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).

Description

BACKGROUND OF THE INVENTION[0001]1. Field of the Invention[0002]The present invention is broadly concerned with improved extruders and methods for extrusion processing of comestible products with very low specific mechanical energy (SME) inputs as compared with conventional methods. More particularly, the invention is concerned with extruders having specially configured screws designed to permit addition of very high quantities of steam, so that the amount of SME required to for complete cooking is maritally reduced. The resultant feeds have very high cook values and expansion characteristics.[0003]2. Description of the Prior Art[0004]Extrusion processing of comestible products such as human foods and animal feeds has long been practiced and is a highly developed art. In general terms, food extruders of the single or twin screw variety are employed, having elongated, tubular barrels with inputs adjacent one end thereof and restricted orifice dies at the outlet thereof, and one or tw...

AI Technical Summary

Benefits of technology

[0012]In more particular aspects ofthe invention, substantially fully cooked comestible products selected from the group consisting of pet feeds and aquatic feeds are fabricated using SME input levels of up to about 18 kWhr / T (more preferably from about 10-18 kWhr / T) for pet feeds, and SME input levels of up to about 16 kWhr / T (more preferably from about 8-16 kWhr / T) for aquatic feeds. The methods of the invention can be controlled for giving desired expansions of the final products. Expansions of up to about 50%, more preferably from about 15-35% can readily be achieved.

[0013]The invention allows extrusion equipment to be produced at lower cost, owing to the fact that acceptable products can be produced with lower SME inputs. That is, the equipment may be made with smaller motors and drives than would otherwise be necessary. The internal extruder components such as barrels, linings, and screws are also subjected to less wear. Furthermore, utility costs are reduced in as much as energy inputs derived from steam are much less expensive than those from SME.

[0014]The preferred extruders of the invention are twin-screw extruders wherein the screw assemblies are of a special and unique design. The screw assemblies include first and second elongated, axially rotatable screws each having an elongated shaft with outwardly extending helical flighting along the length of each shaft and with the flighting of each shaft being intermeshed with the flighting of the other shaft. Each flighting presents a pair of axially spaced apart sections of short pitch length, and an intermediate section between the short pitch length sections and having a pitch length greater than the pitch lengths of either of the short pitch length sections. Also, the flighting of the intermeshed intermediate sections has a very large axial gap distance therebetween of from about 0.1-0.4 inches. This has been shown to enhance distributive mixing of the material being processed within the extruder barrel.

[0015]Other features of the preferred screw assemblies include the ratio of the pitch length of the intermediate, long pitch length section to the pitch lengths of either of the first and second short pitch length sections. This ratio should be from about 1-7. Additionally, the ratio of the pitch length of the intermediate section to the flight depth ratio should be from about 0.4-0.9. These features, particularly in combination, allow for very high barrel steam injection without blowback through the barrel inlet and with complete mixing of the injected steam into the material being processed. Consequently, an increased proportion of the total energy input needed to fully cook and expand the extrudate is derived from steam rather than SME.

Problems solved by technology

Conventional extrusion systems are limited in the amount of steam which can be injected into the extruder barrel, typically no more than about 5% by weight.

Consequently, the extrusion equipment must have a more robust and therefore construction than would otherwise be necessary, the extent of extruder component wear is higher than desirable, and utility costs are increased.

Skilled artisans understand that a ratio of 1.3 is too small, in that the screw(s) lack free volume, creates excessive shear inputs and consumes too much power.

Similarly, a ratio above 1.8 is deemed too large, in that the screw has too much free volume and which will prevent barrel fill and the ability to achieve a desired cook value.

Any significant axial gap or clearance between the adjacent flighting sections is considered to be detrimental in that it could create dead zones of accumulated product, and also would decrease the extent of product mixing within the extruder barrel.

However, these types of extruders will be deficient in that cook values will be unacceptably low and significant expansion cannot be achieved.

Alternately, an aggressive extruder design can be used, which will assure adequate cook and expansion of products, but this will inevitably result in high shear and SME inputs with resultant higher costs.

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