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===Refermentation===
Whenever fruit is added to a a beer that hasn't been pasteurized, a re-fermentation will occur because new sugars from the fruit are introduced to the microbes that are still alive in the beer. This re-fermentation can take some time to being, and some time to finish, depending on the beer. In general, the fruit should be allowed to finish fermenting before packaging. Regular beers and kettle sour beers often contain living ''S cerevisiae'' as long as they are not aged for too long (less than a month). [[Mixed Fermentation]] sour beers where ''Brettanomyces'' was pitched will most likely still contain living ''Brettanomyces'' cells even after a year or two of aging. In the case of ''Brettanomyces'', only a small population of surviving cells is enough to kick off a re-fermentation. For beers that have aged for a very long time in very sour conditions, fresh ''Brettanomyces'' or wine yeast can be pitched to ensure re-fermentation of the fruit.
When adding fruit to a non-pasteurized beer that does '''not''' contain living ''[[Brettanomyces]]'' but does contain living ''S. cerevisiae'' (for example, kettle sours), a re-fermentation will often occur a day or two after adding the fruit. This re-fermentation can last 1-2 weeks until the sugars from the fruit are fermented. Sours such as kettle sours that do not contain living ''Brettanomyces'' can be treated like adding fruit to any other type of "clean" beer (see [https://www.homebrewersassociation.org/how-to-brew/how-to-add-fruit-to-beer/ this article], for example). Beers that are fully pasteurized, for example from a flash pasteurizer, or aged for longer than 6 months or so, will not re-ferment the fruit because all of the yeast is dead.