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Fermentors: Glass vs. Plastic

Historically glass has been seen as a better material for fermentors because it does not “breathe,” or allow oxygen to pass through.

Oxygen permeating the vessel can cause an oxidized off-flavor in the beer, which is not desirable.

This is less of a concern during primary fermentation because the fermentation process fills any open space in the vessel with carbon dioxide, blocking out oxygen.

If the beer is transferred into a secondary fermentor to help improve beer clarity after fermentation is complete oxidation can be more of a concern.

In recent times better plastics have been used in the production of plastic carboys, meaning it is possible to buy a plastic fermentation vessel that will prevent oxygen permeation just as well as a glass vessel. Carboys like the Better Bottle have these characteristics.

Given that there are now both glass and plastic vessels that will adequately block oxygen from your beer, what are the pros and cons of each type of vessel?

The Benefits of Glass Carboys

Glass fermentation vessels have many benefits. In addition to being impermeable to oxygen they are theoretically easy to clean and sanitize and will not pick up the odor of their contents. Glass carboys are also very difficult to scratch when cleaning, and they will not deform when you pick them up.

The Problems With Glass Fermentors

If glass carboys have a major downside it is that they are heavy and they can shatter. As if five or six gallons of beer isn’t heavy enough, you have to haul around a few extra pounds of glass. Carboy handles (which you have to buy separately) only make them slightly less awkward to carry than they already are.

Additionally, let’s face it: glass carboys are actually a nightmare to clean. While they can be cleaned more vigorously than a plastic carboy, their narrow mouths make it almost impossible to get at every nook and cranny inside.

The Benefits of Plastic Carboys

The biggest reasons to buy a plastic carboy fermentor are its resistance to shattering, light weight, and the availability of different shapes.

Where the glass carboys have a weakness, the plastic carboys have a strength. If you knock one of these vessels against a granite countertop while setting up a transfer into a bottling bucket it will not break and ruin all of the beer.

The weight of a plastic carboy is dramatically lower than with glass. In fact, plastic carboys weigh roughly 80 percent less than their glass counterparts.

Finally, and perhaps most importantly, plastic carboys can be manufactured with an extra-wide mouth to dramatically lessen the burden of cleaning them. Examples like the Big Mouth Bubbler have revolutionized fermentors in recent years.

The Problems with Plastic Fermentors

Plastic can scratch, allowing bacteria to hide and grow inside, and infect beer. That is the number one concern with plastic carboys. Also, they will deform a little bit if you pick them up while full, which shouldn’t cause too much of a problem but can throw things off balance.

As long as you are using a brewing-specific PET plastic carboy, as opposed to an empty that you took from the office water cooler, oxygen permeation should not be a problem.

What kind of fermentation vessel do you use? What has been your experience with it?

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