A sanitary, disposable dispensing assembly especially useful in producing a substantially contaminant-free reconstituted mixture of diluent and pure, preservative-free concentrate includes an inner member linearly slidable between open and closed positions in an outer member provided with a concentrate inlet. The dispensing assembly features a vent arrangement for enabling a complete draining of the mixture and preventing bacterial growth. A rib arrangement improves mixing of the combined diluent and concentrate, and prevents dispersion of the mixture when dispensed. The dispensing assembly is utilized in mechanical pump type and non-mechanical pump type applications on a post mix food and beverage dispenser.