Kettle souring

"Kettle souring" is a method of creating sour beer. It is a form of pre-souring — souring the wort before yeast fermentation. Souring is achieved by adding lactic acid bacteria (most often Lactobacillus) to the wort in the kettle, allowing the bacteria time to create lactic acid, and then boiling before the subsequent yeast fermentation. This process is designed to avoid possible contaminants from coming into contact with cold-side equipment.

We consider kettle souring to be an antiquated process without any benefits compared to modern souring methods.
See here: Comparison of fast souring methods.

Pre-sour method (kettle souring):

1. Make unhopped wort.
2. Optional/recommended: Bring the wort to boiling temperature for up to 10 minutes.
3. Chill the wort to 100°F (37°C) or less.
4. Optional/recommended: Pre-acidify the wort to between pH 4.0-4.5 (measured at room temperature), which helps with head retention and helps avoid growth of certain contaminating microbes.
5. Pitch the bacteria.
   • A very high pitch rate is recommended, preferably using a Lacto starter.
   • L. plantarum and L. brevis are the preferred species.
6. Seal the kettle to prevent airborne contaminants from getting into the wort.
   • With good sanitation, pre-acidification, and pre-boiling, it is not necessary to "purge" with CO₂.
7. Allow it to sour for 12-48 hours.
   • L. plantarum requires maintaining temperature above 70°F (21°C). Higher temperatures up to 98°F (37°C) will speed up souring.
   • Other Lacto species require maintenance of a higher temperature (refer to recommendations for the particular culture).
8. Optional/recommended: Monitor pH. Generally the target pH is within 3.1 to 3.8.
9. Optional/recommended: Measure specific gravity after souring. If the s.g. drops more than a couple points, that's a sign of yeast contamination.
10. Boil the sour wort as normal, adding any hops desired. Alternately, the wort may be brought up to 180°F (83°C) to pasteurize.
11. Chill and aerate as normal.
12. Higher yeast pitch rate is recommend because lactic acid inhibits yeast growth.
    • Dry yeast should be rehydrated before pitching.
    • Liquid yeast should be pitched from a starter.
13. Ferment and package as normal.

The Good

The kettle souring process can be used to make a particularly flavorless sour beer to serve as a base for other flavor adjuncts. It can also be a useful method if you cannot obtain L. plantarum and don't want to risk using a more hop-tolerant species on your cold-side gear.

Kettle souring is purported to reduce the risk of contaminating future clean beers. That is only true if you use wild microbes or a very hop-tolerant Lacto species as your bacteria culture. However, when using L. plantarum for souring, there is NO risk of cross-contamination even if it is used in cold-side equipment (i.e. not kettle soured).

• L. plantarum is extremely hop-sensitive, meaning that it will have no effect on any beer with hops.
• L. plantarum is easy to remove with normal cleaning and sanitation processes.

The Bad

Pre-souring and then boiling has notable effects on the resulting beer:

• Boiling the wort after souring removes the large majority of flavor compounds produced by the bacteria (besides lactic acid).
• The low pH environment inhibits ester production by yeast, greatly limiting any yeast flavor contribution.

This combination of factors often results in a "one note" sour beer without any complexity. This lack of flavor is considered undesirable by most craft beer drinkers.

Furthermore:

• The kettle souring process (described above) is much more complicated than other souring methods.
• Kettle soured beers take a couple days longer to produce due to the separate souring phase and longer fermentation (from the acid inhibiting the yeast).

The Ugly

Mold on a kettle sour

During the pre-souring phase (in the kettle), the wort is particularly vulnerable to contamination. Unhopped wort is an excellent growth medium for wild microbes — it easily becomes contaminated and sometimes ruined during this phase of the process.

Contaminated kettle sours are common. Here are some examples:

• Is it yeast contamination?(during kettle souring) HBT thread
• Pellicule in my sour kettle?? HBT thread
• White film on 1 gallon all grain? HBT thread
• Fixing a kettle sour with a yeast infection... HBT thread
• Lacto Kettle Sour Yeast Contamination HBT thread
• Saving a kettle sour HBT thread
• Kettle Sour (surprise! low gravity) HBT thread
• 1st kettle sour spoiled? HBT thread
• goodbelly ate insane amount of sugar? HBT thread
• Everyone throw your ideas on maintaining a lacto temp on a Kettle sour please! HBT thread
• Kettle sour infection question HBT thread
• 60+ hours since yeast pitch and nary a bubble HBT thread
• Kettle Sour Issues HBT thread
• Bad smell with omega yeast labs lacto starter? HBT thread
• what in the world took over my kettle sour? HBT Thread
• Kettle sour probably got infected- with yeast. what to do? HBT thread
• Post your infection HBT thread
• White Labs Lacto Brevis HBT thread
• question on my kettle sour, fermenting with the lacto HBT thread
• Berliner Weisse - butyric acid HBT thread
• What to do with a kettle sour contamination? Reddit thread
• Sour beer wort smells bad HBT thread
• First kettle sour, how bad is the smell supposed to be? Reddit thread
• Kettle sour dumper batch Reddit thread
• Kettle Sour gone wrong ProBrewer thread
• Berliner vomit: dump, or reinfect? HBT thread
• Etc. Hopefully that's enough examples to make the point.

See also

• Sour beer
• Milk the Funk Wort Souring wiki article
• Matt Miller's Lactobacillus 2.0 – Advanced Techniques for Fast Souring Beer blog entry