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Bacteria in beer: Lactobacillus and Pediococcus - how sourness is made

The sour taste of beer, that refreshing, lemony tartness of beers like gose or Berliner Weisse, comes neither from the hops, nor from added acid, nor even from yeast. It comes from bacteria. Specifically from lactic acid bacteria, mainly two genera: Lactobacillus and Pediococcus. They eat the sugars from the wort and produce lactic acid, giving the beer its characteristic, clean sourness. It is a completely different mechanism from the classic alcoholic fermentation by yeast. Understanding the role of these bacteria is the key to grasping where sour beer comes from at all. Here is a guide to bacteria in beer: what Lactobacillus and Pediococcus are, how they produce acid, what a kettle sour is and why some sour beers are made in a week and others in a year.

Where sourness in beer comes from

Let us start with a surprise. Most beers are created so as not to be sour, and sourness in them is even considered a fault. But a whole family of styles, like gose, Berliner Weisse or lambic, rests precisely on a refreshing, deliberate sourness. Where does it come from? Not from the hops, which give bitterness, not from the yeast, which gives alcohol, but from lactic acid bacteria. These bacteria eat the sugars from the wort and turn them into lactic acid, giving a clean, refreshing sour taste. It is a completely different track from ordinary alcoholic fermentation. Understanding that the sourness of beer is the work of bacteria, and not of the hops or yeast, is the starting point for all the rest: getting to know the main species, the mechanism and the methods of producing sour beer.

Lactobacillus - the fast souring agent

The first and most important genus of bacteria is Lactobacillus, Lacto for short. These are lactic acid bacteria that eat the sugars from the wort and turn them into lactic acid, giving the beer a clean, refreshing sourness. Lactobacillus is relatively fast: an active culture, pitched into the wort without competition and held warm, can sour a beer in less than a day. It gives a clean, nice sour taste, ideal for styles like gose or Berliner Weisse. This is why Lactobacillus is the basis of fast methods of producing sour beer. Its advantage is precisely the pace and the cleanness of the sourness, without additional wild notes. Lactobacillus is the workhorse of modern sour brewing, letting a sour taste be reached quickly and predictably, without waiting months.

Pediococcus - slow and complex

The second genus is Pediococcus, Pedio for short. These are also lactic acid bacteria, but of a completely different character from Lactobacillus. Pediococcus works far more slowly and is typical of the secondary fermentation of sour beers ageing in barrels, where it works for months, even years. It gives a more complex, deep profile, often in the company of the wild yeast Brettanomyces, with which it forms mixed cultures. It is said that Lactobacillus can do in a week what Pediococcus needs a year for. Pediococcus is also trickier to handle, because at a certain stage it can give the beer an unpleasant, slimy consistency, which only other microorganisms fix over time. It is a bacterium for the patient, giving the depth and complexity of traditional, barrel-aged sour beers, like lambic.

A table: Lactobacillus versus Pediococcus

Let us gather the two genera in one place:

The table shows the heart of the difference: Lactobacillus is speed and clean sourness, Pediococcus is slow depth and the complexity of traditional beers.

Kettle sour - the fast method

The most popular modern method of souring is the kettle sour, that is souring in the kettle. It consists of souring the wort before the actual alcoholic fermentation. To the wort, usually with little hops or none, a Lactobacillus culture is pitched and held at a high temperature until it reaches the desired level of acidity, measured as pH. The bacteria eat the sugars and produce lactic acid, giving a sour taste. Only after the desired sourness is reached is the wort boiled, to stop the bacteria, and yeast added for normal alcoholic fermentation. The kettle sour lets a beer be soured in a day or two, instead of months. This is why it has become so popular in modern craft brewing: it gives a clean sourness quickly, predictably and without the risk of spreading wild bacteria around the brewery.

Why hops get in the bacteria way

There is an important technical nuance: lactic acid bacteria do not like hops. The bittering substances from hops act on Lactobacillus and Pediococcus inhibitingly, making their work harder. This is why sour beers are made with lightly hopped wort or even no hops at the souring stage. The more hops, the harder it is for the bacteria to develop sourness. It is a practical explanation of why sour beers, like gose or Berliner Weisse, are usually low in hops and gently bitter. Hops and sourness stand here in a certain opposition. The brewer has to take this into account, choosing the hopping so as not to stifle the bacteria. It shows how the different ingredients of beer interact with each other, and creating a sour beer requires understanding that the classic, heavily hopped approach will not work here.

Bacteria, yeast and Brett

It is worth ordering who is responsible for what in sour beer. Yeast, mainly Saccharomyces, makes alcohol from sugar. Lactic acid bacteria, Lactobacillus and Pediococcus, make lactic acid, giving sourness. And the wild yeast Brettanomyces adds funk, that is wild, animal and fruity notes. In traditional, barrel-aged sour beers, like lambic, all these microorganisms work together in a mixed culture, each adding its contribution over months and years. This is why the most complex sour beers are so rich: they combine the acid of the bacteria, the funk of Brett and the alcohol of the yeast. We cover the role of Brett more in Brettanomyces in beer, and the whole family in sour beers. Understanding the division of roles between bacteria, yeast and Brett is the key to grasping how complex sour beer is made.

How to sense it in the beer

The influence of bacteria is easiest to sense in sour beers. A clean, refreshing, lemony sourness without wild notes is often a sign of Lactobacillus and the kettle sour method, as in a simple gose or Berliner Weisse. A more complex, deep, slightly wild sourness with funk is the domain of Pediococcus and mixed cultures ageing in barrels, as in lambic. If a beer is simply cleanly sour and refreshing, it was probably made by the fast method; if the sourness intertwines with a wild depth, it is the longer, barrel road. It is worth comparing a simple gose with a complex lambic, to feel the difference. We cover the whole process more in how beer is made. Over time you will start to recognise whether the sourness is simple and clean or complex and wild, and link it to particular bacteria.

The essentials in brief

Let us gather it up. The sour taste of beer comes not from hops or yeast but from lactic acid bacteria, which eat sugars and produce lactic acid. Lactobacillus is fast, gives a clean sourness and can sour a beer in less than a day, which is why it stands behind styles like gose and Berliner Weisse and behind the kettle sour method. Pediococcus is very slow, works for months in barrels, gives a complex, deep sourness and is often accompanied by Brettanomyces, as in lambic. Bacteria do not like hops, so sour beers are low in hops. In traditional sour beers bacteria, yeast and Brett work together. Now you know where sour beer comes from and why some are made in a week and others in a year.

Note every beer in GustoNote - the style, the kind of sourness and the character you sense. Over time you will start to recognise clean and complex sourness and understand more deeply how bacteria build the flavour of sour beer.