Structural-mechanical (rheological) characteristics of dough for various bakery products.
pastry dough
The use of wheat flour of different quality, a large set of raw materials, changing their ratio and the use of certain technological parameters and techniques makes it possible to obtain dough and products that differ in physical, chemical and rheological properties.
The rheological properties of the dough depend on the degree of swelling of the proteins.
Depending on these properties, confectionery dough is divided into three types:
plastic - viscous(sugar, shortbread, rich, gingerbread dough), perceives and retains its shape well;
elastic - plastic - viscous(prolonged, cracker, biscuit), poorly perceives and poorly retains its shape;
semi-structured(wafer, biscuit dough for biscuit semi-finished products and cakes), has a liquid consistency.
Plastic dough is formed under conditions of limited swelling of flour colloids, therefore, the duration of dough kneading should be minimal and the temperature lower than the temperature of dough with elastic-plastic-viscous properties.
In accordance with GOST "Confectionery. Terms and definitions" there are two types of dough depending on its structure:
Biscuit - rich, sugar, oatmeal, from which products of various shapes with well-developed uniform porosity are obtained,
Layered dough - for long biscuits, crackers, biscuits, from which products of various shapes of layered structure are produced.
The formation of dough with certain rheological properties is associated with:
With the type of products, recipe, with the correct selection of flour grades, with the optimal content and quality of gluten, grinding coarseness,
With the right choice of dough moisture,
With the right choice and maintenance of the technological parameters of dough kneading (temperature, duration, intensity of kneading).
The noted factors affect the degree of swelling of wheat flour and thus the rheological properties of the dough, its plasticity, elasticity, elasticity, viscosity.
Increasing the temperature of the dough during kneading, lengthening the duration of the process from the sugar plastic dough as a result of a more complete swelling of the colloids, you can get a protracted dough with elastic-plastic-viscous properties. The plasticity of sugar dough is close to 1. In order to be able to mold a protracted dough to blanks, eliminating their deformation, its plasticity must be increased to 0.5. For this purpose, such an operation as aging the test is used, or enzyme preparations of proteolytic action are used. For a semi-structured wafer dough, from the rheological characteristics, dough viscosity and elasticity are of great importance. They determine the uniformity of the distribution of the dough on the surface of the waffle iron, as well as the fragility of the waffle sheet.
Confectionery dough, like all pasty masses, is structured disperse system and consists of three phases: solid, liquid and gaseous.
solid phase represent lyophilic flour colloids. These are water-insoluble protein complexes and wheat flour starch.
Liquid phase is a multicomponent aqueous solution of the substances specified in the dough recipe (invert syrup, water, sugar solution, molasses, salt, sodium bicarbonate, ammonium carbonate, milk, etc.). The composition of the liquid phase includes all water-soluble organic and mineral substances of flour.
The ratio between solid and liquid phases depends on the type of dough, its moisture content, quantity and quality of gluten.
gaseous phase makes up the air that is captured during dough kneading, dispersed and retained in the dough. In addition, air enters with flour, water and other types of raw materials and semi-finished products. The gaseous phase can reach 10% in the test.
The degree of leavening of the dough depends on the rheological properties of the dough and on the uniform distribution of chemical leavening agents in the dough. Especially increases the porosity and volume of blanks from plastic dough - sugar, gingerbread. Long and biscuit dough, which has significant elasticity, resists the expansion of gas bubbles. These products have a slight rise and underdeveloped porosity.
The structural and mechanical (rheological) characteristics (effective viscosity h eff, plastic viscosity h pl, modulus of elasticity E 1 , modulus of elasticity E 2 , stress relaxation time t rel, relative plasticity P, etc.) for the test of various bakery products (bread wheat, rich products, lamb, bagel, straw, puff yeast and puff unleavened, flat cakes, etc.). The influence on the rheological characteristics of various factors is shown: the quality of raw materials, the method of technological processing, the degree of mechanical impact on the dough (dough mixing, sheeting machines, screw presses, etc.), dough resting, forming dough pieces, as well as such technological factors as temperature, humidity test, recipe, inclusion of additives and improvers. Examples of the use of rheological characteristics to assess the quality of semi-finished products and finished products are given.
The presented material can be used by employees of design and design bureaus, engineers of the baking industry in the modernization of old and creation of new mechanical equipment, as well as scientists and students in research and graduation theses.
Recipe, main and additional raw materials
Viscosity value for different types of dough
The average values of the viscosity of various types of dough at 30 ° C and atmospheric pressure are given in Table. 6.19.
Table 6.19. Average viscosity values of various types of dough at 30 °C and atmospheric pressure
| Type of test | Rheological body | Shear rate, s –1 | Humidity, W t % | Effective viscosity, h eff, Pa s |
| Opara | Visco-plastic | 2,0 | ||
| flour bakery | ||||
| I grade | – | 5,0 | 44,5 | 6.5 10 2 |
| II | – | 5,0 | 45,7 | 5.5 10 2 |
| For Bulgarian bread | Shvedov–Bingham | 2,0 | 42,6 | 8 10 2 |
| For bagels | Same | 0,5 | 33,5 | 3 10 5 |
| For sugar donuts | –‘’– | 0,3 | 31,6 | 2 10 6 |
| For vanilla bagels | –‘’– | 0,5 | 31,8 | 8 10 5 |
| For crispbread | - | 1,0 | 38,0 | 6 10 2 |
| For cakes | Elastic-viscous-plastic | 2,0 | 41,0 | 1 10 4 |
The viscosity of flour dough is in the range from 0.5 to 2000 kPa·s at a moisture content of 17.0 to 45.7%. Different types of dough belong to different classes of rheological bodies, which makes it necessary to choose the appropriate calculation equation in each case when describing the flow of this type of dough in technological machines.
Yeast-free dough
In the production of test semi-finished wafers, batter is used, which differs from ordinary bakery dough in the absence of yeast and the presence of a large amount of sugar and milk.
Studies () were carried out on a reconstructed viscometer
PB-8 with the following parameters: shear rate 0-9 s−¹, dough moisture 31.8 - 44.3%, dough temperature 15 - 40ºC.
The obtained dependences of the effective viscosity on the shear rate are typical for most types of flour dough. An increase in humidity and temperature leads to a decrease in viscosity.
The nonlinearity of the dependences obtained allows us to conclude that the studied dough has an abnormal viscosity and is a non-Newtonian liquid. At shear rates up to 6 s −¹, this dependence is described by a power law, above the specified value - by a linear one. Processing of experimental data made it possible to obtain an equation describing the dependence of viscosity on shear rate, humidity and temperature,
h=108.8-3.985g+0.25gІ+1.13T-0.032TІ-4.043W+0.0359WІ.(1)
Equation (1) is valid for the following intervals of argument change: 0.5 s –1 £g£7.0 s - 1; 31.8% £W£40.0%; 15°C£T£30°C.
During the development of systems for automatic control and regulation of technological processes, it is necessary to know the correlation between individual technological parameters and the structural and mechanical characteristics of the product under study.
For this purpose, experiments were carried out (12) to determine the viscosity of the test at different moisture content. To prepare the dough, commercial wheat flour of the highest and I grades was used. The experiments were carried out with yeast-free dough with humidity from 44.5 to 65% at a temperature of 30°C. The choice of this range is explained by the following: the upper limit (44.5%) is equal to the value of the moisture content of wheat dough from flour of the I grade adopted at the bakery, the lower limit (65%) is chosen due to the fact that many works note the prospects for the method of preparing wheat dough for liquid sponges, which has a number of advantages.
The viscosity was determined on a rotational viscometer "Reotest-RV" (GDR). The strain rate was changed in the range from 0.167 to 1.8 s -1 . The average results are shown in Fig.59.
Rice. 59. Dependence of the viscosity of the dough from flour of the I grade on its moisture content at various shear rates (in s-1):
I - 0,167; 2 - 0,333; 3 - 0.6; 4 - 1.0; 5 -1.8.
As can be seen from the graphs, the dependences are exponential. With an increase in the moisture content of semi-finished products, their viscosity decreases significantly. So, for a shear rate of 0.167 s -1 with a change in humidity from 46 to 50%, the viscosity decreased by about 3.5 times. As the shear rate increased, the intensity of the viscosity change decreased significantly. For example, at a shear rate of 0.167 s-1 and a change in humidity from 46.0 to 65.0%, the viscosity decreased from 1385 to 42 kPa * s, and at 1.8 s -1 and the same change in humidity, the viscosity decreased only from 284 up to 20 Pa s, i.e. the intensity of viscosity change decreased by 5 times. Here, the anomaly of the viscosity of the bread dough plays a significant role.
The processing of the obtained experimental data made it possible to propose the following form of correlation:
h= c + e a W b , (3-13) a
where a, b, c are empirical coefficients having the following values: for dough from flour of grade I a = 50.26, b = -12.47, c = 0.1; for dough made from premium flour a=52.77, b=-13.17, c=0.1.
Equation (3-13) is valid for a shear rate of 0.167 to 1 s? and dough humidity ranging from 44 to 62%.
Grinding size of wheat flour
Table. The dependence of the elastic-plastic characteristics of the dough on the coarseness of grinding wheat flour
| Grinding fractions | Crude gluten content, % | Elastic modulus, E | E, with | |
| after 30 minutes | ||||
| Passage through a sieve 43 | 43/39,5 | 4,2/9,1 | 7,0/6,9 | 60/132 |
| Passage through a sieve 38 | 38/39,3 | 3,2/8,4 | 3,5/4,7 | 91/179 |
| Passage through a sieve 25 | 25/38,1 | 3,0/6,8 | 3,3/4,3 | 91/157 |
| Departure from the sieve | 25/37,5 | 2,6/6,4 | 2,9/4,0 | |
| The inverse dependence of the viscosity and shear moduli of the dough on the size of the flour particles was established. This pattern depends in part on the increase in gluten protein content with decreasing flour particle size. |
Right side of table 6.2
| Plastic viscosity, η 10 –5 , Pa s | Elastic modulus, E 10 -3 , Pa ??? Recalculate the numbers | Stress relaxation time, η/ E, with | Thinning ratios | |
| K η | K E | |||
| after 3 hours | ||||
| 2,6/6,2 | 4,2/6,5 | 62/95 | 38/32 | 40/6 |
| 2,4/4,4 | 3,3/3,9 | 73/13 | 25/47 | 6/17 |
| 2,2/3,1 | 3,2/3,15 | 71/91 | 27/53 | 7/19 |
| 1,6/2,9 | 2,1/3,2 | 76/91 | 39/51 | 28/20 |
Table 6.20. Structural and mechanical properties of pastry with different sugar and fat content (at 20 °C)
| Dough | Humidity, % | E, Pa | η, Οa s | η/ E, with | P, % | E, % | D, s –1 |
| Control | 30,2 | 3.0 10 3 | 5.0 10 5 | 0,0015 | |||
| With sugar: | |||||||
| 5% | 30,6 | 1.1 10 3 | 2.0 10 5 | 0,0030 | |||
| 10% | 5.1 10 2 | 8.8 10 4 | 0,0045 | ||||
| 20% | 30,3 | 2.7 10 2 | 2.7 10 4 | 0,0090 | |||
| 50% | 30,5 | 1.4 10 2 | 1.6 10 4 | 0,0045 | |||
| Control | 30,6 | 3.6 10 3 | 6.2 10 5 | 0,0015 | |||
| With margarine: | |||||||
| 5% | 30,3 | 1.9 10 3 | 2.9 10 5 | 0,0030 | |||
| 10% | 28,0 | 1.8 10 3 | 2.4 10 5 | 0,0030 | |||
| 20% | 28,0 | 1.5 10 3 | 1.8 10 5 | 0,0040 | |||
| 50% | 30,4 | 4.8 10 3 | 7.9 10 4 | 0,0045 | |||
| With 50% sugar | 20,8 | 5.7 10 3 | 4.3 10 4 | 0,0075 | |||
| With 50% margarine | 20,4 | 4.9 10 3 | 2.8 10 5 | 0,0090 | |||
| With 50% sugar and 50% margarine | 20,0 | 6.1 10 3 | 3.6 10 4 | 0,0030 |
The effect of sugar and fat additives on the mechanical properties of flour dough depends on its moisture content. Significant additions to wheat dough from high-quality flour of protein compounds, sugars and fats significantly change its structural and mechanical characteristics. By adding from 5 to 50% sugar to flour, plasticization of the wheat dough structure is achieved - a decrease in the values of shear moduli and viscosity; there is an elasticization of the dough in the form of a more significant decrease in modules.
Table 6.21. Structural and mechanical characteristics of non-fermenting and fermenting dough made from grade I flour with added sugars
| Sample number | Test samples | Humidity, % | Е 10 –2, Pa | η 10 –4 , Pa s | η/ E, with | P, % | E, % | K E, % | K η , % |
| Non fermenting dough | |||||||||
| Without additives | 44,0 | 8,5/3,5 | 5,9/1,9 | 69/53 | 72/78 | 74/82 | |||
| With 5% sucrose | 43,7 | 4,7/2,4 | 3,5/1,6 | 74/62 | 71/74 | 77/82 | |||
| With 5% glucose | 44,0 | 5,4/2,8 | 4,0/2,0 | 74/68 | 71/72 | 73/77 | |||
| With 10% sucrose | 43,3 | 3,3/1,7 | 2,7/1,3 | 84/74 | 73/71 | 77/82 | |||
| With 10% glucose | 44,1 | 3,1/1,6 | 3,1/1,8 | 99/108 | 64/62 | 91/76 | |||
| With 15% sucrose | 43,4 | 1,5/1,0 | 1,5/1,3 | 100/130 | 67/55 | 85/78 | |||
| With 15% glucose | 43,5 | 1,9/1,2 | 2,5/1,6 | 140/140 | 58/55 | 76/77 | |||
| With 20% sucrose | 43,0 | 1,0/0,6 | 1,3/1,1 | 130/180 | 58/52 | 75/76 | |||
| With 20% glucose | 43,0 | 1,0/0,9 | 1,5/1,7 | 145/180 | 53/48 | 64/67 | |||
| fermenting dough | |||||||||
| Without additives | 44,2 | 6,0/2,9 | 5,4/6,2 | 90/214 | 67/45 | 64/65 | –12 | ||
| With 5% sucrose | 44,0 | 3,5/1,6 | 3,2/4,4 | 92/277 | 66/42 | 67/67 | –38 | ||
| From 10% » | 43,8 | 1,8/1,4 | 1,7/2,9 | 100/207 | 65/46 | 59/60 | –71 | ||
| From 15" | 44,0 | 0,9/0,8 | 0,8/1,4 | 96/178 | 65/50 | 67/63 | –75 | ||
| From 20" | 44,1 | 0,2/0,25 | 0,25/0,37 | 125/135 | 59/56 | 74/74 | –25 | –48 |
The structure of non-fermenting dough without the addition of sugars, due to the increased content of water-soluble compounds, has increased plasticity, liquefies. The dough with an exposure of 2 hours has a low viscosity of the dough, its relative elasticity increases. The addition of 5–20% sugars to the dough significantly reduces its viscosity and the shear moduli are even more noticeable: the relative elasticity increases, and the plasticity decreases; with an increase in the dosage of sugar, this effect increases. The effect of sugar additions on the structure of a non-fermenting dough aged for 2 hours is similar to their effect on the structure without aging. At the same time, sugar additions gradually change the nature of the influence of the duration of dough exposure on its elastic-elastic, plastic-viscous properties.
Table 6.22. Influence on the structural and mechanical characteristics of the test from flour of the I grade of the joint addition of sugar and fat
| Experience Variant | Sample | Humidity, % | Е 10 –2, Pa | η 10 –4 , Pa s | η/ E, with | P, % | E, % | K E, % | Gradient E | K η , % | Gradient η |
| Non-fermenting dough | |||||||||||
| Control | 43,6 | 10/4 1 | 6,8/2,8 | 68/68 | 73/73 | 73/82 | - | - | |||
| With 5% sugar and 2.5% fat | 43,3 | 5,2/2,7 | 4,0/1,5 | 76/55 | 71/77 | 80/80 | 0,2 | 0,2 | |||
| With 10% sugar and 5% fat | 44,3 | 1,7/1,4 | 1,6/0,7 | 94/45 | 66/78 | 76/68 | 0,2 | 0,1 | |||
| With 20% sugar and 10% fat | 44,1 | 0,7/0,8 | 0,6/0,3 | 85/50 | 68/65 | 75/86 | –11 | 0,1 | 0,1 | ||
| fermenting dough | |||||||||||
| Control | 43,8 | 8,2/4,5 | 7,4/11,0 | 91/240 | 67/44 | 70/75 | - | –15 | - | ||
| With 5% sugar and 2.5% fat | 43,8 | 3,0/2,0 | 3,6/4,1 | 120/209 | 60/47 | 75/76 | 0,3 | –11 | 0,9 | ||
| With 10% sugar and 5% fat | 44,7 | 1,3/0,8 | 1,3/2,0 | 100/250 | 64/42 | 70/67 | 0,3 | –15 | 0,6 | ||
| With 20% sugar and 10% fat | 44,2 | 0,3/0,25 | 0,4/0,5 | 133/200 | 63/51 | 74/77 | 0,1 | –12 | 0,3 |
Note. The numerator shows data on a freshly mixed dough, the denominator - on a two-hour exposure test.
Sugars reduce the shear moduli and viscosity of both types of dough more; more significantly than fats, increase the ratio of viscosity to the modulus of non-fermenting dough; in comparison with fats, they less actively reduce this important characteristic of the fermenting dough. The joint addition of sugar and fat will have the most significant effect not only on the elastic-plastic, but on the relaxation properties of the fermenting wheat dough. The joint addition of sugar and fat to non-fermenting dough does not improve, but worsens its baking properties; and in the fermenting it slightly increases the viscosity and reduces the shear moduli.
FEATURES OF THE STRUCTURE AND MECHANICAL PROPERTIES OF THE WAGING DOUGH
Non-fermenting flour dough should be considered a material designed to evaluate the technological properties of grain and flour. Fermenting dough is less suitable for this purpose, since it contains yeast, sourdough, gaseous substances, mainly carbon dioxide, and organic acids formed during fermentation. It is a structural analogue and predecessor of the bread crumb structure, unfixed by heat treatment. The amount of carbon dioxide formed in a 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 yeast, and the conditions of their vital activity. The size of carbon dioxide bubbles and their number in the volume are determined by the gas permeability of the dough (according to 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 also in the dependence of their volumetric expansion coefficient on temperature. Their presence in the structure of the dough increases the volume, reduces its density, complicates the structure. Elastic-plastic deformations of the fermenting dough occur in the walls of the pores of its structured mass. In order to consider the influence of the gaseous phase on the mechanical properties of the fermenting dough, let us consider the diagram of its structure shown in Fig. 21. In it, surfactants, proteins, lipoids, etc. are schematically shown with round-ended rods. Their rounded part represents the polar, and the straight "tail" - the non-polar group of atoms in the molecule.
The most probable centers for the formation of primary bubbles of CO 2 in the fermenting dough are the points of adhesion of non-polar groups of surfactant molecules bound by the weakest forces of dispersion interactions. The gaseous products formed in the dough during its fermentation (CO 2 and others) dissolve in free water and adsorb 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 a breakthrough of the walls, diffusion and leakage of gas from the dough surface.
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 foamy fermenting dough structure having double walls formed by surfactants. They are filled with a mass of hydrated hydrophilic substances of the test, associated with the polar groups of surfactants of the walls of the bubbles by secondary chemical bonds. The dough has a significant viscosity and elastic properties, providing its foam 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 starting to ferment rapidly, which lowers its density, occurs at corresponding reduced stresses, therefore, the initial moduli of elasticity-shear elasticity and the viscosity of such a dough should not be higher than that of a non-fermenting dough. However, in the process of its fermentation and an increase in the 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 dough viscosity. Decreasing the density of the fermenting dough during fermentation allows proteins to more fully realize their elastic properties - to lower the modulus of elasticity-shear elasticity. With increased viscosity, reduced modulus, the fermenting dough should have a significantly greater ratio of these characteristics, have a more solid system than the non-fermenting one.
Owing to the permanent formation of carbonic acid and the increase in volume in this way, the fermenting dough, in contrast to the non-fermenting one, is a doubly strained system. The gravitational forces of its mass during fermentation are inferior, equal to or greater than the energy of chemical reactions of CO 2 formation, which creates forces that develop and move gas bubbles upwards according to the Stokes law (motion 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.
P=π rp (4.1)
and compressive
P =2π rσ (4.2)
where π, r , R , σ - respectively, the ratio of the circumference to the diameter (3, 14), the radius of the bubble, excess pressure and surface tension.
It follows from the equality conditions for equations (4.1) and (4.2) that
P =2 σ / r (4.3)
Equation (4.3) shows that at the initial moment of formation of a gas bubble, when its dimensions, determined by the radius, are very small, the excess pressure must be significant. As the bubble radius increases, it decreases. Neighborhood of gas bubbles of different radii should be accompanied by CO 2 diffusion through the walls in the direction from higher to lower pressure and its equalization. In the presence of a certain overpressure and the average size of gas bubbles, it is easy to calculate, knowing the viscosity of the dough, the rate of their rise according to the Stokes law mentioned.
According to this law, the force that lifts gas bubbles is
P =4/3π rg ( ρ - ρ ) (4.4)
overcomes the force of their friction
P =6 prηυ (4.5)
where g is the gravitational constant;
ρ and ρ are the densities of gas and dough;
η-effective structural viscosity of the dough;
υ - the speed of the vertical movement of gas bubbles in the test
arising in the dough mass when a spherical body (gas bubble) moves in it.
From the equality of equations (4.4) and (4.5) it is easy to determine the value of the velocity
V =2 gr ( ρ - ρ )/9 η (4 .6)
This equation is of great practical importance, making it possible 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 flour of grade I with a density of 1.2 with an average pore radius of 1 mm and a viscosity of about 110 4 Pas is about 10 mm/min. Practical observations show that such a dough has an average rise rate of 2 to 7 mm/min. The highest rate is observed in the first hours of fermentation.
If there are neighboring pores in the test, having different sizes and gas pressures, their walls break and the pores merge (coalescence); this phenomenon also depends on the rate of fermentation and the mechanical properties of the dough; apparently, most of the pores of the dough and bread crumb are not closed, 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 with its surface: taking the cost of dry substances (sugar) for the fermentation of the dough, equal to an average of 3% of the mass of flour, with alcoholic fermentation per 1 kg of flour (or 1, 5 kg of bread) releases about 15 g, or about 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 loss during the fermentation of the dough.
In the fermenting dough, many other organic acids and alcohols are also formed that can change the solubility of grain compounds. Thus, all of the above shows that the structure of the fermented dough is more complex than that of the non-fermented one. It should differ from the latter in smaller: density, elasticity-elasticity modulus, higher viscosity and η / E (greater ability to retain shape), a permanent increase in volume and acidity during fermentation.
For almost a long time, bakers have characterized the baking properties of fermented dough by its ability to exhibit elastic-elastic deformations after stress relief: “live” (or elastic-elastic) “moving” dough after deformation always gave bread products of good volume, shape and structure of the porosity of the crumb, in contrast from a motionless (plastic) dough, devoid of these properties.
The structure of the fermenting dough, its mechanical properties are mutually dependent on the sugar-forming ability of the 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 equal average gas-forming capacity of wheat flour of the first and second groups, the lower absolute and relative gas-retaining capacity 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 the dough (and the volumetric yield of bread) from the wheats of the third group in comparison with these characteristics of the dough (and bread) from the wheats of the second and first groups can partly be attributed to their lower gas-forming capacity.
Their relative (in % to gas formation) gas-retaining capacity was higher than that of wheat dough of the second and first groups, which can be attributed to the highest content of gluten proteins in 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 the flour. It seemed appropriate to investigate and compare the structure of non-fermenting and fermenting dough. The latter is the actual material from which bread products are made from flour of different varieties, differing in physical quality indicators. It was of interest to compare the mechanical properties of non-fermenting and fermenting dough from flour of different grades, as well as to carry out an approximate rationing of them in the latter.
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 wheat flour of the 1st grade with a moisture content of 44%
Note. The numerator shows the data on the non-wandering test, the denominator - on the roaming one. |
Dough made from grade I wheat flour is 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 the non-fermenting dough made from grade I flour should be the most distinct.
As the results of Table. 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 was smaller than that of the fermented dough. After 2 hours of fermentation, the dough viscosity and η/E did not decrease, as in a non-fermenting dough, but, on the contrary, increased, and plasticity decreased. For this reason, the index To had a negative value, characterizing not liquefaction, but an increase in the viscosity of the structure.
The results of comparing the mechanical properties of non-fermenting and fermenting wheat dough from two samples of grade II flour are given in table. 3.1, basically fully confirm the patterns established for the dough from flour of grade I; however, they are of undoubted interest because the aging process lasted up to 24 hours. It is known that the fermentation of pressed baker's yeast at their usual dosage (about 1% to flour) usually ends at a time interval of 3-4 hours (duration of fermentation of dough) . After this time, the dough is replenished with a fresh portion of flour and mixed, after which the fermentation in it resumes. In the absence of flour additives and mixing, alcoholic fermentation is inferior to acid fermentation. Such a dough, acquiring excessive amounts of ethyl alcohol and acids, dissolves gluten proteins (dilutes), losing carbon dioxide - reduces volume, becomes more dense. From Table. 3.1 it can be seen that the fermenting dough after 6 hours and especially after 24 hours of fermentation in terms of shear moduli, 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 the fermenting dough from its non-fermenting structure. Experiments have established that samples of fermenting wheat dough made from flour of I and II grades have a structure that has more perfect properties of elasticity-elasticity (lower shear modulus), greater viscosity and dimensional stability (η / E), as well as greater stability over time in comparison with the structure non-fermenting test. 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 THE MECHANICAL PROPERTIES OF THE FEMINATION DOUGH AND THE QUALITY OF THE BREAD ON THE TYPE AND TYPE 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 nominated by GOSTs.
The structure of the fermenting dough is the direct material from which bread products are obtained by heat treatment in an oven. It was of interest to study the biochemical and structural-mechanical properties of fermenting wheat dough depending on the type of flour. For this purpose, seven samples of soft red wheats were ground in a laboratory mill with a three-grade grinding with a total yield of 78% on average. Then we studied the gas-forming and gas-holding ability of flour, the structural and mechanical characteristics of the fermented dough after proofing, as well as raw gluten proteins and their content in flour, the specific volume (in cm GOST 9404-60. The results are shown in table. 4.2. They showed that the yield of high-quality flour, even under conditions of laboratory experimental grinding, fluctuates significantly and the stronger, the higher its grade. Thus, the grain grinding technology should influence the chemical composition and, consequently, the structure of the dough. It is one of the significant numerous factors affecting 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, in the denominator - on the test.
Technological properties of grain and flour of each grade 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 the movement of the fermenting dough, overcoming the inertia of its mass. The determination of the gas-forming ability of flour is accompanied by taking into account the amount of released CO 2 . Its amount, delayed by the test, determines it. gas retention by volume increase. This physico-chemical indicator characterizes by its inverse value the gas permeability of the test for carbon dioxide. The latter depends on the structure and magnitude of the main elastic-plastic (E, η, η/E) test characteristics. Experiments have shown 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-retaining ability of the dough is directly dependent on the gas-forming ability; despite this, it did not increase in absolute and relative (in % to gas formation) values, but noticeably and regularly decreased with a decrease in the 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 respective properties of the raw gluten proteins and their content in the dough.
Experiments have shown that the content of raw gluten proteins naturally increased with a decrease in grain strength and moisture capacity (viscosity) of flour and its varieties. The protein structure of premium flour had higher shear modulus and, on average, viscosity than the protein structure of grade I flour. This indicates their higher statistical molecular weight. Flour proteins of grade I had a shear modulus and viscosity lower than these characteristics of flour proteins of grade II, but exceeded them in value η/E. This characterizes their great 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 relaxation period for the stresses of gluten proteins and dough, or η/E. The ratio of viscosity to the modulus of gluten proteins of grade II flour was significantly lower than that of proteins of premium and grade I flour.
The gas-holding capacity of dough made from varietal wheat flour depended on the respective values of its shear modulus and viscosity. These characteristics with a decrease in the grade of flour decreased similarly to the ability of gas retention.
It has been established that the fermenting dough from premium flour with a moisture content of 44%, like raw gluten proteins of this flour, had the most significant values of shear moduli, viscosity and viscosity-to-modulus ratio, and the lowest relative plasticity. From this test, bread products of the highest porosity, specific volume of molded bread, as well as 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 were obtained from this flour. High values of viscosity and η/E contributed to the production of hearth bread with the highest N/A.
Dough made from flour of grade I with a moisture content of 44% in terms of gas retention, mechanical characteristics and bread quality was slightly inferior to the quality of dough made from flour of the highest grade; This indicates that the decrease in the viscosity of the dough made from grade I flour contributed both to the development of the specific volume of the molded bread and to the increase in the spreadability of the hearth bread.
The 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 I grades of flour in terms of gas retention and viscosity. The ratio of viscosity to modulus of this test, like that of gluten proteins, was lower, and the relative plasticity was higher than that of the test from flour of the highest and I grades. The quality of the resulting bread products was much lower than the quality of products made from flour of the highest and I grades.
In order to clarify the influence of the structural and mechanical characteristics of the fermenting dough on the physical properties of bread products, we differentiated the results of the experiments into two groups. The first group of samples of each grade had, on average, higher than the arithmetic mean, shear moduli and viscosity, the second group had lower ones. The characteristics of the gas retention of the dough and the elastic-plastic properties of raw gluten proteins were also taken into account (Table 4.3).
Table 4.3
 |
Average characteristics of high and low viscosity dough
From Table. 4.3 it can be seen that the specific volume of bread made from premium flour does not depend on the gas-retaining capacity of the dough, which turned out to be almost the same for both groups of samples. The specific volume of bread from flour of I and II grades depended on a slightly higher value of the 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 quality of bread.
The viscosity of the dough from flour of the highest grade of both groups of samples turned out to be inversely related, and the ratio of viscosity to modulus was in direct proportion to the corresponding indicators of their raw gluten proteins, for dough from flour of I and II varieties of both groups of samples - on the contrary.
From this we can conclude that the main characteristics of the 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 the tin bread as well as the H/D of the hearth bread within each of the three types of wheat flour depend on the viscosity and the ratio of the viscosity to the modulus of the fermented 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 characteristics of bread quality.
The degree of influence of viscosity and the ratio of viscosity to modulus on the physical and mechanical indicators of the quality of bread can be unequal and mutually directed. It depends both on the value of these characteristics of the dough structure and on the modes of its technological processing. Despite this, the data in Table 4.3 allow us to explain the results obtained not only by the type of flour, but also by the dependence on the values of viscosity and the ratio of viscosity to dough modulus. Thus, a significant difference in the specific volume of pan and H/D hearth bread made from flour of the highest, I or II grades with approximately the same dough viscosity should be explained primarily by the unequal values of their ratios of viscosity to modulus. The results obtained by us 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 flour of three-grade grinding. Viscosity and viscosity-to-modulus ratio of fermenting dough made from varietal wheat flour can be used as characteristics that predetermine the physical and mechanical properties of pan and hearth bread. Therefore, it seemed expedient to determine and standardize them for a simple dough made from marketable flour of the main varieties, obtained at Moscow enterprises under the conditions of existing technological production regimes.
By mass measurements of the elastic-plastic characteristics of the fermented, ready-to-cut dough and statistical processing of the results, the average optimal (M ± δ) values of viscosity and the ratio of viscosity to modulus were established for three varieties of wheat and rye marketable flour (Table 4.4).
Table 4.4
Average optimum viscosity and η/E fermenting dough (D=0.003 s)
| Dough moisture,% |
|||
| Wheat I grade |
|||
| peeling |
Comparing the data in Table. 4.4. and 3.14, it can be seen that the fermenting dough made from grade I wheat flour has, as in Table. 3.1 and 4.1 are significantly larger, and the rye dough of both varieties is smaller than that of the non-fermenting dough, the values of viscosity and the ratio of viscosity to modulus.
The main reason for the decrease in viscosity and the ratio of viscosity to modulus of fermented dough from rye wholemeal flour should be considered the dissolution of its compounds by dough acids.
Studies of the effect of lactic acid acidification of non-fermenting dough from three samples of rye wholemeal flour showed that all samples of the acidified (to the norm of fermenting) dough had a lower viscosity and viscosity to modulus ratio than that of the unacidified one. This should be attributed to the partial peptization of swelling proteins and other rye compounds with solutions of organic acids.
INFLUENCE OF MODERN METHODS OF TESTING ON THE MECHANICAL PROPERTIES OF THE DOUGH AND THE QUALITY OF BREAD PRODUCTS
PRODUCTS
In recent years, in the USSR and abroad, work has been carried out that has shown the possibility of reducing the consumption of flour and time for the preparation of bread products. This is achieved by using technological schemes that provide for a mechanical effect on the dough and dough, activating their fermentation. Such schemes are based on the use of large liquid (about 70% moisture) or thick (40-50% moisture) doughs.
Liquid sponges have a viscosity that is 1-2 decimal orders lower than thick ones; the latter are difficult to pump up; they are diluted with water after fermentation. It has been established that diluted sourdoughs have a viscosity significantly lower than undiluted ones of the corresponding moisture content; during fermentation, the viscosity of the dough decreases.
Reducing the duration of the fermentation of dough and dough is achieved by a longer intensive effect in the kneading process. At the same time, the amount of gluten proteins washed from 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. These processes increase the volumetric yield of dough and bread, improve the structure of the porosity of the crumb, the shape of the hearth products.
These characteristics of bread products are also improved by additional mechanical processing of the dough in the process of cutting it. However, excessive machining can lead to a deterioration in the physical and mechanical characteristics of products, so its optimization is necessary. As a criterion for the degree of mechanical impact on the dough during kneading, the value of specific work is proposed. It varies depending on the moisture capacity of flour from 12 to 50 J/g.
Based on the foregoing, the following conclusions can be drawn.
Fermenting dough, unlike non-fermenting, is a more complex doubly strained colloidal dispersed system, including a gas phase, which therefore has a reduced density. Its foamy porous mass, continuously forming CO 2 , increases the volume - coalesces due to equalization of the pressure of neighboring pores of various sizes, forming an open structure; in it, according to the Stokes law, the movement of the largest pores upwards to the surface of the dough and the release of carbon dioxide continuously occur. In the process of formation of pores, increase in volume by small stresses and slow shear deformations, the structure of the fermenting dough is elasticized, increases the viscosity and η/E.
Fermented dough made from wheat flour of grades I and II differs from non-fermenting dough in lower shear moduli, relative plasticity (higher elasticity), higher viscosity and viscosity-to-modulus ratio, as well as stability and increase in these characteristics during fermentation after kneading. More significant differences were established for dough made from grade I flour, which has a 3-4% lower moisture content than dough made from grade II flour, and a different chemical composition.
The fermenting dough made from wholemeal and peeled rye flour differs from the non-fermenting dough in greater shear moduli, lower viscosity and viscosity-to-modulus ratio. This is due to the influence of a significant concentration of organic acids in it, which partially dissolve swelling proteins and other grain polymers.
Structural and mechanical properties of fermenting 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-retaining ability of the dough, the volumetric yield of the tin, as well as H/D of hearth bread. With a decrease in the flour grade, the viscosity and the ratio of viscosity to the modulus 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 I and II flour grades.
Within each grade, the viscosity of the fermenting dough has an inverse effect on the development of its volume (gas retention), the volumetric yield of the bread and a direct effect on the H/D of the bread. The ratio of viscosity to dough modulus has a direct effect on both indicators of bread. Grain variety in some cases affects the structural and mechanical properties of the dough from flour of each variety.
The listed properties of the fermenting dough in order to control and manage them, it is advisable to normalize and regulate. As approximate norms for dough made from grade I wheat flour, rye wholemeal and peeled flour, you can use the results of Table 4.4.
THE EFFECT OF HEATING ON THE MECHANICAL PROPERTIES OF THE DOUGH. MECHANICAL PROPERTIES OF BREAD
The process of production of 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 conformations 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 tension of the gravitational forces of the mass. Then the plastic-elastic structure of the dough turns into an elastic-brittle plastic jelly-like structure of the bread crumb. It should be assumed that its plastic deformations take place mainly at low strain rates due to stress relaxation, and at high rates as a result of brittleness, destruction of the continuity of the walls of the pores of the concentrated protein-starch jelly - crumb in the elastic region. In this regard, when studying the mechanical properties of a bread crumb, one should limit oneself to possibly small values of its deformations and their speeds. Instead of shear deformations, it is advisable to use deformations of uniaxial compression of the porous foamy structure of the crumb.
Heating enhances the thermal movement of the 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 is the 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 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, change the mechanical properties of the dough. Therefore, one would expect that the application of the Arrhenius equation for the structure of the dough is valid 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 a bread crumb proceeds in two main stages. In the initial stage of heating the dough to 50-60°C, the enzymatic systems of the dough are activated, the content of water-soluble compounds in it increases, which can plasticize the structure and, simultaneously with an increase in molecular-thermal movement, reduce viscosity, enhance its adhesive properties. At this stage, the main processes of bread baking also begin: starch gelatinization 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 its enzyme systems are also inactivated.
Union of Soviet
Socialist
Republics (697926 (51) M. Cl. 2
G 01 N 33/10 a 01 S 11/1B
State Committee
USSR for isothere and discoveries (53) UDC 532. 137. (ОЯ8.8) (72) Inventors
P.V. Kazakov, V.I., Denisov, F,.N. Lukach and G. A. Alpatova (71) APPLICANT All-Union Scientific Research Institute of the Bakery Industry (54)
The invention relates to a method for determining the viscosity of the dough and can be used in the baking industry.
A known method for determining the viscosity of the product by immersing in it the sensing elements of a tuning fork with a given frequency of self-oscillations and amplitude and measuring the attenuation frequency by the difference of oscillations at the beginning and end of a certain period of time (1).
However, it is not possible to accurately measure the viscosity of the dough in the known manner.
The aim of the invention is to increase the measurement accuracy. To do this, the sensitive elements are immersed in the dough à 30-33% of their length, the frequency is measured within 2-3 minutes after they are immersed, while the frequency of self-oscillations is chosen within 10-250 Hz, and the amplitude is ” 2-3 mm.
Example A sample of dough 25 with a mass of 150 r is taken and placed in a metal beaker, which is then placed in a thermostat to hold the dough for a long time at 30-32 C and relative humidity 80-85%. The test is kept for 7 minutes to align its structure. After that, elastic steel rods with a cross section of 0.8 mm are immersed in the test dough, which act as sensitive elements that are attached to the ends.
V-shaped tuning fork operating in self-oscillation mode. The rods at a frequency of self-oscillations of 250 Hz and an amplitude of 3 mm are immersed, - in the dough for
1/3 of their length. The change in the frequency of self-oscillations is recorded immediately after the sensing elements are immersed in the dough and after 3 minutes of their being in the test. The time of 3 minutes was chosen based on the condition that the fermenting dough changes its structure over time, being saturated with carbon dioxide, but remains stable in the first 3 minutes.
Then find the difference in the readings of the device at the initial moment n after 3 minutes.
The viscosity of the test is judged by the obtained difference in the frequencies of self-oscillations in relative units. An empirical relationship has been established between the indices of changes in the frequency of self-oscillations and dough viscosity.
For example, let us assume that the value of the self-oscillation period of the tuning fork immediately after the sensing elements are immersed in the dough is TI = 0.005427 s, 697926
Claim
Compiled by I. Vyrazheikina
Editor V. Trubchenko Techred Z, Fanta Proofreader I, Pojo
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Branch of PPP "Patent, Uzhgorod, Proektnaya st., 4 a after 3 minutes of keeping them in the test T = 0.005207 s, i.e., T = T" - T =
220 10 s, which corresponds to the viscosity u = 4.8 Pas.
This method for determining the viscosity of the dough can also be used for determining in-line with continuous testing.
1, A method for determining the viscosity of a dough by immersing in it the sensitive elements of a tuning fork with a given frequency of self-oscillations and amplitude and measuring the attenuation frequency by the difference in oscillations at the beginning and end of a certain period of time, characterized in that, in order to increase the measurement accuracy, the sensitive elements are immersed in the dough 30-33% of their length, the frequency is measured within 2-3 minutes after their immersion, while the frequency of self-oscillations is chosen within 10
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