BBM SourTree 1.0a

© 2020, Randy Mosher / Craft Beer & Brewing Magazine

While sour beers are unlikely to ever dominate our beer landscape, they’ve emerged from the shadowy backwaters and taken their rightful place on the shelves alongside so-called “normal” beers.

Sour beer can be a lot of things, brewed and fermented in countless ways. Rather than run through them all in text, I thought it would be easier to just diagram their relationships (above). Broadly, they break into three large groups: brett beers, which aren’t generally sour; cultured sour beers of various types; and spontaneously fermented beers. Since we’re mainly focused on flavor here, we’ll not dive into their history, fascinating as it is. We’ll stay focused on the huge range of aromas, microbial and otherwise, along with nuances of mouthfeel and the particular differences in sour taste contributed by different microbes. 

Most sour beers start with a conventional yeast-driven fermentation. While wild beers can have hundreds of different species contributing to their flavor profile, three microbial families are most important: Lactobacillus, Acetobacter and the wild yeast, Brettanomyces. While some beers are produced using brett or lacto as their only non-standard microbes, Acetobacter is never the star of the show and is always part of an ensemble of other microbes, along with specific conditions, especially extended wood aging. 

Lactobacillus, a bacterial genus, along with the closely related Pediococcus, produce lactic acid as a byproduct of their metabolism. Found in cultured diary products like cheese and yogurt, various species of lacto provide souring for Germanic sour beers like Berliner weisse, gose and a handful of others. They can also produce buttery-smelling diacetyl, which you’ll find in a lot of California wines, both red and white, where malolactic fermentation softens the wine and knocks months or years off its aging. 

Acetobacter mainly oxidizes alcohol into acetic acid, the dominant sensory chemical in vinegar. Being a functional aerobe, it needs oxygen to do its thing. It can’t survive in stainless tanks, which is why it’s rarely a contamination problem in conventional beers. Wood, however, offers an oxygen-permeable medium, which explains its presence in barrel- and foudre-aged beers. Because wood-aged beers often have some Brett present, as long as it is alive and active it will scavenge any available oxygen and keep the Acetobacter in check. When brett dies, O2 levels rise, enabling the Acetobacter to start producing acetic acid. A touch can be lovely, but it can quickly dominate and become unpleasant. 

Brettanomyces is a yeast family, not too distantly related to brewer’s yeast, but with some important differences in its flavor profile and preferred diet. Its scientific name means “British fungus,” memorializing its discovery in wood-aged English old ale. It may have been responsible to the prized “stale” quality in such beers, as sourness was generally regarded as a flavor defect. Brett generally produces little or no obvious acidity, but it does add interesting aromas. As an uninvited guest, it is a pernicious beer spoilage organism, slowly fermenting starches that normal brewer’s yeast can’t metabolize, sometimes adding harshly smoky or Band-Aid phenolic aromas in the process. 

In terms of sour/funky beer flavor, let’s start with taste. With the exception of brett beers, which have taste profiles similar to normal ones, acidity is the dominant taste. Fermentation mainly produces two acids, acetic and lactic, while others like citric, malic and others may come from the fruits commonly used in many sour styles. Each has its own set of characteristics. 

pH, a logarithmic scale of proton (hydrogen ion) concentration, is the basis for acidity/alkalinity, and is the common measure of acidity in brewing, scientific endeavors and many other applications. pH, however, is a poor yardstick of perceived sourness. Our sense of taste measure sourness by counting intracellular protons, but not all have equal access to our sour receptors inside taste cells. Oddly, protons not bound to molecules have a hard time getting through the cell membrane. As a result, strong mineral acids like hydrochloric have less access to our sour receptors than the organic acids in sour beers, which come across as more sour than their pH would suggest. Each different type of organic acid has a different permeability, and therefore a different perceived sourness relative to pH. 

All strong acids create irritating trigeminal/mouthfeel sensations as a defense, whether swallowed or inhaled. As with aromas, organic acids like acetic have differential access to the sensory cell’s interiors, and can be irritating even at fairly low levels. This is why you may choke up if something like salad dressing or a Belgian sour red ale goes down the wrong way, triggering coughing. Further, there are two different trigeminal receptor types for this: TRPV1 and TRPA1, the capsaicin and mustard oil receptors, adding another layer of differences between acid types. 

A final complication is that citric and malic (apple) acids may enhance a dry, puckering astringency in some fruited, wood-aged sours. All of these variables alter the overall character of an acid, making acetic especially pungent and sometimes harsh, while citric and malic have a bit of a sharp and raspy edge, respectively. So regard any numerical figure of a beer’s acidity with suspicion.

Tannic character is much beloved in wine, forming an important balance element against the acid in wines—and in wood-aged sour beers as well. I have seen a wine vocabulary wheel that lists 19 different tannin-specific vocabulary terms, mostly falling into two camps: smooth and harsh. The science, as it is known right now, suggests that there might be two mechanisms, roughly corresponding to these two categories; mix the two and you get a continuum. It hardly needs to be said that the smooth type is preferred. Wood does a number of other things for sour beer, harboring microbes in its porous structure, and allowing a diffusion of oxygen for organisms like acetobacter and brett. 

The normal interplay of sweet malt vs. bitter hoppiness goes out the window in many of these beers, especially the very sour ones. What constitutes balance in sour beer is impossible to quantify, but it’s important just the same. It might just be easier to describe the opposite: lopsided, incoherent, overwhelmed by (acidity, bitterness, tannin…).

Now, aroma. People often refer to the “funk” in many of these beers, and it’s a fine word as long as you see it as a neutral term and not invariably positive. There are a lot of funky smelling things I would not want to put in my mouth. 

Let’s start with Brettanomyces aromas, which can be found in many wild and other complex sour beers as well as in simpler brett-focused ones. Most common are a distinct barnyard scent called “horsey” by people who know what a horse actually smells like, and a sort of roasted pineapple candy note that always brings up a childhood memory of a Dum-Dum sucker offered as a reward upon leaving the doctor’s office. Mousy or Cheerio character can come from brett, lacto, Maillard browning and likely other sources as well, due to a family of chemicals called tetrahydropyridines (THPs). Most of these, fortunately, will age out over a period of months. 

Lactic acid from Lactobacillus can offer up a pleasant sour cream or yogurt note that can add to the richness of kettle-soured Weisse or gose. While we discussed acetic acid in the context of taste and mouthfeel, it is also a meaningful aromatic. While it’s clearly vinegary, I usually detect it as a whiff of pickle. 

A chemical called methanethiol (methyl mercaptan) smells of old sewer pipe or garbage truck. It can add an unsettling note to kettle-soured beers, especially if the souring process is not well protected from oxygen. As a sulfur chemical, it’s super potent, but not everyone can pick it up. 

Continuing with the stench theme, butyric acid is usually described as either vomit or Parmesan cheese and there is, in fact, a famous scientific study on how people perceive it based on which descriptor is used. Some liken it to rancid butter, but my perfect descriptor is the transparent handle of a quality screwdriver, commonly made from a stinky plastic called butyrate. This scent is easily confused with isovaleric acid, which smells like body odor and stinky cheese because it’s produced by bacteria inhabiting both ecosystems. 

Acetaldehyde is a common beer off-flavor, most commonly associated with “green,” or incompletely conditioned beer. I frequently find it in barrel-aged sours, sometimes at quite high levels. While it’s commonly described as either green or bruised apple, I find it chameleonic: wet grass, raw pumpkin, latex paint and drunk breath all capture aspects of its essence. 

Let’s not forget to look for the kinds of things you’d find in a conventional beer: esters, hop aromas, malty/cooked notes and yeast phenols, because they’re likely to be there, even though they may play a secondary role. 

Oak brings some aroma as well, adding sweet-smelling vanillin as its lignin degrades over time and sometimes, especially in spirits barrels, a toasted coconut smell as well. Fresh oak can present dill and butyric notes, most strongly in American white oak barrels. 

Quality is always an elusive characteristic in sour beers, and the extreme nature of these beasts makes them somewhat polarizing. And all sour beers are by no means created equal. The path to better appreciating them is to learn their sensory parts and pieces and observe how they’re assembled, then decide what suits your own personal taste and dive in.