A rotary introducer for introducing a trackable element and/or contaminant on or into a flow path of a food-transit system without or substantially without adjusting a flow-rate, the rotary introducer comprising: an inlet for receiving a trackable element and/or contaminant; an outlet which is positioned radially inwardly of the inlet and which is communicable with a food-transit conduit to discharge the trackable element and/or contaminant thereinto; a rotatable driving member having a first device-contact surface which is movable between a driving-member start position at or adjacent to the inlet and a driving-member stop position; and a rotatable drivable member having a second device-contact surface which is movable between a drivable-member start position at or adjacent to the inlet and a drivable-member stop position at or adjacent to the outlet; the driving-member start position and the drivable-member start position being angularly displaced from each other; the driving-member stop position and drivable-member stop position being coincident or substantially coincident; the first and second device-contact surfaces (72, 74) opposing each other and defining a volume-adjustable device cavity; wherein, in a trackable-element receiving condition, the inlet is open to receive a trackable element and/or contaminant in the device cavity due to the first and second device-contact surfaces (72, 74) being at the angularly displaced driving-member and drivable-member start positions respectively, and the outlet is closed by at least one of the driving and drivable members (14, 16); and in a trackable-element discharge condition, the outlet is open with the second device-contact surface being at the drivable-member stop position and the first device-contact surface being urged towards the driving-member stop position such that a trackable-element discharge force is directed radially inwardly through the outlet to discharge the trackable element and/or contaminant from the device cavity, thus enabling the first device-contact surface to continue to the driving-member stop position causing the outlet to close. an inlet for receiving a trackable element and/or contaminant; an outlet which is positioned radially inwardly of the inlet and which is communicable with a food-transit conduit to discharge the trackable element and/or contaminant thereinto; a rotatable driving member having a first device-contact surface which is movable between a driving-member start position at or adjacent to the inlet and a driving-member stop position; and a rotatable drivable member having a second device-contact surface which is movable between a drivable-member start position at or adjacent to the inlet and a drivable-member stop position at or adjacent to the outlet; the driving-member start position and the drivable-member start position being angularly displaced from each other; the driving-member stop position and drivable-member stop position being coincident or substantially coincident; the first and second device-contact surfaces (72, 74) opposing each other and defining a volume-adjustable device cavity; wherein, in a trackable-element receiving condition, the inlet is open to receive a trackable element and/or contaminant in the device cavity due to the first and second device-contact surfaces (72, 74) being at the angularly displaced driving-member and drivable-member start positions respectively, and the outlet is closed by at least one of the driving and drivable members (14, 16); and in a trackable-element discharge condition, the outlet is open with the second device-contact surface being at the drivable-member stop position and the first device-contact surface being urged towards the driving-member stop position such that a trackable-element discharge force is directed radially inwardly through the outlet to discharge the trackable element and/or contaminant from the device cavity, thus enabling the first device-contact surface to continue to the driving-member stop position causing the outlet to close.