A sour ale is made by letting acid-producing microbes into the beer rather than keeping them out, and there are two ways a brewer does it. In the first, the wort is soured in the kettle before any alcohol exists, then boiled and fermented normally, which gives a bright, one-note tartness in a couple of weeks. In the second, the beer is put into wood and left to a mixed culture of yeast and bacteria for a year or more, which gives something drier and stranger. The gap between those two routes explains almost everything about why a lemon-fresh sour in a can and an aged lambic both sit under the heading of sour ale and taste like different beverages.
What To Know Before You Buy A Sour
- Understand that sourness is an acid rather than a flavouring, because the bacteria make lactic or acetic acid and the wild yeast adds dryness and a barnyard note on top.
- Judge a sour by how acidic it measures rather than by its colour, because a normal pale ale sits near 4.0 to 4.5 on the scale while a sour lands between 3.0 and 3.8.
- Treat the two routes as different drinks, because a kettle sour is a single bright line of lactic acid while a barrel sour is years of layered organisms working the sugar out.
- Serve these beers colder than a stout but warmer than a lager, somewhere in the 7 to 10 C range, in a stemmed glass rather than a pint.
Sourness Is an Acid Rather Than a Flavouring
Three kinds of organism do the work. Lactobacillus and Pediococcus are bacteria that produce lactic acid as they feed, the same acid that appears in yoghurt and sauerkraut, and it lands on the tongue as a clean sharpness at the front of the mouth. Acetobacter is a bacterium that turns alcohol into acetic acid, which is vinegar, and it needs oxygen to do it, which is why a barrel beer can slide from pleasantly tart to pickle liquor if the headspace in the wood is left unmanaged. Brettanomyces is a yeast rather than a bacterium, and it contributes comparatively little acid; what it brings is dryness, a faint note of leather or barnyard, and the ability to ferment the longer sugar chains that ordinary brewer's yeast leaves behind.
The measurable result is acidity. A normal pale ale sits at roughly 4.0 to 4.5 on the pH scale, while a sour beer usually lands between 3.0 and 3.8, and how tart either one tastes also depends on residual sugar, body and carbonation, so two beers with the same pH can differ a good deal in the glass.
Kettle Souring, the Fast Route
Kettle souring, also called pre-souring, separates the acid from the alcohol. The brewer makes wort, chills it to the range Lactobacillus likes rather than the range brewer's yeast likes, doses it with a culture, and holds it warm for a few days with the vessel blanketed in carbon dioxide to keep oxygen out. When the acidity is where the brewer wants it, the wort goes back on the fire, and that boil both stops the souring at a chosen point and sterilizes the liquid. Only then does the yeast go in.
- Mash and boil the wort as usual, then cool it to roughly 35 to 45 C instead of to normal fermentation temperature.
- Pitch a lactobacillus culture, or sour the wort the older way by stirring in crushed malt, which carries the bacteria on its husk.
- Blanket the vessel with carbon dioxide and hold it warm for one to three days, tasting as it goes until the tartness is right.
- Bring the wort back to a boil, which halts the acidification and kills the bacteria, then add the hop charge and chill it.
- Ferment with the ale yeast the style calls for, and package it knowing that nothing live from the souring step survives in the can.
The advantages are control and speed, since the brewer can stop the acidity at a chosen point instead of waiting to see what the barrel does, and a kettle sour can be on shelf within a month. The cost is depth, because lactic acid on its own is a single line rather than a chord. Hops also get in the way of the souring step, since the iso-alpha acids that make beer bitter suppress lactic bacteria, which is why a kettle sour is often brewed with barely any bittering charge and hopped afterwards instead.
The Slow Route of Air, Wood and Time
Spontaneous fermentation is the older method and the one that still defines the classic sour beers of Belgium. Cooled wort is run into a coolship, a shallow open vessel with a wide surface, and left overnight with the roof vents open so that whatever is living in the air of the valley settles into it. Lambic, made in the Senne valley south-west of Brussels, is the surviving example, and the fermentation that follows is not one event but a succession, with different organisms taking over as the acidity rises and the alcohol appears. The beer is then racked into old oak barrels, where it sits for one, two or three years, and the blender's job is to marry young beer for fizz and sugar with old beer for acid and complexity. Still lambic blended and bottle-conditioned becomes gueuze, and fruit added to a barrel becomes kriek with cherries or framboise with raspberries.
Flanders red and brown ale reach the same destination by a slightly different road. The wort is inoculated rather than left to the air, and the beer ages in large oak vats where a mixed culture of yeast, Lactobacillus, Pediococcus and Acetobacter works slowly enough that the brewery is blending one and two year old beer for the next release. The result is rounder and more wine-like than a kettle sour, with an acidity that has an edge of vinegar in it rather than being all lactic.
Which Style Comes From Which Route
| Style | How it gets soured | What to expect in the glass |
|---|---|---|
| Berliner Weisse | Lactobacillus, now usually in the kettle | Very pale, low in alcohol, sharp and clean, often served with syrup |
| Gose | Lactobacillus with salt and coriander | Soft and lemony with a savoury edge, nearly always kettle soured today |
| Lambic and gueuze | Airborne microbes in a coolship, then barrel ageing | Dry, complex, barnyard and citrus, blended by age |
| Flanders red ale | Mixed culture in large oak vats | Tart and vinous, sweet against sour, blended young and old |
| American wild ale | Whatever the brewer pitches, often a blend | No rules, Brett-led, dry, frequently finished with fruit |
| Sour IPA | Kettle soured, then hopped hard | Citrus and pineapple sitting over a bright acidity |
Why the Two Routes Taste Nothing Alike
A kettle sour is lactic acid plus whatever hops and yeast you chose, and nothing else, because the souring step is finished before fermentation begins. A barrel sour is the product of years of overlapping activity in which the same organisms strip out nearly all of the residual sugar, pull tannin and a little oxygen through the wood, and leave Brettanomyces faintly working in the bottle. That is why the barrel beer tastes dry and layered while the kettle beer tastes fruity and forward, and it is the reason a drinker who likes one should not be pushed toward the other without a warning.
What Goes Wrong
The organisms that make a sour beer are the same ones that ruin an ordinary one, which is why breweries running both keep the equipment separate and never share a hose. The common failures are instructive. Pediococcus working in an open vessel throws a large amount of diacetyl, the slippery buttery compound that a clean lager brewer would treat as a fault. Oxygen reaching a barrel with Acetobacter in it produces vinegar rather than beer. A kettle left too long at too cool a temperature can throw a flat, yoghurt character instead of a clean tartness. And a fruit addition that goes into a bottle with live culture still in it will ferment further, which is how a bottle becomes a grenade and a brewery learns to put the fruit in a barrel instead.
Serving and Food
Serve these beers cooler than a stout but not at lager temperature, somewhere in the 7 to 10 C range, in a stemmed tulip or a flute rather than a pint glass, since the aroma of a sour ale is mostly fruit and hay and a wide glass throws it away. Gueuze and lambic need a steady pour that leaves the sediment in the bottle unless you want it, and a Flanders red poured at the warmer end of that range will show more of its sweetness. Food follows the acidity, since what a sour beer does to a rich dish is what a squeeze of lemon does: mussels, fried fish, goat cheese, roast chicken with pan juices, and anything fatty all get cut by it, while the fruit-forward kettle sours go with dessert rather than dinner.
Frequently Asked Questions
Are sour ales healthy?
No, and the claim that they are a source of probiotics does not survive the process that makes them. In a kettle-soured beer the bacteria are boiled to death before fermentation even starts, so there is nothing live left in the can, and in a barrel-aged beer the alcohol is the larger fact about what the drink does to you whatever is floating in it. The acidity is also the reason some people find these beers uncomfortable to drink on an empty stomach, so the honest summary is that a sour ale is a beer, and it should be chosen because it tastes good.
Are sour ales gluten free?
Almost none of them are. Lambic is brewed with a large proportion of wheat alongside barley, Berliner Weisse and gose are wheat beers, and Flanders red is barley, so all four carry gluten through fermentation. A beer treated with an enzyme to break gluten down is a different product making a different claim, and it is not something a coeliac reader should treat as safe on the strength of a style name.
How long does it take to make a sour ale?
Between three weeks and three years, depending on the route. A kettle sour spends one to three days souring, a week fermenting and a short conditioning period, which is why a brewery can put a fruit sour on shelf in the same season it was dreamed up. A lambic is a year old before anyone tastes it and a gueuze is usually a blend of one, two and three year old beer, so the brewery that sells it has to have decided what it wanted three years earlier.
Is a fruited sour the same thing as a lambic?
That equation has mostly broken down in favour of the word fruit. Fruit lambic is a real and specific thing, made by adding whole cherries or raspberries to a barrel of lambic so that the sugars in the fruit feed whatever is still working in the wood, and the fruit is gone within a year leaving acid, colour and flavour behind. Most of what a Canadian shelf calls a fruit sour is a kettle-soured base beer with fruit purée added after fermentation, which is a faster and cleaner way to get a mango or passionfruit beer that happens to be tart.
Can a homebrewer make a sour beer?
Kettle souring is the practical route, since it uses a purchased lactobacillus culture, a pot and a thermometer, and it ends with a boil that puts the souring under your control. A mixed culture in a plastic bucket is the version to avoid, because the bacteria that carry through a home brewery's equipment will follow it into every clean batch you brew afterwards, and a homebrewer who has not separated the two sets of gear has usually created an infection rather than a program.
Start on the Kettle Side of the Style
If you want to understand how a sour beer gets its acid, brew the fast one first and drink the slow one while you wait. A kettle-soured gose is a weekend of work and will teach you more about where your own taste for tartness stops than any number of bottles bought blind, and the barrel beer you drink in the meantime will show you what the extra two years of organisms and oak are actually buying. That comparison, rather than any single style, is the thing worth knowing about sour ales, because most disappointment in this corner of the shelf comes from somebody expecting one route and being handed the other.
