I am curious to know the correct usage of these words as it seems to be misused often. See https://homebrew.meta.stackexchange.com/q/202/59 for a related question.

3 Answers 3


From Dictionary.com and the Free Online Dictionary: fermenter: any agent or substance, such as a bacterium, mold, yeast, or enzyme, that causes fermentation; or, (also fermentor) an apparatus that maintains optimal conditions for the growth of microorganisms, used in large-scale fermentation and in the commercial production of antibiotics and hormones.

So fermenter can mean either the enzyme or the fermentation apparatus, whereas fermentor can only be used to mean the apparatus.

  • 1
    @Joe: I checked up on the Homebrewing site, and found while brewchez is right in proposing a fermentor tag as a less ambiguous option than the current fermenter tag, you are right in saying that fermenter is not incorrect from a solely English-concerned standpoint. That said, you homebrewers can decide for yourselves whether the current ambiguity in that tag poses enough of a problem to be worth changing.
    – Daniel
    Jun 25, 2011 at 17:37

Fermentor is a container in which fermentation takes place; fermenter can be that container, or an organism that causes fermentation.


The -er ending in a word usually designates a person who performs some sort of function related to a verb. For example, someone who fishes is a fish-er. Someone who performs carpentry is a carpent-er. Someone who doctors is a doctor-er, but the extra -er isn't usually used.

The -or ending in a word usually designates a machine or piece of equipment associated with a verb. For example, a machine that processes food is a food process-or. A machine that calculates is a calculat-or.

So, when talking about the fermentation of something, the container the fermentation takes place in is a piece of equipment, or a ferment-or. The person (or organism) doing the fermentation is the ferment-er. The final judge of the fermentation is the drink-er:-)

  • Which verb stems get which suffixes to make agent nouns is primarily a matter of the etymological origin of the stem. A third such suffix is -ist, from the Greek -ίστης, though that does not mean it is only for Greek verb stems (e.g., rapist, from Latin rapio). The -er suffix is also used to form comparatives of adjectives. Jun 12, 2014 at 10:23

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