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Why Does Beer Have to Be Fermented Twice?

Homebrew forums are a great source of information if you are trying to figure out how to make your own beer. They are packed with a lot of great questions and equally great answers. One of the more common questions is this: why does beer have to be fermented twice? This post will answer that question.

Truth be told, the question is a red herring. Beer is not actually fermented twice. People get confused by two common industry terms: ‘primary fermentation’ and ‘secondary fermentation’. They do not realize that ‘primary’ and ‘secondary’ do not indicate separate fermentation processes. The terms describe two stages of the same process.

What is fermentation?

Before getting into the differences between primary and secondary fermentation, it is important to establish just what fermentation is. From a scientific standpoint, fermentation is any metabolic process capable of producing chemical changes within an organic substance by way of enzyme action.

A simple way to understand fermentation in beer and wine making is to discuss it in terms of yeast and glucose. When you add yeast to wort or mulch, it immediately begins interacting with the sugars it encounters. It converts the sugars into alcohol. That is pretty much it in a nutshell. Alcohol fermentation is the process of converting glucose into ethanol and oxygen.

What is primary fermentation?

The reaction between yeast and sugars occurs quickly and aggressively. In the primary fermentation stage, about 80% of the alcohol is produced. Wort foams up as the yeast quickly digests the sugars and convert them to alcohol and oxygen. Depending on the type of brew being made, primary fermentation can take anywhere from four days to two weeks.

During this time, the yeast multiplies at a fairly rapid rate. At some point though, reproduction peaks. The yeast then start to die off because there isn’t enough food (sugar) to sustain all of it. The dead yeast sinks to the bottom of the fermentation tank and becomes part of the sediment.

The conclusion of primary fermentation leaves a beer product with heavy sediment but not full alcohol content. At this point, it can be transferred into a new tank for secondary fermentation. Some breweries use a uni-tank breweries, which Houston-based CedarstoneIndustry says accommodates both primary and secondary fermentation without the need to transfer.

What is secondary fermentation?

Secondary fermentation is not a separate process. Instead, it completes primary fermentation in an environment with less oxygen and more pressure. The secondary process takes at least two weeks to complete. Some lagers need up to eight weeks for secondary fermentation. The reason it takes so long is because the yeast is running out of sugar to feed on.

By now you might be wondering why a brew master doesn’t simply let primary fermentation go until it’s complete. The answer lies in the sediment. Allowing fermentation to run its full course without separating it from the sediment would both ruin the taste and make it more difficult to condition the beer.

When a separate fermentation tank is used, the beer is transferred to it free of the sediment. When a uni-tank is used, sediment is removed from the bottom of the tank before secondary fermentation commences. Either way, removing the sediment is key to secondary fermentation.

Why does beer have to be fermented twice? It doesn’t. And in fact, it’s not. Primary and secondary fermentation are merely two phases of the same process. Sediment is produced during primary fermentation. It is removed before secondary fermentation occurs. When all is said and done, you have a beer that is fully fermented and ready for conditioning.

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