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In wood or steel (or glass or plastic).
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 refermentation 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 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.
Things change a little when adding fruit to aged sours that contain living ''Brettanomyces''. When adding fruit to an aged, non-pasteurized sour beer that contains ''Brettanomyces'', a secondary fermentation will generally occur after a few days of adding the fruit, and will last 1-2 months. A seven-day lag time is not unheard of, as this follows the growth cycle of many ''[[Brettanomyces]]'' strains. The visual indicators of this fermentation will depend on the strain(s) of ''Brettanomyces'' and other microbes that have survived the previous aging process, and can range from active fermentation to developing a pellicle to having no visual fermentation at all. Active fruit fermentation may cause fruit particles to clog the airlock or bung and prevent the vessel from venting pressure. This may cause the airlock to shoot off or worse. The Rare Barrel prevents this problem in vigorously re-fermenting beers by stabling foil over the bung holes of barrels of actively fermenting fruit beer rather than using a bung<ref name='Jester King on the Sour Hour pt. 1'>[http://www.thebrewingnetwork.com/the-sour-hour-episode-14/ The Sour Hour #14: with Jester King, pt. 1]</ref> (~52 minutes in). Raf Soef uses stainless mesh from a tea ball on the bottom of bungs to prevent fruit pieces from clogging the bung or airlock <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1156796387681907/ Raf Soef's tea strainer modification]</ref>. Raf also suggests filling only 80% of the capacity of the vessel with fruit and beer, and then topping up after the re-fermentation of the fruit <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1186697904691755/?comment_id=1186735661354646&comment_tracking=%7B%22tn%22%3A%22R%22%7D Conversation with Raf Soef on MTF regarding re-fermentation of fruit and dealing with blow off. 11/30/2015.]</ref>.