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Fermentation in drinks: yeast and microbes in beer, wine, kombucha and coffee

Beer, wine, kombucha and coffee seem like drinks from completely different worlds, and yet they are linked by one fundamental process: fermentation. It is thanks to the invisible work of yeast and bacteria that sugars turn into alcohol, acids, gases and a whole wealth of aromas. The same principles of microbiology stand behind the head of beer, the depth of wine, the tang of kombucha and part of the flavour of coffee. Understanding fermentation reveals the surprising kinship of these drinks. In this cross-niche post we will look at fermentation in beer, wine, kombucha and coffee: you will learn the role of yeast and bacteria, the mechanism of flavour creation, malolactic fermentation and the SCOBY and the difference between wild and controlled fermentation. It is a journey through one process that links four different drinks and makes them what they are.

What fermentation is

Fermentation is a natural process in which microorganisms, such as bacteria and yeast, break down sugars and other components, producing alcohol, acids and gases. It is one of the oldest techniques known to humanity, used for millennia to create and preserve food and drinks. Fermentation not only transforms the raw material, but also enriches its nutritional value and flavour. In drinks its essence is simple: invisible microbes eat sugars and in return produce new compounds, including alcohol, carbon dioxide, acids and hundreds of aroma compounds. That is why fermentation is such a powerful tool: it turns a simple, sweet raw material into a complex, aromatic drink. Without it there would be no beer, wine, kombucha or fermented coffee. Understanding that fermentation is simply the controlled work of microorganisms on sugars is the key to grasping what links these four drinks. It is a common, fundamental process that, despite different raw materials and traditions, works everywhere according to the same biological principles.

Yeast - the main heroes

The main heroes of the fermentation of alcoholic drinks are yeast. They are single-celled fungi that eat sugars and turn them above all into ethyl alcohol and carbon dioxide. It is thanks to yeast that beer and wine contain alcohol, and beer additionally bubbles. But yeast does far more than just alcohol: along the way it produces a whole range of aroma compounds, including esters giving fruity and floral notes and other flavour-shaping compounds. That is why the type of yeast used and the conditions of fermentation so strongly affect the character of the drink. Different yeast strains give different flavour profiles: some clean and neutral, others fruity, spicy or downright wild. Yeast is also key in kombucha, where it cooperates with bacteria. Understanding that yeast is not only a producer of alcohol, but also a creator of flavour, is the foundation of appreciating fermentation. It is they, invisible to the eye, who do most of the work turning a sweet raw material into an aromatic drink. Yeast are true, if microscopic, masters in the world of fermented drinks.

Bacteria - acids and complexity

Alongside yeast, the second great group of fermentation microbes is bacteria, which bring above all acids and complexity to drinks. While yeast specialise in alcohol, bacteria often produce organic acids, like lactic or acetic acid, giving drinks tang and character. It is lactic acid bacteria that stand behind the sour taste of many sour beers and pickles, and acetic acid bacteria behind the vinegary note of kombucha. In wine bacteria are responsible for the important malolactic fermentation, of which more in a moment. Bacteria can also add depth, funky notes and unexpected complexity to drinks, though in uncontrolled conditions they are also a source of faults. That is why some drinks, like kombucha or sour beers, rest on the cooperation of yeast and bacteria. Understanding the role of bacteria completes the picture of fermentation: it is not only yeast and alcohol, but also bacteria with their acids. Together these two groups of microbes create the full spectrum of fermentation possibilities, from pure alcohol to complex tang. Bacteria are often underrated, yet key co-creators of the flavour of fermented drinks.

Fermentation in beer

Beer is one of the classic fruits of fermentation. After brewing a sweet wort from malt, yeast eats the sugars contained in it, turning them into alcohol and carbon dioxide, and along the way into aroma compounds. This is the basic fermentation of beer. The type of yeast decides the style: top-fermenting yeast gives ale-type beers, often fruity and spicy, and bottom-fermenting yeast lagers, clean and dry. But beer is also a world of bacteria: sour beers, like lambics or gose, are created thanks to lactic acid bacteria and wild yeast, like Brettanomyces, which give a sour, funky character. Brettanomyces, wild yeast, is famous for quickly eating sugars and creating complex, wild notes in sour beers. Fermentation in beer is thus a whole spectrum: from the clean work of noble yeast to the wild cooperation of yeast and bacteria. It is what gives beer alcohol, bubbles and a huge part of its flavour. We write more about beer yeast in our post on beer yeast.

Fermentation in wine

Wine is the second great world of fermentation, here based on sugars from grapes. Yeast, naturally present on the skins or added, eats the fruit sugar, turning it into alcohol and aromas. This is the basic alcoholic fermentation of wine. But in wine a second, less known but important fermentation also takes place: malolactic. It is a process in which lactic acid bacteria turn the sharp malic acid, naturally present in grapes, into milder lactic acid, making the wine smoother and softer. Malolactic fermentation is standard in almost all red wines and some whites, like buttery Chardonnay. It is a great example of how bacteria, rather than only yeast, shape a drink. Wine thus shows two faces of fermentation: the yeast one, giving alcohol, and the bacterial malolactic one, softening acidity. Both processes are key for the final character of the wine. Understanding their role reveals how complex the microbiology in a glass of wine is. We write more about this in our post on malolactic fermentation.

Fermentation in kombucha

Kombucha is a fascinating example of fermentation based on the close cooperation of yeast and bacteria. Its heart is the SCOBY, that is the symbiotic culture of bacteria and yeast. The process is two-stage and takes place in sweetened tea. First the yeast contained in the SCOBY metabolises the sugar, producing alcohol and carbon dioxide. Then the acetic acid bacteria oxidise this alcohol into acetic acid and other organic acids, which give kombucha its characteristic tang and lower its pH. It is precisely this two-stage cooperation, yeast create alcohol, bacteria turn it into acids, that makes kombucha what it is. The diverse microorganisms in the SCOBY give kombucha a complex flavour profile. Kombucha is an excellent example of symbiotic fermentation, in which two kinds of microbes cooperate, giving a drink different from each of them alone. It links the world of tea with fermentation, much as beer links malt with yeast. We write more about this drink in our post on kombucha.

Fermentation in coffee

Coffee is the least obvious but real example of fermentation. It takes place at the processing stage, when freshly picked coffee cherries are processed into green beans. During processing, especially washed and natural, naturally present yeast and bacteria ferment the sugars contained in the fruit pulp surrounding the bean. This fermentation affects the final flavour of the coffee. What is more, deliberate, controlled fermentations with the addition of specific yeast or bacteria cultures are increasingly used, for example in anaerobic processing. It has been shown that the use of bacteria or yeast as starter cultures in the fermentation of coffee beans enhances aroma, flavour, phenolic content and antioxidant properties. This opens a fascinating world of experiments, in which fermentation becomes a tool for shaping the flavour of coffee, much as in beer or wine. Coffee shows that fermentation is not only alcoholic drinks, but also a process key for the flavour of the bean. We write more about this in our post on anaerobic coffee processing.

Microbes create flavour

Fermentation is not only the production of alcohol or acids, but above all the creation of flavour. Microbes guide the process and influence its aroma, and different microorganisms produce distinct flavour compounds. Yeast, along with producing alcohol, create esters, giving fruity and floral notes, and other aroma compounds. Bacteria produce acids giving tang and freshness. That is why the same raw material, fermented by different microbes, gives drinks of extremely different flavour. The choice of yeast or bacteria strain is for the brewer, winemaker or kombucha producer one of the most important decisions, because it is the microbes that largely create the aromatic profile of the drink. Wild cultures give complex, unpredictable notes, and selected ones clean, repeatable flavours. Understanding that fermentation is above all the creation of aromas, rather than only alcohol, reveals its true role. Microbes are invisible cooks who, from a simple, sweet raw material, create a complex bouquet of flavours. It is they, rather than the raw material itself, that decide a huge part of the character of a fermented drink.

Four drinks, one process

Let us gather fermentation in four drinks in one place, to see the common pattern:

The table shows the common denominator: in all four drinks microorganisms process sugars, creating alcohol, acids, gases and aromas. They differ in raw material and the proportion of yeast to bacteria, but the principle is the same. It is proof of how one biological process links seemingly distant drinks, from beer to coffee.

Wild versus controlled

An important distinction in fermentation is the division into wild and controlled. Wild fermentation rests on naturally present, spontaneous microorganisms in the environment, without adding specific cultures. This is how spontaneous lambics, some wild-yeast wines or traditional kombuchas are created. It gives complex, unpredictable, unrepeatable flavours, but also a greater risk of faults and less control. Controlled fermentation consists in adding selected, known cultures of yeast or bacteria, which gives a repeatable, predictable and cleaner effect. Most modern beer, wine and coffee is fermented in a controlled way, for certainty and quality. Both approaches have their supporters: wild is prized for character and authenticity, controlled for reliability. This tension between wildness and control runs through all four drinks. Understanding this difference helps grasp why some drinks are so unpredictable and unique, and others stable and repeatable. It is another dimension of fermentation, common to beer, wine, kombucha and coffee, showing how much depends on which microbes we hand the steering of the process to.

The key points in a nutshell

Fermentation is a process in which microorganisms, yeast and bacteria, break down sugars, creating alcohol, acids, gases and above all aromas. It links four drinks: in beer yeast give alcohol, CO2 and esters, and bacteria the acidity of sour beers; in wine, alongside yeast fermentation, malolactic takes place, in which bacteria soften acidity; in kombucha a SCOBY combines yeast and bacteria, giving acids and light alcohol; in coffee microbes ferment the fruit pulp, shaping the flavour of the bean. Different microbes create different flavours, and fermentation can be wild or controlled. It is one biological process linking seemingly distant drinks. Want to record how fermentation shapes the flavour of your drinks? Keep notes in the GustoNote app. See also our posts on beer yeast and malolactic fermentation.