High temperature kneading.
dough preparation technology.
In pasta production, depending on a number of factors, several types of dough kneading are used.
Depending on the moisture content of the dough being kneaded, there are three types of kneading:
- hard - with a dough moisture content of 28 to 29%;
- medium - with dough moisture from 29.1 to 31%;
- soft - with a dough moisture content of 31.1 to 32.5%.
The type of kneading in terms of humidity depends on the type of flour, the quantity and quality of gluten, the coarseness of grinding, the type of drying surface, the matrix material, and the shape of the products.
When using flour with a low gluten content, it is desirable to use soft kneading, and if the gluten of the flour is sticky, stretching, it is desirable to use hard kneading.
In the manufacture of short products and pasta with cassette drying, it is better to use a hard or medium batch to prevent sticking of products to each other during drying.
In the production of long products with hanging drying (on bastuns), medium or soft kneading is used to impart greater plasticity to the products, which excludes cracking of raw products in places of inflection on bastuns. When using semi-grains or bread flour, the moisture content of the dough should be 1-1.5% higher than when using grits.
When using matrices with Teflon inserts, the moisture content of the dough is reduced by 1-1.5%.
Soft kneading is used for flexible products with figured laying (in a skein, bow, nest), and hard kneading is used for stamped products of complex shape.
Depending on the temperature of the water added when kneading the pasta dough, there are also three types of kneading:
- hot - at a water temperature of 75-85 ° C;
- warm - at a water temperature of 55-65 ° C;
- cold - at a water temperature of at least 30 ° C.
The type of kneading by temperature is chosen depending on the quality of the flour and the shape of the products. The warm type of kneading is most common in pasta production; it is used for normal quality flour with a gluten content of at least 28%. With warm water, the kneading process is faster than with cold water. If flour with a low gluten content is processed in production, it is recommended to use water with a temperature of 30-45 °C.
Cold kneading is used when the content of weak gluten is low, in the presence of warm flour (in summer) or when forming products of complex shape to obtain a very viscous and elastic dough.
Hot kneading is used for durum wheat flour with a gluten content of more than 38% and an excessively elastic quality.
Heating pasta dough during kneading to temperatures of 60 ... 65 ° C (high-temperature kneading mode), without leading to a deeper change in the properties of protein and starch in the process of pressing the dough on a screw press than with traditional kneading modes, increases the plasticity of the extruded dough. As a result, the high-temperature mode of kneading, along with maintaining the normal quality of pasta, increases the productivity of the press by an average of 15%, reduces energy consumption for pressing by at least 10%, and prevents whitish products from being pressed out; in addition, the water consumption for cooling the screw chamber is eliminated, the drying time is reduced and the formation of sticky products during drying is prevented.
Heating matrices with fluoroplastic dies to temperatures of 75...85°C leads to fixation of the structure of the outer layer of extruded products, to an increase in the plasticity of their inner layers and to the elimination of dough adhesion to the metal surface of the forming channel. As a result, the high-temperature molding mode improves the uniform yield of pasta, and compared to the traditional molding mode, it increases the productivity of the press by an average of 10% and allows you to achieve other advantages, similar to the high-temperature kneading mode.
Dough preparation steps
Pasta dough preparation is carried out continuously in two stages in continuous pasta presses.
At the first stage, flour and liquid components are mixed in dough mixers, at the second stage, compaction is carried out in the screw chamber of the press.
Dosing of ingredients i.e. the supply of flour and water to the dough mixer in the required ratio, corresponding to the recipe, is carried out using flour and water dispensers, which, being components of the pasta press, work synchronously.
Additives after dissolving in water or preparing an aqueous emulsion enter the dough mixer through a water dispenser.
Test batch. After adjusting the dispensers, turn on the dough mixer. For good mixing, the mixer trough should be filled with dough by about 2/3 of the volume. After the dough enters the next trough (in multi-tub presses), where further kneading of the dough is carried out, or into the screw chamber (in single-tub presses).
During the kneading of the dough, there is a gradual swelling of starch grains and protein lumps of flour, as well as a uniform distribution of moisture throughout the mass of the dough.
The processes that occur during the kneading of pasta dough. Dough kneading is a complex process of mixing flour components with water. As a result of this mixing, deep physico-chemical changes occur in the main components of flour: starch and proteins.
When preparing pasta dough, complex biochemical, colloidal, enzymatic and physical processes occur. The main role in the preparation of pasta dough is played by colloidal processes.
The main components of flour protein and starch have different water absorption capacity. The interaction of water with starch and protein proceeds in two stages. At the first stage, adsorption binding of water occurs, as a result, the thinnest water films are formed on the surface of the flour particles. The amount of water bound by adsorption is small. The wetting process is accompanied by the release of heat. At the second stage, water is absorbed by the flour particles. It is due to the presence of water-soluble fractions inside the protein and starch. At this stage, the absorption of water under the action of osmotic pressure (osmotic binding of moisture) takes place.
Under the temperature conditions for obtaining pasta dough, its starch part has a weak swelling ability.
Starch grains retain moisture mainly by their surface, i.e. adsorption. Osmotic swelling of starch grains begins at a temperature of 70°C and above, i.e. at a temperature which under normal conditions in pasta dough is not observed.
Gluten (protein) reaches its maximum swelling in the temperature range from 20 to 30°C, at higher temperatures its swelling decreases. The swelling of flour when kneading pasta dough is mainly due to the hydration ability of gluten.
The gluten in soft wheat flour swells quickly but absorbs less water. And the gluten of their durum wheat flour, on the contrary, swells slowly, but absorbs more water.
Gluten is the main binder in pasta dough. It binds moistened starch grains and the remains of flour grains.
Pasta dough after kneading is a three-phase dispersed system. The solid dispersed phase is moistened starch grains and the remains of flour grains. The dispersion medium is plasticized gluten. The third gaseous phase is air inclusions captured during kneading.
This three-phase heterogeneous system has the ability to densify and harden. The degree of this compaction depends on the mode of mixing.
Pasta kneading mode. The kneading mode of pasta dough is characterized by two parameters: duration and intensity of kneading (speed of rotation of the kneading body).
Intensive kneading leads to strengthening of the pasta dough structure due to denser packing of the solid phase particles in the dispersion medium and an increase in the adhesive ability of gluten, which sticks the solid phase particles together more firmly.
Strengthening the mechanical impact on the dough leads to an increase in the water absorption capacity of gluten. As a result, the amount of raw gluten, the amount of reducing sugars and water-soluble nitrogen increases, i.e. the ripening process of pasta dough is accelerated.
Excessive mechanical processing causes a weakening of the elastic-viscous properties of the pasta dough due to the partial destruction of the protein structure, i.e. mechanical denaturation of gluten.
Dough made from pasta flour (grains or semi-grains) requires a longer kneading time than dough made from bread flour, since the penetration of moisture into dense grainy particles is much slower than into small particles of baking flour. The duration of kneading dough from pasta flour is 20 minutes.
Test seal. In the screw cylinder, the mass of moistened dough lumps and crumbs is picked up by the blades of the rotating screw and moves along the cylinder to the matrix. The auger in this part performs the work of a transporting mechanism that moves the bulk product.
The dough particles, in close contact with each other, are gradually compressed. There is a compaction of the mass, turning it into a steep plastic dough. The air that fills the pores and gaps between the dough particles is displaced towards the feed opening of the screw chamber. The pressure rises from zero by 5-12 MPa.
Further, the dense bound dough mass is injected into the pre-matrix chamber (press head) and, overcoming the resistance of the matrix, is forced through the forming holes.
When the pressed dough is moved to the matrix, the dough rubs against the inner surface of the screw chamber and the blades of the bacon, as well as intensive grinding of the dough layers against each other. This is explained by the fact that about 20% of the dough injected by the screw to the matrix is forced through its holes. The rest of the dough mass is twisted in the pre-matrix chamber, trying to return to the interturn space of the screw. As a result of this grinding, the temperature of the dough rises by 10-20 °C. The temperature of the dough in front of the matrix is 55-60 °C. The dough at this temperature is plastic, it easily slides through the holes of the matrix. Technological instructions provide for cooling the dough by supplying cold water to the water jacket of the screw chamber.
Structural and mechanical properties of the dough
The compacted pasta dough supplied to the matrix is an elastic-plastic-viscous material.
The elasticity of the test is the ability of the test to restore its original shape after a quick removal of the load, it manifests itself under small and short-term loads.
Plasticity is the ability of dough to deform. Under prolonged and significant loads (above the so-called elastic limit), pasta dough behaves like a plastic material, i.e. after removal of the load, it retains the shape given to it, deforms. It is this property that makes it possible to form raw pasta of a certain type from dough.
Viscosity - is characterized by the magnitude of the forces of adhesion of particles to each other (cohesive forces). The greater the value of the cohesive forces of the dough, the more viscous (strong) it is, the less plastic it is.
Plastic dough requires less energy for molding, it is easier to mold. When using metal matrices from a more plastic dough, products with a smoother surface are obtained. With an increase in plasticity, the dough becomes less elastic, less durable, more sticky, adheres more strongly to the working surfaces of the screw chamber and the screw, and raw products from such dough stick together more strongly and do not retain their shape well.
The rheological properties of compacted dough, i.e. the ratio of its elastic, plastic and strength properties are determined by the following factors.
With an increase in the moisture content of the dough, its plasticity increases and strength and elasticity decrease.
With an increase in the temperature of the dough, an increase in its plasticity and a decrease in strength and elasticity are also observed. Such a dependence is also observed at temperatures above 62.5 °C, i.e. above the gelatinization temperature of wheat starch. This is because the pasta dough does not have enough moisture to fully gelatinize the starch at that temperature.
With an increase in the gluten content, the strength properties of the dough decrease and its plasticity increases. The dough has the highest viscosity (strength) when the flour contains about 25% of raw gluten. When the content of raw gluten is below 25%, with a decrease in the plastic properties of the dough, its strength also decreases. Sticky, highly stretchy raw gluten increases the plasticity of the dough and significantly reduces its elasticity and strength.
With a decrease in the size of the flour particles, the strength increases and the plasticity of the dough from it decreases: the dough from baking flour is stronger than from semi-grains, and from semi-grains is stronger than from grains. The optimal ratio of strength and plastic properties is typical for particles of the original flour with a size of 250 to 350 microns.
With an increase in pressing pressure, the density and strength of the dough increases and its plasticity decreases.
Dough shaping methods
The purpose of molding is to give the pasta dough a certain shape.
Forming pasta by pressing is carried out by pressing out compacted plastic dough through the holes of the matrix. The conditions for the process of molding raw pasta from dough determine the productivity of the press, as well as the quality of finished products (color, surface roughness, density and strength, cooking properties).
Matrices are made of corrosion-resistant alloys with sufficient strength, wear resistance and low adhesion. These requirements are met by phosphorous brass, bronze, stainless steel. To increase the anti-adhesion ability, the forming channels of the matrices are covered with fluoroplastic (Teflon).
The shape of the matrix is divided into round (disk) and rectangular. Rectangular dies are installed for forming long products (spaghetti) on automated lines with hanging drying, and disk dies are used for forming other types of long and short products.
The productivity of press dies. The performance of the matrix is characterized by the number of raw products that are pressed out through its holes per unit of time. The performance of the press matrix depends on the speed of pressing out, the area of the live section of the matrix.
The speed of extrusion of pasta dough through the forming holes of the matrix is determined by the plasticity of the dough and the value of the pressing pressure. The plasticity of the dough depends mainly on its humidity and temperature. With an increase in dough moisture content up to 33%, plasticity and pressing speed increase. A further increase in humidity contributes to the formation of coarse-lumpy dough after kneading, which poorly fills the screw chamber, sticking of the dough to the screw and the walls of the chamber - pressing pressure drops. All this leads to a decrease in the pressing speed.
As the temperature of the dough rises to about 70°C, the pressing speed will increase. A further increase in temperature leads to an increase in the pressing speed: a decrease in the resistance of the matrix to punching through a very plastic dough leads to a drop in the pressing pressure.
The cross-sectional area of the matrix or the area of the matrix in the light depends on the shape of the holes and on the number of holes in the matrix. During the operation of clogged matrices, the free cross-sectional area of the matrix decreases, which leads to a decrease in the performance of the matrices. Products made from dough heated to temperatures above 70°C have poor cooking properties due to profound changes in the properties of protein substances. The optimal dough temperatures in front of the matrix should be considered: in the production of long products - 60 ° C, in the production of short ones - 70 ° C. However, modern presses, in the absence of special heating of the dough, can rarely provide such temperatures.
Cutting raw pasta
The cutting of raw pasta is carried out immediately after pressing out; its purpose is to prepare products for drying.
Cutting consists in blowing, cutting and laying out (or hanging) molded raw pasta. Such indicators as the productivity of drying equipment, the consumption of raw materials and the quality of finished pasta depend on the correctness of cutting.
Blowing of raw products. Pressed out raw pasta at the exit from the matrix is a plastic, rather easily deformable material. To facilitate cutting and prevent sticking, raw products are intensively blown with air when they exit the molding holes of the matrix. This leads to the formation of a dried layer on their surface, which prevents the products from sticking together when they are dried on conveyors (short products) or in tray cassettes (pasta), as well as from sticking to bastuns (hanging drying of long products).
Blowing is carried out with the air of the molding department at a temperature of about 25°C and a relative humidity of 60-70%. At the same time, the relative humidity of raw products decreases by 1-2% under traditional kneading and molding modes, by 3-4% under high-temperature conditions.
When using hanging drying of long products (on bastuns), blowing must be carried out carefully, avoiding excessive drying of the surface of the products, since the destruction of the surface layer of products in the places of inflection and their fall from the bastuns during hanging or during drying is possible.
Layout. The purpose of the layout is to place the cut semi-finished product on the drying surfaces. The raw product is fed into the dryer by means of a mechanical spreader (crusher). The spreader's pipe or conveyor oscillates over the moving belt of the dryer's upper conveyor, distributing the product on it in an even layer. The thickness of the product layer is adjusted by changing the speed of the belt. The layer thickness is between 2 and 5 cm, depending on the range of products. To prevent the formation of ingots of products, it is necessary to maintain a minimum layer thickness on the upper belts of the dryers (while the products are still plastic and can stick together).