Brewing Sour Beers: A Kettle Souring Primer

by Todd Stephens

Brewing sour beers has always been a challenge due to the potential of contaminating one’s brewing equipment with the microbes used to sour wort. Many brewers dedicate separate equipment for fermenting, transferring, bottling, or kegging clean and sour beers, which can get expensive and take up a lot of space. Although the risk of cross-contamination can be minimized by using strict sanitation procedures, it only takes a small number of living microbes from a sour batch to ruin a clean beer, so the margin of error is very small. Also, the time required for traditional development of acidity in a sour beer can range from several months to years, requiring space in a somewhat temperature-controlled area to store batches while they develop.

Kettle souring trades complexity and flavor development for quick turn-around. (photo by Ron Caspi)
Kettle souring trades complexity and flavor development for quick turn-around. (photo by Ron Caspi)

The technique of kettle souring has become more popular with its ability to turn around a batch of sour beer in just a few weeks without the risk of cross-contamination, using equipment most brewers already own. All the souring is done in the boil kettle prior to boiling, so the bacteria are killed before the wort is transferred into the fermenter. These quick-soured beers do not display the range of complexity of an aged Belgian Lambic or Flanders red, and do not evolve over time in the bottle, but sometimes a simple, light, tart beer can be the perfect thing on a hot summer afternoon.

Kettle souring is not an advanced or difficult process. There are a couple of tricks to improve the final product, but it’s really a fairly simple task. At a high level, the process of kettle souring starts with the production of wort like any other all-grain batch through mashing and sparging grains, or for an extract brewer, by dissolving the malt extract in the appropriate amount of water. The wort is then inoculated with lactic acid bacteria and allowed to sour until the desired sourness is reached over the course of several hours to a few days. The wort is boiled to kill off the lactic acid bacteria and finished in the fermentor with clean brewer’s yeast. Since the fermentation is done exclusively with brewer’s yeast, the beer can be packaged and served in the same time frame as any other clean beer. Also, the sourness level is locked in at the time of the boil, so the brewer has greater control over the finished product.

Beer was traditionally soured pre-boil via sour mashing, which uses the naturally occurring bacteria on the grain husks to create lactic acid. This process has a well-deserved reputation for creating horrible smells, due to the multitude of other microbes present on the grain which could lead to spoilage before the lactic acid bacteria could drop the pH sufficiently to prevent off-flavors and toxins from forming in the wort. In the presence of oxygen, naturally occurring bacteria on the grain can produce isovaleric acid, which has a distinct aroma of parmesan cheese or sweaty socks, or butyric acid, which has an aroma of bile. If done properly these issues can be avoided, but the process of kettle souring wort eliminates these risks by controlling the microbes used to sour.

For quickly souring pre-boil wort, a culture of lactobacillus must be used. A culture can be made from grain, probiotic drinks and capsules, or a cultured dairy product containing live cultures, such as sour cream or yogurt. A pure culture can also be purchased from any of the major yeast laboratories. Commercially available lactobacillus species include lactobacillus delbrueckilactobacillus buchnerilactobacillus brevis, and lactobacillus plantarum. Of the four, l. brevis and l. plantarum are probably the most reliable for quick souring. The metabolism of lactic acid bacteria is not as well documented as for brewer’s yeast, but there is some evidence that lactobacillus multiplies until about pH 3.8, at which point the lactic acid inhibits cell growth. As such, making a small 1-liter starter to increase the number of active cells will help ensure quick souring of the wort.

Kettle souring beer a style that is meant to be consumed young and utilizes a fairly easy technique. (photo by Ron Caspi)
Kettle souring beer a style that is meant to be consumed young and utilizes a fairly easy technique. (photo by Ron Caspi)

What do heteros and homos have to do with beer?

Lactic acid bacteria can be classified as heterofermentative or homofermentative. In reality it is a little more complicated than this, but as long as the bacteria are not exposed to oxygen during growth, then they can be considered one or the other. Homofermentative means that the bacteria eat sugar in the wort and produce only lactic acid. Heterofermentative means that the bacteria convert sugar to both lactic acid and ethanol. Many brewers have noticed large krausen and significant gravity drop from using pure cultures of lactobacillus, and it was assumed that this was due to heterofermentative lactobacillus. Some new research, however, has shown that pure cultures of heterofermentative lactobacillus under controlled conditions can only create fractions of a percent of alcohol. The natural explanation for this is that many commercial bacteria cultures available to homebrewers are often contaminated with yeast, which is responsible for the drop in gravity and the creation of alcohol.

It’s important to know whether your lactic acid bacteria culture creates alcohol, because if it does it will produce carbon dioxide and a large krausen in the souring vessel, and may require a blow-off tube. Also, ethanol produced by yeast in a contaminated lactobacillus culture will evaporate during the boil, so this may affect your decision to boil the wort after souring or just pasteurize it. If your bacteria starter shows any bubbling or airlock activity, then it is safe to assume that there is yeast in the culture. Although not guaranteed, lactic acid bacteria cultured from grain husks will more than likely be contaminated with wild yeast, so some ethanol will probably be produced during souring.

Pre-acidifying the wort to a pH of 4.5 has some advantages when kettle souring. This can be done with food-grade lactic acid, and should happen after collecting all the wort in the boil kettle so that the mash pH can be kept in the proper range. Ideally this would be measured with a calibrated pH meter, but brewing water calculators can be used to get close. When using wild lactic acid bacteria from grain, dropping the pH to 4.5 will inhibit the growth of unwanted microbes and reduce the risk of off-flavors and spoiling the wort before the lactobacillus can get to work. In addition, some lactic acid bacteria produce an enzyme that can break down the proteins that aid in foam retention. These enzymes are only active around a pH of 5, so pre-adjusting the pH to around 4.5 will help in producing a sour beer with good head retention.

But that’s enough science and background information.

Here’s how to do it.

The pre-boil wort can be collected in the boil kettle the same way it is done on any other brew day, through lautering and sparging. A mash-out in the 170-180F temperature range should bring the grain bed up to pasteurization temperatures, but if a completely sanitary wort is desired it can be brought up to a short boil in the kettle, or held at 170F for 15 minutes or so to pasteurize the wort. Keeping the wort below 180F will also prevent any DMS from forming. Don’t add any hops yet – hops only serve to inhibit the growth of lactobacillus, which is counter-productive in quick souring. If some hop presence is desired, they can be added during the boil later.

The wort should be cooled to the optimal growth temperature for the type of lactobacillus used, which is 80-100F for l. plantarum, or 110-120F for other lactobacillus cultures. Especially for wild cultures, keeping the temperatures at the higher end of the range will favor the growth of the lactobacillus and inhibit the growth of spoilage bacteria. Third, the wort should be adjusted to pH 4.5 and the lactic acid bacteria culture pitched.

At this point:
Oxygen is the enemy. If a wild lactobacillus culture is used, oxygen exposure can lead to butyric or isovaleric acid. Some pure cultures, such as l. plantarum, can produce acetic acid if exposed to too much oxygen, so it’s best to take steps to minimize it. If left in the kettle, the headspace can be purged with carbon dioxide and sealed with tape and plastic wrap, and a layer of plastic wrap placed directly on the surface of the wort to keep out oxygen. The wort can also be transferred to a carboy with an airlock. Wrapping the vessel with a thick blanket or sleeping bag can help maintain temperature during the souring period, although the mass of the wort may be high enough that it will hold temperatures long enough to sour. Depending on the lactobacillus species, this may be anywhere from 12-24 hours for l. brevis and l. plantarum, or a few to several days for l. buchneri or l. delbruecki. Knowing whether your variety of lactobacillus is heterofermentative will also help avoid a mess from a blown-out lid or airlock.

Once the desired sourness is reached, the souring is stopped by heating to pasteurization temperatures to kill the bacteria, and can be boiled and hopped as any other brew. If a lactic acid bacteria culture containing some yeast was used, the presence of ethanol in the wort after souring may affect whether you boil the wort after souring or just pasteurize it, as the ethanol will certainly evaporate during the boil. If a significant gravity drop is measured, pasteurizing at 165-170F for 15 minutes will minimize the amount of ethanol lost, as ethanol boils at 173.1F. For styles such as Berliner Weisse and Gose that do not have a significant hop presence, pasteurizing the soured wort and cooling to yeast-pitching temperatures is a good approach, and boiling the wort is not necessary. For styles that do require some hop presence, such as porter or saison, the wort can be boiled and hopped as normal.

Since the low pH and oxygen level creates a somewhat hazardous environment to yeast, pitching the yeast at double the normal rate is a good idea and will help ensure a good fermentation of the sour beer. Some yeast varieties are more tolerant than others of acidic environments. For liquid yeasts, acid tolerant strains include saison and kolsch yeasts, and for dry yeast US-05 works well. This is certainly not a complete list – many varieties can perform well in an acidic environment, but if in doubt, these examples should work fine.

And that’s it. Kettle soured beers can usually be bottled or kegged after two or three weeks, but as always, let the hydrometer be the real guide. Had the souring bacteria not been killed by boiling the wort, the fermentation time could easily stretch out to several months as wild yeast and bacteria can slowly ferment longer-chain sugars over time leading to over-carbonation and possibly fracturing bottles. The real trade-off with kettle souring is complexity and flavor development for quick turn-around. For a style of beer that is meant to be consumed young, though, this is a fairly easy technique in your toolbox to add a different dimension in your homebrewed beers.