Learn & Discover
Learn & Discover
Kettle Souring
If you’ve visited a pub, bar or brewery taproom that serves craft beer, you may have seen the beers described as ‘kettle sour’ on the menu. But what exactly is a kettle sour? And what should you expect if you order one? In this technical and sensory guide Ruvani de Silva explains what kettle-souring is, how it differs from how traditional sour beers are made, why a brewer might choose to kettle-sour, impacts on flavour and provides examples of US and UK breweries implementing the technique to great effect.
Ruvani de Silva
Ruvani brings a unique voice to the world of craft beer. A travel-loving beer writer, with a host of bylines, Ruvani blogs about beer in Central Texas and beyond, as Craft Beer Amethyst. A vocal advocate for diversity, equity and inclusion in beer.
Kettle Souring
If you’ve visited a pub, bar or brewery taproom that serves craft beer, you may have seen the beers described as ‘kettle sour’ on the menu. But what exactly is a kettle sour? And what should you expect if you order one? In this technical and sensory guide Ruvani de Silva explains what kettle-souring is, how it differs from how traditional sour beers are made, why a brewer might choose to kettle-sour, impacts on flavour and provides examples of US and UK breweries implementing the technique to great effect.
Ruvani de Silva
A travel-loving beer writer, Ruvani blogs about beer in Central Texas and beyond, as Craft Beer Amethyst.
What is kettle souring?
A ‘kettle sour’ beer is a beer that has been made sour by the brewer adding lactobacillus, the primary probiotic bacteria that is used to make beer sour, to the wort before the start of the boil. This is a relatively modern method of souring that is very quick, and offers the brewer a significant degree of control over the level of sourness.
The term ‘kettle souring’ refers to the process by which the beer is made sour – it is not a style of beer. This process can be applied to most beer styles, making it a swift and adaptable way to brew sour beers, and ideal for brewers who are keen to experiment with sour versions of different beer styles. Without kettle souring we would not have Sour IPAs, Sour Stouts or Milkshake Sours.
While traditional sour beers are aged in wooden barrels, allowing yeasts and bacteria to slowly, naturally change their flavour, kettle sours are created by a controlled infection of lactobacillus which is killed off in the boil once the beer has reached the brewer’s desired level of sourness. Think of kettle souring as the ‘nurture’ to the ‘nature’ of Belgian spontaneous-fermentation sours.
What is kettle souring?
If you’ve visited a pub, bar or brewery taproom that serves craft beer, you may have seen the beers described as ‘kettle sour’ on the menu. But what exactly is a kettle sour? And what should you expect if you order one?
A ‘kettle sour’ beer is a beer that has been made sour by the brewer adding lactobacillus, the primary probiotic bacteria that is used to make beer sour, to the wort while it is still in the steel mash tun (the ‘kettle’). This is a relatively modern method of souring that is very quick, and offers the brewer a significant degree of control over the level of sourness..
The term ‘kettle souring’ refers to the process by which the beer is made sour – it is not a style of beer. This process can be applied to most beer styles, making it a swift and adaptable way to brew sour beers, and ideal for brewers who are keen to experiment with sour versions of different beer styles. Without kettle souring we would not have Sour IPAs, Sour Stouts or Milkshake Sours.
While traditional sour beers are aged in wooden barrels, allowing yeasts and bacteria to slowly, naturally change their flavour, kettle sours are created by a controlled infection of lactobacillus which is killed off in the boil once the beer has reached the brewer’s desired level of sourness. Think of kettle souring as the ‘nurture’ to the ‘nature’ of Belgian spontaneous-fermentation sours.
“A ‘kettle sour’ beer is a beer that has been made sour by the brewer adding lactobacillus, the primary probiotic bacteria that is used to make beer sour…”
— Ruvani de Silva
How to Kettle-Sour a beer
To kettle sour a beer, a brewer will add lactobacillus into the wort after the mash but before the boil. This is the probiotic bacteria that you find in Greek yoghurt, and adds this same tangy sharp lemony tartness to the beer. The beer will be stored at a warm temperature (110–120°F/43–49°C) for 1-3 days before returning it to the brewing process and beginning the boil. The rest of the brew will proceed as normal, with any excess lactobacillus boiled off to prevent spoilage. It is important to keep the beer at a stable temperature during the lactic fermentation, using an electric blanket or water bath, and to prevent the wort coming into excess contact with the air, which may carry unwanted microbes – carbon dioxide or nitrogen can be used to flush the air away from the beer. Home-brewers can re-use wort samples from previous batches in the same way bakers use a sourdough starter.
How to Kettle-Sour a beer
To kettle sour a beer, a brewer will add lactobacillus into the wort after the mash but before the boil. This is the probiotic bacteria that you find in Greek yoghurt, and adds this same tangy sharp lemony tartness to the beer. The beer will be stored at a warm temperature (110–120°F/43–49°C) for 1-3 days before returning it to the brewing process and beginning the boil. The rest of the brew will proceed as normal, with any excess lactobacillus boiled off to prevent spoilage. It is important to keep the beer at a stable temperature during the lactic fermentation, using an electric blanket or water bath, and to prevent the wort coming into excess contact with the air, which may carry unwanted microbes – carbon dioxide or nitrogen can be used to flush the air away from the beer. Home-brewers can re-use wort samples from previous batches in the same way bakers use a sourdough starter.
How to Kettle-Sour a beer?
To kettle sour a beer, a brewer will add lactobacillus into the wort after the mash but before the boil. This is the probiotic bacteria that you find in Greek yoghurt, and adds this same tangy sharp lemony tartness to the beer. The beer will be stored at a warm temperature (110–120°F/43–49°C) for 1-3 days before returning it to the brewing process and beginning the boil. same way bakers use a sourdough starter.
The rest of the brew will proceed as normal, with any excess lactobacillus boiled off to prevent spoilage. It is important to keep the beer at a stable temperature during the lactic fermentation, using an electric blanket or water bath, and to prevent the wort coming into excess contact with the air, which may carry unwanted microbes – carbon dioxide or nitrogen can be used to flush the air away from the beer. Home-brewers can re-use wort samples from previous batches in the
Why Kettle-Sour your beer?
If you are familiar with traditional sour beer styles from Belgium and Germany, you may be aware that the majority of these beers take a long time to brew – from months to years – to allow the wild yeasts and bacteria that turn the beer sour to ferment to the desired flavour. This requires a lot of time and space, and often specific environmental conditions. These can be expensive to maintain, and one of the primary reasons many brewers choose to kettle-sour their beer is to keep costs down. Kettle-souring offers brewers the opportunity to create a specific level of sourness with minimal risk of unwanted off-flavours, with excess bacteria being killed off in the boil, leaving kettle-soured beer hygienically pasteurised, unlike its ‘living’ unpasteurised traditional counterparts. In fact, some traditional sour styles like Berlinner Weiss and Gose are now often kettle-soured to keep production costs down..
Why Kettle-Sour your beer?
If you are familiar with traditional sour beer styles from Belgium and Germany, you may be aware that the majority of these beers take a long time to brew – from months to years – to allow the wild yeasts and bacteria that turn the beer sour to ferment to the desired flavour. This requires a lot of time and space, and often specific environmental conditions. These can be expensive to maintain, and one of the primary reasons many brewers choose to kettle-sour their beer is to keep costs down.
Kettle-souring offers brewers the opportunity to create a specific level of sourness with minimal risk of unwanted off-flavours, with excess bacteria being killed off in the boil, leaving kettle-soured beer hygienically pasteurised, unlike its ‘living’ unpasteurised traditional counterparts. In fact, some traditional sour styles like Berlinner Weiss and Gose are now often kettle-soured to keep production costs down
What about the flavour?
For aficionados of traditional European sour styles such as Lambic and Gueze, a kettle-soured beer may taste disappointing. This is because the deep, earthy, funky flavours that are created by the yeast and bacteria combinations used to brew and age these styles cannot be replicated by kettle-souring. The sour flavour the beer gets from the short lactobacillus fermentation is much cleaner and simpler. This means that brewers have the freedom to add other flavour-producing adjuncts to kettle-soured beers, such as fruit and spices. It also means that the beers are often seen as more approachable, ‘gateway sours’, suitable as an introduction to sour beer for drinkers who aren’t familiar with traditional sour styles. Fruited sours are becoming increasingly common on both sides of the Atlantic, with ever more bold and unusual flavour profiles, including the recent trend for heavily fruited ‘milkshake’ or ‘smoothie’ sours, and the growing popularity of Sour IPAs and Sour Stouts. These styles are only possible because of the process of kettle-souring, because hops, which do not interact well with lactobacillus, can be added at the end of the boil, after any remaining bacteria has been boiled off.
“Sour mashing is a quick-souring technique similar to kettle-souring. The difference is that sour-mashing involves adding the lactobacillus during the mash itself as opposed to before the boil.”
— Ruvani de Silva
What about the flavour?
For aficionados of traditional European sour styles such as Lambic and Gueze, a kettle-soured beer may taste disappointing. This is because the deep, earthy, funky flavours that are created by the yeast and bacteria combinations used to brew and age these styles cannot be replicated by kettle-souring. The sour flavour the beer gets from the short lactobacillus fermentation is much cleaner and simpler. This means that brewers have the freedom to add other flavour-producing adjuncts to kettle-soured beers, such as fruit and spices. It also means that the beers are often seen as more approachable, ‘gateway sours’, suitable as an introduction to sour beer for drinkers who aren’t familiar with traditional sour styles.
Fruited sours are becoming increasingly common on both sides of the Atlantic, with ever more bold and unusual flavour profiles, including the recent trend for heavily fruited ‘milkshake’ or ‘smoothie’ sours, and the growing popularity of Sour IPAs and Sour Stouts. These styles are only possible because of the process of kettle-souring, because hops, which do not interact well with lactobacillus, can be added at the end of the boil, after any remaining bacteria has been boiled off.
What’s the difference between Sour Mashing and Kettle Souring?
Sour mashing is a quick-souring technique similar to kettle-souring. The difference is that sour-mashing involves adding the lactobacillus during the mash itself as opposed to before the boil. This involves a slightly greater risk of spoilage, as it is more difficult to limit air exposure. Some brewers may sour-mash, kettle-sour and if necessary add a final injection of lactic acid after the boil to bring up the level of sourness in their beer. They may also use a combination of these techniques, but kettle-souring is the most common method currently used for ‘quick-souring’, as it is a relatively forgiving technique. Small divergences in time and temperature are usually not critical, making it particularly attractive to home-brewers.
What’s the difference between Sour Mashing and Kettle Souring?
Sour mashing is a quick-souring technique similar to kettle-souring. The difference is that sour-mashing involves adding the lactobacillus during the mash itself as opposed to before the boil. This involves a slightly greater risk of spoilage, as it is more difficult to limit air exposure.
Some brewers may sour-mash, kettle-sour and if necessary add a final injection of lactic acid after the boil to bring up the level of sourness in their beer. They may also use a combination of these techniques, but kettle-souring is the most common method currently used for ‘quick-souring’, as it is a relatively forgiving technique. Small divergences in time and temperature are usually not critical, making it particularly attractive to home-brewers.
Who kettle-sours?
This is such a big trend in the US now that there are a plethora of breweries around the country specialising in kettle sours, particularly of the fruited/milkshake variety, including Imprint in Pennsylvania, Drekker in North Dakota, plus Urban South HTX and Ingenious in Texas. Austin’s Blue Owl Brewing, until very recently, brewed only kettle-soured beers, often also sour-mashing, creating a rich, exciting landscape of sour beers ranging from a Czech Pilsner to an Imperial Oatmeal Stout. Blue Owl’s co-owner and executive brewer Jeff Young fell in love with the science of kettle-souring. “The process of it was sort of a mystery (and not well researched) and I was obsessed with mastering it” he says, “We also saw an opportunity in the ever-increasingly crowded beer scene to focus solely on styles produced through this process and stand out from the crowd.” A key part to this experimentation for Blue Owl is ensuring that each beer retains key elements of its original style – otherwise they would all taste the same. This is where the brewer’s expertise is critical in delivering a really exciting, innovative and tasty kettle-sour experience.
At Urban South HTX, there’s a strong focus on kettle-souring as a way to create a canvas for using exciting adjuncts to experiment with unusual flavour combinations. Brewer and General Manager Dave Ohmer says “We look at kettle souring as just clean acid production to set a base for us to add other fun ingredients. It’s more like squeezing a lemon or lime over your food so that slight acidity cuts through some of the bigger flavours and makes it more refreshing and quaffable.” This demonstrates the potential that kettle-souring offers for creating beers like Urban South’s Ice Cream Paint Job, a collaboration with J Wakefield Brewing, combining raspberry and orange puree plus lemon and lime for a rainbow sherbet effect – impossible using any other method of brewing.
Who kettle-sours?
This is such a big trend in the US now that there are a plethora of breweries around the country specialising in kettle sours, particularly of the fruited/milkshake variety, including Imprint in Pennsylvania, Drekker in North Dakota, plus Urban South HTX and Ingenious in Texas. Austin’s Blue Owl Brewing, until very recently, brewed only kettle-soured beers, often also sour-mashing, creating a rich, exciting landscape of sour beers ranging from a Czech Pilsner to an Imperial Oatmeal Stout.
Blue Owl’s co-owner and executive brewer Jeff Young fell in love with the science of kettle-souring. “The process of it was sort of a mystery (and not well researched) and I was obsessed with mastering it” he says, “We also saw an opportunity in the ever-increasingly crowded beer scene to focus solely on styles produced through this process and stand out from the crowd.” A key part to this experimentation for Blue Owl is ensuring that each beer retains key elements of its original style – otherwise they would all taste the same. This is where the brewer’s expertise is critical in delivering a really exciting, innovative and tasty kettle-sour experience.
At Urban South HTX, there’s a strong focus on kettle-souring as a way to create a canvas for using exciting adjuncts to experiment with unusual flavour combinations. Brewer and General Manager Dave Ohmer says “We look at kettle souring as just clean acid production to set a base for us to add other fun ingredients. It’s more like squeezing a lemon or lime over your food so that slight acidity cuts through some of the bigger flavours and makes it more refreshing and quaffable.” This demonstrates the potential that kettle-souring offers for creating beers like Urban South’s Ice Cream Paint Job, a collaboration with J Wakefield Brewing, combining raspberry and orange puree plus lemon and lime for a rainbow sherbet effect – impossible using any other method of brewing.
In the UK, Reading’s Double-Barrelled Brewing are leading the kettle-souring charge. “We did quite a bit of travel to help bring something interesting to the UK; as part of that we discovered kettle sours being made within the States” says Co-Founder and Operations Director Mike Clayton-Jones. Control and flavour are also key reasons Double-Barrelled choose to kettle sour. “By employing kettle-souring over a specified timeframe, we get a much greater depth of flavour from the bacteria being used, which we can then go on to accentuate with various additions of fruit etc” says Clayton-Jones. His favourite Double-Barrelled kettle sours include mango-passion fruited The Big Fruit Heist, a raspberry-peach Rosella Gose. Balance is crucial in perfecting these delicate brews with complex adjuncts. Clayton-Jones explains “We have learnt over time about varying those quantities of fruit additions and ensuring that they carry well on that sour base.”
Other UK breweries who currently make kettle-soured beer include Siren, Neon Raptor, Brew York, Wander Beyond, Brick, North and Dolphin. With UK beer trends often following closely behind industry shifts in the US, we can expect to see kettle-souring becoming increasingly common in the UK, so look out for a kettle-soured beer in your next beer-delivery box or on your next bar or taproom visit.
In the UK, Reading’s Double-Barrelled Brewing are leading the kettle-souring charge. “We did quite a bit of travel to help bring something interesting to the UK; as part of that we discovered kettle sours being made within the States” says Co-Founder and Operations Director Mike Clayton-Jones.
Control and flavour are also key reasons Double-Barrelled choose to kettle sour. “By employing kettle-souring over a specified timeframe, we get a much greater depth of flavour from the bacteria being used, which we can then go on to accentuate with various additions of fruit etc” says Clayton-Jones. His favourite Double-Barrelled kettle sours include mango-passionfruited The Big Fruit Heist, a raspberry-peach Rosella Gose. Balance is crucial in perfecting these delicate brews with complex adjuncts. Clayton-Jones explains “We have learnt over time about varying those quantities of fruit additions and ensuring that they carry well on that sour base.”
Other UK breweries who currently make kettle-soured beer include Siren, Neon Raptor, Brew York, Wander Beyond, Brick, North and Dolphin. With UK beer trends often following closely behind industry shifts in the US, we can expect to see kettle-souring becoming increasingly common in the UK, so look out for a kettle-soured beer in your next beer-delivery box or on your next bar or taproom visit.
