole and when fermented into 2 molecules of carbon dioxide and 2 molecules of ethanol, yields 88 grams of carbon dioxide. In other words, 2.05 grams of glucose yield 1 gram of carbon dioxide (180 ÷ 88 = 2.05). Hold onto that number for just a second, please!

Sucrose (C12H22O11) has a molecular weight of 342 grams per mole and when fermented into 4 molecules of carbon dioxide and 4 molecules of ethanol, yields 176 grams of carbon dioxide. In other words, 1.94 grams of sucrose yield 1 gram of carbon dioxide (342 ÷ 176 = 1.94).

If we go back to the problem at hand, we are looking to produce 40 grams of carbon dioxide during bottle conditioning. Using glucose as the priming sugar we need 82 grams of glucose (40 grams CO2 x 2.05 grams glucose/ gram CO2) or 77.6 grams of sucrose (40 grams CO2 x 1.94 grams sucrose/gram CO2). These numbers are right in line with the online calculators I found. For this calculation to be completely correct, the moisture content of the sugar (typically 5-8%) needs to be factored in. Assuming 5% moisture bumps these weights to 86 grams and 82 grams.

There are a few difficulties with all of this. The first thing is being able to estimate the initial carbon dioxide content of your beer. All of the online calculators require you to input your beer temperature. This is used to determine the initial carbon dioxide content. Since carbon dioxide solubility increases as beer is cooled, decreasing your beer temperature will increase the carbon dioxide content. This is true if the headspace above the beer contains carbon dioxide. If you take beer at 68 °F (20 °C), for example, and cool it down after fermentation is complete, the carbon dioxide content cannot and will not increase if the headspace above the beer is not carbon dioxide. Most homebrewers who ferment in carboys or plastic buckets do not flow carbon dioxide gas into the headspaces of their fermenters upon crash cooling and what happens is that the fermenter sucks in air from the atmosphere as the beer chills and very little carbon dioxide pick-up is possible because there is simply not much (by weight) in the headspace to begin with.

If you use Corny kegs or a stainless steel fermenter for cold crashing and allow carbon dioxide to flow into the vessel during crash cooling things are different. But to assume that beer temperature is a good indicator of carbon dioxide content is a faulty assumption because it takes time for the beer below the carbon dioxide headspace to equilibrate with the headspace. This is why it takes so much longer to force carbonate beer using headspace pressure versus bubbling carbon dioxide through a stone or shaking the keg during carbonation. And keep in mind the online calculators are basing the initial carbon dioxide content on no head pressure above atmospheric (0 psig carbon dioxide pressure). I am not criticizing the assumptions used by these calculators because there must be a method to estimate initial carbon dioxide content and temperature absolutely affects this value, and all of the calculators that explain the methodology discuss this conundrum.
Without actually measuring the carbon dioxide content of your beer before bottle conditioning, all you can do is make an educated guess. It seems to me that using the fermentation temperature as the baseline for this makes the most sense as this is when the beer’s initial carbon dioxide concentration was established. If the beer is warmed or cooled for a few days you know the content is going to change, but you just don’t know how much. One online calculator suggests “tweaking” the temperature to be some sort of reflection of a weighted average. I can see doing this if you ferment a lager at 50 °F (10 °C) and warm it to 68 °F (20 °C) for a diacetyl rest for a week because this condition results in a reduction in carbon dioxide. Assuming that an ale picks up carbon dioxide when cold crashed is another story, as explained above, and you are probably best off not assuming much carbon dioxide gain when cold crashing unless you are adding carbon dioxide to the headspace and holding the beer cold for several days.

We do reader surveys here at BYO and I know that there are some scientists and engineers gritting their teeth about some of my rounded numbers earlier and my typical aversion to using decimals, although I have used a few in the above examples! My results are so, so close to the results from the online calculators that I am satisfied that my explanation of the problem you present is solid. And I know some people are thinking “what about the headspace volume!” Well, just don’t grossly under-fill your bottles or kegs and this can be blissfully ignored. And this brings us to that loosey-goosey stuff I referenced earlier.

What drives me bonkers is when a recipe makes the assumption that a known volume of beer is being carbonated. And add to this the arcane volumetric measurements often used to express the amount of priming sugar required! In order to have any hope of consistently bottle conditioning beer you absolutely must have two tools in your battery of brewing implements; a calibrated vessel and a decent scale capable of displaying grams to a tenth of an ounce.

The easiest, and perhaps the cheapest, way to calibrate a vessel is to slap a piece of tape on the side of a carboy or bottling bucket and add water one liter a time so that you can place volume marks on your tape. This sort of calibration tape will last a very long time if you are careful with how you handle your vessel. Knowing your beer volume and the amount of sugar required, coupled with an accurate means of weighing your priming sugar, is going to get you far closer to your end result.

And the very last bit of advice about bottle conditioning is to consider the age of your beer/yeast at the time of bottling. If you have used extending aging in the secondary your yeast viability may be low. A little pinch of happy yeast added to the bottling bucket is something to consider, and is certainly something that nearly all commercially brewed, bottle-conditioned beers have had added before bottling.

Q There is a local brewery that kettle sours some of their beers and I have enjoyed their sours for their clean taste. I was looking to brew a Gose and thought about trying this technique. Is kettle souring just as simple as brewing up through the sparge, then pitching the Lactobacillus into the kettle? Also, how will I know when it is done?

Dan Green
Boston, Massachusetts

A It seems that many sour beers are being brewed these days that are clean, tart, and a great base on which to layer other flavors. At the 2015 Great American Beer Festival held in Denver, Colorado, I had some really nice, clean sours flavored with a wide range of fruits, herbs, and spices. I know for sure that some of these beers were brewed using the kettle sour method that you succinctly describe. That is, wort inoculated with Lactobacillus, held warm for a period of time to sour and then boiled. One of the alluring facets of this method is that the bacteria used to sour the wort are killed by boiling, and before the wort is cooled and transferred to fermentation. This is especially appealing to commercial brewers who want to brew sour beers, yet are not keen on turning their entire cellar operations into a funk factory.

At a very basic sense, kettle souring has a lot in common with making yogurt because both methods benefit from an incubator of sorts to keep things in the 85–110 °F (29–43 °C) range. I suggest reading about home yogurt making to get ideas on cost-effective incubators. My experience with kettle souring is on a larger scale and I will pass on some of the techniques being used by commercial breweries.

I know of three craft breweries using the same basic method to produce kettle sour-type beers. These brewers all have a special fermenter equipped with tank heating using hot water/glycol flowing through the tank jacket, much like a normal fermenter being chilled with glycol. The technique is to produce unhopped wort as if the plan was to boil the wort and add hops. But instead of bringing the unhopped wort to a boil, it is cooled to about 100 °F (38 °C), inoculated with Lactobacillus and maintained at the ideal temperature for the strain being used (usually in the 100–110 °F/ 38–43 °C range) for 24–72 hours. By the way, hops inhibit the growth of lactic acid bacteria and it is important to sour unhopped wort.

During this time period lactic acid is produced and the pH of the culture drops. Since the principle product of the reaction is lactic acid, pH is a useful method to monitor progress. Tasting is also very useful to monitor progress. The soured wort can be used as the sole source of fermentables in a beer or it can be blended in with normal wort. A commercial example that explains this process on their website is New Belgium’s Snapshot Wheat.

There are a couple of things about Lactobacilli that help explain how brewers use these organisms in brewing. The first is that Lactobacillus species are anaerobic bacteria that are not harmed by oxygen and are considered aerotolerant anaerobes. In contrast, facultative anaerobes, such as yeast, are able to use aerobic metabolic pathways in the presence of oxygen and anaerobic metabolic pathways in the absence of oxygen. The fact that Lactobacilli are aerotolerant anaerobes means that soured food products, such as sauerkraut, kimchi, and cultured dairy products are easy to ferment in an open container. When wort is soured by adding malt to wort in the 86–113 °F (30–40 °C) range, aerobic spoilage bacteria can also grow and produce volatile fatty acids, such as butyric acid, along with other unpleasant aromas. The easiest way to suppress the growth of aerobic wort spoilers is to create an anaerobic environment.

Another important thing to know about Lactobacilli is that some are homofermentative, meaning that lactic acid is the sole product of glucose fermentation, and some are heterofermentative, meaning that lactic acid is not the only product of fermentation. Heterofermentative species produce lactic acid, alcohol, and carbon dioxide from glucose and oftentimes kick out acetic acid. Lactobacillus delbrueckii is a homofermentative strain and Lactobacillus brevis is a heterofermentative strain.

In the German brewing tradition, Lactobacillus delbrueckii is the primary lactic acid producer in Berliner weisse and is also used to produce sour mash to adjust mash pH in the biological acidification process and to produce lactic acid for sauermalz (sour malt or acidified malt). The latter two uses specifically relate to mash pH adjustment and the Reinheitsgebot. The traditional way of brewing Berliner weisse is to make sour wort in a dedicated vessel and then blend with regular wort to a target pH prior to boiling, so that the final beer pH is between 3.2 and 3.4. Not all brewing yeast strains tolerate this low pH and it is important to consider this when selecting yeast.

Lactobacillus delbrueckii will also produce that clean canvas to paint your Gose atop. If you make sour wort using the method described above and add the spices and salt to the kettle boil you have laid the groundwork for a solid Gose. Yeast selection with this style is important and all of the normal rules apply downstream of wort production.

Heterofermentative species are more commonly found in funkier styles as these bacteria produce more than just lactic acid. I am not sure what lactics traditionally grow in Gose wort, but using a heterofermentative bacteria could make an interesting contribution to your Gose. It seems that heterofermentative species are more popular than species that only produce lactic acid.

Q I want to carbonate my beer in my chest freezer kegerator, but I do not know which temperature to use for the carbonation chart. The air temperature at the bottom of the kegerator reads 42 °F (5.5 °C) while the top reads 46 °F (8 °C). Which temperature do I use when carbonating? Alternatively, should I use the temperature found on the outside of my keg instead?

J.R. Gradel
Hartland, Wisconsin

A When you carbonate beer you need to use the actual beer temperature to determine the head pressure required to hit your target. The easiest way to do this in a small keg is to measure the skin temperature of your keg somewhere in the middle. This will get you really close to the average temperature of the contents.

Things are pretty simple in a small environment with minimal variation in temperature from top to bottom. Larger tanks have a larger temperature variation from top to bottom and the hydrostatic head at the bottom of a tall tank can be a real factor since 33 feet (10 m) of liquid represents 15 pounds of pressure. This is a hairier problem that you fortunately are not wrestling!

Issue: December 2015