Abstract:
The present invention relates to apparatus and methods for manufacturing food, in particular pet food, and in particular apparatus and methods involving extruders and cavity transfer mixers.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority to GB 0601353.6 filed Jan. 23, 2006 which is fully incorporated herein.  
       TECHNICAL FIELD  
       [0002]     The present invention relates to apparatus and methods for manufacturing food, in particular pet food, and in particular apparatus and methods involving extruders.  
       BACKGROUND OF THE INVENTION  
       [0003]     Cavity transfer mixers (CTMs) are known and have been employed, particularly in the plastics industry, for a number of years. CTMs allow efficient mixing of additives, e.g. colorings, into a product and are particularly useful in extrusion processes. Typically a CTM is bolted on to the end of an extruder screw in order to mix an additive into the stream of product being output from the extruder screw.  
         [0004]     EP 0 048 590 discloses a CTM which comprises a hollow cylindrical stator member, and a cylindrical rotor member positioned for rotation within the stator. The facing cylindrical surfaces of the rotor and stator carry respective pluralities of parallel, circumferentially extending rows of cavities. The cavities in adjacent rows on the stator are circumferentially offset, as are the cavities in adjacent rows on the rotor. The rows of cavities on the stator and rotor are also axially offset.  
         [0005]     EP 0 234 889 discloses apparatus for treating a viscous material, particularly an insulating plastics material for coating wires. Material is fed from an extruder to a gear pump. The output from the gear pump is divided into at least two streams, one of which is fed to a CTM. Colorant is injected into one of the streams either before or at the mixer and the two streams are recombined downstream of the mixer. The CTM and the extruder are driven independently.  
         [0006]     U.S. Pat. No. 4,750,842 also discloses a mixing apparatus employing a CTM. Material is fed from an internal mixer to an extruder and then on to a CTM. Material traversing the length of the extruder passes directly into the CTM and an additive is injected into the CTM for mixture with material output from the extruder. A motor operates the internal mixer, extruder and CTM and separately drives and varies the speed of the CTM whereby the CTM can be driven at a higher speed than the extruder.  
       BRIEF SUMMARY OF THE INVENTION  
       [0007]     The present invention provides a method of manufacturing food comprising:  
         [0008]     extruding first and second streams of food product respectively from first and second outputs of an extruder; and  
         [0009]     passing the first stream of food product through a first cavity transfer mixer.  
         [0010]     The present invention also provides an apparatus for manufacturing food comprising:  
         [0011]     an extruder having first and second outputs for outputting first and second streams of food product respectively; and  
         [0012]     a first cavity transfer mixer having a first input connected to the first output of the extruder.  
         [0013]     The extruder may comprise a twin screw extruder.  
         [0014]     The second output may be arranged such that the second stream of food product passes through a hollow channel through the centre of the first cavity transfer mixer. In such a way, the first cavity transfer mixer may be arranged so that the first stream of food product exits the first cavity mixer and coats the second stream of food product so as to produce a co-axial food product.  
         [0015]     Alternatively, the second output may be arranged such that the second stream of food product passes around the outside of the first cavity transfer mixer. In such a way, the second stream of food product may be arranged to coat the first stream of food product exiting the first cavity transfer mixer so as to produce an alternative coaxial food product.  
         [0016]     Alternatively, the apparatus may be arranged so that the first stream and second stream are output as two side-by-side streams, each stream producing a separate product.  
         [0017]     The first cavity transfer mixer may be capable of reducing the density of the first stream of product. In order to reduce the density of the first stream of product, the first cavity transfer mixer may comprise a hollow cylindrical stator and a cylindrical rotor arranged for rotation within the stator, each of the facing cylindrical surfaces of the rotor and stator including a plurality of cavities arranged such that the temperature and pressure exerted on the first stream of food product is increased to such an extent that the first stream of food product has a lower density upon exiting an output of the apparatus than upon entering the cavity transfer mixer. The first cavity mixer may be used to produce a foamed material for coating a second stream of material extruded through the centre of the cavity transfer mixer and thus producing a dual-texture coaxial product having a dense central portion and a relatively less dense coating. Alternatively, the apparatus may be arranged so that the first stream and second stream are output as two side-by-side streams to produce two separate products of different textures. Alternatively, the foamed material may be coated with the second stream of material which is more dense than the foamed material.  
         [0018]     The first cavity transfer mixer may be adapted to reduce the density of the product by minimizing the tolerances between the stator and the rotor in order to increase the temperature and pressure exerted on the first stream of food product.  
         [0019]     The apparatus may include means for varying the product throughput rate. Preferably, the means allows real time adjustment so that the product throughput rate can be changed whilst the apparatus is running. The means may be either internal or external to the first cavity transfer mixer. The means may comprise a choke ring positioned at the input of the first cavity transfer mixer.  
         [0020]     The speed at which the rotor of the first cavity transfer mixer rotates may be variable so as to change the product throughput rate.  
         [0021]     Independent heating and/or cooling may be applied to the first cavity transfer mixer to decrease or increase the density of the product and change the texture of the product.  
         [0022]     The first cavity transfer mixer may have an input through which an additive can be added to the first stream of food product. The additive may comprise, for example, a food coloring, a food flavoring or a fat or oil.  
         [0023]     The additive can be chosen so that the final product is a nutritionally complete product. For example, an oil or fat could be added or a vitamin or mineral supplement. Traditionally it has been difficult to add an oil or fat to an extrudate mixture because the added oil or fat has a lubricating effect which causes the mixture to shoot through the extruder without being thoroughly mixed or processed. The prior art has attempted to address this problem by spraying the oil or fat onto the extruded product as a separate step. However, this adds another step to the process and is consequently inefficient. The present invention allows an oil or fat to be added to the food product effectively and efficiently.  
         [0024]     By adding an additive to the first stream, the first stream can be output with a first property (e.g. a first color) and the second stream can be output with a second property (e.g. a second color). The two streams can be combined into a single product, for example a coaxial product, or can be retained as separate products.  
         [0025]     The apparatus may further comprise a second mixer, such as a second cavity transfer mixer or a second extruder, having a second input connected to the second output of the first extruder. The second cavity transfer mixer may have any of the functions described above in connection with the first cavity transfer mixer. The function of the second cavity transfer mixer may be the same as, or different to, that of the first cavity transfer mixer.  
         [0026]     In addition, the apparatus may have a third output for providing a third stream of food product. One embodiment of the present invention provides a means to simultaneously extrude three different food products using two CTMs and a twin screw extruder. Thus, three streams of extrudate exit the extruder, a first and second stream which enters a first and second cavity transfer mixer, as described above, and a third stream which exits the extruder without further treatment.  
         [0027]     A third mixer, such as a third cavity transfer mixer or a third extruder, with a third input may optionally be connected to the third output of the extruder. The third cavity transfer mixer may have any of the functions described above in connection with the first cavity transfer mixer. The function of the third cavity transfer mixer may be the same as, or different to, that of the first and/or second cavity transfer mixer.  
         [0028]     Where the apparatus is provided with more than one cavity transfer mixer, it may be possible to set the speed of rotation of the rotor of at least one of the cavity transfer mixers to be different to the speed of rotation of the rotor of at least one of the other cavity transfer mixers so that the product throughput rate of each cavity transfer mixer can be chosen.  
         [0029]     Each stream of food product may be extruded directly from its respective output of the extruder.  
         [0030]     In a further embodiment, the apparatus comprises a fourth output stream. The fourth output stream may comprise extrudate which exits the extruder without further treatment. In this embodiment, the third and fourth output streams of untreated extrudate may be combined with the first and second output streams which have passed through the first and second mixers, respectively, to produce two bi-phase product, optionally wherein the two phases of each product are in coaxial arrangement.  
         [0031]     The second cavity transfer mixer may have an input through which an additive can be added to the second stream of food product, as described in respect of the first mixer. A third cavity transfer mixer may have an input through which an additive can be added to the third stream of food product, as described in respect of the first mixer.  
         [0032]     When the apparatus includes three outputs, the three extruded food products may be combined into a single final product or may be retained as three separate final products. For example, the three extruded food products may be intertwined into a single product. Alternatively, three separate products, for example beef, lamb and chicken flavored dog chews, could be produced from the same apparatus.  
         [0033]     In a further alternative embodiment, only two of the three streams may be combined. Thus, in one embodiment a first output stream passes through a first mixer, as described above, and is combined into a single food product with a second output stream that has passed through a second mixer. A third output stream is retained a separate final product.  
         [0034]     Alternatively, the first output stream passes through a first mixer, as described above, and is combined into a single, optionally coaxial, food product with the third output stream of untreated extrudate. The second output stream passes through a second mixer and is output as a separate final product. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0035]     Embodiments of the present invention will now be described with reference to the enclosed drawings in which:  
         [0036]      FIG. 1  shows an apparatus according to a first embodiment of the present invention;  
         [0037]      FIG. 2  shows a cut-through portion of a cavity transfer mixer for use in the present invention;  
         [0038]      FIG. 3  shows an apparatus according to a second embodiment of the present invention;  
         [0039]      FIG. 4  shows an apparatus according to a third embodiment of the present invention; and  
         [0040]      FIG. 5  shows an example of a food product which can be produced using an apparatus according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0041]      FIG. 1  shows apparatus according to an embodiment of the present invention in which the output  12  of a twin screw extruder (TSE)  10  is split into three streams. A first stream  14  of product output from the TSE  10  is connected to a first cavity transfer mixer  20  and a second stream  16  of product output from the TSE  10  is connected to a second cavity transfer mixer  30 . A third stream  18  of product output from the TSE  10  is not channeled through a cavity transfer mixer.  
         [0042]     The first and second cavity transfer mixers  20  and  30  may be used to mix additives into the first and second streams of product output from the TSE  10 . Alternatively, the first and second cavity transfer mixers  20  and  30  may be used to exert a pressure on the first and second streams of product output from the extruder in order to change the density of the first and second streams of product. For example, pressure exerted on the product as it passes through the cavity transfer mixer may cause the product to foam as it exits the cavity transfer mixer, thereby creating variations in texture.  
         [0043]     The three streams of product exiting the apparatus may be retained as separate products of the apparatus. Alternatively, two or more of the three streams of product may be combined into a single product with an appealing appearance. For example an intertwined strand of two or three different color materials.  
         [0044]     As can be seen from  FIG. 2 , each of the cavity transfer mixers  20 ,  30  comprises a hollow cylindrical stator  22  and a cylindrical rotor  24  arranged for rotation within the stator  22 , each of the facing cylindrical surfaces of the rotor  24  and stator  22  includes a plurality of cavities  26 ,  28 . In order to output a foamed product, the rotor  24  and stator  22  are arranged such that the temperature and pressure exerted on the stream of food product passing through the cavity transfer mixer is increased. The pressure on the food product is increased by minimizing the clearance between the stator  22  and the rotor  24  in order to increase the temperature and pressure exerted on the first stream of food product. For example, the distance between the cylindrical surfaces of the stator  22  and rotor  24  may be set at approximately 1 mm.  
         [0045]      FIG. 3  shows an alternative embodiment of the present invention in which the output of an extruder  10  is attached to a trinocular barrel  50 . Each of the three outputs of the trinocular barrel is connected to a separate cavity transfer mixer  20 ,  30  and  40 .  
         [0046]      FIG. 4  shows an alternative embodiment of the present invention similar to that shown in  FIG. 3  in which the output of an extruder  10  is attached to a binocular barrel  60 . Each of the two outputs of the binocular barrel  60  is connected to a separate cavity transfer mixer of which only one,  20 , is shown (the second cavity transfer mixer is behind the mixer  20 ). A dump channel  70  from the input of the two cavity transfer mixers provides a path for product from the extruder to be fed into a third cavity transfer mixer  40 .  
         [0047]     At least one of the three cavity transfer mixers  20 ,  30  and  40  in  FIGS. 3 and 4  can be used to mix in an additive such as a food coloring or a food flavoring. Alternatively or in addition, at least one of the three cavity transfer mixers  20 ,  30  and  40  can be used to change the density and hence the texture of the product passing therethrough. For example, as described above, the pressure in the cavity transfer mixture can be increased by minimizing the distance between the cylindrical surfaces of the rotor and stator. Such an increase in pressure can result in foaming of the product exiting the cavity transfer mixture.  
         [0048]     Each of the three cavity transfer mixers can function with different tolerances and sizes and at different temperatures, resulting in different products. One or more of the cavity transfer mixers can be independently heated or cooled to change the density of the product exiting the cavity transfer mixer. Such independent heating and/or cooling could be used to enhance textural differences between the streams of product.  
         [0049]     One or more of the three cavity transfer mixers can function at different rotation speeds to the other of the three cavity transfer mixers, thus allowing the texture of the product to be altered. For example, in  FIG. 4 , drive  45  of cavity transfer mixer  40  can be rotated at a different rate to the rotors of cavity transfer mixers  20  and  30  allowing for the product texture from barrel  40  to differ significantly from that of the products from barrels  20  and  30 .  
         [0050]     One or more of the inputs of the three cavity transfer mixers can be provided with a choke ring  80  to alter the cross-sectional area of the input to the cavity transfer mixer and affect the product throughput level of the cavity transfer mixer.  
         [0051]     The embodiments shown in  FIGS. 3 and 4  can be used to produce a single multi-component product have multiple colors, flavors and/or textures such as that shown in  FIG. 5 . For example, one of the cavity transfer mixers may operate at a higher temperature or higher rotation speed which would allow a “marshmallow” type product to result involving a layer akin to the foam, a layer akin to the biscuit and a “chocolate” coating.  
         [0052]     Alternatively the embodiment shown in  FIG. 3  can be used to produce three different products which can be packaged together in one pack. The three different products may differ in their colors, flavors, textures and/or shapes. Therefore, a single pack of food products can be produced from a single apparatus which contains three varieties of product.  
         [0053]     Accordingly, the present invention provides for simultaneous production of multiple varieties of product. No separate storage of each product type is required before packaging, and no downstream mixing of products for a multi-variety pack is required; this leads to consistent varieties in packs. The present invention also provides for triple component products from one extruder which has low resource and space requirements.  
         [0054]     It will be understood that the present invention has been described above purely by way of example, and that modifications of detail can be made within the scope of the invention as defined by the claims.