Figure 8. The two types of die designs used for co-extrusion. "A" demonstrates the extrudate being turned 90° and "B" shows the center filling being turned 90° (Abbott, 1987).

Figure 8 demonstrates the two methods available for die design. Both are based on turning the product 90 degrees in the final design. The design shown in Figure 8b is more preferred in that the smaller of the two components is turned 90 degrees as compared to designing a entire extruder to flow downward towards the floor. The overall principles behind co-extrusion is shown in Figure 9. Briefly, the crispy expanded outside of the product is kneaded into a cooked gelatinized dough forming around the smaller barrel inside the extruder that contains the filling. The inside barrel protrudes further out from the larger extruder barrel and fills the expanded outside with the desired filling. The product is then cooled enough to cut with a rotary blade before going to the dryer. To achieve the most space for remixing, it is desirable to have the center entry point as far away from the final die as possible. This, however, leads to significant passage of heat form the extrudate to the center filling, giving problems with cooling of the inner wall of the extrudate and excessive temperature rise of the center. A reasonable compromise would be a distance of 40 mm (Abbott, 1987).

Figure 9. Cross section showing the expansion effect of a aerated filling in the process of co-extrusion. "D" refers to the diameter of the extruded outside layer and "d" is the diameter of the filling extruder barrel (Abbott, 1987).

Extrusion cooking technology has made great advancements in producing snack type products which are inexpensive to produce, made continuously and have a wide range of food applications. Co-extrusion cooking extruders can only enhance this field and continue to meet the demand for innovative snack, cereal, pet foods and candy products (Abbott, 1987).

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