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What is Secondary Fermentation? And Glass vs Plastic Carboys

Sometimes when you pick up a beer ingredient kit at the local home brewing store it recommends a secondary fermentation. What does that mean?

Why Use Secondary Fermentation?

Secondary fermentation is the practice of transferring your beer out of its original fermenting vessel into a different vessel. Somewhat ironically, this should only be done after the actual fermentation of the beer is mostly complete.

Transferring the beer to a secondary fermentor gets it off of the trub that is sitting in the bottom of the primary fermentor and allows any residue, such as hops that were not filtered out, to settle out of the beer. It makes for a more clear and cleaner tasting finished product.

Secondary fermentation also allows you to make adjustments to the flavor of the beer. It gets the beer off of the dead yeast from the primary fermentation to prevent that flavor from overtaking the beer, and it allows you to add spices, fruits, or dry hops to the beer to get the flavor where you want it to go.

The Secondary Fermentation Vessel

Most home beer brewers use glass carboys like this one for their secondary fermentor. Glass is generally seen as best for secondary fermentation because it keeps excess oxygen out. If beer becomes overly oxygenated it can pick up some off flavors.

The size of the secondary fermentor comes in to play also. In general you do not want much head space at the top of the secondary fermentor. Fermentation in the primary fermentor fills the head space with carbon dioxide, but usually there is not much fermentation in the secondary vessel so this space will be filled with oxygen. Once again, too much oxygen will cause problems with the beer.

If you are adding anything with fermentable sugar to the beer, though, such as dried fruits or some type of syrup, you will want some space at the top of the vessel to accommodate any fermentation that might take place as a result.

Have you tried using secondary fermentation? What was your experience with it? Let us know in the comments below!

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