Centrifugal extrusion

Centrifugal extrusion is another encapsulated technique that has been investigated and is currently used by some vitamin manufacturers for the encapsulation of vitamin A acetate. The device used in this encapsulation technique consists of a concentric feed tube through which coating material and core material are pumped separately to the many nozzles mounted on the other surface of the device. Core material flows through the centre of the tube; coating material flows through the other tube. The entire device is attached to a rotating shaft such that the head rotates around its vertical axis. As the head rotates, the core material and coating material are co-extruded through the concentric orifices of the nozzles as a fluid 'rot' of core sheathed in coating material. Centrifugal force impels the rod outward, causing it to break into tiny particles. By the action of surface tension, the coating material envelops the core material, thus accomplishing encapsulation. The capsules are collected on a moving bed of fine-grained starch, which cushions their impact and absorbs unwanted coating moisture. Particles produced by this method have diameters ranging from 150 to 2000 m (Harlowe, 1988; Lyle et al., 1984).

A number of innovative, food approved coating systems have been formulated to encapsulate products such as flavourings, seasonings, vitamins, and many others.

Another extrusion-based development from Southwest Research Institute is a process for encapsulating of water-insoluble liquids as particles of 1 to 15 mm. In this process, a core material is fed down a vertical tube. Simultaneously, the coating material, a viscous solution of sodium alginate, flows through a ring-shaped opening around the base of the tube, forming a membrane across the bottom of the device. The exuding core material bulges the membrane until it eventually breaks off as an unshaped blob, carrying with it a portion of the membrane. Spinning, the particles assume a spherical shape and become encapsulated. Passage through a bath of aqueous calcium acetate (or calcium glutamate or calcium lactate) finishes the film- forming process by converting the coating to water-insoluble calcium salt.