WO1996036242A1

WO1996036242A1 - Improvements in and relating to textured wheat gluten protein products

Info

Publication number: WO1996036242A1
Application number: PCT/GB1996/001179
Authority: WO
Inventors: Oliver John Barnes; Ashley Peter Stone
Original assignee: United Biscuits (Uk) Limited
Priority date: 1995-05-19
Filing date: 1996-05-17
Publication date: 1996-11-21
Also published as: EP0831716A1; AU5770396A

Abstract

A method of making a textured food product, comprises introducing gluten and water into a twin-screw conveyor (1), mixing the gluten and the water to form a dough whilst at the same time conveying the material towards and through an extrusion outlet, and ensuring that the material reaches a temperature at which it melts and, subsequently, cooling the material before it exits from the extrusion outlet. The product may be used, if appropriate after hydration, as a meat analogue suitable for consumption by vegetarians, or as a meat extender.

Description

IMPROVEMENTS IN AND RELATING TO TEXTURED WHEAT GLUTEN PROTEIN PRODUCTS

The invention relates to edible proteinaceous products. More especially, the invention relates to proteinaceous products suitable for consumption by vegetarians.

Throughout the specification, the term "edible" is used to indicate that a product can (for example, because it is not poisonous) be orally ingested. It is not used to mean that a product is necessarily ready to be eaten. According to European Patent Specification No. 0 262 276A, protein products having a fibrous structure may be made by a process in which a mixture of protein and water is prepared, the mixture is heated and is allowed to expand during the heating, and then the expanded mixture is subjected to a form of treatment that is described as exerting pressure on it in two dimensions while permitting the mixture to move in the third dimension. That treatment causes orientation of the protein molecules and of the macroscopic fibrous struc¬ tures that form as a result of the orientation of the protein molecules.

The only form of the process that is described is a batch process, in which the dough is introduced into the main, generally spherical, portion of a flask, from which an aperture leads to a long, narrow neck portion.

A quantity of dough is introduced into the flask, the quantity being such that the dough can expand by at least 35% before any of the dough is forced into the neck portion of the flask. The flask is then heated, causing the dough to expand and dough to be forced out of the flask through the neck portion. It is the forcing of the dough through the neck portion that effects the orientation.

In Examples 5, 6 and 7, the desired " icrofibrils" were not observed in the product, presumably because in none of those Examples were the prescribed heating and expansion step, and the prescribed orienting step, both carried out. In Example 7, a homogeneous dough was fed to a Brabender food extruder of which the barrel was heated to a temperature of 160°C and of which the die plate was heated to a temperature of 166"C. The product lacked apparent macroscopic fibres.

Accordingly, European patent specification No. 0 262 276A teaches that a fibrous vegetable protein with suitable properties for use as a meat substitute can be made, but that the heating, and consequent expansion, of the dough must take place in a region in which the dough is free to expand (by at least 35% by volume) in all directions.

The present invention provides a method of making a textured food product, comprising: introducing vital wheat gluten into a twin-screw conveyor; introducing water into the conveyor, the gluten and the water being introduced at or in the vicinity of the upstream end of the conveyor; mixing the gluten and the water to form a dough whilst at the same time conveying the material towards and through an extrusion outlet; and ensuring that the material reaches a temperature at which it melts and, subsequently, cooling the material before it exits from the extrusion outlet. It has been found that, surprisingly, the method of the invention can be used to obtain reliable texturisation of doughs comprising vital wheat gluten.

The term "textured" is used herein to refer to structures having an oriented fibrous structure, and the term "texturisation" is to be understood accordingly.

The mixing of the dough and the conveying of the mixed dough towards the extension outlet is effected by the twin-screw conveyor, which is preferably one in which the screw assemblies are co-rotating. When the twin-screw conveyor is one in which the screw assemblies are co-rotating, each screw assembly will normally comprise a plurality of elements arranged along the length of a shaft and, advantageously, adjacent to the upstream end of each screw assembly, there is provided an element or elements arranged to effect relatively rapid movement of the gluten away from the position at which it is introduced into the conveyor. That arrangement reduces the risk of bridging of the solid material/gluten immediately before it enters the conveyor. Advantageously, elements of each screw assembly make up groups of elements, each of which groups is made up of a first element or elements arranged to impart a high degree of mixing to the dough together with some force tending to cause the dough to continue to move in a downstream direction followed, in a downstream direction, by a second element or elements arranged to impart a high degree of mixing to the dough without exerting any significant force on the dough in an axial direction. That arrangement gives good mixing combined with a force tending to cause the dough to continue moving in a downstream direction. Preferably the elements of each screw assembly are so arranged that at least some of the said groups also include a third element or elements, situated downstream of the second element or elements and arranged to impart a high degree of mixing to the dough together with some force tending to cause the dough to move in an upstream direction. The third elements increase the rate of input of mechanical work into the dough and tend to promote stability by increasing the back pressure. Advantageously, the elements of each screw assembly include fourth elements arranged to impart a force to the dough tending to cause it to move in the downstream direction, without imparting to the dough a significant degree of mixing, the fourth elements being so located that they separate adjacent ones of the said groups of elements. The fourth elements serve to increase significantly the force tending to cause the dough to move in a downstream direction and serve to maintain the movement of the dough in that direction. Preferably, the elements of each screw assembly include a fifth element or elements arranged to impart a force to the dough tending to cause it to move in the downstream direction without imparting to the dough a significant degree of mixing, the fifth element or elements of each screw assembly being situated adjacent to the downstream end of the screw assembly. The fifth element or elements materially assist the overcoming of the retarding effect of stationary parts of the apparatus that are encountered by the dough after it has left the conveyor.

Within the twin-screw conveyor, shear and agitation are imparted to the material in many directions by the rotating screw assemblies. This multi-directional shear is important at this stage of the process since it promotes efficient and even heating of the material. On heating the material to a temperature sufficiently high for it to melt, protein molecules denature and become dissociated. The conveying of the material towards and through the extrusion outlet is effected by the pressure developed in the conveyor and without imparting further mixing to the material. Any such further mixing would inhibit the establishment of laminar flow and, to the extent that laminar flow was established, would disturb it.

It is believed that the formation of oriented fibres of proteinaceous materials occurs, at least in the main, after the material has left the cavity of the twin-screw conveyor. After the material has left that cavity, and as it moves towards the extrusion outlet, the shear forces within the material derive from the retarding effect of the stationary boundaries of the region through which the material flows, that is to say, from the retarding effect of the inner surfaces of the walls of the apparatus. The shear is imparted predominantly in one direction and the flow of the material is believed to be essentially laminar. The shearing action in the direction of flow promotes alignment of the melted components of the material. Where, as in a cooling die (the use of such a die in the process of the invention being discussed below) , the walls of the apparatus are cooled, the viscosity of an outer layer of the material is increased so that, in that region, the rate of increase (in a radially inward direction) of the velocity of the material decreases. At the same time, as required by continuity considerations, the velocity of the material in a central region increases. Thus, between the outer region and the central region, there is a region (which is of annular cross-section when the cooling die is of circular cross-section) where the velocity gradient, and hence the shear applied to the material, is increased. That effect becomes more pronounced towards the downstream end of the cooling die as the radial thickness of the region of increased viscosity increases, with the result that the velocity of the material in a central region increases towards the downstream end of the cooling die.

The aligned molecules associate to form elongate aggregates which become stabilised on cooling, the aggregates being observable in the product (with the assistance of an optical microscope) as a multiplicity of fibres, extending in a substantially common direction. Unidirectional shear will start to predominate as soon as the material leaves the rotating section of the twin- screw conveyor. It is believed that, although fibre formation may start to develop in those regions, it at least predominantly occurs within the cooling die.

The term "melts" is used herein in relation to the material heated in the conveyor to refer to conversion of the material to a hot, flowable mass. It is not to be taken to imply that all solid matter present in the material is converted to liquid form; solid components of relatively high melting point which, under the process conditions, are not converted into liquid form, may be present in the hot flowable mass provided that the amounts of any such components are small enough for their presence not to prevent the desired texturisation from taking place. In general, in order to achieve texturisation, the conditions must be such that at least a major proportion of, and preferably substantially all, the gluten protein in the mixture melts.

The melting temperature will depend on the pressure generated in the extruder and on the water content of the dough, as well as on the composition of the ingredients other than water. Under the pressures that obtain in the extruder used in the process of the invention, it will usually be found advantageous to ensure that the mixtures used in accordance with the invention reach a temperature of at least 130°C, and preferably at least 135^βC. The pressure to which the material is subjected during its passage through the conveyor may reach at least 250 psi (17.5 x 10⁵ Pa), and the maximum pressure reached is advantageously within the range of from 400 to 1,000 psi (27 x 10⁵ to 69 x 10⁵ Pa).

The heating of materials during extrusion has been used previously to form vegetarian meat analogues from, for example, soy protein. Doughs comprising vital wheat gluten have high viscoelasticity compared with doughs based on other proteins of vegetable or cereal origin, and as a result it has not been possible to obtain reliably a product with the desired textured structure by the heating of such doughs during extrusion. The method of the present invention offers the possibility of obtaining reliably the desired textured product from doughs comprising vital wheat gluten.

As is well-known, vital wheat gluten is normally obtained by washing wheat flour with water to separate starch from the proteins. In practice, the vital wheat gluten so obtained typically contains some residual starch (for example, about 15 to 25% by weight based on the total weight of the vital wheat gluten) and minor amounts of other insoluble materials in addition to the proteinaceous material. The expression "vital wheat gluten" used herein includes vital wheat glutens containing such residual starch and other materials.

The material is cooled before it passes out of the extrusion outlet in order to prevent rapid expansion of the heated material. Such a rapid expansion (which typically occurs on extrusion of a heated, moist food material as a result of the sudden vaporisation of the water contained in the material that occurs as the material leaves the high-pressure environment of the extrusion apparatus) is undesirable because it disrupts the structure of aligned fibres that has been developed. Cooling is advantageously effected by causing the material from the conveyor to flow through a cooling die of substantially uniform cross-section, which may be circular, and of which the wall is cooled. In that manner, lateral expansion of the material during cooling is constrained. As is explained above, it is thought that, in addition to preventing the disruption of already-formed fibrous structure, cooling of the material in that way promotes the formation of oriented fibres. The cooling die is believed also to assist in maintaining the alignment until the material has cooled sufficiently for its structure to have stabilised.

During the course of its flow through the cooling die, and before it ceases to be constrained against lateral expansion, the material is advantageously cooled to a temperature not greater than 110°C and, preferably, not greater than 100°C. It will be appreciated that the temperature may not be uniform throughout the cross- section of the product when the product leaves the extrusion outlet. The temperatures of 110"C and 100°C refer to the outer regions of the product. The fact that inner regions are at higher temperatures will generally be found not to give rise to significant disruption of the fibrous structure.

It will be appreciated that further cooling may occur after the product has passed through the extrusion outlet. In addition to undergoing further cooling, there will usually be some reduction in the moisture content of the product through evaporation. For example, where as in the Example below the moisture content of the material on mixing (which may generally for practical purposes be assumed to correspond to the amount of added water) is about 42% by weight based on the total weight of water and other ingredients, the moisture content of the product may be reduced by such evaporation to, for example, about 40% by weight.

The downstream end of the cooling die may itself constitute the extrusion outlet.

Before it is cooled, the heated material is advantageously passed through a breaker plate having a plurality of apertures, each aperture having a diameter smaller than the diameter of the extrusion outlet. It is believed that the breaker plate may perform at least two functions.

The shear to which the mixture is subjected within the apertures of the breaker plate will be largely unidirectional and it is thought that its magnitude may be great enough for it to make a contribution to the required orientation of the protein molecules.

A probably more important function of the breaker plate is to promote stable extrusion. Thus, it is believed that the small apertures in the breaker plate, which give rise to a considerable pressure drop across it, reduce the significance of any pressure variation in a lateral direction immediately upstream and/or downstream of the breaker plate. That decreases the magnitude of any fluctuations there may be in the rate of extrusion, which in turn reduces the risk of blockage, and reduces variations in residence time and variations in degree of cooling. In the case of gluten-containing doughs, stable extrusion is very important and, because of the high elasticity of the dough, especially difficult to obtain. The breaker plate may also promote stable extrusion by increasing the back pressure on the mixture in the conveyor.

The amount of added water may be from 35 to 70% by weight, based on the total weight of water and other ingredients, and advantageously does not exceed 55% by weight. If the viscosity of the dough is reduced, for example, by increasing the amount of added water or reducing the amount of solid ingredients, the viscous dissipation of mechanical energy involved in mixing and conveying the dough and hence the rate at which heat is supplied to the dough by mechanical action, is reduced. It will generally then be found necessary to increase the rate at which heat is supplied other than mechanically, preferably, by heating the walls of the conveyor (for example, electrically) . A possible additional consequence of increasing the moisture content of the dough is an impairment of the stability of the process. Those consequences of increasing the moisture tend to become more significant when the moisture content exceeds 55% by weight. It is preferred that the moisture content be from 40 to 50% by weight, based on the total weight of water and other ingredients, doughs with those moisture contents giving products with an excellent fibrous structure. The amount of added water may be from 50 to 335 parts, and is advantageously from 60 to 200 parts, by weight water per 100 parts by weight gluten. Preferably, the amount of added water is from 65 to 150 parts, and more preferably from 80 to 115 parts by weight water per 100 parts by weight gluten. Advantageously, at least one further edible ingredient is introduced into the conveyor, the at least one further edible ingredient constituting not more than 30%, and preferably not more than 20%, of the total weight of the ingredients, excluding added water. The gluten and any other solid ingredients used (referred to hereafter as "dry ingredients") may include some moisture. In practice, however, those moisture contents will be small, for example less than 15% by weight based on the total weight of the respective ingredient. If the total moisture content of the ingredients other than water is greater than 15% by weight based on the total weight of those ingredients, however, the amount of added water may need to be reduced accordingly. The at least one further ingredient may be, or may include, one or more dry ingredients, for example, an ingredient selected from cereal flours, soya protein and flavouring. Advantageously, the at least one further ingredient comprises wheat flour. Where further dry ingredients are present as well as gluten, they may expediently be mixed with the gluten and introduced into the conveyor in admixture with.the gluten. Having introduced the solid particulate material, of which at least the major part is vital wheat gluten, into the conveyor, that material is mixed with the water to obtain the dough. If desired, cutting means, for example, a blade, may be mounted at or close to the extrusion outlet for cutting the extrudate into pieces, preferably, into lengths. If, when it leaves the extrusion outlet, the extrudate is allowed to drop sufficiently far before it meets a supporting surface (for example, the upper run of an endless belt conveyor arranged to convey it away from the extruder) the extrudate will break up under its own weight. Further in the case of many of the dough formulations that may be used in accordance with the invention, the extrudate will be found to break up under the influence of the material's own weight into pieces of a suitable size for vise in pies, stews or the like, so that the provision of cutting means is then unnecessary. The extrudate can be used, after hydration if that be found necessary or desirable, as a meat substitute, for example, as a chicken meat substitute, in vegetarian ready meals, for example, in pies or casseroles. Where the extrudate is to be hydrated, that is effected by immersing it in boiling water for a period of from 30 minutes to two hours. The pieces obtained can be reduced in size, for example, by shredding, for use as a substitute for minced meat. If desired, however, the extrudate may instead be used as an extender in meat dishes. In that case meat-derived material, for example, meat-derived flavouring, may be introduced into the conveyor and mixed with the other ingredients, the presence of that material enhancing the meat-like properties of the extrudate. The products substantially retain their textured structure when incorporated in such dishes. In comparison with certain known meat analogue products, the product of the invention has a relatively high moisture content. In contrast to, for example, the known textured vegetable protein products, it may be unnecessary to hydrate the product obtained in accordance with the invention before it is consumed, at least when further processing (for example, incorporating the product into a pie) is involved, providing the pos¬ sibility of using the product as a chilled meat substitute without the further processing that would otherwise be necessary.

One form of apparatus suitable for carrying out the method of the invention, and one form of method according to the invention and using that apparatus, will now be described, by way of example, with reference to the accompanying drawings, in which:

Fig. 1 is a diagrammatic, vertical longitudinal section taken through the apparatus on the line I-I of

Fig.2, with one of the shafts omitted in the interests of clarity; Fig. 2 is a diagrammatic, horizontal axial cross- section taken through the apparatus, on the line II-II of Fig. 1 with the shafts omitted in the interests of clarity; Fig. 3 is a diagrammatic transverse cross-section taken through the apparatus on the line III-III of Fig. 1;

Fig. 4 is a diagrammatic plan view, on a larger scale than Fig. 1, of a plate forming a part of the apparatus shown in Fig. 1;

Fig. 5 is a diagrammatic plan view, on a larger scale than Fig. 1, of a paddle which forms a part of the apparatus; and

Fig. 6 is a diagrammatic axial cross-section, on a larger scale than Fig. 1, taken through a jacketed tube which forms a part of the apparatus shown in Fig. 1.

Referring to the accompanying drawings, which are not to scale, the apparatus is made up of a twin-screw conveyor, which is indicated generally by the reference numeral 1, a cooling die, which is indicated generally by the reference numeral 2, and a thick plate 3 having within it a tapered aperture 3a, the thick plate being interposed between the conveyor and the cooling die.

The hollow interior 4 of the screw conveyor 1, which has a figure-of-eight cross-section (see Fig. 3) can be regarded as being made up of a feed section 4a, a mixing section 4b, a heating section 4c and a metering section 4d. Fig. 1 being diagrammatic and not to scale, the relative lengths of the sections 4a to 4d may differ from those shown.

The conveyor 1 has two co-rotating screw assemblies, the axes of which are parallel to one another and lie in the same horizontal plane. The screw assemblies comprise shafts 5a and 5b, respectively, on which are mounted screw sections 6a and 6b, respectively (see Fig. 3) and paddle sections discussed below. The screw sections 6a and 6b mesh with each other. As can be seen in Fig. 3, the shafts 5a and 5b are mounted coaxially within the part circular lobes (as seen in transverse cross-section of the hollow interior 4 of the conveyor 1) . The journalling of the shafts 5a and 5b, and the means for driving them, are omitted in the interests of clarity.

In the barrel or wall 7 of the conveyor 1, there is provided a first port 8 for the introduction of dry material into the feed section 4a, and a second inlet port 9, which is of smaller diameter than the first inlet port 8, is provided downstream of the first inlet port for the introduction of water into the mixing section 4b.

A dry ingredient feeder, for example, a volumetric screw conveyor (not shown) may be used to introduce dry ingredients through the first inlet port 8. A piston pump (also not shown) may be provided to introduce water through the second inlet port 9.

In what follows, the letter D is used to denote the diameter of each of the shafts 5a and 5b, and the lengths referred to, for example, the lengths of screw sections and of paddle sections (see below) , and of paddle sections are the axial lengths of the items in question. In the feed section 4a of the conveyor 1, each shaft 5a and 5b is provided with a spacer having a length of O followed (in the direction of flow of the material) by two screw sections, of which the first is a feeder screw and the second is a single lead screw. The feeder screw has a length of 3D and a relatively large pitch, so that it serves to convey the mixture of dry ingredients relatively rapidly away from the inlet 8. The single lead screw has a length of ID, and is of considerably smaller pitch than the feeder screw. In the mixing section 4b, each shaft 5a, 5b is provided with a paddle section made up of six 45° forwarding paddles 10 (see Fig. 5) , each of which has a length of \Ω, and a single lead screw of length ID. The expression "paddle" is used in the art to refer to a member of which the central plane extends at right angles to the shaft and which is not circular. Thus, a paddle might, for example, be elliptical when viewed along the axis of the shaft on which it is mounted. In the apparatus shown in Fig. 1, however, the paddles 10 each have the external shape shown in Fig. 5. An individual paddle 10 is not of itself "forwarding". What makes a section composed of a plurality of paddles 10 "forwarding" is the change in azimuthal angle from one paddle to the next, the sense of the change being such that, in operation, the forwarding paddles collectively operate to facilitate forward movement of the dough. In fact, because of the cohesive nature of the dough, they exert a force on the dough tending to cause it to move in a downstream direction. Here the change in azimuthal angle between adjacent paddles 10 is 45°. Thus, the paddle sections made up of the six 45° forwarding paddles 10 can together be regarded as approximating to an interrupted screw having thick flights but, unlike a screw, the paddles do not have surfaces that are inclined at an acute angle to the axis of the shaft on which they are mounted; they have surfaces that are perpendicular to that axis and a cylindrical (but not a circular cylindrical) surface. Because of the thickness of the paddles 10 and the interruption of the notional screw, the paddles each exert a substantial circumferential force on the material, which provides a higher degree of mixing and of shear and hence a higher input of mechanical work than would be provided by a corresponding continuous screw element. Of course the forwarding paddles 10 "facilitate" the forward movement of the dough only after allowing for the inevitable retarding effect that results from the fact that the mere presence of such paddles reduces the free cross-sectional area within the part of the conveyor where they are provided. In the heating section 4c, each of the two shafts 5a and 5b is provided with paddles 10 and lead screws as set out in Table 1. In that table, the elements are listed in the order in which they are arranged in the direction in which the material is conveyed in use.

In Table 1, the letters FP stand for "forwarding paddles", the meaning of which is explained above. The letters RP stand for "reversing paddles". Individual reversing paddles 10 are indistinguishable from forwarding paddles, but they differ collectively in that the 45° change in azimuthal angle from one paddle to the next within a section of reversing paddles is in a sense opposite to that in the case of the paddles in a section of forwarding paddles. Thus, considered collectively, the reversing paddles 10 can be regarded as approximating to an interrupted screw that would, in operation, tend to force the material being conveyed back in an upstream direction. The actual reversing paddles 10 serve to inhibit the forward movement of the dough. In fact, because of the cohesive nature of the dough, they exert a force on it tending to cause the dough to move in an upstream direction, but without actually causing any such upstream movement to occur. The letter P stands for "paddle", and denotes one of a section of adjacent paddles 10 in which the change in azimuthal angle from one paddle to the next is 90°. The 90° paddles 10 neither facilitate forward movement of the dough nor significantly impair such forward movement of the dough (save by reason of the reduction in cross-sectional area of the flow passage that results from their presence) . The paddles P do not tend to exert any force on the dough in an axial direction. Each paddle, of whichever type, has a length of ^D.

The reversing paddles 10 act both to increase the pressure within the interior 4 of the conveyor 1 in the region upstream of them and to effect a relatively large input of mechanical work, and hence of heat, into the dough. In addition, they serve to impart shear and hence to effect mixing and to impart shear as do the forwarding paddles.

The 90° paddles 10 also induce considerable shear in the material and hence effect mixing. Thus, they increase the amount of mechanical work needed to convey unit mass of material, and consequently increase the heat input that derives from the mechanical work done by the conveyor on the material, although to a lesser extent than do the reversing paddles 10.

Table 1

Structure of screw sections and sections of paddles in the heating section 4c of the conveyor 1:

6 x 45° FP 3 x 90° P

3 X 45° RP 1*_D Single Lead Screw 6 x 45° FP 3 x 90° P

1*_D Single Lead Screw 5 x 45° FP

3 x 90° P

4 x 45° RP l^lD Single Lead Screw

6 X 45° FP 3 X 90° P

3 X 45° RP FP - Forwarding Paddle RP - Reversing Paddle P - Paddle

In the metering section 4d, each of the two shafts 5a and 5b is provided with a single lead screw of length 2^D.

Individual electrical heating means (not shown) are provided in the wall 7 of the conveyor 1, one such heating means being situated within each of the zones SI to S9 (see Fig. 1) . Each heating means can be controlled separately from the other heating means, and the heating means can be individually switched off.

Similarly, individual cooling means are provided for each of the zones SI to S9. The need for cooling means arises, at least in part, from the thermal conduction along the length of the wall 7. Each cooling means (not shown) comprises a passage within the wall 7 and means for causing a cooling liquid (which is conveniently water) to flow through the passage. Each cooling means can be brought into, and taken out of, operation separately from the other cooling means. Also, the degree of cooling effected by any cooling means that is in operation can be controlled separately from the other such cooling means, the control of the cooling means being effected by controlling the temperature of the cooling liquid entering the cooling means or, more usually, by controlling the rate of flow of the cooling liquid through the cooling means.

For each of the zones SI to S9, there is provided a temperature controller, which can be set manually to a desired temperature. Each temperature controller serves to control both the electrical heating means and the cooling means in the zone. Thus, the electrical heating means and the cooling means are so operated as to maintain the temperatures of sensors embedded in the wall 7 in the zone at a temperature that is approximately equal to the temperature to which the temperature controller is set.

The mechanical work done on the material being conveyed arises because the material is viscous and is subjected to shear. The conveyor 1 subjects the material to a high rate of shear, and an input of mechanical work also arises from the shear to which the material is subjected after it has ceased to be acted on directly by the conveyor. Within the conveyor 1, the back pressure is increased by the reversing paddles 10. Because the viscosity of the material changes along the length of the conveyor 1, different screw and paddle elements impart different degrees of shear to the material and the back pressure of the material varies along the length of the conveyor 1, the rate of input of mechanical work (and hence the rate of generation of heat in the material) , per unit length of the conveyor, varies along the length of the conveyor.

In general, the overall rate at which heat is generated in the material by the mechanical work done on it is considerably less than that required to bring the material to the required temperature, so that additional heat has to be supplied at a significant rate. Further, even if the overall rate of heat generation in the material by the mechanical work done on it were to be sufficient, it would not in general be possible so to arrange the various screw and paddle elements 6a, 6b and 10, respectively, as to achieve the desired axial temperature profile. In order to bring the material up to the required temperature, the provision of heating means, for example, the electrical heating means described above, will be found to be necessary. At the same time, it may also happen that, in one or some of the zones SI to S9, even switching off the heating means in those zones will not suffice to prevent the material from reaching a temperature that is undesirably high. Where that occurs, it may be because of the conduction of heat through the wall 7 in a longitudinal direction. The problem can be overcome by the provision of cooling means, for example, the cooling means described above. Because the operation of the heating elements is controlled with reference to the temperature within the wall 7, and not in response to the temperature of the material being conveyed through the interior 4 and because it is to the temperature within the wall 7 that the settings refer, it may be found that the temperature to which an individual heating means is set differs from, for example, is some 5°C higher than, the temperature of the material in the corresponding zone. In order to enable the temperature of the material in a downstream part of the interior 4 of the conveyor 1 to be monitored, three thermocouples are provided. The hot junctions of the three thermocouples are located in recesses in the wall 7 at positions along the length of the conveyor 1 indicated by the references Tl, T2 and T3, respectively. The recesses are located at positions in the wall 7 that are adjacent to the screws 6a and 6b where they are enmeshed.

In use, it is commonly found that some of the material becomes entrapped by the recesses, and other portions of the material may become attached to the entrapped material in the immediate vicinity of the recesses. In those circumstances, if in the zone in which the respective thermocouple is located the temperature of the wall 7 is higher than that of the main body of the material, the material with which the hot thermocouple junctions are in contact may be at a higher temperature than the main body of the material, for example, at a temperature about 5°C higher, than that of the main body of the material. Temperatures specified herein in relation to the conveyed material are as measured by the thermocouples at Tl, T2 and T3, respectively.

Within the hollow interior 4 of the conveyor 1, and at its downstream end (so that it is downstream of the downstream ends of the single lead screws) , there is provided a plate 11 which extends transversely across the width of, and fills the whole cross-sectional area of the hollow interior of the conveyor to the wall 7 of which it is securely fixed, and which is formed with a plurality of circular apertures. Such a plate is known in the art as a "breaker plate" and is referred to as such throughout the specification.

The form of the breaker plate 11 is shown in Figure 4, in which some of the apertures are denoted by the reference numeral 12. It will be seen that the breaker plate 11 is rectangular, and that the apertures 12 are symmetrically disposed with respect to a central, vertical axis through the plate. Further, the apertures 12, which are forty-three in number, are arranged in vertical and horizontal rows. The diameter of each of the apertures 12 is the same. The separations between adjacent rows of apertures 12 are the same, for both vertical and horizontal rows, the spacing being taken as the separation between lines through the centres of the apertures in each of the rows concerned. The function of a breaker plate, such as the breaker plate 11, is explained above.

Affixed to the downstream end of the wall 7 of the conveyor 1 is the thick plate 3 formed with the central aperture 3a. At the upstream face of the thick plate 3, the aperture is in register with the downstream end of the hollow interior of the conveyor 1.

To the downstream face of the thick plate 3 there is secured a cooling die 2 of which the upstream end abuts the thick plate 3. The cross-sectional area of the aperture 3a decreases in a downstream direction, and the shape of the transverse cross-section of the aperture changes so that, at the downstream face of the thick plate 3, the aperture is in register with the upstream end of the hollow interior of the cooling die 2, which is of circular cross-section.

Located within the aperture 3a is a pressure transducer, which serves to provide a measure of the pressure within the material when it is about to enter the cooling die 2.

The cooling die 2 is made up of a plurality of jacketed tubes joined end-to-end. One of the jacketed tubes, which is indicated generally by the reference numeral 13, is shown in Fig. 6.

As shown in Fig. 6, each jacketed tube 13 comprises an inner tube 14, which is of circular cross-section and which is surrounded by a cooling jacket 15 which is of annular cross-section and through which a cooling fluid, for example, water (or a mixture of water and glycol) that has been chilled, may be circulated as indicated by the arrows in Fig. 6.

Each individual jacketed tube 13 is joined to the or each adjacent jacketed tube, for example, by screw- threaded and flanged connector means (not shown) . As an example of suitable dimensions for the conveyor 1 and the cooling die 2, the parts of the shaft 5a and 5b on which the screw sections 6a and 6b, the paddle sections and any spacer or spacers are mounted, may have a length of 1,265 mm. Then, if the shafts 5a and 5b each have a diameter of 50mm, the length-to- diameter ratios of those parts of the shafts will be 25:1. A Baker-Perkins MPF50 co-rotating twin-screw extruder provides such a conveyor having the dimensions stated above to be suitable. The screw and paddle elements can be selected from a range of such elements and can be mounted on the shafts of the extruder in the desired order, including in the order set out in detail above. The breaker plate 11 may have a thickness of 10mm, the diameter of the apertures 12 in the breaker plate may be 3.1mm, and the spacing between the rows of apertures may be 10mm. Each of the inner tubes 14 of the jacketed tubes 13 that make up the cooling die 2 may have an internal diameter of 10mm. The number and length of the inner tubes 14 may be such that together they define a continuous passage with a total length of 900mm.

In operation, the shafts 5a and 5b are driven at a constant rate and in the same sense (so that they are co- rotating) , and the dry ingredients are first thoroughly mixed and then continuously introduced into the hollow interior 4 of the conveyor 1 through the first inlet port 8 at a constant rate. At the same time, water is introduced through the second inlet port 9 at a constant rate that bears a predetermined relation to the rate of introduction of the dry ingredient or mixture of dry ingredients, which in turn bears a predetermined relation to the rate at which the shafts 5a and 5b are driven.

The dry ingredient or mixture of dry ingredients is transported relatively rapidly away from the first inlet port 8 by the two feeder screws, mounted on, and close to the upstream ends of the two shafts, 5a and 5b, thus preventing the first inlet port 8 from becoming blocked. As the initial mixture of dry ingredient(s) and the water are conveyed along the conveyor 1 by the action of the various screw elements 6a and 6b, the configuration and function of which are explained above, they are further mixed to form a dough. The mixing occurs both within each lobe of the hollow interior 4 of the conveyor 1 and between the two lobes.

The temperature of the dough rises and, when it reaches a temperature of about 100°C, it tends to expand and, were it allowed to expand freely, it would typically increase considerably in volume. The fact that the warm dough is confined within the interior 4 of the conveyor 1, however, prevents such an expansion from taking place. Not only is the dough physically prevented from expanding to such an extent, but the tendency of the dough to expand in a confined space results in an increase in pressure, of which one effect is to raise the boiling point of the water, which itself retards further expansion. It is believed that it is in the cooling die 2 that the major part of the formation of fibres occurs. Further, the water vapour pressure declines sufficiently to prevent disruption of the fibrous structure when it leaves the cooling die 2. The product so obtained exhibits a fibrous character on both a microscopic scale and a macroscopic scale; thus, on examination under an optical microscope the product may be seen to consist of substantially parallel aligned fibres, which for the most part are present in bundles, and on examination with the naked eye individual macroscopic "fibres" (each of which is believed to correspond to a bundle of microscopic fibres) may be discerned.

At the exit of the cooling die 2, the product is allowed to fall through a distance sufficient to cause it to break into lengths under its own weight. Commonly, it will be found that the lengths into which the product breaks are such as to render it suitable for incorporation into a food product without the need for it to be cut into shorter lengths. Where that is not the case, however, a cutter may be provided at or near the exit from the cooling die 2 to cut the product into pieces of the required size.

It will be appreciated that the construction of the apparatus may be varied in a number of ways. For example, instead of providing electrical heating elements located in recesses in the inner surface of the wall 27 of the conveyor 1, passages through which a heating fluid, for example, a hot oil, may be passed, may be provided in the wall. Also, it is not essential for the cooling die 2 to be in sections arranged end-to-end, although the ability to apply different, and individually adjustable, degrees of cooling at different points along the length of the cooling die will generally be found to be advantageous. The inner tubes 14 of the jacketed tubes 13, which constitute the wall of the cooling die 2, may be of other than circular cross-section.

In the apparatus described with reference to the accompanying drawings, the cooling die 2 is separated from the conveyor 1 only by the thick plate 3, which is described above only in general terms. In fact, the thick plate 3 may take the form of a conventional die head, to which the cooling die 2 may be connected, if appropriate by means of a coupling. Where the diameter of the die in the said die head is smaller than the internal diameter of the cooling die 2, the coupling may expediently be tapered.

The following Example illustrates the invention: Example. In this Example, references to percentages are to be understood as being references to percentages by weight.

The extruder used was a Baker-Perkins MPF 50 co- rotating twin screw extruder, which was as described above with reference to Figs. 1 to 5 of the accompanying drawings. In particular, the arrangement of the various screw and paddle elements on the shafts of the conveyor was as described above. The two shafts 5a and 5b were each driven at a constant rate of 150 r.p. ..

The dry ingredients were composed of 83.8% vital wheat gluten, 11.5% wheat flour and 4,7% flavouring based on the total weight of the dry ingredients. They were pre-blended in 15 kg batches using a Hobart mixer, and the resulting dry mix was introduced into the interior 4 of conveyor 1 through the first inlet port 8 at a constant rate of 13.8 kg/h using a dry ingredient feeder. Within the extruder, the dry ingredients were conveyed to the mixing section 4b where water was introduced into the interior 4 of the conveyor 1 through the second inlet port 9 at a constant rate of 10 kg/h using a piston pump. Thus, the total mass flow rate into the extruder was 23.8 kg/h. In the mixing section 4b of the extruder, the dry ingredients were hydrated to form a homogeneous, visco- elastic dough of which the total water content based on the weight of the dough was approximately 48%. That moisture content was made up of 42% added water and 6% from the initial moisture content of the dry ingredients, each of those two percentages being based on the total weight of the dough.

In the mixing section 4b of the extruder, the six forwarding paddles 10 served, in addition to facilitating the forwarding flow of the dough, to impart a high degree of shear to the dough. The shear served both to ensure homogeneity of the dough and to cause heating of the dough by reason of the input of mechanical work needed to overcome the viscous drag exerted by the dough on the paddles.

The temperatures to which the temperature controllers for the zones SI to S9 were set are shown in Table 2, as are the temperatures recorded by the thermocouples at Tl, T2 and T3 in zones S7, S8/9 and S9, respectively.

It will be noted that the temperature controllers for the two zones SI and S2 in the feed section 4a of the conveyor 1 were each set to temperatures below 100°C, the actual settings being 40°C and 70°C, respectively. Those relatively low temperature settings were used because of the desirability of avoiding the formation of steam before all the added water had been incorporated in the dough.

If steam were to be present in the feed section 4a of the conveyor, it would be liable to cause aggregation of the particles of the dry ingredients, which would probably lead to bridging and a consequent possible interruption of the flow of material along the conveyor.

In the heating section 4c of the conveyor 1, the dough was brought up to a temperature within the range of from approximately 150°C to approximately 160°C. The increase in the temperature of the material in the heating section 4c of the conveyor 1 resulted partly from the input of mechanical work and partly from the heat output of the electrical heating elements in that section. The hot material was conveyed through the breaker plate 11 and into the cooling die 2. Pressure readings obtained from the pressure transducer immediately downstream of the breaker plate 11 (and hence before the material entered the cooling die 2) varied during the duration of the run, but remained within the range of from 700 to 1000 psi (48 x 10⁵ to 69 x 10⁵Pa) . Before it emerged from the cooling die 2, the dough was cooled to a temperature below 100°C to prevent disruption of its fibrous structure on exit from the cooling die, where the extrudate was subjected to ambient air pressure only. Chilled water was circulated, counter-current to the flow of the mixture, through the jackets 15 of the jacketed tubes 13 making up the downstream 0.45m of the cooling die 2. Before it entered the jackets 15, the water was chilled to a temperature of 7°C to 8°C. The rate of water circulation through the jackets 15, and hence the degree of cooling, was controlled by a regulating valve. The jackets 15 of the jacketed tubes 13 making up the upstream 0.45m of the cooling die 8 were open to ambient air. The temperature of the product immediately after leaving the cooling die 2 varied within the range of from 85°C to 99°C. After leaving the cooling die 2, the extrudate broke into lengths under the influence of its own weight.

The average residence time of the ingredients from the introduction of the dry ingredients into the feed section 4a until the processed dough left the cooling die 2 was found using a dye tracer to be six minutes.

After the product had left the die 2 and then been allowed to cool to ambient temperature, it was observed to have a substantially uniform textured structure, made up of fibres having diameters within the range of from 1.5 to 10 microns. The fibrous structure could conveniently be observed using an optical microscope with a magnification (the magnification being the product of the primary magnification produced by the objective and the power of the eyepiece) within the range of from 100X to 2OOX. The fibres were typically present both singly and in bundles, the fibres and bundles being of various lengths and generally arranged in an oriented manner substantially parallel to the direction in which the material had flowed through the cooling die 2. The macroscopic appearance of the product was also generally fibrous in nature and resembled a piece of meat or poultry in that the fibres were substantially aligned in a common direction. ^' The product was suitable, after hydration, for use as a vegetarian meat analogue.

Table 2

Temperature Settings (°C)

Zone Temp. (°C)

SI 40

S2 70

S3 100

S4 110

S5 120

S6 130

S7 140

S8 160

S9 163 Measured Temperatures (°C)

Zone Temp. (°C)

S7 136

S8/9 156

S9 145

Claims

1. A method of making a textured food product, comprising: introducing vital wheat gluten into a twin-screw conveyor; introducing water into the conveyor, the gluten and water being introduced at or in the vicinity of the upstream end of the conveyor; mixing the gluten and the water to form a dough whilst at the same time conveying the material towards and through an extrusion outlet; ensuring that the material reaches a temperature at which it melts and, subsequently, cooling the material before it exits from the extrusion outlet.

2. A method as claimed in claim 1, wherein the twin- screw conveyor comprises two co-rotating screw assemblies.

3. A method as claimed in claim 2, wherein each screw assembly of the twin-screw conveyor comprises a plurality of elements arranged along the length of a shaft and, adjacent to the upstream end of each screw assembly, there is provided an element or elements arranged to effect relatively rapid movement of the gluten away from the position at which it is introduced into the conveyor.

4. A method as claimed in claim 3, wherein elements of each screw assembly make up groups of elements, each of which groups is made up of a first element or elements arranged to impart a high degree of mixing to the dough together with some force tending to cause the dough to continue to move in a downstream direction followed, in a downstream direction, by a second element or elements arranged to impart a high degree of mixing to the dough without exerting any significant force on the dough in an axial direction.

5. A method as claimed in claim 4, wherein the elements of each screw assembly are so arranged that at least some of the said groups also include a third element or elements, situated downstream of the second element or elements and arranged to impart a high degree of mixing to the dough together with some force tending to cause the dough to move in an upstream direction.

6. A method as claimed in claim 5, wherein the elements of each screw assembly include fourth elements arranged to impart a force to the dough tending to cause it to move in the downstream direction, without imparting to the dough a significant degree of mixing, the fourth elements being so located that they separate adjacent ones of the said groups of elements.

7. A method as claimed in any one of claims 3 to 6, wherein the elements of each screw assembly include a fifth element or elements arranged to impart a force to the dough tending to cause it to move in the downstream direction without imparting to the dough a significant degree of mixing, the fifth element or elements of each screw assembly being situated adjacent to the downstream end of the screw assembly.

8. A method as claimed in any one of claims 1 to 7, wherein the temperature of the material reaches at least 130°C.

9. A method as claimed in claim 8, wherein the material reaches a temperature of at least 140°C.

10. A method as claimed in any one of claims 1 to 9, wherein the pressure to which the material is subjected during its passage through the conveyor reaches at least 250 psi (17.5 x 10⁵ Pa) .

11. A method as claimed in claim 10, wherein the maximum pressure reached is within the range of from 400 to

1000 psi (27 X 10⁵ to 69 X 10⁵ Pa) .

12. A method as claimed in any one of claims 1 to 11, wherein cooling is effected by causing the material from the conveyor to flow through a cooling die of substantially uniform cross-section and of which the wall is cooled.

13. A method as claimed in claim 12, wherein the die is of circular cross-section.

14. A method as claimed in claim 12 or claim 13, wherein the downstream end of the cooling die constitutes the extrusion outlet.

15. A method as claimed in any one of claims 1 to 14, wherein, before said cooling, the heated material is passed through a breaker plate having a plurality of apertures, each aperture having a diameter smaller than the diameter of the extrusion outlet.

16. A method as claimed in any one of claims 1 to 15, wherein the amount of added water is from 35 to 70% by weight, based on the total weight of water and other ingredients.

17. A method as claimed in claim 16, wherein the added water does not exceed 55% by weight, based on the total weight of water and other ingredients.

18. A method as claimed in claim 17, wherein the amount of added water is from 40 to 50% by weight, based on the total weight of water and other ingredients.

19. A method as claimed in any one of claims 1 to 18, wherein the amount of water added is from 50 to 335 parts by weight water per 100 parts by weight gluten.

20. A method as claimed in claim 19, wherein the amount of water added is from 60 to 200 parts by weight water per 100 parts by weight gluten.

21. A method as claimed in claim 20, wherein the amount of water added is from 65 to 150 parts by weight water per 100 parts by weight gluten.

22. A method as claimed in claim 21, wherein the amount of water added is from 80 to 115 parts by weight water per 100 parts by weight gluten.

23. A method as claimed in any one of claims 1 to 22, wherein at least one further solid ingredient is introduced into the conveyor, the at least one further solid ingredient constituting not more than 30% of the total weight of the ingredients, excluding added water.

24. A method as claimed in claim 23, wherein the at least one further ingredient comprises a cereal flour.

25. A method as claimed in claim 24, wherein the cereal flour is wheat flour.

26. A method of making a textured food product, in which water is mixed with at least one dry ingredient comprising a cereal gluten, and during said mixing the mixture is conveyed towards an extrusion outlet, the mixture being heated and, subsequently, being cooled before exit from the extrusion outlet.

27. A method as claimed in any one of claims 1 to 26, wherein the extrudate is hydrated.

28. A method as claimed in claim 27, wherein the hydration is effected by immersing the extrudate in boiling water for a period of from 30 minutes to 2 hours.

29. A method of making a textured food product substantially as described in the Example herein.

30. A textured food product made by a method as claimed in any one of claims 1 to 29.

31. A vegetarian food product comprising a textured food product obtainable by a method as claimed in any one of claims 1 to 29.