WO2005025341A1

WO2005025341A1 - Method and apparatus for continuous production of extruded and/or coextruded potato and/or cereal flour-based snacks

Info

Publication number: WO2005025341A1
Application number: PCT/IB2003/005903
Authority: WO
Inventors: Giovanni Guidolin
Original assignee: Fen Food Enterprise S.R.L.
Priority date: 2003-09-12
Filing date: 2003-12-12
Publication date: 2005-03-24
Also published as: ITVI20030175A1; AU2003304465A1

Abstract

A method for continuous production of extruded and/or coextruded potato and cereal flour-based snacks, including the steps of preparing a dough (D) from food flours (F) having a predetermined residual moisture, extruding the dough (D) to form one or more continuous strips (2), coupling said one or more extruded strips (2) for forming an elongate continuous semifinished product (3), extending along a substantially longitudinal axis (L), deforming in a controlled manner the continous semifinished product (22), cutting the continuous semifinished product (3) into segments having a predetermined length to form semifinished snacks (1), drying the semifinished snacks (1). After the extrusion step and before the cutting step, the extruded strips (2) are subjected to a spinning and controlled deformation action, in such a manner as to take a substantially helical shape, and to provide the continuous semifinished product (3) with a substantially twisted or multiple thread screw shape, with a wavy and spiraled outer surface (4).

Description

METHOD AND APPARATUS FOR CONTINUOUS PRODUCTION OF EXTRUDED AND/OR COEXTRUDED POTATO AND/OR CEREAL FLOUR- BASED SNACKS Field of the Invention

This invention generally finds application in the field of food products, and particularly addresses a method for continuous production of extruded or coextruded between-meal snacks made from potatoes, cereals and mixtures of food flours.

The method of the invention may be also used in the continuous production of food products for catering purposes, such as breadsticks, food bars and other similar products.

The invention also relates to an apparatus for implementing the method, as well as a food snack obtained by using said method and apparatus.

Prior art

A variety of between-meal snacks are currently widespread over the market, a particular importance being given to those obtained by extrusion of semifinished products, to be finished with several treatments, such as, for example, warm drying, baking, frying in oil, seasoning, sweet and salty covering.

The finished products are sold in sealed packages in food stores, supermarkets, public concerns, such as bars, restaurants, refectories, etc.

Extruded and/or direct expanded snacks are increasingly widespread, which are preferably not fried in oil, with seasoning and post-extrusion treatments for nutritionally enriching the product and improving its appearance by using novel shapes and colors, in addition to traditional sweet and salty coverings. Recent studies, as well as market trends, show the need of developing non-fried between-meal products, such as baked, hot-dried snacks.

The traditional semifinished snack frying technology is still used worldwide over a large scale, as it is inexpensive, easy to use, providing rather important and typical organoleptic and functional product characteristics, such as the fried oil flavor and the typical honeycomb structure of the finished product.

The latter characteristic, which is imparted to the product during the frying process, is a crucial aspect for between-meal snacks, wherein texture and crispiness are basic consumer appreciated quality features.

Prior art snack manufacturing technologies provide various, sometimes complex shaped semifinished products (pellets), which are later subjected to the above described treatments.

These technologies provide high quality products, but involve excessive investments and production costs, as they require a double manufacturing process, i.e. for obtaining semifinished products (pellets) and subsequently cooking them by frying, hot air baking, microwaving, puffing, etc, wherein the term cooking is intended as the step in which the structure of the semifinished product is changed from a compact to a porous and honeycomb structure (expansion), thereby providing the final product with a pleasant chewability and crispiness (snack).

One of prior art alternative technologies consists in the production of snacks that are directly expanded during extrusion, and in possibly finishing the extruded product with a sweet covering or a sweet or salty seasoning.

The process for expanding/cooking the snacks obtained from semifinished products is based on the fast transformation of pellet's residual moisture (about 10% in the semifinished product) into steam. The amount of steam which is readily formed within the pellet structure causes micro-expansions and cavities in the blank, thereby increasing its size by expansion of such cavities, and providing the texture of the blank structure, which will be crispy and crumbly after cooling.

The conventional pellet-snack expansion/cooking processes are employed with frying, microwaving, hot air baking, gun puffing and other similar technologies.

Conversely, the extrusion technology that provides direct expansion of the blank provides the texture of snack structure by quick steam generation due to the pressure drop generated in the product output from the die.

This technology, which is incidentally widespread in developing countries, is highly cost-effective and convenient, thanks to the lack of an intermediate processing step (pellet), a facilitated management of production lines and product manufacturing, a high return on investment, a simple mechanical fabrication and plant installation, and a low cost of spare parts.

However, this snack production technology by extrusion with direct expansion of the blank, considerably limits the possibility of obtaining snacks having new sizes or complex and convoluted shapes, which is highly desirable and is an important requirement in the field of between-meal snacks which, as mentioned above, are subjected to a continuous evolution of formats, flavors, colors, tastes, etc.

Summary of the invention

The main object of the present invention is to provide a method and an apparatus for continuous production of three-dimensional snacks, made from potatoes, cereal flours or similar products, by a direct or indirect expansion extrusion technology, which provide special formats for quality and aesthetic improvement of finished products.

A further object of the present invention is to provide a method and an apparatus providing products having particular shapes, that might not be otherwise obtained by using prior art extrusion technologies, particularly direct expansion extrusion.

Yet another object is to provide a method and an apparatus providing various particular formats consisting of the combination of semifinished products having different characteristics.

Another object of the invention is to provide a method and apparatus allowing for the production of more tasty and aesthetically attractive snacks.

Another important purpose is to provide a method and an apparatus allowing for the production of attractive snacks, possibly having a three-dimensional shape, with competitive investment and production costs as compared with traditional technologies.

These objects, as well as others that will be apparent hereinafter, are achieved, according to claim 1 , by a method for continuous production of extruded potato and cereal flour-based snacks, which include the steps of preparing a food flour dough having a predetermined residual moisture, extruding said dough to form one or more continuous strips, coupling together said one or more extruded strips to form an elongated continuous semifinished product, which extends along a substantially longitudinal axis, cutting said semifinished product into segments having a predetermined length to form semifinished snacks, drying the semifinished snacks, wherein, after the extrusion step and before the cutting step, said one or more extruded strips are subjected to a spinning and controlled deformation action to take a substantially helical design and provide said continuous semifinished product with a substantially twisted or multiple thread screw shape, with a wavy and spiraled outer surface.

Yet according to the invention, the controlled strip deformation step is performed with the strips being in high plasticity and stickiness conditions and possibly with the help of a food grade adhesive, to enhance coupling therebetween, by bonding and/or mutual penetration.

Preferably, the extrusion step is followed by a strip expansion step. Depending on the residual moisture content of the dough, expansion occurs immediately after extrusion or after forming and cutting the continuous semifinished product and possibly after drying thereof.

Thanks to this particular method, snacks will be obtained, either directly expanded, or to be later expanded, which have a highly particular shape, providing a quality and aesthetic improvement of finished products.

According to another aspect of the invention, there is provided an apparatus for implementing the above method according to claim 13.

According to yet another aspect of the invention, there is finally provided a food snack according to claim 26.

Brief Description of the Drawings

Further features and advantages of the invention will be more apparent from the detailed description of a few preferred, non-exclusive embodiments of a method, an apparatus and a snack according to the invention, which are described as non- limiting examples with the help of the annexed drawings, in which: FIG. 1 is a flow chart of a process of a first embodiment of snack food manufacturing in accordance with the invention; FIG. 2 is a second flow chart of a second embodiment of the snack food manufacturing method in accordance with the invention; FIG. 3 is a general schematic view of an embodiment of a snsack food manufacturing apparatus in accordance with the invention; FIG. 4 is an enlarged view of a subassembly of the apparatus of FIG. 3, in partial section as taken along an axial vertical plane; FIG. 5 is a schematic view of a further enlarged detail of the subassembly of FIG. 4; FIG. 6 is a further enlarged schematic view of the detail of FIG. 4 in an alternative embodiment; FIG. 7 is an enlarged axonometric view of a further enlarged detail of an embodiment of the subassembly of FIG. 3; FIG. 8 is a top schematic view of a detail of FIG. 7 in a first operating position; FIG. 9 is a top schematic view of a detail of FIG. 7 in a second operating position; FIG. 10 is a schematic view of a second embodiment of the subassembly of

FIG. 3; FIG. 1 1 is an enlarged sectional view of a detail of the subassembly of the apparatus of FIG. 10, as taken along a transverse plane, perpendicular to the longitudinal axis; FIG. 12 is a schematic axonometric view of a third embodiment of the method; FIG. 13 is a sectional view of the subassembly of FIG. 12, as taken along a transverse plane, perpendicular to the longitudinal axis; FIG. 14 is a schematic perspective view of a fourth embodiment of the method; FIG. 15 is a top schematic enlarged, partly sectional view of a fifth embodiment of the subassembly of FIG. 3; FIG. 16 is a general top, partly sectional view of an alternative embodiment of the apparatus according to the invention; Fig. 17 shows a few schematic views of a few food snacks that may be made by using the method and apparatus according to the invention, designated by letters A to L.

Detailed description of a few preferred embodiments

With reference to the above figures, the method of the invention may be used for continuous production of extruded and/or coextruded potato and cereal flour- based snacks, in various sizes and shapes, which are schematically shown, by way of example and without limitation in figures 17A to 17L.

Particularly, a food snack, generally designated by the reference numeral 1 may be formed by at least one or more food flour strips 2 or strands, which are suitably coupled or deformed to define a substantially elongate body or semifinished product 3.

With particular reference to FIG. 1 , the method for continuous production of food snacks 1 may include at least the following steps.

After mixing, the food flours F are kneaded with other food compounds to obtain a dough D having a percentual residual moisture H, depending on the desired expansion type.

Particularly, by controlling the parameters of the kneader, a dough D1 may be obtained having a relative residual moisture H1 in a range of 5% to 25%.

Alternatively, a dough D2 may be obtained having a residual moisture H2 in a range of 25% to 45%.

The dough D is then extruded to form one or more continuous strips 2, similar to cords or strands having a circular, polygonal or other different cross-sections.

When a dough D1 is used having a relatively low residual moisture H1 of 5% to 25%, direct expansion will occur in the strips 2, immediately after extrusion thereof, which is caused by the almost instantaneous formation of steam in the base material, resulting in the formation of microcavities within the blank, which provides the texture of the blank structure.

In this case, the extruded and directly expanded strips will be submitted to subsequent treatments for forming a final product known as "direct expanded snack", which will be crispy and crumbly, after being cooled and dried to change its residual moisture to about 2%.

If a dough D2 is used, having a relatively high residual moisture H2, the extruded strips 2 will not be readily expanded but may have a compact and deformable consistence, said strips may be subsequently processed until semifinished products 1 or finished snacks are obtained. The latter may be submitted to further processing steps and then to a drying process, to change their residual moisture to a value close to 10% (pellets). In this case, the expansion required to obtain the proper texture of the product will occur thanks to traditional cooking-expansion process, by frying, hot air baking, microwaving etc., thereby obtaining a so-called pellet-snack, having a final moisture of about 2%, which is ready to be consumed.

Alternatively, by using the same dough D2, having a relatively high residual moisture D2, the strips may be submitted to subsequent processing steps until a semifinished snack 1 is obtained, which may be submitted to an expansion step by direct frying, or another equivalent process of the wet blank. In this case, a "wet fried" snack will be obtained, having a residual moisture of about 2%. The snack obtained thereby may be packaged and sold to the final user.

In all the above cases, the extruded strips will be submitted to a sequence of processing steps according to this invention, which will be described hereafter.

Particularly, the extruded strips 2 may be joined together in a coupling step to form the continuous elongate semifinished product 3, which extends along a longitudinal axis L.

The continuous semifinished product 3 may be submitted to a cutting step to be divided into segments of a predetermined length to form the semifinished snacks 1.

The semifinished snacks 1 may be dried to various extents, as the case may be, with the traditional procedure applied to pellets, which is possibly followed by a blank cooking-expansion step and subsequent packaging in sealed ready-for-sale packages.

A peculiar feature of this invention consists in that the after the mutual coupling step and before the blanking step, the extruded strips 2 are subjected to a spinning and controlled deformation action, in such a manner as to take a substantially helical design, and to provide the continuous semifinished product 3 with a substantially twisted or multiple thread screw shape, with a wavy and spiraled outer surface 4.

Then, the strand-like continuous helical semifinished product will be cut to the desired length to obtain the semifinished snack 1.

It shall be noted that the controlled deformation step is performed on the strips 2 with the dough D in high plasticity and stickiness conditions, to allow for mutual bonding and penetration thereof into a single continuous body 3.

Optionally, the strips 2 may be previously coated with a layer of food grade adhesive to facilitate the mutual coupling of the strips 2 by bonding and/or mutual penetration.

Preferably, the strips 2 may be previously covered with a layer of food grade materials having coloring and/or seasoning characteristics, adapted to enrich the organoleptic properties of the semifinished product 1.

In order to perform the spinning and controlled deformation of the strips 2, the continuous semifinished product 3 may be submitted to a combined drawing and twisting action about the longitudinal axis L.

For instance, such combined twisting and drawing action may be performed by exerting on the outer peripheral surface of the semifinished product at least two forces acting along predetermined directions with respect to the longitudinal axis L of the semifinished product.

The position of the area 22 in which the strips 2 are twisted and deformed is adjusted as a function of the consistency of the strips, which is variable and increases with the transit time downstream from the die.

Preferably, the strength and direction of the two forces may be adjusted as a function of the speed at which the strips 2 are extruded and the semifinished product 3 is formed, to control the final size thereof.

Also, the direction of the two forces that act on the semifinished product 3 may be inclined relative to the longitudinal axis L. The inclination angles of the directions of forces relative to the axis L may be varied to change the pitch and inclination of the strips 2. The variation of the inclination of forces, as well as the resulting speed whereat the semifinished product 3 is drawn, determines the blank spinning degree, and defines the shape of the final product.

During the controlled deformation step, the strips 2 may be compressed radially in such a manner as to mutually penetrate, thereby defining a substantially compact and uniform continuous semifinished product 3.

Alternatively, the action of radially compressing the strips 2 is such that a surface bond between the latter is obtained, whereby a longitudinal cavity is formed therein, to define a substantially tubular and internally hollow continuous semifinished product FIG. 17A and 17L.

Finally, by controlling the speed of the drawing devices and the crossed inclination thereof, the resulting semifinished product drawing capacity is determined, as well as the thickness or outside diameter of such semifinished product by partially squeezing and/or stretching it in a continuous and uniform lamination processing. By adjusting radial compression on the semifinished product, the possible partial lamination capacity is determined, and the final size of the blank is defined.

By controlling the speed whereat the strand 3 is drawn, the blank size may be more easily defined.

If the continuous semifinished product 3 is formed by a single strip 2 or strand, the latter may be internally hollow and have an appropriate cross section, e.g. have the shape of a star with three or more points, a rounded, square, triangular, elliptical, flat shape, etc. An example is the blank that is schematically shown in FIG. 17F, which has the shape of a simple tube, which is obtained by extrusion and direct expansion, has a porous and crumbly consistence, wherein a filling cream 5 is preferably injected.

As a rule, if the semifinished product 4 is made in such a manner as to have a central cavity, the filler 5 may be preferably obtained by coextrusion with the same strip or strand of dough.

In order to obtain tastier snacks, at least one of the strips 2 or all of them may be made from doughs having different organoleptic properties, as schematically indicated in FIG. 171.

In order to provide the snacks with a more attractive appearance, strips of different colors may be provided in separate extrusion dies, as schematically shown in FIG. 17H.

The processing method as described above may be used for two or more strands obtained by direct expansion extrusion, generated by the same die or by two or more dies, thereby actually obtaining a twisted blank, consisting or multiple combined and twisted strands by using belt conveyors or equivalent means, with a crossed drawing action. In this step, a partial melting or bonding of the twisted strands occurs, the latter being joined in the vicinity of the output die, where the product is still hot, wet and sticky.

The method may be used for two or more direct expanded strands, generated by two or more different dies 15, i.e. two or more separate extruders, as shown in FIG. 6, which can jointly feed the group of preferably twisted strips, which form one or more continuous semifinished products, which are later subjected to controlled deformation according to this invention.

In such arrangement, the extruders are able to process different mixtures, and to generate composite strips or strands, having different colors, flavors, etc.

Referring to Figures 3 to 16, an apparatus for continuous production of extruded snacks for carrying out the method of the invention is shown.

The apparatus, generally designated by reference numeral 10 sequentially comprises at least one unit 12 for mixing food flours F to prepare a substantially uniform dough D, having a predetermined moisture content.

The dough D is introduced in an extruder unit or head 13 having a power-operated extrusion screw 14 for processing the dough D and a plurality of extrusion dies 15 for forming one or more adjacent strips 2.

Downstream from the extrusion dies 15, a station or area 16 is provided for coupling the strips 2 and form the elongated continuous semifinished product 3 extending along the longitudinal axis L, which is preferably but not necessarily horizontal.

Downstream from the coupling station 16, a cutting station 17 is provided, for instance having one or more rotating blades 18, for cutting the continuous semifinished product 3 into segments of a predetermined length to form the semifinished snacks 1.

After cutting 1 , the snacks are conveyed through the hopper 19 and the elevator 20 to a drying unit 21 and then to a conventional belt conveyor,

According to the invention, downstream from the extruder head 13, controlled deformation means are provided, generally designated by numeral 22, which are used to impart a substantially helical shape, i.e. of a spiral, to the strips 2 thereby substantially imparting a twisted or multiple thread screw shape, with a wavy and spiraled outer surface 4, to the continuous semifinished product 3 .

Directly downstream from the extruder dies 15, and before the controlled deformation step 22, a natural direct strip expansion area is provided, if the dough to be extruded D is prepared with a residual moisture H1 of about 5%-25%.

By using a dough to be extruded D, prepared with a relative residual moisture of about 25-45%, strips of semifinished blanks (pellets) may be obtained which have a compact consistency and are prepared, after the controlled deformation step 22 and the cutting step 17, for a traditional pellet-snack drying process, in which the semifinished snacks 1 are dried to achieve a residual moisture of about 10% and are finally prepared for their final transformation with a cooking-expansion processes by frying, hot air baking, microwaving, etc.

Alternatively, after the cutting step 17, the semifinished extruded pellet is directly conveyed to the cooking-expansion step by frying or another equivalent process.

Particularly, the controlled deformation means 22 include means for combined drawing/twisting of the continuous semifinished product 2 about the longitudinal axis 1.

As a rule, the combined drawing-twisting means comprise at least two opposed movable surfaces which are fed along directions T', T" relative to the longitudinal axis L, in such a manner as to exert on the semifinished product a resulting force with a drawing component T in the longitudinal direction and a twisting component R in the transverse direction.

Still in general terms, adjusting means, not shown in the drawings, are associated to the drawing-twisting means 22, for controlling the strength of forces with respect to the strip 2 feeding speed, to accordingly change the pitch and inclination of the spirals that form the cord.

In a first embodiment of such drawing-twisting means, which is schematically shown in FIGS. 7, 8, 9, the movable surfaces are two power operated belt conveyors 23, 24, which are subtended by corresponding pairs of end rolls, designated by numerals 25, 26 and 27, 28, which are stationary relative to the extruder dies 15.

At least one of the end rolls, 25, 27 respectively is coupled to a respective motor 29, 30, which is supplied with power from an electronic central unit, not shown in the drawings, and is part of the adjustment means.

The belt conveyors 23, 24 have respective operating portions 31 , 32, interacting with the outer surface 4 of the continuous semifinished product 3.

In the design of the drawing-twisting means as shown in FIG. 9, the angles a', a" of the belt conveyors 23, 24 relative to the longitudinal axis L are close to 45°, therefore the drawing force T and the twisting force R are approximately equal and provide helical strips 2 and a twisted semifinished product 3, which have predetermined pitch, inclination angle and maximum diameter settings.

In the design of the drawing-twisting means as shown in FIG. 8, the angles a', a" of the belt conveyors 23, 24 relative to the longitudinal axis L are smaller than 45°, hence the twisting component R' is larger than the drawing component T', whereby the pitch, inclination angle and maximum diameter settings are different from the previous ones.

In the embodiment of the drawing-twisting means that is schematized in FIGS. 10 and 11 , the movable surfaces are formed by the outer periphery of at least two, preferably three appropriately shaped and rubber coated power operated driving wheels 33, 34, 35, which act on the outer surface of the continuous semifinished product 3.

The lower wheels 33 and 35 have parallel axes of rotation which may be adjusted with an inclination a relative to the longitudinal axis L, whereas the upper wheel 34 has a contact surface whose axis of rotation is substantially inclined through the same angle a, relative to the axis L, but positioned opposite to the two lower wheels 33 and 35.

In the embodiment of the drawing-twisting means as schematized in FIGS. 12 and 13, the movable surfaces are formed by at least one, preferably two pairs of opposed power operated rollers, which act on the continuous semifinished product 3 from opposite sides, designated by numerals 36, 37 and 38, 39 respectively. All the rollers, with their respective motors, are secured to a mandrel 40 which is rotatably fitted on an axle which is stationary relative to the frame of the apparatus 10 and coincides with the longitudinal axis L.

The mandrel 40 has a central hole 41 , which is actually formed by the gap resulting from the opposed arrangement of the rollers 36, 37 and 38, 39, that are spaced from each other to allow the passage of the semifinished product 3 while it is drawn. The mandrel is coupled to one or two external motors 41 , which set it into rotation at a predetermined speed in the direction of arrow G.

Each extruder die 15 is stationary in such a manner that the relative rotation in the direction of arrow G allows the strips and the semifinished product 3 to be twisted while the rotation of the rollers 36, 37, 38 and 39 provides a drawing action along the axis L. In the embodiment of the drawing-twisting means as shown in FIG. 14, the movable surfaces are formed by the outer periphery of at least one driving roller 43 and at least one belt conveyor 44 subtended by end rolls.

The roll 43 and the conveyor 44 are placed on opposite sides from the continuous semifinished product 3, are both power driven and have axes of rotation and unwinding that are inclined relative to each other and to the longitudinal axis L.

In the embodiment as schematized in FIG. 15, two power operated rolls 45, 46 are provided, which act from opposite sides of the semifinished product 3. Each roll 45, 46 has a smooth or roughened portion 47, 48 which performs a combined drawing-twisting action and a specially shaped portion 49, 50 which performed a combined guiding and twisting action on the semifinished product 3.

The embodiment of the apparatus as schematically shown in FIG. 6 is different from the one of FIGS. 12 and 13 essentially for the kinematic inversion of the pulling-twisting means. In fact, in the latter case, the rollers 36, 37 have stationary axes (designed for simply drawing the continuous twisted semifinished strand), whereas the extruder dies 15 may rotate about the longitudinal axis L. Particularly, the rotation of the extruder head 13 about the axis L causes the strips to be twisted, whereas the adjustment of the drawing speed of the rolls 36, 37 or the equivalent apparatus causes the blank size to be defined in terms of thickness and degree of twisting of the strips that compose the semifinished strand.

In all the above mentioned drawing-twisting means, the speeds, distances and inclinations of the rolls, the rubber coated wheels and the rollers may be varied and adjusted to control the shape and size of the semifinished products 3 and the snacks 1.

The method, the apparatus and the snack food of the invention are susceptible to a number of changes or variants, within the inventive concept disclosed in the appended claims. All the details thereof may be replaced by other technically equivalent parts, and the materials may vary depending on different needs, without departure from the scope of the invention.

While the invention has been described with particular reference to the accompanying figures, the numerals referred to in the disclosure and claims are only used for the sake of a better intelligibility of the invention and shall not be intended to limit the claimed scope in any manner.

Claims

1. A method for continuous production of extruded and/or coextruded potato and/or cereal flour-based snacks, including the following steps: - preparing a dough (D) from food flours (F) having a predetermined residual moisture (H); extruding said dough (D) to form one or more continuous strips (2); coupling said extruded strips (2) for forming an elongate continuous semifinished product (3), extending along a substantially longitudinal axis (L), having an outer surface (4); cutting the continuous semifinished product (3) into segments having a predetermined length to form semifinished snacks (1); possibly drying the semifinished snacks (1); wherein, after the extrusion step and before the cutting step, said one or more extruded strips (2) are subjected to a spinning and controlled deformation action, in such a manner as to impart a substantially helical shape thereto, and to provide the continuous semifinished product (3) with a substantially twisted or multiple thread screw shape, with a wavy and spiraled outer surface (4).

2. A method as claimed in claim 1 , characterized in that said coupling and controlled deformation steps are performed on said one or more strips (2) with the strips being in high plasticity and stickiness conditions and optionally with the help of a food grade adhesive, to enhance coupling therebetween, by bonding and/or mutual penetration.

3. A method as claimed in claim 2, characterized in that, prior to the cutting step, said continuous semifinished product (3) is subjected to a combined drawing- twisting action about said longitudinal axis (L).

4. A method as claimed in claim 3, characterized in that said combined drawing and twisting action is performed by exerting on the outer peripheral surface (4) of said semifinished product (3) forces (T, R) acting along predetermined directions with respect to said longitudinal axis (L).

5. A method as claimed in claim 4, characterized in that the strength and direction of said forces (T, R) may be adjusted as a function of the speed at which said semifinished product (3) is extruded, to control the final size thereof.

6. A method as claimed in claim 5, characterized in that said forces (T, R) are inclined with respect to said longitudinal axis (L) with substantially equal inclination steps (a¹, a"; a', a") with respect to said longitudinal axis (L), for varying the pitch and inclination of said strips (2).

7. A method as claimed in claim 2, characterized in that said strips (2) are compressed radially in such a manner as to mutually penetrate, thereby defining a substantially uniform continuous semifinished product (3).

8. A method as claimed in claim 2, characterized in that said strips (2) are compressed radially in such a manner that their surfaces bond and form a central cavity that defines a substantially tubular continuous semifinished product (3).

9. A method as claimed in claim 2, characterized in that said continuous semifinished product consists of a single hollow strip (2).

10. A method as claimed in claim 1 , characterized in that said extrusion step is followed by an expansion step.

11. A method as claimed in claim 10, characterized in that said expansion step is a direct expansion which occurs directly downstream from the extrusion of said strips (2) which are made of a dough (D1) having a residual moisture (H1) of 5% to 25%.

12. A method as claimed in claim 1 , characterized in that said expansion step occurs in semifinished snacks (1) which are formed by strips made of a dough (D2) that have a residual moisture (H2) of 20% and 45% and directly downstream from a drying step to change the relative residual moisture of snacks to a final value of about 2% by frying, hot air baking, microwaving and the like.

13. A method as claimed in claim 12, characterized in that said expansion step occurs in semifinished snacks (1) which are formed by strips made of a dough (D2) that have a residual moisture (H2) of 15% and 45% with no intermediate drying and by direct wet frying, to change the relative residual moisture of snacks to a final value of about 2% (wet fried snacks).

14. A method as claimed in any of the preceding claims, characterized in that said continuous semifinished product is made with a central cavity filled with semisolid or creamy products (5), by coextrusion thereof.

15. A method as claimed in claim 1 , characterized in that at least one of said strips (2) is made from doughs having organoleptic properties and/or colours that are different from those of the other strips.

16. A method as claimed in claim 9, characterized in that said continuous semifinished product consists of a plurality of internally hollow strips (2).

17. A method as claimed in claim 15, characterized in that said continuous semifinished product (3) is obtained by coupling two or more strips (2) which are produced by separate extrusion (13 and 13A) of two or more doughs, having different organoleptic properties.

18. An apparatus for continuous production of extruded potato and cereal flour- based snacks, for carrying out the method according to any of the preceding claims, characterized in that it comprised a line including at least the following units: a food flour kneader unit (12) for preparing a substantially uniform dough (D); a dough extruder unit (13), having one or more extruder dies (15), for forming one or more adjacent strips (2); a station for coupling said adjacent strips (2) for forming an elongate continuous semifinished product (3), extending along a substantially longitudinal axis (L); a station for cutting (17) said continuous semifinished product into segments having a predetermined length to form semifinished snacks (1); a unit for drying said semifinished snacks (1); wherein, downstream from the extruder head (13), means (22) are provided for controlled deformation of said strips (2), to impart a substantially helical shape thereto and a substantially twisted or multiple thread screw shape, with a wavy and spiraled outer surface (4) to the continuous semifinished product (3).

19. An apparatus as claimed in claim 18, characterized in that said controlled deformation means comprise means (22) for combined drawing-twisting (22) of said continuous semifinished product about said longitudinal axis (L).

20. An apparatus as claimed in claim 19, characterized in that said combined drawing-twisting means (22) comprise at least two opposed movable surfaces which are fed along predetermined directions relative to the longitudinal axis (L), in such a manner as to exert on the semifinished product a resulting force with a drawing component (T) in the longitudinal direction and a twisting component (R) in the transverse direction.

21. An apparatus as claimed in claim 19, characterized in that it comprises means for adjusting said combined drawing-twisting means (22) with respect to the strip extrusion or feeding speed to control the pitch and inclination thereof.

22. An apparatus as claimed in claim 20, characterized in that said movable surfaces are inclined with respect to said longitudinal axis, with substantially equal inclination angles.

23. An apparatus as claimed in claim 22, characterized in that said movable surfaces are formed by at least two power operated conveyor belts (23, 24), subtended by substantially parallel end rolls (25, 26; 27, 28), with respective operating portions (31 , 32) interacting with the outer surface (4) of said continuous semifinished product (3).

24. An apparatus as claimed in claim 23, characterized in that the end rolls (25, 26; 27, 28) of said belt conveyors (23, 24) are stationary relative to said extruder dies (15).

25. An apparatus as claimed in claim 22, characterized in that said movable surfaces are formed by the outer periphery of at least two power operated driving rolls (36, 37, 38, 39), acting on the outer surface of the continuous semifinished product and having axles transverse to said longitudinal axis.

26. An apparatus as claimed in claim 25, characterized in that the axles of said at least one pair of power operated rolls (36, 37, 38, 39) are fitted on a mandrel (40) having a through cavity (41) for the passage of said continuous semifinished product (3).

27. An apparatus as claimed in claim 26, characterized in that said extruder dies (15) are stationary, said mandrel (40) being coupled to first external motor means (42) to rotate at a predetermined speed about said longitudinal axis relative to said stationary extruder dies (15).

28. An apparatus as claimed in claim 26, characterized in that said power operated rolls (36, 37) are stationary, said extruder dies (15) being coupled to second external motor means to rotate at a predetermined speed about said longitudinal axis (L) relative to said power operated rolls (36, 37).

29. An apparatus as claimed in claim 22, characterized in that said movable surfaces include a plurality of appropriately shaped, preferably rubber coated wheels (33, 34, 35), with axles being inclined relative to said longitudinal axis and angularly staggered, said wheels acting on the outer surface (4) of said semifinished product (3) from different sides.

30. An apparatus as claimed in claim 22, characterized in that said movable surfaces include at least one idler roll (43) and at least one belt conveyor (44) subtended by end rolls, said at least one roll (43) and said at least one conveyor (44) being placed on opposite sides of said continuous semifinished product, both being motor driven and with inclined feed direction and relative to said longitudinal axis (L).

31. An extruded potato and cereal flour-based food snack, that may be obtained with the method and the apparatus as claimed in any of claims 1 to 17, and in any of claims 18 to 30, comprising one or more mutually coupled adjacent strips (2) to form an elongated continuous semifinished product (3), which is cut and dried and possibly covered with sweet or salty products, characterized in that said continuous semifinished product (3) has a substantially helical shape and that said semifinished product (3) has a substantially twisted or multiple thread screw shape, with a wavy and spiraled outer surface (4).

32. A food snack as claimed in claim 31 , characterized in that said strips (2) are direct expanded or may be expanded by frying, hot air baking, microwaving or equivalent technologies.

33. A food snack as claimed in claim 31 , characterized in that said semifinished product (3) comprises a single strip or a plurality of mutually coupled adjacent strips (2) to define a substantially tubular semifinished product having a central cavity that may be filled with semisolid or creamy fillers (5).

34. A food snack as claimed in claim 33, characterized in that said strips (2) and said fillers (5) are coextruded to define said semifinished product (3).

35. A food snack as claimed in claim 34, characterized in that said continuous semifinished product (3) consists of a single internally hollow strip.

36. A food snack as claimed in claim 35, characterized in that said strip (2) is made with a central cavity filled with semisolid or creamy products (5), by coextrusion thereof.

37. A food snack as claimed in claim 32, characterized in that the semifinished product (3) consists of two or more strips having different organoleptic and aesthetic properties.

38. A food snack as claimed in claim 31 , characterized in that it has an elongate, substantially straight shape of the expanded blank.

39. A food snack as claimed in claim 32, characterized in that the strips (2) are subjected to surface treatment with seasoning and/or coloring substances.

40. A food snack as claimed in claim 38, characterized in that the strips (2) are subjected to surface treatment with adhesive substances.