Ich kenne dafür nur accidental fermentation als häufigen Begriff. Ob das auch ein Fachbegriff ist, weiß ich nicht. Gesprochen wird auch von unwanted/undesirable fermentation.

Vielleicht ist ja jemand aus einer typischen Branche (außer Backhandwerk z.B. auch Brautechnik oder Weinherstellung) besser vertraut mit der Terminologie.

I am wondering about this. To this non-baking expert, "accidental fermentation" sounds like unplanned fermentation, which doesn't seem to stress the same information that "Fremdgärung" does - there, the emphasis seems to be on the fact that it is fermentation involving the action of non-culture microbes (undesired fermentatives) in a context where fermentation by the culture microbes is desired. I can see, though, "Fremdgärung" would always be accidental, as you said it leads to spoilage, so perhaps it is a distinction without a difference.

EP0346909A2 - Fermentative procedure and additive for preserving fodder - Google Patents

Rhizopus Microsporus Var. Oligosporus - an overview | ScienceDirect Topics

Thank you all very much.

I think for colloquial use depending on context #1 accidental or unplanned fermentation is perfect. The right word in a more scientific context should definitely be #4 misfermentation.