Extrusion technology has offered food manufacturers exciting new opportunities in markets where consumers clamour to try new exciting eating experiences. Snack and breakfast cereals created using twin screw extrusion technology are already massively produced in U.K and America and has very vast potential in India and other developing countries. Confectionery and petfood are also equally common products of this process. The extrusion process is continous- the materials fed to extruder inlet constantly being conveyed, compacted & sheared to carry out cooking/gelatinisation of the ingredients and being extruded out though suitably shaped die fitted at the outlet of extruder, a constantly rotating knive cutting the extruded product into small beads for further treatment ahead.

A typical example of the use of cooker extruder is shredded cereal production. Typical raw-materials used for extrusion cooking are maize, rice, potato, wheat and other flours premixed with proteins, vitamins and moisture (water). The latter is most important constituent of the raw-material to be fed to the extruder. The Twin screw extruder (TSE) is usually fitted with twin co-rotating, self wiping screws which intermesh fully. The Extrusion barrel is provided with electrical heating and also cooling jackets to avoid excessive gelatinisation and proper temperature control of the dough. The TSE processing elements are individually assembled onto central shafts and the modular feature enables to vary the profile and shear input, easily as per product requirements. In comparision to single screw cooking extruders, the twin screw extruder with state-of-the-art self-wiping and corotating screws performs several unit operations within a narrow span of time, the total dwell time of the material in the extruder being approx. 35-40 seconds. The TSE cooks and gelatinizes the feed materials by combined action of heat supplied from the heaters around the barrel, and also be mechanical shearing by the screws.

The initial section of the twin screws and barrel act as feed zone to take the pre-conditioned and premixed material from the feeder above. This is followed by a compression zone which compacts the free flowing material coming from feed zone, into a colloidal dough. There are kneading paddles on the screw shaft which improves the mechanical working of the dough. The material continues to be positively conveyed & transported forward towards the die. The temperature of this moist dough is rapidly elevated in the last 3-5 seconds seconds of the total 35-40 seconds total dwell time within the extruder barrel resulting in a viscoelastic homogenous matrix. For this purpose the last zone on screw consists of screw elements with reduced pitch and volume to compress the material and increase shear. Heat is also added to the material through electric heaters fitted round the barrel and the water cooling jackets serve to remove excessive heat from the barrel and process for proper temperature control of dough. It is critical that dough quality be consistently maintained behind the die to achieve good final product bowl life, texture and nutritional quality. To this end, the modular construction of the twin screws and barrel permit changing of screw profile, necessaciated due to changes in raw-material or product specifications and moisture contents, for desired final dough quality. For RTE cereals of Direct expanded type, the shape of the die orifice determines the shape of product. The continuously extruded product is chopped at the die, by a suitably rotating knives into desired lengths before drying-toasting and coating with flavours, etc.

The corotating twin-screw extruder used to process RTE cereal generally utilize a barrel L/D of 16:1. This configuration is same for producing both flaked and directly expanded cereals.

Typical extrudate moisture content at the die in direct expanded cereals is 10-15 per cent, while flaked product will exit at 25-30 per cent moisture. Direct expanded RTE cereals will require only drying-toasting and coating (if desired) to complete their processing. Flaked product will require further processing through Flaking Roller getting compressed and flattened as it enters the nip of the rollers. It then passes through the toaster-dryer to reduce moisture content and giving the product the crunchy and toasted appearance, and may be subsequently coated with flavours and sugar, etc.

We summarize our discussions as under :

1. The twin screw extruder TSE can effectively & efficiently be used for producing RTE snack food and cereals compared to single screw cooker extruder.
   

2. The TSE cooker extruder can produce snack food of both Direct expanded type and Flaked product. Direct expanded RTE snack food would just require Toasting/Drying and coating with additional flavours, vitamins, sugar, etc (if desired) to complete processing. Flaked product will pass through Flake Roll Mill where shredded, thin flakes are produced from relatively thick dough as the product is made to pass through nip of rollers before being cut into pieces and Toasting/Drying and coating with flavours, vitamins, sugar, etc.
   

3. The raw-material compound, before being fed to the TSE should be carefully premixed and preconditioned in suitable pre-conditioning chamber (blender) which mixes all ingredients viz: flour, proteins, moisture, etc. in required proportions. The single most important ingredient is moisture (water) which determines the degree of cooking (gelatinisation) of compound. Direct expanded cereals typically contain not more than 10-15 percent moisture while Flaked product contain about 25-30 percent moisture.
   
   

4. The TSE converts the free flowing material from preconditioner into colloidal dough (visoelastic mass) by taking heat from heaters and mechanical working by the twin screws. The continuously extruded material could be of direct expanded type or flaked product- continuously cut into small beads (pieces) by the action of rotating knives against the die.
   

5. Different die forms (orifices) can be accommodated at extruder discharge to get various shapes.
   

6. A typical snack food line would essentially consist of a Preconditioner, Twin screw cooker extruder & cutter, Dryer-toaster oven, Flaking Roll Mill, Coating/Drying unit & packaging units.