Method and apparatus for producing fully cooked extrudates with significantly reduced specific mechanical energy inputs

Abstract

Improved extruders and methods for the extrusion cooking of comestible products such as human foods or animal feeds are provided wherein the products may be produced with very low specific mechanical energy (SME) inputs as compared with conventional processing. The methods preferably involve introduction of very high levels of steam into the extruder barrel ( 12 ) during processing, which concomitantly reduces necessary SME inputs required to achieve desired cook and expansion levels in the products. In accordance with the invention, fully-cooked pet foods can be fabricated with SME inputs of up to about 18 kWhr/T, whereas aquatic feeds can be fabricated with SME inputs of up to about 16 kWhr/T. In preferred forms, the extruder ( 10 ) includes specially configured, intermeshed extrusion screws ( 18, 20 ) and a plurality of obliquely oriented steam inlets ( 48, 50 ), and an upstream, dual-shaft preconditioner ( 90 ) is used having individual variable frequency drives ( 98, 100 ) which allow infinite variablility in the speed and rotational direction of the preconditioner shafts ( 94, 96 ).

1 . A method of extruding a comestible material through a cooking extruder having an elongated, tubular barrel with an inlet and a restricted orifice die outlet and presenting a longitudinal axis, and an elongated, axially rotatable, helically flighted screw within said barrel, said method comprising the steps of:
 passing said material into said barrel inlet, and conveying said material along the length thereof toward and through said restricted orifice die outlet;   during such passage of said material through the barrel, creating at least one zone within the barrel between said inlet and said outlet and having free volume not occupied by said material; and   injecting steam into said barrel at said at least one zone, with the total amount of steam injected into said barrel being from about 10-25% by weight of steam into said material, based upon the weight of material within said barrel taken as 100% by weight.   
     
     
         2 . The method of  claim 1 , including the step of creating steam flow restriction zones on opposite sides of said one zone in order to inhibit passage of the injected steam past the flow restriction zones. 
     
     
         3 . The method of  claim 1 , including the step of injecting said steam into said zone at an angle oriented such that an orthogonal resolution of said angle gives a component parallel with the longitudinal axis of said barrel and extending toward said barrel outlet. 
     
     
         4 . The method of  claim 1 , said angle being from about 30-60° relative to said barrel longitudinal axis. 
     
     
         5 . The method of  claim 1 , substantially all of said steam being injected into said at least one zone. 
     
     
         6 . A method of extruding a comestible material using a cooking extruder having an elongated, tubular barrel with an inlet and an outlet and presenting a longitudinal axis, a restrictive orifice die plate disposed across said outlet, and an elongated, axially rotatable, helically flighted screw within said barrel, said method comprising the steps of:
 passing said material into said barrel inlet and rotating said screw to convey said material along the length thereof toward and through said die plate, and imparting an SME input into said material;   during said passage of said material through said barrel, creating at least one zone within the barrel having free volume not occupied by said material; and   injecting steam into said zone at an oblique angle relative to said barrel longitudinal axis, and causing said injected steam to mix with said material,   said injecting and mixing steps serving to add in excess of 6% by weight steam, based upon the total dry weight of said material taken as 100% by weight,   said SME input being up to about 22 kWhr/T of said material.   
     
     
         7 . The method of  claim 6 , said steam being added at a level of from about 10-25% by weight. 
     
     
         8 . The method of  claim 6 , said barrel including a plurality of injection ports oriented such that orthogonal resolutions of the longitudinal axes thereof have components parallel with said barrel longitudinal axis and extending toward said barrel outlet. 
     
     
         9 . The method of  claim 6 , said oblique angle being from about 30-60 ° relative to said barrel longitudinal axis. 
     
     
         10 . In a method of processing comestible materials in order to produce an extrudate, said method comprising the steps of passing a comestible material into and through a twin-screw cooking extruder having an elongated, tubular barrel with an inlet and an outlet and presenting a longitudinal axis, a restrictive orifice die plate disposed across said outlet, and a pair of elongated, axially rotatable, helically flighted screw within said barrel, the improvement which comprises creating a zone within said barrel between said inlet and said outlet having free volume not occupied by said material, and injecting steam into said barrel at said zone, wherein the step of injecting steam into said zone includes the step of injecting said steam into said zone at a level of from about 7-25% by weight of steam into said material, based upon the weight of material within said barrel taken as 100% weight, and injecting the steam at an angle oriented such that an orthogonal resolution of said angle gives a component parallel with the longitudinal axis of said barrel and extending toward said barrel outlet. 
     
     
         11 . A screw assembly for use in a twin-screw extruder and comprising:
 first and second elongated, axially rotatable screws each having an elongated shaft with outwardly extending helical fighting along the length of each shaft, the fighting of each shaft being intermeshed with the fighting of the other shaft,   the fighting of each of the first and second shafts presenting a pair of axially spaced apart sections of short pitch length, and an intermediate section having a pitch length greater than the pitch lengths of either of said short pitch length sections,   the fighting of said intermeshed intermediate sections having an axial gap distance therebetween of from about 0.1-0.4 inches.   
     
     
         12 . The screw assembly of  claim 12 , said gap distance being from about 0.15-0.35 inches. 
     
     
         13 . The screw assembly of  claim 11 , said screws having an S D /R D  ratio of from about 1.9-2.5. 
     
     
         14 . The screw assembly of  claim 11 , the ratio of the pitch length of said intermediate section to the pitch lengths of either of said first and second sections being from about 1-7. 
     
     
         15 . The screw assembly of  claim 11 , the ratio of said long pitch length to the flight depth ratio in the long pitch length section being from about 0.4-0.9. 
     
     
         16 . A screw assembly for use in a twin-screw extruder and comprising:
 first and second elongated, axially rotatable screws each having an elongated shaft with outwardly extending helical flighting along the length of each shaft, the flighting of each shaft being intermeshed with the flighting of the other shaft,   the flighting of each of the first and second shafts presenting a pair of axially spaced apart sections of short pitch length, and an intermediate section having a pitch length greater than the pitch lengths of either of said short pitch length sections,   the ratio of the pitch length of said intermediate section to the pitch lengths of either of said first and second sections being from about 1-7.   
     
     
         17 . The screw assembly of  claim 16 , the ratio of said long pitch length to the flight depth ratio in the long pitch length section being from about 0.4-0.9. 
     
     
         18 . A screw assembly for use in a twin-screw extruder and comprising:
 first and second elongated, axially rotatable screws each having an elongated shaft with outwardly extending helical flighting along the length of each shaft, the flighting of each shaft being intermeshed with the flighting of the other shaft,   the flighting of each of the first and second shafts presenting a pair of axially spaced apart sections of short pitch length, and an intermediate section having a pitch length greater than the pitch lengths of either of said short pitch length sections,   the ratio of said long pitch length to the flight depth ratio in the long pitch length section is from about 0.4-0.9.