International Journal of Applied and Basic Research. Abstract: Structural and mechanical properties of fermented dough Baking properties of rye flour

FEATURES OF THE STRUCTURE AND MECHANICAL PROPERTIES OF FERMENTING DOUGH

Non-fermented flour dough should be considered a material designed to evaluate the technological properties of grain and flour. Fermented dough is less suitable for this purpose, since it contains yeast, starter cultures, gaseous substances, mainly carbon dioxide, and organic acids formed during fermentation. It is a structural analogue and precursor of the structure of bread crumb, not fixed by heat treatment. The amount of carbon dioxide formed per unit volume of dough depends on the content and distribution of yeast cells in it, the energy of their fermentation, determined by the mass of the yeast, and the conditions of their vital activity. The size of carbon dioxide bubbles and their quantity in volume are determined by the gas permeability of the dough (by CO 2), which depends on its structural and mechanical properties.

Gaseous substances, as is known, differ significantly from solids and liquids in their lower density, greater compressibility, and the dependence of their coefficient of volumetric expansion on temperature. Their presence in the dough structure increases the volume, reduces its density, and complicates the structure. Elastic-plastic deformations of fermenting dough occur in the pore walls of its structured mass. In order to consider the influence of the gaseous phase on the mechanical properties of fermenting dough, consider the diagram of its structure shown in Fig. 21. In it, sticks with a round end schematically show surfactants, proteins, lipoids, etc. Their rounded part represents a polar group, and the straight “tail” represents a nonpolar group of atoms in a molecule.

The most likely centers for the formation of primary CO 2 bubbles in fermenting dough are the adhesion points of non-polar groups of surfactant molecules bound by the weakest forces of dispersion interactions. The gaseous products (CO 2 and others) formed in the dough during its fermentation dissolve in free water and are adsorbed on the surfaces of hydrophilic polymer molecules. Their excess forms gas bubbles in the fermenting dough. The walls of the bubbles form surfactants. An increase in the amount of gaseous products causes a corresponding increase in the number and volume of gas bubbles, a decrease in the thickness of their walls, as well as wall breakthrough, diffusion and gas leakage from the surface of the dough.

This complex process of formation of the fermenting dough structure is naturally accompanied by an increase in the volume of its mass and shear deformations. The accumulation of many bubbles of gaseous products leads to the formation of a foam-like structure of the fermenting dough, having double walls formed by surfactants. They are filled with a mass of hydrated hydrophilic dough substances connected to the polar surfactant groups of the bubble walls by secondary chemical bonds. The dough has significant viscosity and elastic-elastic properties, providing its foam-like structure with sufficient strength and durability, a certain ability to flow and retain gaseous substances (air, steam, carbon dioxide).

Elastic-plastic shear deformations of such a structure as a result of a permanent increase in the volume of gas bubbles and dough lead to a decrease in the thickness of the walls, their rupture and merging (coalescence) of individual bubbles with a decrease in the total volume.

The development of elastic-plastic shear deformations in the mass of dough that begins to quickly ferment, reducing its density, occurs at correspondingly low stresses, therefore the initial shear elasticity moduli and viscosity of such dough should not be higher than that of non-fermenting dough. However, during its fermentation and increase in volume, deformation of the spherical walls of its gas pores should be accompanied by the orientation of proteins and other polymers in the direction of shear and flow, the formation of additional intermolecular bonds between them and an increase in the viscosity of the dough. Reducing the density of fermenting dough during fermentation allows proteins to more fully realize their elastic properties - lowering the shear modulus of elasticity. With increased viscosity and decreased modulus, fermenting dough should have a significantly higher ratio of these characteristics and have a more solid system than non-fermenting dough.

Due to the permanent formation of carbon dioxide and thus increasing the volume, fermenting dough, unlike non-fermenting dough, is a doubly stressed system. The gravitational forces of its mass during fermentation are inferior to, equal to or greater than the energy of chemical reactions of the formation of CO 2, which creates forces that develop and move gas bubbles upward according to Stokes' law (movement of spherical bodies in a viscous medium). The number and size of gas bubbles in the dough are determined by the energy and rate of yeast fermentation, the structural and mechanical properties of the dough, and its gas permeability.

The size of the carbon dioxide bubble formed during fermentation at any given moment will depend on the balance of its tensile forces

Р=π rp (4.1)

and compressive

P =2π rσ (4.2)

where π, r , R , σ - respectively, the ratio of circumference to diameter (3, 14), bubble radius, excess pressure and surface tension.

From the equality conditions of equations (4.1) and (4.2) it follows that

P =2 σ / r (4.3)

Equation (4.3) shows that at the initial moment of gas bubble formation, when its dimensions, determined by the radius, are very small, the excess pressure must be significant. As the bubble radius increases, it decreases. The proximity of gas bubbles of different radii should be accompanied by the diffusion of CO 2 through the walls in the direction from higher to lower pressure and its equalization. In the presence of a certain excess pressure and the average size of gas bubbles, it is not difficult to calculate, knowing the viscosity of the dough, the rate of their rise according to the mentioned Stokes law.

According to this law, the force that raises gas bubbles is

P =4/3π rg ( ρ - ρ ) (4.4)

overcomes the force of their friction

P =6 πrηυ (4.5)

where g is the gravity constant;

and ρ - gas and dough densities;

η is the effective structural viscosity of the dough;

υ - speed of vertical movement of gas bubbles in the dough

arising in the dough mass when a spherical body (gas bubble) moves in it.

From the equality of equations (4.4) and (4.5), the speed value is easily determined

V =2 gr ( ρ - ρ )/9 η (4 .6)

This equation is of great practical importance, allowing us to establish the dependence of the rate of increase in the volume of fermenting dough on its density and viscosity, the size of individual pores, which is also determined by the energy of fermentation of microorganisms. Calculated by the equation, the rate of increase in the volume of wheat dough from grade I flour with a density of 1.2 with an average pore radius of 1 mm and a viscosity of the order of 1

10 4 Pass is about 10 mm/min. Practical observations show that such dough has an average rising speed of 2 to 7 mm/min. The highest speed is observed in the first hours of fermentation.

If there are neighboring pores in the dough that have different sizes and gas pressures, their walls rupture and the pores merge (coalescence); this phenomenon also depends on the fermentation rate and the mechanical properties of the dough; Apparently, most of the pores of the dough and bread crumb are unclosed, open. Due to the phenomena of diffusion of CO 2 through the walls of the pores and their rupture by excess pressure, the fermenting dough loses carbon dioxide on its surface: taking the consumption of dry substances (sugar) for dough fermentation equal to an average of 3% of the flour mass, with alcoholic fermentation per 1 kg of flour (or 1. 5 kg of bread) releases about 15 g, or approximately 7.5 liters of CO 2 . This amount at atmospheric pressure is several times greater than the volume of gaseous products in the specified volume of bread and characterizes their losses during dough fermentation.

Fermenting dough also produces many other organic acids and alcohols that can change the solubility of grain compounds. Thus, everything stated above shows that the structure of fermented dough is more complex than that of non-fermented dough. It should differ from the latter in lower density, modulus of elasticity, higher viscosity and η/E (greater ability to retain shape), a permanent increase in volume and acidity during fermentation.

The dough is a polydisperse colloidal solid-liquid system, which has both elastic-elastic and visco-plastic properties, on the surface of which adhesion properties appear. The physical properties of rye dough are largely determined by the properties of its very viscous liquid phase. Rye dough is characterized by high viscosity, plasticity, low stretchability, and low elasticity.

The viscosity of rye dough changes during the fermentation process (Table 2.6).

Table 2.6 – Dependence of the viscosity of baking dough (in kPa∙s) on the duration of fermentation and shear rate

Shear rate, s -1	Fermentation duration, min
Shear rate, s -1

As can be seen from Table 2.6, with increasing shear rate, the viscosity of the dough at any fermentation duration decreases, which is typical for most dough masses. As fermentation time increases, viscosity also decreases. Note that with fermentation durations of 120 and 150 minutes at all speeds, the viscosity is almost the same.

2.1.2.3 Baking properties of rye flour

The baking properties of rye flour are determined by the following indicators:

gas-forming ability;

the power of torment;

the color of the flour and its ability to darken;

grinding coarseness.

Gas-forming ability of flour. The gas-forming ability of flour is the ability of dough prepared from it to form carbon dioxide.

During alcoholic fermentation, which is caused by yeast in the dough, the saccharides contained in it are fermented. Most of all, ethyl alcohol and carbon dioxide are formed in the process of alcoholic fermentation, and therefore it is by the amount of these products that one can judge the intensity of alcoholic fermentation. Therefore, the gas-forming ability of flour is characterized by the amount of carbon dioxide per ml formed during 5 hours of fermentation of dough prepared from 100 g of flour, 60 ml of water and 10 g of yeast at a temperature of 30 ° C.

The gas-forming ability depends on the content of intrinsic sugars in the flour and on the sugar-forming ability of the flour.

The flour's own sugars (glucose, fructose, sucrose, maltose, etc.) are fermented at the very beginning of the fermentation process. And to obtain the best quality bread, it is necessary to have intensive fermentation both during the ripening of the dough, and during the final proofing and during the first period of baking. In addition, monosaccharides are also necessary for the reaction of melanoid formation (formation of the color of the crust, taste and smell of bread). Therefore, what is more important is not the sugar content of flour, but its ability to form sugars during the dough maturation process.

The sugar-forming ability of flour is the ability of a water-flour mixture prepared from it to form a certain amount of maltose at a set temperature and over a certain period of time. The sugar-forming ability of flour is determined by the action of amylolytic enzymes on starch and depends both on the presence and amount of amylolytic enzymes (a- and β-amylases) in flour, and on the attackability of flour starch. Normal ungerminated rye grain contains a fairly large amount of active α-amylase. During grain germination, α-amylase activity increases many times. In rye flour, β-amylase is approximately 3 times less active than in wheat flour, and α-amylase is more than 3 times active.

All this leads to the fact that the crumb of rye bread always has increased stickiness compared to bread made from wheat flour, which is of lower quality. This is due to the fact that active α-amylase easily hydrolyzes starch to a significant amount of dextrins, which, by binding moisture, reduce its connection with protein and starch grains; a large amount of water is in a free state. The presence of some free moisture not bound by starch will make the bread crumb moist to the touch.

Knowing the gas-forming ability of flour, you can predict the intensity of fermentation of the dough, the course of the final proofing and the quality of the bread. The gas-forming ability of flour affects the color of the crust. The color of the crust is due in large part to the amount of unfermented sugars before baking.

The power of flour. The strength of flour is the ability of flour to form a dough that has certain structural and mechanical properties after kneading and during fermentation and proofing. Based on strength, flour is divided into strong, medium and weak.

Strong flour contains a lot of protein substances and gives a large yield of raw gluten. Gluten and dough made from strong flour are characterized by high elasticity and low plasticity. The protein substances of strong flour swell relatively slowly when kneading dough, but generally absorb a lot of water. Proteolysis in the dough occurs slowly. The dough has a high gas-holding capacity, the bread has the correct shape, large volume, and porosity that is optimal in size and structure. It should be noted that very strong flour produces bread with a smaller volume. The gluten and dough of such flour are too elastic and insufficiently extensible.

Weak flour forms inelastic, overly extensible gluten. Due to intense proteolysis, dough made from weak flour has low elasticity, high plasticity, and increased stickiness. The formed dough pieces spread out during the proofing period. Finished products are characterized by low volume, insufficient porosity and vagueness (hearth products).

Medium flour produces raw gluten and dough with good rheological properties. The dough and gluten are quite elastic and elastic. The bread has a shape and quality that meets the requirements of the standard.

The color of flour and its ability to darken during the baking process. The color of the crumb is related to the color of the flour. Dark flour will produce bread with a dark crumb. However, light flour can in certain cases produce bread with a dark crumb. Therefore, to characterize the baking quality of flour, not only its color, but also its ability to darken is important.

The color of flour is mainly determined by the color of the endosperm of the grain from which the flour is ground, as well as the color and amount of peripheral (bran) particles of the grain in the flour.

The ability of flour to darken during processing is determined by the content of phenols, free tyrosine in flour and the activity of the enzymes O-diphenoloxidase and tyrosinase, which catalyze the oxidation of phenols and tyrosine with the formation of dark-colored melanins.

Size of rye flour particles. The sizes of flour particles are of great importance in baking production, significantly influencing the rate of biochemical and colloidal processes in the dough and, as a result, the properties of the dough, the quality and yield of bread.

Both insufficient and excessive grinding of flour worsens its baking properties: excessively coarse flour will produce bread of insufficient volume with a coarse thick-walled crumb porosity and often with a pale colored crust; Bread made from overly ground flour results in reduced volume, with an intensely colored crust, often with a darkly colored crumb. Hearth bread made from this flour may be mushy.

The best quality bread comes from flour with the optimal particle size. The grinding optimum, apparently, should be different for flour made from grains with different amounts and especially quality of gluten.

Structural-mechanical, or rheological, properties of food products characterize their resistance to external energy, determined by the structure and structure of the product, as well as the quality of food products and are taken into account when choosing the conditions for their transportation and storage.

Structural and mechanical properties include strength, hardness, elasticity, elasticity, plasticity, viscosity, adhesion, thixotropy, etc.

Strength- the property of the product to resist deformation and mechanical destruction.

Under deformation understand the change in body shape and size under the influence of external forces. The deformation can be reversible and residual. With reversible deformation, the original shape of the body is restored after the load is removed. Reversible deformation can be elastic, when there is an immediate restoration of the shape and size of the body, and elastic, when recovery requires a more or less long period of time. Residual (plastic) deformation is the deformation that remains after the cessation of external forces.

Food products, as a rule, are characterized by a multicomponent composition; They are characterized by both elastic deformation, which disappears instantly, and elastic, as well as plastic deformation. However, for some, elastic properties predominate over plastic ones, for others, plastic properties predominate over elastic ones, and for others, elastic properties predominate. If food products are not capable of permanent deformation, then they are fragile, for example refined sugar, dryers, crackers, etc.

Strength is one of the most important indicators of the quality of pasta, refined sugar and other products.

This indicator is taken into account when processing grain into flour, when crushing grapes (in the production of grape wines), when crushing potatoes (in the production of starch), etc.

Hardness- the ability of a material to resist the penetration of another harder body into it. Hardness is determined when assessing the quality of fruits, vegetables, sugar, grains and other products. This indicator plays an important role in the collection, sorting, packaging, transportation, storage and processing of fruits and vegetables. In addition, hardness can be an objective indicator of their degree of maturity.

Hardness is determined by pressing a hard tip shaped like a ball, cone or pyramid into the surface of the product. The hardness of the product is judged by the diameter of the hole formed: the smaller the size of the hole, the harder the product. The hardness of fruits and vegetables is determined by the amount of load that must be applied in order for a needle or ball of a certain size to enter the pulp of the fruit.

Elasticity- the ability of bodies to instantly restore their original shape or volume after the action of deforming forces ceases.

Elasticity- the property of bodies to gradually restore shape or volume over some time.

Indicators of firmness and elasticity are used to determine the quality of dough, gluten content of wheat flour, and the freshness of meat, fish and other products. They are taken into account in the manufacture of containers and in determining the conditions for transportation and storage of food products.

Plastic- the ability of a body to deform irreversibly under the influence of external forces. The property of raw materials to change their shape during processing and retain it later is used in the production of food products such as cookies, marmalade, caramel, etc.

As a result of prolonged external influence, elastic deformation can turn into plastic. This transition is associated with relaxation - the property of materials to change stress at a constant initial deformation. The production of some food products, such as sausages, is based on relaxation. From meat characterized by elastic deformation, minced meat is prepared, and from it sausage, which has the properties of a plastic material. Certain relaxation values are characteristic only for products with a solid-liquid structure - cheese, cottage cheese, minced meat, etc. This property of food products is taken into account during the transportation and storage of bakery products, fruits, vegetables, etc.

Viscosity- the ability of a liquid to resist the movement of one part of it relative to another under the influence of an external force.

There are dynamic and kinematic viscosities .

Dynamic viscosity characterizes the force of internal friction of the medium that must be overcome to move a unit surface of one layer relative to another with a displacement velocity gradient equal to unity. The unit of dynamic viscosity is taken to be the viscosity of a medium in which one layer, under the action of a force equal to 1 Newton per square meter, moves at a speed of 1 m/s relative to another layer located at a distance of 1 m. Dynamic viscosity is measured in N-s/m 2 .Kinematic viscosity is called a value equal to the ratio of dynamic viscosity to the density of the medium, and is expressed in M 2 / C.

The reciprocal of viscosity is called fluidity.

The viscosity of products is affected by temperature, pressure, humidity or fat content, solids concentration and other factors. The viscosity of food products decreases with increasing humidity, temperature, fat content and increases with increasing concentration of solutions and the degree of their dispersion.

Viscosity is a property characteristic of food products such as honey, vegetable oil, syrups, juices, alcoholic beverages, etc.

Viscosity is an indicator of the quality of many food products and often characterizes the degree of their readiness during processing of raw materials. It plays an important role in the production of many products, as it actively influences technological processes - mixing, filtering, heating, extraction, etc.

Creep- the property of a material to continuously deform under the influence of a constant load. This property is typical for cheeses, ice cream, cow butter, marmalade, etc. In food products, creep appears very quickly, which has to be taken into account during their processing and storage.

Thixotropy- the ability of some dispersed systems to spontaneously restore a structure destroyed by mechanical action. It is characteristic of dispersed systems and is found in many semi-finished products and food industry products.

A special place among the structural and mechanical properties is occupied by surface properties, which include adhesion, or stickiness.

Adhesion characterizes the force of interaction between the surfaces of the product and the material or container with which it comes into contact. This indicator is closely related to the plasticity and viscosity of food products. There are two types of adhesion: specific (adhesion itself) and mechanical. The first is the result of adhesive forces between material surfaces. The second occurs when the adhesive penetrates into the pores of the material and retains it due to mechanical jamming.

Adhesion is characteristic of food products such as cheese, butter, minced meat, some confectionery products, etc. They stick to the knife blade when cutting, to the teeth when chewing.

Excessive adhesion complicates the technological process, and losses during product processing increase. This property of food products is taken into account when choosing the method of processing, packaging material and storage conditions.

Sample number

Duration of exposure, h

E 10 ,

η 10

Pa With

η/E, s

P, %

E, %

TO , %

8,5/6,0

3,5/2,9

12,0/7,6

6,4/3,8

5,9/5,4

1,9/6,2

6,4/5,4

3,2/8,4

69/89

53/220

50/71

50/221

72/67

78/45

77/73

78/45

74/64

82/65

78/67

76/70

59/52

47/50

68/-15

50/-55

Note. The numerator shows data on non-fermenting dough, and the denominator shows data on fermenting dough.

Dough made from grade I wheat flour has a less complex labile structure than dough made from grade II flour: it contains less active hydrolysis processes, contains less sugars and other compounds that change the elastic properties of the structure over time. For this reason, the differences in the structure of unfermented dough made from grade I flour should be most distinct.

As the results of Table 1 show. 4.1, immediately after kneading, the non-fermenting dough of both samples had shear moduli and viscosity, relative plasticity and elasticity were large, and η/E less than that of fermenting dough. After 2 hours of fermentation, the viscosity of the dough and η/E did not decrease, as in non-fermented dough, but on the contrary, increased, and plasticity decreased. For this reason, the indicator TO had a negative value, characterizing not liquefaction, but an increase in the viscosity of the structure.

The results of comparison of the mechanical properties of unfermented and fermented wheat dough from two samples of grade II flour are given in Table. 3.1, basically completely confirm the patterns established for dough made from grade I flour; they, however, are of undoubted interest because the process of aging lasted up to 24 hours. It is known that the fermentation of pressed baker's yeast at its usual dosage (about 1% of flour) usually ends within a period of 3-4 hours (the duration of fermentation of the dough) . After this time, the dough is replenished with a fresh portion of flour and mixed, after which fermentation in it resumes. In the absence of flour additives and stirring, alcoholic fermentation is inferior to acidic fermentation. Such dough, acquiring excessive amounts of ethyl alcohol and acids, dissolves gluten proteins (liquefies), losing carbon dioxide - reduces the volume and becomes denser. From the table 3.1 it is clear that fermenting dough after 6 hours and especially after 24 hours of fermentation in terms of shear modulus, viscosity, relative plasticity and elasticity approaches these indicators of non-fermenting dough. This shows that yeast fermentation processes lasting up to 6 hours are the main reason for significant differences in the structure of fermented dough from its non-fermented structure. Experiments have established that samples of fermented wheat dough from flour of grades I and II have a structure that has more advanced elasticity properties (lower shear modulus), greater viscosity and dimensional stability (η/E), as well as greater stability over time compared to the structure of unfermented dough. The main reason for these differences should be considered the process of alcoholic fermentation of baker's yeast in fermenting dough, the formation of gas-filled pores in it, causing a permanent increase in volume, the development of elastic-plastic deformations and strengthening of the structure due to the orientation of polymers in shear planes. Acid fermentation in it is less significant and, as shown below, affects these properties by changing the processes of swelling and dissolution of flour compounds.

DEPENDENCE OF MECHANICAL PROPERTIES OF FERMENTING DOUGH AND QUALITY OF BREAD ON THE TYPE AND GRADE OF FLOUR

The quality of bread products - their volumetric yield, shape, porosity structure and other characteristics are determined by the type of flour and are accordingly rated by GOSTs.

The structure of fermenting dough is the direct material from which bread products are produced by heat treatment in the oven. It was of interest to study the biochemical and structural-mechanical properties of fermented wheat dough depending on the type of flour. For this purpose, seven samples of soft red grain wheat were ground in a laboratory mill using three-grade grinding with a total yield of 78% on average. Then we investigated the gas-forming and gas-holding ability of flour, the structural and mechanical characteristics of fermented dough after proofing, as well as raw gluten proteins and their content in flour, specific volume (in cm 3 /d) molded, as well as HID round hearth bread baked according to GOST 9404-60. The results obtained are shown in table. 4.2. They showed that the yield of varietal flour, even under laboratory experimental grinding conditions, fluctuates significantly and the more strongly, the higher its grade. Thus, the grain grinding technology should influence the chemical composition, and therefore the structure of the dough. It is one of the significant numerous factors influencing the quality indicators of flour, dough and bread products.

Table 4.2

Biochemical and structural-mechanical characteristics

gluten proteins of fermented dough and bread

(average data)

Note. The numerator contains data on proteins, the denominator contains data on the test.

The technological properties of grain and flour of each variety are characterized primarily by their gas-forming ability. This property characterizes the ability of grain and flour to convert the chemical energy of carbohydrate oxidation into thermal and mechanical energy of movement of fermenting dough, overcoming the inertia of its mass. Determination of the gas-forming ability of flour is accompanied by taking into account the amount of CO released 2 . The amount retained by the test determines it. gas retention by volume increase. This physicochemical indicator characterizes by its inverse value the gas permeability of the test to carbon dioxide. The latter depends on the structure and size of the main elastic-plastic (E, η, η/E) test characteristics. Experiments showed that the gas-forming ability of flour increased significantly from the highest to the first and second grades, while the volumetric yield of bread, on the contrary, decreased.

The gas-holding capacity of the dough is directly dependent on the gas-forming ability; despite this, it did not increase in absolute and relative (% of gas formation) values, but noticeably and naturally decreased with decreasing flour grade. There is a close direct relationship between the absolute value of CO retained by the dough and the volumetric characteristics of bread (volume Yield, specific volume). The foregoing allows us to conclude that these characteristics of bread quality are determined mainly not by biochemical, but by physicochemical (gas permeability) and mechanical properties (η, E Andη/E) test. The latter depend mainly on the corresponding properties of raw gluten proteins and their content in the dough.

Experiments have shown that the content of crude gluten proteins naturally increased with a decrease in grain strength and moisture-holding capacity (viscosity) of flour and its variety. The protein structure of premium flour had higher values of shear modulus and, on average, viscosity than the structure of proteins of first grade flour. This indicates their higher statistical molecular weight. The proteins of grade I flour had a shear modulus and viscosity lower than these characteristics of proteins of grade II flour, but exceeded them in value η/E. This characterizes their greater elasticity and dimensional stability.

The gas-holding capacity of the dough and the volumetric yield of bread products directly depend on the duration of the stress relaxation period of gluten proteins and dough, or η/E . The ratio of viscosity to modulus of gluten proteins of grade II flour was significantly lower than that of premium and grade I flour proteins.

The gas-holding capacity of dough made from high-quality wheat flour depended on the corresponding values of its shear modulus and viscosity. These characteristics decreased with decreasing flour grade, similar to the gas retention ability.

It was established that fermenting dough from premium flour with a moisture content of 44%, like the raw gluten proteins of this flour, had the most significant values of shear modulus, viscosity and viscosity-to-modulus ratio, and the lowest relative plasticity. From this dough, bread products with the highest porosity, specific volume of molded bread, and the ratio of height to diameter of hearth bread were obtained. Thus, despite the significant viscosity, the least gas formation due to the high η/E Dough and bread of high volumetric yield are obtained from this flour. High viscosity values and η/E contributed to the production of hearth bread with the highest N/A .

Dough made from grade I flour with a moisture content of 44% was slightly inferior in gas retention, mechanical characteristics and bread quality to the quality of dough made from premium flour; it had a viscosity reduced by 14-15%, η/E dough, N/A . This indicates that a decrease in the viscosity of dough made from grade I flour contributed to both the development of the specific volume of molded bread and the increase in the spreadability of hearth bread.

Dough made from grade II flour had a higher moisture content (45%). Despite the greatest gas formation, it was significantly inferior to the dough of the highest and first grade flour in terms of gas retention and viscosity. The viscosity-to-modulus ratio of this dough, like that of gluten proteins, was lower, and the relative plasticity was higher than that of dough made from premium and grade I flour. The quality of the resulting bread products was much lower than the quality of products made from premium and first grade flour.

In order to clarify the influence of the structural and mechanical characteristics of fermenting dough on the physical properties of bread products, we differentiated the experimental results into two groups. The first group of samples of each variety had, on average, higher shear moduli and viscosity than the arithmetic mean, while the second group had lower ones. The characteristics of gas retention of the dough and elastic-plastic properties of raw gluten proteins were also taken into account (Table 4.3).

Table 4.3

Average characteristics of dough with high and low viscosity

From the table 4.3 it is clear that the specific volume of bread made from premium flour does not depend on the value of the gas-holding capacity of the dough, which turned out to be almost the same for both groups of samples. The specific volume of bread made from flour of grades I and II depended on the slightly higher gas-holding capacity of the dough of the second group of samples. The amount of raw gluten in both groups of samples for all types of flour turned out to be approximately the same and could not affect the bread quality indicators.

The viscosity of dough made from premium flour of both groups of samples turned out to be inversely related, and the ratio of viscosity to modulus was directly dependent on the corresponding indicators of their raw gluten proteins; for dough made from flour of grades I and II of both groups of samples, it was the opposite.

FEATURES OF THE STRUCTURE AND MECHANICAL PROPERTIES OF FERMENTING DOUGH

The size of the carbon dioxide bubble formed during fermentation at any given moment will depend on the balance of its tensile forces

Р=π rp (4.1)

and compressive

P =2π rσ (4.2)

where π, r , R , σ - respectively, the ratio of circumference to diameter (3, 14), bubble radius, excess pressure and surface tension.

From the equality conditions of equations (4.1) and (4.2) it follows that

P =2 σ / r (4.3)

According to this law, the force that raises gas bubbles is

P =4/3π rg ( ρ - ρ ) (4.4)

overcomes the force of their friction

P =6 πrηυ (4.5)

where g is the gravity constant;

ρ and ρ - gas and dough densities;

η is the effective structural viscosity of the dough;

υ - speed of vertical movement of gas bubbles in the dough

arising in the dough mass when a spherical body (gas bubble) moves in it.

From the equality of equations (4.4) and (4.5), the speed value is easily determined

V =2 gr ( ρ - ρ )/9 η (4 .6)

This equation is of great practical importance, allowing us to establish the dependence of the rate of increase in the volume of fermenting dough on its density and viscosity, the size of individual pores, which is also determined by the energy of fermentation of microorganisms. The rate of increase in the volume of wheat dough made from grade I flour with a density of 1.2 with an average pore radius of 1 mm and a viscosity of about 110 4 Pas, calculated by the equation, is about 10 mm/min. Practical observations show that such dough has an average rising speed of 2 to 7 mm/min. The highest speed is observed in the first hours of fermentation.

Bakers have almost long been characterizing the baking properties of fermenting dough by its ability to exhibit elastic-elastic deformations after stress relief: “living” (or elastic-elastic) “moving” dough after deformation always produced bread products of good volume, shape and crumb porosity structure, in contrast from immobile (plastic) dough devoid of these properties.

The structure of fermenting dough and its mechanical properties are mutually dependent on the sugar-forming ability of flour, as well as the gas-forming and gas-retaining (gas permeability) abilities of the dough. They also depend on the type, age and fermentation ability of microorganisms - fermentation generators.

This is confirmed by the data on the values of gas formation and retention of dough from varietal wheat flour, given in Table. 3.10. With the average gas-forming ability of wheat flour of the first and second groups being equal, the lower absolute and relative gas-retaining ability of the dough (and the volumetric yield of bread) of the first is explained by its higher elastic-plastic properties. At the same time, the lower gas-holding capacity of dough (and volumetric yield of bread) from wheat of the third group in comparison with these characteristics of dough (and bread) from wheat of the second and first groups can be partly attributed to their lower gas-forming ability.

Their relative (in % of gas formation) gas-retaining capacity turned out to be higher than that of the wheat dough of the second and first groups, which can be attributed to the highest content of gluten proteins in the wheat of this group. Thus, when considering the gas-holding capacity of the dough and the volumetric yield of bread, it is necessary to take into account not only the mechanical characteristics of the dough, but also the named properties of flour. It seemed appropriate to investigate and compare the structure of unfermented and fermented dough. The latter is the actual material from which bread products are made from different types of flour, differing in physical quality indicators. It was of interest to compare the mechanical properties of non-fermenting and fermenting dough made from different types of flour, as well as to approximately standardize them for the latter.

The structural and mechanical properties of non-fermenting and fermenting dough prepared from two samples of commercial wheat flour of grades I and II are given in Table. 3.1 and 4.1.

Table 4.1

Structural and mechanical characteristics of dough made from 1st grade wheat flour with a moisture content of 44%

Sample number	Duration of exposure, h

Note. The numerator shows data on non-fermenting dough, and the denominator shows data on fermenting dough.

Dough made from grade I wheat flour has a less complex labile structure than dough made from grade II flour: it contains less active hydrolysis processes, contains less sugars and other compounds that change the elastic properties of the structure over time. For this reason, the differences in the structure of non-fermented dough made from grade I flour should be most distinct.

As the results of the table show. 4.1, immediately after kneading, the non-fermenting dough of both samples had shear moduli and viscosity, the relative plasticity and elasticity were large, and η/E was smaller than that of the fermenting dough. After 2 hours of fermentation, the viscosity of the dough and η/E did not decrease, as in non-fermented dough, but on the contrary, increased, and plasticity decreased. For this reason, the indicator TO had a negative value, characterizing not liquefaction, but an increase in the viscosity of the structure.

The results of comparison of the mechanical properties of unfermented and fermented wheat dough from two samples of grade II flour are given in Table. 3.1, basically completely confirm the patterns established for dough made from grade I flour; they, however, are of undoubted interest because the process of aging lasted up to 24 hours. It is known that the fermentation of pressed baker's yeast at its usual dosage (about 1% of flour) usually ends within a period of 3-4 hours (the duration of fermentation of the dough) . After this time, the dough is replenished with a fresh portion of flour and mixed, after which fermentation in it resumes. In the absence of flour additives and stirring, alcoholic fermentation is inferior to acidic fermentation. Such dough, acquiring excessive amounts of ethyl alcohol and acids, dissolves gluten proteins (liquefies), losing carbon dioxide - reduces the volume and becomes denser. From the table 3.1 it is clear that fermenting dough after 6 hours and especially after 24 hours of fermentation in terms of shear modulus, viscosity, relative plasticity and elasticity approaches these indicators of non-fermenting dough. This shows that yeast fermentation processes lasting up to 6 hours are the main reason for significant differences in the structure of fermented dough from its non-fermented structure. Experiments have established that samples of fermented wheat dough from flour of grades I and II have a structure that has more advanced elasticity properties (lower shear modulus), greater viscosity and dimensional stability (η/E), as well as greater stability over time in comparison with the structure non-fermentable dough. The main reason for these differences should be considered the process of alcoholic fermentation of baker's yeast in fermenting dough, the formation of gas-filled pores in it, causing a permanent increase in volume, the development of elastic-plastic deformations and strengthening of the structure due to the orientation of polymers in shear planes. Acid fermentation in it is less significant and, as shown below, affects these properties by changing the processes of swelling and dissolution of flour compounds.

DEPENDENCE OF MECHANICAL PROPERTIES OF FERMENTING DOUGH AND QUALITY OF BREAD ON THE TYPE AND GRADE OF FLOUR

The quality of bread products - their volumetric yield, shape, porosity structure and other characteristics are determined by the type of flour and are accordingly rated by GOSTs.

The structure of fermenting dough is the direct material from which bread products are produced by heat treatment in the oven. It was of interest to study the biochemical and structural-mechanical properties of fermented wheat dough depending on the type of flour. For this purpose, seven samples of soft red grain wheat were ground in a laboratory mill using three-grade grinding with a total yield of 78% on average. Then we investigated the gas-forming and gas-holding capacity of flour, the structural and mechanical characteristics of fermented dough after proofing, as well as raw gluten proteins and their content in flour, the specific volume (in cm 3 /g) of molded bread, as well as the HID of round hearth bread baked using GOST 9404-60. The results obtained are shown in table. 4.2. They showed that the yield of varietal flour, even under laboratory experimental grinding conditions, fluctuates significantly and the more strongly, the higher its grade. Thus, the grain grinding technology should influence the chemical composition, and therefore the structure of the dough. It is one of the significant numerous factors influencing the quality indicators of flour, dough and bread products.

Table 4.2

Biochemical and structural-mechanical characteristics

gluten proteins of fermented dough and bread

(average data)

Note. The numerator contains data on proteins, the denominator contains data on the test.

The technological properties of grain and flour of each variety are characterized primarily by their gas-forming ability. This property characterizes the ability of grain and flour to convert the chemical energy of carbohydrate oxidation into thermal and mechanical energy of movement of fermenting dough, overcoming the inertia of its mass. Determining the gas-forming ability of flour is accompanied by taking into account the amount of CO 2 released. The amount retained by the test determines it. gas retention by volume increase. This physicochemical indicator characterizes by its inverse value the gas permeability of the test to carbon dioxide. The latter depends on the structure and size of the main elastic-plastic (E, η, η/E) test characteristics. Experiments showed that the gas-forming ability of flour increased significantly from the highest to the first and second grades, while the volumetric yield of bread, on the contrary, decreased.

The gas-holding capacity of the dough is directly dependent on the gas-forming ability; despite this, it did not increase in absolute and relative (% of gas formation) values, but noticeably and naturally decreased with decreasing flour grade. There is a close direct relationship between the absolute value of CO retained by the dough and the volumetric characteristics of bread (volume Yield, specific volume). The foregoing allows us to conclude that these characteristics of bread quality are determined mainly not by biochemical, but by physicochemical (gas permeability) and mechanical properties (η, E and η/E) of the dough. The latter depend mainly on the corresponding properties of raw gluten proteins and their content in the dough.

Experiments have shown that the content of crude gluten proteins naturally increased with a decrease in grain strength and moisture-holding capacity (viscosity) of flour and its variety. The protein structure of premium flour had higher values of shear modulus and, on average, viscosity than the structure of proteins of first grade flour. This indicates their higher statistical molecular weight. The proteins of grade I flour had a shear modulus and viscosity lower than these characteristics of proteins of grade II flour, but exceeded them in terms of η/E. This characterizes their greater elasticity and dimensional stability.

The gas-holding capacity of the dough and the volumetric yield of bread products directly depend on the duration of the stress relaxation period of gluten proteins and dough, or η/E. The ratio of viscosity to modulus of gluten proteins of grade II flour was significantly lower than that of premium and grade I flour proteins.

The gas-holding capacity of dough made from high-quality wheat flour depended on the corresponding values of its shear modulus and viscosity. These characteristics decreased with decreasing flour grade, similar to the gas retention ability.

It was established that fermenting dough from premium flour with a moisture content of 44%, like the raw gluten proteins of this flour, had the most significant values of shear modulus, viscosity and viscosity-to-modulus ratio, and the lowest relative plasticity. From this dough, bread products with the highest porosity, specific volume of molded bread, and the ratio of height to diameter of hearth bread were obtained. Thus, despite the significant viscosity, the least gas formation due to the high η/E is obtained from this flour into dough and bread of high volumetric yield. High values of viscosity and η/E contributed to the production of hearth bread with the highest N/A.

Dough made from grade I flour with a moisture content of 44% was slightly inferior in terms of gas retention, mechanical characteristics and bread quality to dough made from premium flour; it had viscosity, η/E of dough, N/A reduced by 14-15%. This indicates that a decrease in the viscosity of dough made from grade I flour contributed to both the development of the specific volume of molded bread and the increase in the spreadability of hearth bread.

Dough made from grade II flour had a higher moisture content (45%). Despite the greatest gas formation, it was significantly inferior to the dough of the highest and first grade flour in terms of gas retention and viscosity. The viscosity-to-modulus ratio of this dough, like that of gluten proteins, was lower, and the relative plasticity was higher than that of dough made from premium and grade I flour. The quality of the resulting bread products was much lower than the quality of products made from premium and first grade flour.

In order to clarify the influence of the structural and mechanical characteristics of fermenting dough on the physical properties of bread products, we differentiated the experimental results into two groups. The first group of samples of each variety had, on average, higher shear moduli and viscosity than the arithmetic mean, while the second group had lower ones. The characteristics of gas retention of the dough and elastic-plastic properties of raw gluten proteins were also taken into account (Table 4.3).

Table 4.3

Average characteristics of dough with high and low viscosity

From the table 4.3 it is clear that the specific volume of bread made from premium flour does not depend on the value of the gas-holding capacity of the dough, which turned out to be almost the same for both groups of samples. The specific volume of bread made from flour of grades I and II depended on the slightly higher gas-holding capacity of the dough of the second group of samples. The amount of raw gluten in both groups of samples for all types of flour turned out to be approximately the same and could not affect the bread quality indicators.

The viscosity of dough made from premium flour of both groups of samples turned out to be inversely related, and the ratio of viscosity to modulus was directly dependent on the corresponding indicators of their raw gluten proteins; for dough made from flour of grades I and II of both groups of samples, it was the opposite.

From this we can conclude that the main characteristics of fermenting dough - viscosity and the ratio of viscosity to modulus - depend not only on the corresponding characteristics of gluten proteins, but also on the influence of other grain compounds.

The volumetric yield of molded bread, as well as the H/D of hearth bread within each of the three types of wheat flour, depend on the viscosity and the ratio of viscosity to the modulus of fermenting dough. Viscosity has an inverse effect on the volumetric yield and a direct effect on the H/D value. The ratio of viscosity to modulus has a direct impact on both of these bread quality characteristics.

The degree of influence of viscosity and the ratio of viscosity to modulus on the physical and mechanical indicators of bread quality can be unequal and mutually directed. It depends both on the magnitude of these characteristics of the dough structure and on the modes of its technological processing. Despite this, the data in Table. 4.3 make it possible to explain the results obtained not only by the type of flour, but also by the dependence on viscosity values and the ratio of viscosity to dough modulus. Thus, a significant difference in the specific volume of molded and H/D hearth bread made from premium, I or II grade flour with approximately the same dough viscosity should be explained primarily by the unequal values of their viscosity-to-modulus ratios. The results we obtained allow us to state that the type of grain, ground even according to the same technological scheme, affects the gas retention and structural and mechanical properties of the dough obtained from each type of three-grade flour. Viscosity and the ratio of viscosity to the modulus of fermenting dough made from high-quality wheat flour can be used as characteristics that predetermine the physical and mechanical properties of pan and hearth bread. Therefore, it seemed appropriate to determine and standardize them for a simple dough made from commercial flour of the main varieties, produced at Moscow enterprises under the current technological production conditions.

By means of mass measurements of the elastic-plastic characteristics of fermented, ready-to-cut dough and statistical processing of the results, average optimal (M±δ) values of viscosity and the ratio of viscosity to modulus were established for three types of commercial wheat and rye flour (Table 4.4).

Table 4.4

Average optimal values of viscosity and η/E of fermenting dough (D=0.003 s)

	Dough moisture content,%
Wheat I grade


peeling

Comparing the data in Table. 4.4. and 3.14, it can be seen that fermenting dough made from grade I wheat flour has, as in table. 3.1 and 4.1 are significantly larger, and rye dough of both varieties has lower viscosity values and viscosity-to-modulus ratios than non-fermentable dough.

The main reason for the decrease in viscosity and the ratio of viscosity to modulus of fermenting dough made from rye wallpaper flour should be considered the dissolution of its compounds by dough acids.

Studies of the effect of acidification with lactic acid of non-fermentable dough from three samples of rye wallpaper flour showed that all samples of dough acidified (to fermenting standards) had a lower viscosity and viscosity-to-modulus ratio than that of non-acidified dough. This should be attributed to the partial peptization of swelling proteins and other rye compounds by solutions of organic acids.

INFLUENCE OF MODERN METHODS OF TESTING ON THE MECHANICAL PROPERTIES OF DOUGH AND THE QUALITY OF BREAD

PRODUCTS

In recent years, work has been carried out in the USSR and abroad that has shown the possibility of reducing flour consumption and time for preparing bread products. This is achieved by using technological schemes that provide mechanical impact on the dough and dough, activating their fermentation. The basis of such schemes is the use of large liquid (humidity about 70%) or thick (humidity 40-50%) sponges.

Liquid doughs have a viscosity that is 1-2 decimal orders lower than thick ones; the latter are difficult to pump upward; After fermentation, they are diluted with water. It has been established that diluted doughs have a viscosity significantly lower than undiluted ones of the corresponding humidity; During fermentation, the viscosity of the dough decreases.

Reducing the duration of fermentation of the dough and dough is achieved by longer intense exposure during the kneading process. At the same time, the amount of gluten proteins washed out of the dough decreases, the content of water-soluble nitrogenous compounds and carbohydrates increases, the attackability of starch by amylase and the fermentation activity of yeast increase. The listed processes increase the volumetric yield of dough and bread, improve the porosity structure of the crumb, and the shape of hearth products.

These characteristics of bread products are also improved by additional mechanical processing of the dough during the cutting process. However, excessive mechanical processing can lead to deterioration in the physical and mechanical characteristics of products, so its optimization is necessary. The specific work value is proposed as a criterion for the degree of mechanical impact on the dough when kneading it. It varies depending on the moisture capacity of flour from 12 to 50 J/g.

Based on the above, the following conclusions can be drawn.

Fermenting dough, unlike non-fermenting dough, is a more complex doubly stressed colloidal disperse system, including a gas phase, which therefore has a reduced density. Its foam-like porous mass, continuously forming CO 2, increases the volume - it coalesces due to equalization of the pressure of neighboring pores of different sizes, forming an open structure; in it, according to Stokes' law, the largest pores move continuously upward to the surface of the dough and the release of carbon dioxide. In the process of pore formation, increasing volume by small stresses and slow shear deformations, the structure of the fermenting dough becomes elastic, increasing viscosity and η/E.

Fermented dough made from wheat flour of grades I and II differs from non-fermented dough by lower values of shear modulus, relative plasticity (greater elasticity), higher viscosity and the ratio of viscosity to modulus, as well as the stability and increase of these characteristics during the fermentation process after kneading. More significant differences were established for dough made from grade I flour, which has 3-4% less moisture than dough made from grade II flour, and a different chemical composition.

Fermented dough made from wallpaper and peeled rye flour differs from non-fermented dough in having larger shear moduli, lower viscosity and a lower viscosity-to-modulus ratio. This is explained by the influence of a significant concentration of organic acids in it, which partially dissolve swelling proteins and other grain polymers.

The structural and mechanical properties of fermented wheat dough and raw gluten proteins from flour of the highest, I and II grades, obtained from one grain by three-grade grinding, viscosity, as well as the ratio of viscosity to modulus differ significantly: they determine the gas-holding capacity of the dough, the volumetric yield of the molded dough, as well as H/D of hearth bread. With a decrease in the grade of flour, the viscosity and the ratio of viscosity to the module of gluten proteins and the gas retention of the dough, the volumetric yield of bread, its porosity and H/D decrease. The most significant differences in the indicated characteristics of dough, gluten proteins and bread are observed between grades I and II of flour.

Within each variety, the viscosity of fermenting dough has an inverse effect on the development of its volume (gas retention), the volumetric yield of bread and a direct effect on the H/D of bread. The ratio of viscosity to dough modulus has a direct impact on both bread parameters. The type of grain in some cases influences the structural and mechanical properties of dough made from flour of each type.

It is advisable to normalize and regulate the listed properties of fermenting dough in order to control and manage them. As approximate norms for dough made from grade I wheat flour, rye wallpaper and peeled flour, you can use the results of Table. 4.4.

INFLUENCE OF HEATING ON THE MECHANICAL PROPERTIES OF THE DOUGH. MECHANICAL PROPERTIES OF BREAD

The process of producing bread products is completed by heating the mass of fermenting dough from 30 to 100°C under conditions of large gradients of heat and mass transfer.

Heat treatment during baking in the specified temperature range significantly affects the activity of biochemical processes, changes the conformation of the molecules of the main grain polymers, their hydrophilic properties, as well as the mechanical properties of the dough; the content of free water in the structure decreases, the dough loses its ability to flow under the stress of gravitational forces of the mass. Then the plastic-elastic structure of the dough turns into an elastic-brittle plastic gelatinous structure of the bread crumb. It should be assumed that its plastic deformations occur mainly at low deformation rates due to stress relaxation, and at high rates as a result of the phenomena of fragility, destruction of the continuity of the pore walls of the concentrated protein-starch jelly - crumb in the elastic region. In this regard, when studying the mechanical properties of bread crumb, one should limit oneself to the smallest possible values of its deformations and their speed. Instead of shear deformations, it is advisable to use uniaxial compression deformations of the porous foam-like structure of the crumb.

Warming up enhances the thermal movement of molecules of chemical compounds. In polymer solutions, it reduces the coefficient of internal friction (viscosity). The inverse dependence of the viscosity of polymer solutions on temperature is determined by the well-known empirical Arrhenius equation

η=Ae

where A is a constant depending on the properties of the substance;

e is the base of the natural logarithm;

T - absolute temperature;

K - gas constant;

E - activation energy (work expended on moving particles).

However, this equation is valid only for solutions of low concentration and provided that there are no significant changes in the shape of the polymer molecules. The concentration of the main grain polymers - gluten proteins and starch - in bread dough is very high, and its heat treatment changes the shape of the molecules, as well as the ability of these main grain polymers to interact with the solvent - water. The sizes and shapes of their molecules also change during hydrolysis and fermentation by enzymes of grain and dough microorganisms.

All of these processes can affect the structure and change the mechanical properties of the dough. Therefore, it should be expected that the application of the Arrhenius equation for the dough structure is permissible in a very limited temperature range. The dependence of these dough properties on temperature over a wide range is more complex. Let us consider in more detail its possible influence on these properties: heating the dough during baking and turning it into bread crumb occurs in two main stages. In the initial stage of heating the dough to 50-60°C, the enzyme systems of the dough are activated, the content of water-soluble compounds in it increases, which can plasticize the structure and, simultaneously with increased molecular thermal movement, reduce viscosity and enhance its adhesive properties. At this stage, the main processes of bread baking also begin: gelatinization of starch and denaturation of grain proteins, which proceed most actively and end in the second, final stage of heating the dough from 60 to 100 ° C, when inactivation of its enzyme systems also takes place.