Methodology For Htst Extrusion Cooking

Before beginning the HTST extrusion cooking process, the extruder and its related equipment must be assembled first. All fasteners should be tight and all parts aligned. If there is a torque specification for the retaining bolts, it must be closely followed using a torque wrench. After the barrel and screw(s) are assembled, the screw should be turned manually to ensure that it moves freely. The die plate with its die inserts is then bolted to the end of the extruder barrel. The rotating cutter assembly is installed and the cutting knives are adjusted to provide a cleanly cut product and an extended life for the blades. Water and steam lines are then connected to the extruder barrel as required. Condensate in the steam line must be removed. All feeders and regulators should be checked. If a volumetric feeder is used for dry ingredients, it should be calibrated first for each batch of feed materials. A typical Wenger extrusion cooking system is shown in Figure 1.

To minimize the amount of waste produced during the start-up, the cooking extruder is usually brought to operating condition and equilibrium as quickly as possible. This can be accomplished by preheating the barrel and die plate to the desirable operation temperature. The water pump is then started at a rate about 1.5 times the target operating rate. Once the water is in the extruder, start the extruder and the feed stream at a low rate. Increase the screw speed and feed rate, but reduce the water rate to operating levels gradually. When the barrel and die have reached stable operating temperatures, adjust the water rate, feed rate, and screw speed to obtain the desired product at a moderate torque level (90% or less motor load).

Similar to the start-up procedures, shutdown procedures for HTST cooking extruders must also be followed in sequence. Water is gradually increased first to allow uncooked material to purge the cooked material from the system. Barrel temperatures are reduced to stop cook reactions, which can prevent burning on after shutdown. When the percent load meter readings are below 25%, decrease the screw speed and shut off the extruder; in addition, shut off the feeder and water pump. The die plate is then removed by gradually and carefully undoing the bolts because some pressure may still exist behind the die plate.




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  • Kaarlo
    Is extrusion cooking a htst?
    7 years ago

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