Container assembly for mixing substances

A portion dispensing container assembly includes a container that has an externally threaded finish portion and a closure main body that is mounted to the finish portion. The closure main body has a storage space defined therein in which a concentrate may be stored. A plunger member is mounted for linear movement within the closure main body between a first upper position and a second lower position. In the second, lower position the plunger member causes the storage space two open into communication with the interior of the container. A tamper evident interlock is further provided to prevent movement of the plunger member to the second, lower position until a consumer removes the tamper evident interlock.