TITLE: PROCESS AND SYSTEM FOR CONTINUOUS PREPARATION OF FOOD DOUGHS TO PRODUCE LEAVENED PRODUCTS AND/OR PASTA

DESCRIPTION FIELD OF THE INVENTION The present invention relates to a process and a system for continuous production of food doughs, to produce either leavened products for the bread making and bakery industry, or fresh and dry pasta, providing in little time a dough having a homogeneous and adequately developed gluten structure, ready for the subsequent molding and/or processing steps.

BACKGROUND OF THE INVENTION

Continuous kneaders have already been proposed and made available on the market, but their structural complexity and especially their inadequate kneading action have strongly limited their efficiency and use.

In order that a dough may be formed in a relatively little time, as compared with conventional kneading methods , it is important to develop a gluten structure allowing a subsequent molding step, while withstanding the stresses required for obtaining the final product.

Particularly, the dough mass should be perfectly homogenized, by adequately mixing and hydrating the components; also, during kneading, gluten should develop an adequate protein structure for the dough to achieve optimal rheological properties , allowing it to withstand the stresses caused by the passage of the dough through various equipment, such as for dividing, rounding, exclusion, lamination and other similar processing steps.

Generally, gluten should develop a structure allowing gas retention during leavening and imparting

particular mechanical properties for the subsequent steps of lamination and molding of various shapes of plain or stuffed pasta, or anyway such a structure as to allow subsequent dough processing without applying excessive pressures thereon, which would affect or considerably reduce the useful life of a dough.

In a dough, gluten has a quite complex protein structure, which has not been fully understood; however, such structure is known to influence most of the rheological properties and the physical characteristics of the dough.

To understand its development during resting times in a conventional process, attention should be paid to molecular interactions occurring between gluten amino acids , such as covalent bonds , hydrogen bonds and Van der Walls forces .

With the kneading action, new bonds are created in the dough, i.e. disulphide bridges or sulphur-sulphur groups (-S-S-) deriving from the union of thiol groups or sulphur-hydrogen groups (-SH) .

As mentioned above, wheat proteins and more generally cereal proteins are a complicated matter to investigate; nevertheless, the various protein molecules were found to be able to stabilize the dough through the exchange of the thiol groups and disulphide bridges of gluten proteins . OBJECT OF THE INVENTION

The object of the present invention is to improve this technology by a process and a system for continuous preparation of doughs from a powdery food material, such as flour and/or semolina, by utilizing an adequate preliminary mixing and hydration action, followed by a suitable mechanical kneading step, in a

continuous process.

BRIEF DESCRIPTION OF THE INVENTION

Our tests have shown that suitable hydration and mixing of the powdery food material in a particular pre-mixer provides a pre-mix that can be submitted to a subsequent mechanical kneading step in a continuous screw kneader, which is conceived for the kneading action to occur in little time, of the order of a few minutes . This configuration provides doughs that can retain gas during leavening and have a compact structure, in the case of pasta, i.e. having the required toughness and preventing bond breaking, which would produce an insufficiently tough overkneaded dough having poor qualities . To achieve this result a first food material hydration and pre-mixing step was found to be required, which had to be performed by a centrifugal action under no pressure, in combination with a subsequent mechanical kneading step in a screw kneader, at still a comparatively low maximum pressure on the dough, considerably lower than the pressures exerted by conventional extruder screws .

In a first aspect, the invention provides a process for continuous preparation of food doughs to produce either leavened products or pasta, in which metered amounts of powdery flour and/or semolina-based materials, water, additives and/or other ingredients, if any, are mixed and kneaded to cause development of a gluten having a homogeneous protein structure, characterized by the step of: feeding such metered amounts of food materials and water into a continuous mixer defining a linear path; causing hydration of the food material and

carrying out a pre-mixing step by submitting said food materials and the hydration water to a centrifugal mixing action, as they are freely fed along said linear path until a mass of pre-mixed material is obtained; feeding the pre-mix into a continuous screw kneader; and causing gluten development in the pre-mix by submitting it to a mechanical kneading action in said screw kneader, while exerting a maximum kneading pressure of 8í10 bar or less, preferably of 5 bar or less.

In another aspect, the invention relates to a system for continuous preparation of food doughs using the process as defined in claim 1, wherein the powdery food materials and the hydration water are fed into a pre-mixing unit and subsequently into a kneading unit, characterized in that it comprises : a centrifugal unit for pre-mixing and hydrating the food materials , comprising a cylindrical mixing chamber having radially spaced inlets for food materials and hydration water at one end, and an outlet for the pre-mixed material at one end away from the former; the pre-mixing unit having an impeller with radial blades of such a shape and orientation as to provide centrifugal mixing of the food materials and the hydration water as well as free feed of the pre-mix; and a screw-type kneader unit designed to receive the pre-mixed material directly from the pre-mixing and hydrating unit; the screw kneader being further designed to

provide a mechanical kneading action adapted to cause gluten development, while exerting a maximum pressure of 8í10 bar or less on the dough.

Preferably, the inlet for the powdery food materials is oriented axially with respect to the mixing chamber whereas the hydration water inlet is in a radially external position, near the peripheral surface of the cylindrical mixing chamber.

BRIEF DESCRIPTION OF THE DRAWINGS These and other features and advantages of the invention will be more apparent upon reading of the following description with reference to the description and example of the drawings , in which :

Figure 1 is a schematic view of a system for continuous preparation of food doughs according to a first embodiment of the invention;

Figure 2 shows a second embodiment of the system of the invention;

Figure 3 shows an illustrative flow chart of the operation of the system of Figures 1 and 2, according to the process of the invention.

DETAILED DESCRIPTION OF THE INVENTION

As is known, a cereal milling process provides flour or semolina and more generally a powdery food material of a well-defined particle size; during the milling step, the bonds that normally exist between particles are destroyed. Therefore, in a subsequent kneading step for production of leavened products and/or pasta, new bonds are formed, which tend to promote gluten development adapted to provide the desired processability of the final food product during cooking.

Our tests have shown that the conditions of mixing and hydration of the powdery food material have a considerable effect on the sulphydric concentration of thiol groups , as well as on the deformability modulus of the dough as resting time progresses.

Therefore, the invention is directed to a process and a system having such characteristics as to increase the formation of thiol groups for gluten development and to decrease the deformability modulus of the dough in comparatively shorter times than with conventional processes and systems.

The above may be achieved using a system as schematically shown in Figure 1.

The system comprises a first unit for pre-mixing and hydrating the powdery food material, generally designated by numeral 10, and a second kneading unit which receives the pre-mixed material simply falling from the pre-mixing and hydration unit 10.

Namely, in the example of Figure 1, the pre-mixing unit has a cylindrical chamber 12 horizontally extending along a longitudinal axis between an upstream end inlet for the food materials to be mixed and hydrated and a downstream end outlet, away from the former, for the pre-mixed food material. Particularly, the mixing chamber 12 extends on the upstream side by a smaller diameter portion 13 thereof having a side inlet 14 for the powdery food material, additives and/or other materials to be mixed; in Figure 1, a duct for supplying hydration water, possibly added with other substances is designated with numeral 15, and a side outlet for the pre-mixed food material is designated with numeral 16, the latter being downwardly oriented and communicating with a side inlet 23 of the

underlying kneading unit 11.

The mixing chamber 12 contains a centrifugal blade impeller, substantially formed of a plurality of radially extending flat blades 17, which are shaped and oriented in such a manner as to generate a centrifugal mixing and hydrating action, substantially under no pressure, whereas the pre-mixed material is freely fed towards the outlet 16 until it falls into the underlying kneader 11. The blades 17 of the impeller are attached to a central shaft 18 which extends rearwards beyond the mixing chamber 12 through a chamber 19 in which the powdery food material is axially fed into the mixing chamber 12. To such effect, the feeding chamber 19 contains a rotating screw feeder 20 integral with the shaft 18; the latter is further connected to a gearmotor or a driving motor 21, as shown.

The powdery food material is thus axially fed into the mixing chamber 12, at the central axis of the pre- mixing unit 10, which is defined by the drive shaft 18 of the blade impeller.

According to another aspect of the system of the present invention, hydration water is fed through the duct 15 always in a direction parallel to the axis of the chamber 12 at an injection point 15' which is radially spaced from the inlet for the powdery food material, in the proximity of the peripheral wall of the mixing chamber 12. Axial feed of the powdery food material and water for forming the pre-mix, in combination with the centrifugal mixing action of the radial blade impeller, substantially under no pressure, allows quick and

effective hydration and provides a soft pre-mix in which new bonds begin to form, which will later promote fast development of the protein structure in the kneader 11. Still referring to Figure 1, the continuous screw- type kneader 11 is designed to generate a low-pressure mechanical kneading action, to promote gluten development in a relatively short time while preventing the pre-formed or forming bonds from being broken. Particularly, the kneader 1 is of the type having two or more screws ; it comprises a cylindrical chamber 22 having a first side inlet 23 for the pre-mix which flows directly out of the opening 16 of the mixing unit 10 and an axial outlet 24 for the final dough. In the example of Figure 1, a first kneading screw 25 and at least one second kneading screw 26 rotate in the chamber 22 of the kneader and are connected to the same gearmotor 27 or to separate driving motors which ensure counter-rotation thereof. The kneader screws 25 and 26 extend in close, parallel directions within the chamber 22 and are driven into rotation to exert a mechanical kneading action under a progressively increasing pressure, to a predetermined maximum value at the outlet 24 which is relatively low, e.g. 8í10 bar or less, preferably of 5 bar or less.

The screws 25 and 26 of the kneader 11 may be of any type and have any profile adapted to generate low pressures and an effective kneading action; for instance, they may be in a mirror arrangement and have a constant pitch or a varying pitch to progressively increase the pressure on the dough before ejection

thereof; in this regard, an optimal thread pitch reduction was found to be 1í2 , or approximately so . For instance, screws having a maximum constant thread diameter of 80 mm to 200 mm, may have a thread pitch varying from 90 mm at the pre-mix inlet 23 to 45 mm at the outlet 24.

In Figure 1, the two units 10 and 11 are axially aligned. Figure 2 shows, still by way of example, a different arrangement, in which the pre-mixer 10 is orthogonal to the kneader 11, the two units of Figure 2 being identical to those of Figure 1 in all other respects; therefore, Figure 2 uses the same reference numerals as Figure 1, to designate similar or equivalent parts . As shown in Figure 2 the screws 25 and 26 extend beyond the pre-mix inlet 23 towards a second side inlet 28 upstream from the former, for possible addition of other food materials that do not require any further hydration, such as food dough scraps. Figure 3 shows a flow chart indicating the main steps of the process to be carried out with the system of Figure 1.

As shown, the powdery food material and the mix hydrating water are fed axially and continuously from the upstream end of the chamber 12 of the mixing unit 10 (Steps Sl and S2) .

In the chamber 12 , immediately next to the inlet for the food materials and the water, a centrifugal mixing step (Step 3) is carried out by the blade impeller 17, which progressively continues all along the linear path of the chamber 12 , and during which the powdery food material is fully homogeneously hydrated

and a pre-mix is formed (Step S4) substantially under no pressure.

The pre-mix so formed falls through the outlet of the mixer, into the entry hopper 23 of the underlying kneader 11 (Step S5) , in which it is submitted to mechanical kneading action with gluten formation (Step S6) , whereas the dough being formed is progressively fed towards the outlet 24 under a progressively increasing pressure, to a maximum value that is relatively low, and considerably lower than the pressure exerted by conventional screw kneaders .

Finally, the final dough flows out of the outlet 24 (Step S7) with the desired consistency and toughness, and is then conveyed to the next molding stations.

It shall be noted that the presence of thiol (-S4) and disulphide (-SS-) groups is a critical factor for the rheological features of a dough.

The characteristics of the process and system of the present invention were evaluated by determining the amount of thiol groups with time, in doughs formed using the mixing unit 10 of Figure 1 only and with the unit 10 combined with the screw kneader 11 respectively. The results are shown in the following Tables I and II

TABLE I

The above Table I shows the average values with respective standard deviations of sulphydryl group concentration (μM) with the increase of the resting time , in minutes .

TABLE II

The above Table II shows the average values with respective standard deviations of deformability modulus (in N/mm) with the increase of the resting time, in minutes .

Both tables show time variations of the values obtained, in column A, from the analysis of pre-mix samples prepared using the pre-mixing unit 10 only, and in Column B, from the analysis of dough samples prepared using the combined action of the pre-mixer 10 and the kneader 11.

As shown in Table I , the kinetic characterization of sulphydryl group (-SH) concentration with time shows

that the process and system of the invention provides doughs having a lower sulphydryl content, due to the formation of disulphide bridges in comparatively very short times , which involves the advantage of a considerable reduction of resting and processing times.

Table II further shows that doughs tends to become tougher as resting times increase, due to gluten development occurring with no further energy supply (Column A) . Column B shows that, as resting time increases, in combination with an increase of the time during which the mechanical action is applied to the dough, the deformability modulus further decreases , thereby providing the additional advantage to require a lower force to obtain the same deformation.

Figures 1 and 2 show another feature of the invention. In such figures, the pre-mixer 10 is shown to be very close to the kneader 11; thus, the outlet 16 of the pre-mixer 10 opens directly into the inlet 23 of the kneader 11 thereby preventing the dough from forming bridges or stagnations as it passes therethrough .

It shall be finally noted that further changes or modifications may be made to the present process and system or parts thereof, without departure from the scope as defined by the annexed claims .