Sour Beer: What It Is & How It Is Made

Sour Beer

There’s a saying that goes like this, “Everything likes beer as much as we do.”. While we do our best to keep every microorganism other than our yeast friend out while making traditional beer, for sour beers, some of our acquaintances are allowed in, too, with an occasional open-door policy where we invite complete strangers over because we know they will give our sour beer the flavor that we want. 

Sometimes, this “invitation or open-door policy” can lead to nasty-tasting sour beers that are more like vinegar than anything else. But, at other times, and if we are lucky enough, the beer might run into the kind of strains that make it just the right kind of tart and sour, giving it a great taste. 

Controlled and spontaneous infections

There are two types of infections, controlled and spontaneous. In a controlled infection that skilled brewers have a knack for, they not only keep everything sanitized but know exactly at which point to add bacteria and yeast into the mix, which strains to use, and when to kill them off by raising the temperature. A spontaneous infection lets nature do its work.

What is sour beer?

Regular beers have a pH of around 4. Sour beers, on the other hand, generally have a pH of about 3.4, giving them the classic tart or acidic flavor that we know. This drop in pH is the result of a controlled or spontaneous addition of microorganisms other than traditional yeast, particularly bacteria or wild yeasts, into the mix that converts sugars into acids.

Brettanomyces vs. Lactobacillus and Pediococcus

Three common types of microorganisms used in the additional process of converting traditional beer into the sour beer are Brettanomyces, Lactobacillus, and Pediococcus. 

Brettanomyces is wild yeast that feeds on sugars and converts them into alcohol, carbon dioxide, and other compounds like acetic acid in the presence of oxygen. 

Lactobacillus is a bacterium, commonly used to make yogurt that produces lactic acid when it eats sugar. The problem with Lactobacillus is that it needs a specific environment to survive. Too many hops mean too many isomerized alpha acids resulting in a less-than-ideal condition. Sour beers like Berliner Weisse are great when using Lactobacillus because they use minimal hops. Other possibilities are using it in a sour mash or in kettle souring applications. 

Pediococcus is a nice substitution for Lactobacillus since it tolerates diverse environments well. Pediococcus produces the same acidity as Lactobacillus. However, there is a downside. It takes longer to produce lactic acid, and it always has to be used with Brettanomyces to deal with off-flavors.

Both lactic and acetic acids increase the acidity of the mixture they are in, but acetic acid is more pungent and sour than lactic acid.

Open or spontaneous fermentation – open door policy

Sometimes, wild yeast and bacteria strains can contaminate your wort and either add a desirable flavor to the end product or make it rotten. It all depends on what kind is floating around in your vicinity at that particular moment. The Lambic breweries in Belgium have enjoyed great success, the credit for which goes to the wild strains of yeast in the Senne River valley.

Open fermentation is all about experimentation, and if we aspire to be good beer scientists, we may as well let local microorganisms work their magic and see what we get.  

Kettle souring

Kettle souring uses lactobacillus to sour the beer and lactobacillus plantarum is the most popular strain used because it sours cleanly and quickly. It is added to the wort before boiling begins, more specifically, when the wort is in the brew kettle or vessel, hence “kettle souring.” When the brewer adds bacteria at this stage, the beer is soured within hours as opposed to months. 

After giving the wort 12 to 36 hours to sour, it is then boiled and fermented by adding traditional yeast, a process identical to making a typical beer. The high heat during the boil kills off the bacteria. Hence there is no threat of contamination. Care is taken that the wort gravity is around 1.050. The higher the gravity, the more fermentable sugars, i.e. yeast food. 

Smell during the kettle souring process

If you are a homebrewer making sour beer, you might notice distinct odors released during the fermentation process with Lactobacillus. Your wort can smell like yogurt or lemony. But this is completely normal, and you shouldn’t worry. An off-smelling wort will smell like fecal matter, strong cheese, or vomit. This is when you should be ready to get rid of the batch! An unwanted smell resulting from an unwanted infection might lead to the formation of butyric acid, giving odd nasty odors. 

If you want to keep wild bacterial strains out, you should use pure cultures and make sure your wort is pasteurized. Additionally, while the mix is souring, be sure to close the lid properly. 

Addition of hops in the kettle souring process

Hops should only be added to the wort after it has been soured because it can inhibit the growth of Lactobacillus. Proper souring can be achieved by adding Lactobacillus to a wort that has no hops in it.

Kettle sour beer styles

Berliner Weisse

Nicknamed Champagne of the North, Berliner Weisse, a mildly sour and tart beer with a low alcohol content of about 3% to 4% ABV, originated in Berlin and has a light and fruity flavor. It was most popular in the 19th century when it was produced by 700 breweries. Now, the beverage is produced by two breweries, Schultheiss and Berliner Kindl. These breweries carry out bacterial fermentation separate from yeast fermentation, after which the beer is blended. The resulting beer is slightly hazy, effervescent, lightly tang, and very pale. 


This top fermented wheat beer with an ABV ranging from 4.2% to 4.8% is seasoned with coriander and salt for added flavor. Coriander gives it a dry and spicy flavor, and salt adds sharpness to it. While it originated in Goslar, Germany in the early 13th century, it only became popular in the 1700s. Gose, like Berliner Weisse, is sometimes colored and flavored with syrups to balance the sourness from the lactic acid.

Sour IPA

Commonly understood as a hoppy-tart beer made from grains like wheat or oats in the style of a New England IPA, its primary flavor comes from Lactobacillus, and then it’s additionally flavored by aging it in barrels or from additions of lactose, vanilla, or fruit. It is a relatively new concept that is both complex and refreshing, and one of the leading drivers of this beer style is the Hudson Valley Brewery. 

Fruited sour

Fruited sour has an additional step to a typical sour beer, the addition of fruits, including blackberries, peaches, blueberries, raspberries, passionfruit, plums, mango, pineapples, apricots, cherries, and lychee. Fruit is mostly added during secondary fermentation to create juicy flavors.

Saison and farmhouse ale

Originating from Wallonia, Saison, a French word meaning season, was originally brewed for farmers as a refreshing drink to be had in the warm summer months. But now, it is brewed all year round. It is an unfiltered pale ale with high carbonation with a fruity or hoppy aroma. This complex beer has an ABV ranging from 5% to 8.5% and has an IBU between 20 to 35.

Other sour beer styles


Made in Brussels around the Zenne River valley, lambic beers are fruity and complex with a funky, barnyard aroma and use spontaneous fermentation for souring. After the wort cools down, it is kept in the open air during spring and winter and then placed in barrels for fermentation and maturation.

Flanders red ale

This beverage is fermented with yeast and then put into oak barrels for maturation and aging. For consistency and taste adjustment, mature beer can be blended with younger beer.

American wild ale

This beer brewed in the US uses wild bacteria and yeast strains along with standard yeast to give a unique and funky flavor and aroma. Having an ABV ranging from 6% to 12%, they are best served cool.

Oud Bruin

Originating in Belgium’s Flemish region, Oud Bruin has a darker color than Flanders’s red ale. Oud or old refers to the aging process that takes more than a year to finish. 

Other souring methods

Reusing sour beer barrels

Since barrels that contained sour beer in the past will still have souring bacteria residing in them, they can be perfect for souring the next batch

Fresh fruit additions

Bacteria like Pediococcus and Lactobacillus live naturally on the skin of fresh fruit. If you add this fruit to the beer, it can provide it great souring.

Adding Lactobacillus and Pediococcus after brewing

Once the beer is fermenting, these two strains of bacteria can be added to it. Additions during this time take these strains more time to get the job done but give the beer a well-rounded flavor and tartness. This is quite different from kettle souring where these two strains of bacteria are added during the brewing process.

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