The invention relates to a device (1) for manipulating a beverage dispenser (100). The invention also relates to a beverage dispenser (100) and a method for operating a beverage dispenser (100). A device (1) is proposed for changing the cross-section (2) of a section (3) of an outlet channel (4) of the beverage dispensing system (100) in order to reliably prevent in a simple manner an undesired mixing of liquids to be dispensed in a beverage dispensing system (100), especially for the purpose of briefly generating a negative pressure in the outlet channel (4).
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