GB2297936A

GB2297936A - Extrusion and expansion of cereal products

Info

Publication number: GB2297936A
Application number: GB9502950A
Authority: GB
Inventors: Neil Donaldson Frame
Original assignee: APV UK Ltd; APV Corp Ltd
Current assignee: SPX Flow Technology Crawley Ltd
Priority date: 1995-02-15
Filing date: 1995-02-15
Publication date: 1996-08-21
Also published as: GB9502950D0

Abstract

A die assembly 1 comprises a die body 2 having a bore 3 and a coaxial gas supply pipe 4, the free end 5 of which projects outwardly from the die face 6, an annular die outlet 7 being defined at the die face 6 between the bore and a spacer 8 mounted coaxially of the gas pipe 4. An expandable cereal product, containing air, is supplied to the left hand end of the bore 3, and air is supplied to the pipe 4 through pressure regulator (not shown). On exiting from the die outlet 7 air bubbles in the cereal product expand rapidly so as to thicken the wall of the cereal tube 9 while the pressure differential between the inside and outside of the cereal tube 9 results in inflation of the tube 9, thereby reducing the wall thickness of the cereal tube. Bubbles in the tube 9 are elongated in the circumferential direction of the tube, due to inflation of the tube between regions 10 and 11. A further supply pipe may be provided for feeding an edible filling material within the expanded tube. This further pipe may be annular, surrounding the gas supply pipe 4, or fitted within that gas supply pipe. The expanded product may be crimped by means 15 (Fig 5, not shown).

Description

EXTRUSION OF BAKED PRODUCTS AND THE LIKE This invention relates to the extrusion of expandable products, primarily expandable cereals, and the like.

The invention particularly, but not exclusively, concerns a process of extrusion of an expandable product, such as cereal, as a tube followed by expansion of the tube by a gas, such as air, at super-atmospheric pressure preferably, but not exclusively, followed by the moulding of discrete, finished snack-food products from the extruded product so formed.

The invention is also suitable for non-food applications, such as for the production of a biodegradable substitute for materials such as expanded polystyrene.

Some aspects of the invention relate to the expansion with gas of a co-extruded product.

In the manufacture of snack foods it is well known to use co-extrusion technology in which two materials tusually dissimilar in nature) are combined in an extrusion die to produce a continuous filled tube of product. The two materials may be supplied from two extruders or from one extruder and a pump, such as is the case with an expandable cereal and a non-expandable confection creme filler.

This process provides the manufacturer with the ability to make a product which has two textures, two colours or two flavours. The most common form of ready-to-eat co-extrusion has been the combination of a directly expandable crisp cereal outer tube with a sweet or savoury confection creme filling pumped into the centre of the cereal tube.

Extruded, directly expandable cereal products are characterised by having foamed structures, ie having gas bubbles dispersed in a biopolymer matrix. The texture of the resulting cereal tubes can be varied by raw material formulation and extruder process conditions.

The cereal component in the tube plays an important role in forming the product structure, normally providing a continuous fluid melt phase which binds together all of the other particulate matter in the cereal mix. This helps to retain the gases released during the explosive expansion process of exiting from the extrusion die, thus enabling expanded foam structures to be formed.

The amount of carbohydrate and protein present in the continuous fluid melt phase largely determines the extensibility of the bubble cell walls in the foam and therefore the overall expansion of the extrudate. The texture of the cereal tube can be manipulated to provide a variety of structures, from coarse to fine, by the addition of ingredients such as emulsifiers, fats and oils, sugars proteins and nucleating agents such as fibres or minerals.

We have now found, entirely unexpectedly, that by introducing a gas under super-atmospheric pressure into the bore of the cereal tube the texture of the cereal tube can be modified, without there being any need for substantial alteration to the ingredient formulation, centre filling or the extrusion operating parameters. In addition it has been found that a significant reduction in bulk density of the final product can be achieved.

The effect that the injection of the gas has on the process is to radially expand the cereal tube and to tend to drive the steam, generated in the extrusion of the cereal mix, through the wall of the cereal tube. This appears to cause the formation of relatively elongate bubbles in the wall of the cereal tube. Inspection of the cereal tube of the finished product shows that the elongate bubbles have a relatively thin-walled structure and it is this that provides for an improvement in the texture.

According to a first aspect of the invention a method for the extrusion of an expandable cereal product or the like through an annular extrusion die is characterised by the supply of a gas under super-atmospheric pressure into the bore of the tube of expandable cereal product emitted from the extrusion die.

Preferably the gas is non-reactive with the cereal product and is most preferably air.

The gas is introduced into the cereal tube by means of a pipe, the free end of which is located, preferably coaxially of the extrusion die with the free end projecting outwards from the end of the extrusion die face.

The extent of the projection is preferably between 20mm and 50mm for finished snack diameters of between lOmm and 20mm.

This method provides the cereal producer with an ability to market high volume filled or unfilled snack foods with structures varying from soft, wafer-like to hard, biscuit-like textures. The degree of texture modification is found to be directly related to the pressure of the injected gas.

Preferably the pressure of the injected gas is modulated in order to obtain a desired final texture of the extrudate.

Typical suitable parameters are: gas pressure modulation between 1.5 and 3.0 bar (abs), with an exit area of the gas pipe of between 3 and 4 sq mm to suit a finished snack diameter of between lOmm and 20mm.

Cereal tubes thus formed are usually in a firm but plastic state after expansion following emission from the extrusion die. It is common practice for cereal tubes to be moulded into discrete shapes while still in a plastic state. In accordance with a preferable feature of the present invention, the tube is directly conveyed into a pillow crimping device which entraps the gas, enabling the shape of the pillow to be enlarged to fill a mould. The shape of the pillow mould can be made such as to form discrete, shaped products.

The moulded products formed by processes of known art tend to suffer from a hardness of texture. Such products do not retain the full shape of the mould, in particular there being a loss of surface detail. We believe that the reason for this is the relatively small, round bubbles formed in the cereal, with the corresponding relatively thick walls of cooked cereal product between the bubbles.

This provides for a relatively stiff and resilient material for moulding. By making the structure of the cereal lighter it was expected that these problems could be avoided.

It has been found that products formed by the inventive method have a lighter structure than products of known art and additionally have a crisp but soft, wafer-like consistency in comparison to the harder, biscuit-like consistency produced by the methods of known art. This provides for an improvement in the mould-to-product shape integrity and an improved retention of surface detail.

According to a second aspect of the invention, we provide a method for the extrusion of an expandable cereal product or the like in accordance with first aspect of the invention, characterised in that the extruded tube of product is conveyed directly to a moulding means which, in operation, divides the tube into discrete portions, entraps the gas and enables the pillow so formed to be enlarged to fill the shape of a mould in order to form discrete, shaped products.

The present invention is also applicable to a co-extrusion process in which a second extrusion die is mounted within said extrusion die.

When a creme filler is used in a co-extrusion process, it is known that the ingredients from the creme diffuse, or are forced, into the material forming the cereal tube, changing its texture. This change of texture is usually to the detriment of the finished product, turning an otherwise soft, crisp texture to a harder or more chewy texture.

We have found that, in accordance with a third aspect of the invention, by injecting gas between the creme and the cereal tube such diffusion of the creme into the cereal tube can be reduced sufficiently for the change in texture to be reduced to negligible levels. Additionally, less dense cremes can be used that would otherwise produce unacceptable textures for the outer cereal tube.

In this manner it is possible to reduce the bulk density of the final product by the incorporation of an aerated centre and a lighter outer cereal tube.

According to a fourth aspect of the invention a method for the co-extrusion of an expandable cereal product and a creme comprises the extrusion of the cereal product and creme from co-axial extrusion dies, and with gas under super-atmospheric pressure being supplied to the interior of the resulting tube of cereal product.

Preferably the gas is introduced by pipe leading to an annular gas outlet defined radially between the cereal product extrusion die outlet and the creme extrusion die outlet.

For some products it is desirable to simply coat the inner surface of the cereal tube with the creme. In such cases it can be beneficial to position the gas outlet substantially on the axis of the creme outlet which is then of annular shape. This has the effect of assisting in the dispersal of the creme around the inner surface of the cereal tube.

As the cereal product emerges from the die it rapidly expands, typically, in processes of known art, producing a 3:1 expansion ratio. Thus, in order to produce a final wall thickness for the cereal tube that is acceptable for the final product, it is necessary with the prior art process to have a relatively narrow annulus in the extrusion die through which the cereal is extruded.

The width of the annulus is typically of the order lmm. In operation it is common for these dies to become blocked by particulate matter in the cereal mix.

Using the inventive method, it is possible to utilise a wider annulus for the extrusion die while still maintaining the desired final thickness of the cereal tube. The overall diameter of the die can be smaller because the cereal tube is made to expand under action of the pressurised gas. This expansion also reduces the thickness of the wall of the cereal tube.

The wider annulus of the die has the effect of extending the operational duration for continuous extrusion. The wider annul us has the additional benefit of rendering less critical the positional accuracy of the central tube.

The usual value for the width of the annul us in processes of known art used to produce a 3mm cereal tube wall thickness would be lmm.

For a similar product the annulus width used for the inventive process can be about 2mm.

In accordance with a preferred feature of the present invention, the arrangement is such that the die and tube assembly used to form the cereal tube has an annulus-to-cereal-tube-wall thickness ratio substantially of the value of 2:3.

In one advantageous embodiment of the invention the cereal product is in the form of a ready-to-eat cereal snack food.

In another advantageous embodiment of the invention the cereal product is in the form of a biodegradable cereal product not intended for edible consumption.

Although gases other than air may be supplied to the interior of the extruded product, the gas supplied under super-atmospheric pressure is preferably air.

This process is applicable to the production of any form of expandable cereal product, either in edible or non-edible form.

According to a fifth aspect of the invention, apparatus for producing an expanded cereal product in tubular form comprises an annular extrusion die having an inlet connected to a cereal product supply means capable of feeding expandable cereal product to said unit, a gas supply pipe having a gas inlet connected to a gas supply, and a gas outlet positioned coaxially of the annular outlet of the extrusion die.

The extrusion die may comprise a creme outlet which is preferably a tubular outlet, in which case the gas outlet is an annular gas outlet encircling the tubular creme outlet. Alternatively, the creme outlet may be an annular creme outlet, positioned radially intermediate the annular cereal product outlet and a tubular gas outlet.

The invention also incorporates products made in accordance with any of the methods of the first, second, third or fourth aspects of the invention.

The invention will now be further described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic longitudinal cross-section through an extrusion die assembly for use in producing a tube of expanded cereal product, Figure 2 is an end view of the die assembly of Figure 1, looking from the right in Figure 1, Figure 3 is a section similar to Figure 1 but of an extrusion die assembly suitable for use in producing a filled cereal tube, Figure 4 is an end view of the die assembly of Figure 3, looking from the right in Figure 3, Figure 5 is a side elevation of crimping apparatus being used to crimp an empty cereal tube produced by the die assembly of Figure 1, Figure 6 is a view similar to Figure 5 but of the die of Figure 3 producing a creme-filled cereal tube.

The die assembly 1 of Figure 1 comprises a die body 2 formed with a through-bore 3 in which is supported a co-axial gas pipe 4, the free end 5 of which projects outwardly from the die face 6, an annular die outlet 7 being defined at the die face 6 between the wall of through-bore 3 and a tubular spacer 8 mounted co-axially of the gas pipe 4. A suitable spider, not shown, is arranged in the bore 3 to hold the spacer 8 and pipe 4 co-axial with the bore 3.

Figure 5 shows the die assembly of Figures 1 and 2 in use to produce a tubular cereal product. The crimping apparatus 15 shown in Figure 5 is described in detail in our patent specification no. GB 2238506A.

An expandable cereal product is supplied under pressure to the left hand end of the bore 3.

Suitable cereal mixes for extrusion are well-known to those skilled in this art. Under the extrusion process used to supply the expandable cereal product to the die, the cereal cooks and generates a foamed structure of bubbles dispersed in a biopolymer matrix and this can be fed to the die assembly 1 by the twin-screw cooker-extruder. Air at a pressure of between 1.5 and 3 bar (abs) is supplied to the gas pipe 4 through an adjustable pressure regulator.

On exiting from the annular die outlet 7 the bubbles in the cereal product expand rapidly so as to thicken the wall of the cereal tube 9 in the region 10 immediately downstream of the die face 6. The pressure differential between the inside and outside of the cereal tube 9 results in an inflation of the cereal tube 9 in the region 11, thereby reducing the wall thickness of the cereal tube relative to that in region 10, where the wall thickness is typically 1.5 times the width of the annular die outlet 7.

In the region 11 the cereal tube 9 has almost reached a stable diameter, and wall thickness, and an examination of the structure of the resulting cereal tube shows that the bubbles in the dough have been elongated in the circumferential direction of the tube, due to the inflation of the cereal tube between regions 10 and 11.

Figure 3 shows a modified extrusion die assembly suitable for producing a filled cereal tube product. Parts corresponding to the parts of the die assembly of Figures 1 and 2 have been given corresponding reference numerals. In this die assembly a filler tube 12 is arranged co-axially in bore 3 so as to define with the outer surface of tube 5 an annular orifice 13.

Figure 6 shows one mode of operation of the die assembly of Figures 3 and 4 to co-extrude an expandable dough and a creme filling so as to produce a creme-filled cereal tube product 14. It will be appreciated that any suitable edible filling could be used instead of confection creme.

In the mode of operation shown in Figure 6, the central pipe 4 is connected to the creme supply and the air is fed to pipe 12 at super-atmospheric pressure to exit from annular orifice 13. The air issuing from orifice 13 both expands the cereal tube and helps to provide a separation between the creme and the inside of the cereal wall.

In a second mode of operation of the die assembly of Figure 3, the air is supplied to the central pipe 5 and the creme to the pipe 12 to issue from annular orifice 13. This mode can be used when a small quantity of creme is used to coat the inside of the cereal tube.

Crimping means, shown at 15, may be employed to crimp the cereal tube, thereby producing a series of pillow shapes. The crimper device may also sever the cereal tube.

Alternatively, or in addition, moulding dies may be arranged to mould the pillows as the tube is crimped.

Claims

1. A method for the extrusion of an expandable cereal product or the like through an annular extrusion die, characterised by the supply of a gas under super-atmospheric pressure into the bore of the tube of expandable cereal product emitted from the extrusion die.

2. The method claimed in claim 1, wherein the gas is air.

3. The method claimed in claim 1 or 2, wherein the gas supplied is introduced into the cereal tube by means of a pipe, the free end of which is located within the extrusion die with the free end of the pipe projecting outwards from the end of the extrusion die face.

4. The method claimed in claim 3, wherein the extent of the outward projection of the pipe is between 20mm and 50mm.

5. The method claimed in claim 3 or 4, wherein the free end of the gas supply pipe is coaxially disposed within the extrusion die.

6. The method claimed in claim 3, 4 or 5, wherein the inner surface of the annular extrusion die is defined by the free end of the gas supply pipe.

7. The method claimed in any one of claims 1 to 6, wherein the pressure of the supplied gas is modulated in order to influence the final texture of the extruded cereal product.

8. The method claimed in claim 7, wherein gas pressure modulation is between 1.5 and 3.0 bar (abs), with an exit area of the gas pipe of between 3 and 4 sq mm to suit a finished cereal tube diameter of between lOmm and 20mm.

9. The method claimed in any one of claims 1 to 8, wherein the extruded tube of cereal product is conveyed directly to a moulding means which, in operation, divides the tube into discrete portions, entraps the gas and enables the pillow so formed to be enlarged to fill the shape of a mould in order to form discrete, shaped products.

10. The method of any one of claims 1 to 9, applied to a co-extrusion process in which a second extrusion die is mounted within said first-mentioned extrusion die

11. The method of claim 10, wherein a creme filler is used in the co-extrusion process, and gas is injected between the creme and the cereal tube.

12. A method for the co-extrusion of an expandable cereal product and a creme comprises the extrusion of the cereal product and creme from co-axial extrusion dies, and with gas under super-atmospheric pressure being supplied to the interior of the resulting tube of cereal product.

13. The method claimed in claim 12, wherein the gas is introduced by a pipe leading to an annular gas outlet defined radially between the cereal product extrusion die outlet and the creme extrusion die outlet.

14. The method claimed in claim 13, wherein the gas outlet is positioned substantially on the axis of the creme outlet.

15. The method claimed in any one of claims 1 to 14, wherein the cereal product comprises a foamed structure of bubbles dispersed in a biopolymer matrix.

16. A method for the extrusion of expandable cereal product, substantially as hereinbefore described with reference to Figures 1 and 2 of the accompanying drawings.

17. A method for the extrusion of expandable cereal product, substantially as hereinbefore described with reference to Figures 3 and 4 of the accompanying drawings.

18. A method for the extrusion of expandable cereal product, substantially as hereinbefore described with reference to Figures 5 and 6 of the accompanying drawings.

19. Product manufactured by the method claimed in any one of claims 1 to 18.

20. Apparatus for producing an expanded cereal product in tubular form comprising an annular extrusion die having an inlet connected to a cereal product supply means capable of feeding expandable cereal product to said unit, a gas supply pipe having a gas inlet connected to a gas supply. and a gas outlet positioned within the annular outlet of the extrusion die.

21. Apparatus as claimed in claim 20, wherein the gas supply pipe has a free end projecting outwards from the end of the extrusion die face.

22. Apparatus as claimed in claim 21, wherein the extent of outward projection of the gas supply pipe is between 20mm and 50mm.

23. Apparatus as claimed in claim 20, 21 or 22, wherein the free end of the gas supply pipe is coaxially disposed within the extrusion die.

24. Apparatus as claimed in any one of claims 20 to 23, wherein the inner surface of the die is defined by the free end of the gas supply pipe.

25. Apparatus as claimed in any one of claims 20 to 24, wherein the extrusion die comprises a creme outlet which is of a tubular form, the gas outlet comprising an annular gas outlet encircling the tubular creme outlet.

26. Apparatus as claimed in any one of claims 20 to 24, wherein the extrusion die comprises an annular creme outlet, positioned radially intermediate the annular cereal product outlet and a tubular gas outlet.

27. Apparatus for producing an expanded cereal product in tubular form, substantially as hereinbefore described, with reference to Figures 1 and 2 of the accompanying drawings.

28. Apparatus for producing an expanded cereal product in tubular form, substantially as hereinbefore described, with reference to Figures 3 and 4 of the accompanying drawings.

29. Apparatus for producing an expanded cereal product in tubular form, substantially as hereinbefore described, with reference to Figures 5 and 6 of the accompanying drawings.