Beer 101: Fermentation

I know I’m skipping around a little bit (after all, there are several steps that come before fermentation in the brewing process), but it’s kind of because fermentation is my favorite. After all, it’s responsible for some of the best-tasting things in life: beer, wine, cheese, and chocolate to name a few. It’s a  crucial step, chemically, as it creates the alcohol that we all enjoy (presumably you enjoy alcohol – you are on a beer blog).

Beer 101 Fermentation
Photo from uncommoncaribbean.com

The Basics

Fermentation, at its very simplest, is the chemical process by which yeast converts the malts‘ sugar (glucose) into ethyl alcohol and C02 gas – this yields both the alcohol content and the carbonation that make beer, well, beer.

 

We’ve explained a little bit that temperature is important for this process, depending on the kind of yeast being used and the beer style that is being made – an ale needs to be kept at 68 degrees Fahrenheit for about two weeks and a lager needs to stay at 48 Fahrenheit for about six weeks. This process creates a lot of heat as a byproduct and so the container in which the fermentation happens generally needs to be carefully cooled.

To avoid contamination by stray, wild yeasts, fermentation tanks are generally sealed off from the air with only a small vent for the CO2 buildup to exit the tank.

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The Details

I’m no chemist, so I’m not going to go into that level of detail! However, there are some interesting things that go on during the fermentation process that need to be addressed. Yeast basically works in two stages during fermentation: the primary stage and the anaerobic phase.

In the primary stage, the yeast consumes all of the oxygen in the cooled and aerated wort mixture. During this stage, sterols (which are a type of cholesterol that make up part of the yeast’s cell wall) are produced. These sterols allow the cell wall to be permeable so sugar and alcohol can move in and out of the yeast cells; they also allow the yeast to survive in an increasingly alcoholic environment

Once that oxygen is gone, the yeast moves into the anaerobic phase, during which most of the sugars in the wort are turned into ethanol and CO2. Additionally, flavor compounds like esters (fruity notes) , diacetyl/ketones (butterscotch notes), fusel alcohols (responsible for a hot or burning sensation), and other chemicals that can make or break the flavor of a final beer.

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Other Uses for Fermentation

Cacao seeds must be fermented (often out in the sun, on large tarps) before being dried and then roasted in order to create the chocolate flavor that we know.

In winemaking, fermentation begins naturally when the skin of grapes is broken and the wild yeast on them and in the air begin the primary fermentation stage, turning sugar in to alcohol.

Fermentation happens several times along the journey from milk to cheese, developing flavors and even creating the famous holes in Swiss cheese.

The workhhorse of bread making, yeast-based fermentation creates the textures, flavors, and rise in bread doughs.

Pickles, sauerkraut, kimchee, and more can be fermented during the pickling process, allowing natural bacteria to create acids needed to preserve foods.

 

Beer 101: Yeast

Yeast is one of the four main ingredients that go into making beer, beautiful beer. The others are hops, malt, and water (this post is next in my Beer 101 series). There are certainly other ingredients that can be added to the beer process, but these four are the core pillars that hold up the whole thing.

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These itty bitty single-celled microrganisms are technically classified as a fungi. They reproduce by an asymmetric division process called budding. Their job is to convert fermentable sugars from the malt into alcohol and other byproducts. There are hundreds of varieties and strains of yeast out there, some of which are commonly used to brew beer.

Yeasts are generally put into one of two categories: ale yeast (top fermenting) or lager yeast (bottom fermenting), depending on how they behave during the fermentation process. There’s also a nebulous third category, known as spontaneously fermenting yeasts, which result when beer is left exposed to the air and is literally infected with wild yeast strains as they wander by – this is what creates sour beers.

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Ale yeasts generally sit on top of the beer-to-be, fermenting away between temperatures of 10° to 25°C (though some yeasts won’t activate below 12°C). These guys rise up to the surface, forming a thick raft of a head as they bubble away. These yeasts tend to yield beers higher in esters, which are the chemicals that give fruits their characteristic flavors. In the case of Hefe Weizen beers, the yeast produces the ester iso-amyl acetate, the same one that is found in bananas. Other esters include ethyl acetate, which can be flowery, and ethyl caproate, which is kind of wine-like and fruity. Top-fermenting yeasts are used for brewing ales, porters, stouts, Kölsch, Altbier, and wheat beers.

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Lager yeasts create much less of a head and tend to settle at the bottom of the tank as fermentation nears completion. They grow less rapidly than the ale yeasts and don’t create that layer of thick foam on top of the beer. These yeasts work at lower temperatures, around 7° to 15°C.

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In addition to making beer the alcoholic beverage that we so enjoy, it also has a large impact on the flavor of the final beer. The flavor and aroma of beer is complex and is influenced by  many factors, including malt, hops, and the yeast strain. The synthesis of yeast creates many byproducts, including ethanol (alcohol), CO2 (carbon dioxide), and also some flavor compounds like clove, butterscotch, and green apple.

Yeast may be tiny and invisible to the naked eye, but it plays a huge role in making beer what it is.