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 transform into plastic deformation. 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 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.
The compacted pasta dough entering the matrix is an elastic-plastic-viscous material.
Dough elasticity is the ability of the dough to restore its original shape after quickly removing the load; it manifests itself under small and short-term loads.
Plasticity is the ability of dough to deform. Under long-term and significant loads (above the so-called elastic limit), pasta dough behaves like a plastic material, i.e. after removing the load, it retains its given shape and is deformed. It is this property that allows raw pasta of a certain type to be formed from dough.
Viscosity is characterized by the magnitude of the adhesion forces between particles (cohesion forces). The greater the cohesion forces of the dough, the more viscous (strong) and less plastic it is.
Plastic dough requires less energy to form and is easier to shape. When using metal matrices, more plastic dough produces products with a smoother surface. With increasing plasticity, the dough becomes less elastic, less durable, more sticky, sticks more strongly to the working surfaces of the screw chamber and screw, and raw products from such dough stick together more strongly and do not retain their shape well.
Rheological properties of compacted dough, i.e. the ratio of its elastic, plastic and strength properties is determined by the following factors.
As the humidity of the dough increases, its plasticity increases and strength and elasticity decrease.
With increasing temperature of the dough, an increase in its plasticity and a decrease in strength and elasticity are also observed. This dependence is also observed at temperatures above 62.5 °C, i.e. exceeding the gelatinization temperature of wheat starch. This is because the pasta dough does not have enough moisture to completely gelatinize the starch at the specified temperature.
With an increase in gluten content, the strength properties of the dough decrease and its plasticity increases. The dough has the greatest viscosity (strength) when the flour contains about 25% raw gluten. When the raw gluten content is below 25%, as the plastic properties of the dough decrease, its strength also decreases. Sticky, highly stretchy raw gluten increases the plasticity of the dough and significantly reduces its elasticity and strength.
With a decrease in the size of flour particles, the strength increases and the plasticity of the dough made from it decreases: dough made from bread flour is stronger than from semi-grain flour, and from semi-grain flour it is stronger than from semolina. The optimal ratio of strength and plastic properties is typical for particles of original flour ranging in size from 250 to 350 microns.
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 |
||||
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 largely due 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.
Below we consider the structural-mechanical (rheological) characteristics (effective viscosity h eff, plastic viscosity h pl, elastic modulus E 1, elastic modulus E 2, stress relaxation time t reel, relative plasticity P, etc.) for the dough of various bakery products (bread wheat, butter products, lamb, bagel, straws, 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 press and grease), dough resting, molding of dough pieces, as well as such technological factors as temperature, humidity dough, recipe, inclusion of additives and improvers. Examples are given of the use of rheological characteristics to assess the quality of semi-finished and finished products.
The presented material can be used by employees of design and engineering bureaus, engineers of the baking industry when modernizing old and creating new mechanical equipment, as well as by researchers and students in research and diploma works.
Recipe, main and additional raw materials
Viscosity value for different types of dough
The average viscosity values of various types of dough at 30 °C and atmospheric pressure are given in table. 6.19.
Table 6.19. Average viscosity values of different 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 | ||
| Bread made from flour | ||||
| 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 crispbreads | - | 1,0 | 38,0 | 6 10 2 |
| For flatbreads | Elastic-visco-plastic | 2,0 | 41,0 | 1 10 4 |
The viscosity of flour dough ranges from 0.5 to 2000 kPa s with a humidity of 17.0 to 45.7%. Different types of dough belong to different classes of rheological bodies, which makes it necessary to select in each case the appropriate design equation when describing the flow of a given type of dough in technological machines.
Yeast-free dough
When producing test semi-finished waffles, a liquid dough is used, which differs from conventional bakery dough in the absence of yeast and the presence of a large amount of sugar and milk.
Research () was carried out on a reconstructed viscometer
RV-8 with the following parameters: shear rate 0-9 s−¹, dough humidity 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. Increasing humidity and temperature leads to a decrease in viscosity.
The nonlinearity of the obtained dependencies allows us to conclude that the dough under study 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 indicated value, by a linear law. 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-0.032TI-4.043W+0.0359WІ.(1)
Equation (1) is valid for the following intervals of changes in arguments: 0.5 s –1 £g£7.0 s - 1 ; 31.8%£W£40.0%; 15°C£T£30°C.
When developing 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 being studied.
For this purpose, experiments (12) were carried out to determine the viscosity of the dough at different humidity levels. To prepare the dough, commercial wheat flour of the highest and first grades was used. 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 as follows: the upper limit (44.5%) is equal to the moisture content of wheat dough made from grade I flour accepted at the bakery; the lower limit (65%) was chosen due to the fact that many works note the promise of the method of preparing wheat dough for liquid dough, which has a number of advantages.
Viscosity was determined using a Reotest-RV rotational viscometer (GDR). The strain rate was varied from 0.167 to 1.8 s -1 . The average results are shown in Fig. 59.
Rice. 59. Dependence of the viscosity of dough made from grade I flour on its moisture content at different 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 dependencies are exponential. As the humidity of semi-finished products increases, their viscosity decreases significantly. Thus, for a shear rate of 0.167 s -1, when the humidity changed from 46 to 50%, the viscosity decreased by approximately 3.5 times. With increasing shear rate, the intensity of the change in viscosity 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 viscosity anomaly of the baking dough plays a significant role.
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 made from grade I flour 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 shear rates from 0.167 to 1 s? and dough humidity ranging from 44 to 62%.
Wheat flour grinding coarseness
Table. Dependence of elastic-plastic characteristics of dough on the coarseness of grinding wheat flour
| Grinding fractions | Raw gluten content, % | Elastic modulus, E | E, With | |
| in 30 minutes | ||||
| Pass through sieve 43 | 43/39,5 | 4,2/9,1 | 7,0/6,9 | 60/132 |
| Pass through sieve 38 | 38/39,3 | 3,2/8,4 | 3,5/4,7 | 91/179 |
| Pass through sieve 25 | 25/38,1 | 3,0/6,8 | 3,3/4,3 | 91/157 |
| Escape from the sieve | 25/37,5 | 2,6/6,4 | 2,9/4,0 | |
| An inverse relationship between dough viscosity and shear modulus and flour particle size has been established. This pattern is partly due to an increase in gluten protein content with a decrease in 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 | Liquefaction coefficients | |
| K η | K E | |||
| in 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 butter dough with different sugar and fat contents (at 20 °C)
| Dough | Humidity, % | E, Pa | η, Οа·с | 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 added sugar and fat on the mechanical properties of flour dough depends on its moisture content. Significant additions of protein compounds, sugars and fats to wheat dough made from high-quality flour significantly change its structural and mechanical characteristics. By adding from 5 to 50% sugar to flour, plasticization of the structure of wheat dough is achieved - a decrease in the shear modulus and viscosity; Elastication of the dough is observed 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, % | E·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 added sugars, due to the increased content of water-soluble compounds, has increased plasticity and liquefies. Dough aged for 2 hours has a low dough viscosity and its relative elasticity increases. Adding 5–20% sugars to the dough significantly reduces its viscosity and even more noticeably the shear modulus: the relative elasticity increases, and the plasticity decreases; with increasing dosage of sugar, this effect increases. The effect of added sugars on the structure of unfermented dough aged for 2 hours is similar to their effect on the structure without resting. 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. Effect of combined addition of sugar and fat on the structural and mechanical characteristics of dough made from grade I flour
| Experience Option | Sample | Humidity, % | E·10 –2 , Pa | η·10 –4 , Pa·s | η/ E, With | P, % | E, % | K E, % | Gradient E | K η, % | Gradient η |
| Unfermented 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 for freshly mixed dough, and the denominator shows data for a two-hour test.
Sugars more strongly reduce the shear modulus and viscosity of both types of dough; more significantly than fats, they increase the ratio of viscosity to modulus of non-fermenting dough; Compared to fats, they less actively reduce this important characteristic of fermenting dough. The combined addition of sugar and fat will have the most significant effect not so much on the elastic-plastic properties, but on the relaxation properties of fermenting wheat dough. The combined addition of sugar and fat to non-fermenting dough does not improve, but rather worsens, its baking properties; and during fermentation it slightly increases the viscosity and reduces the shear modulus.
Assessment of baking properties of wheat flour. (1 part)
The term “strength” of flour used is actually synonymous with the quality of flour, its physical properties. Flour is considered strong if it is capable of absorbing a relatively large amount of water during kneading and at the same time forming a dough that stably retains its shape, does not stick to hands and machines, and does not spread when cutting and baking. Good wheat flour produces aromatic, tasty, fluffy bread (of regular shape, covered with a smooth, shiny, browned crust, with an elastic, evenly loosened, finely porous crumb. Predicting and ensuring high quality bread is possible only by taking into account the baking qualities of flour, which depend on protein-doproteinase and carbohydrate-amylase complexes of flour. The term "protein-proteinase complex" means flour proteins (mainly gliadin and glutenin), proteolytic enzymes that hydrolyze them, as well as activators and inhibitors of proteolysis. The concept of "carbohydrate-amylase complex" includes sugar , starch and amylases that hydrolyze it.
Protein-proteinase complex. The protein-proteinase complex, and above all gluten, is the main factor determining the strength of flour. Wheat flour gluten is a highly hydrated complex consisting mainly of the proteins gliadin and glutenin. Their ratio, according to V. S. Smirnov, in gluten from premium flour ranges from 1: 1.6 to 1: 1.8. With an increase in flour yield, it decreases and in gluten from 2nd grade flour it ranges from 1:1.1 to 1:1.2. Both of these proteins are heterogeneous, each consisting of several fractions.
Gliadin has a molecular weight from 27,000 to 65,000. Swelling in water, it forms a relatively liquid syrupy mass, which is characterized by a sticky, viscous, highly extensible and non-elastic consistency.
Glutenin the molecules are larger, their molecular weight ranges from hundreds of thousands to several millions. Hydrated glutenin forms a rubber-like, short-extensible mass with high resistance to deformation, elastic and relatively tough.
Raw gluten combines the structural and mechanical properties of these proteins and occupies an intermediate position: glutenin is the basis, and gliadin is its gluing principle.
In raw gluten, the proportion of water is 64-70%. In addition to water, proteins firmly hold small amounts of starch, sugar, lipids, and mineral elements. In gluten, non-protein substances are (in% of dry matter): from premium flour - 8-10; 1st - 10-12; 2nd-16-22. It has been established that lipids, carbohydrates and mineral elements are in gluten in a chemically bound state - in the form of lylo- and glycoproteins, and starch and shell particles are retained mechanically. The lipids that make up gluten affect its properties. Their action is explained by the fact that unsaturated fatty acids, oxidizing and forming peroxides and hydroperoxides, promote the oxidation of sulfhydryl groups - SH with the formation of disulfide bonds - S - S -, which strengthen the intramolecular structure of the protein, making it more dense. Disulfide bonds are formed both within one protein molecule and between different molecules of gluten-wine proteins. A certain part of the lipids remains unbound with proteins and serves as a lubricant between protein molecules, giving gluten additional elasticity.
The properties of gluten and methods for their determination are regulated by the standard that regulates amount of gluten. The content of raw gluten should be (in % of the weight of flour, no less): in semolina - 30, premium - 28, 1st - 30, 2nd - 25, wallpaper - 20.
Gluten quality characterized mainly organoleptically by color and smell, as well as firmness, elasticity and extensibility. Good quality gluten has a white color with a yellowish or grayish tint and a faint, pleasant floury odor. Reduced quality gluten has a gray color, sometimes with a brownish tint, and an unpleasant odor.
Good quality gluten is elastic, cohesive, after deformation it quickly restores its original shape and does not stick to your hands. Bad gluten is not elastic, sticks to your fingers, and has a smeared, sometimes spongy or crumbly consistency.
Gluten is considered strong if a piece of 4 g stretches less than 10 cm, medium stretch - from 11 to 16 and weak - more than 16 cm.
The standard divides gluten into three groups according to the above indicators: I - good elasticity, long or medium extensibility; II - good elasticity and short elongation or satisfactory elasticity, short, medium or long elongation; III - weak elasticity, strongly stretching, sagging when stretched, breaking in weight under its own weight, as well as inelastic, floating, incoherent.
The quality of gluten can be fairly objectively indicated by its hydration ability. According to G.N. Pronina, it varies (in% of raw gluten): for premium flour - from 175 to 188, 1st - from 172 to 197 and 2nd - from 166 to 186.
Definition dry gluten (in % by weight of flour on dry matter) eliminates the influence of fluctuations in flour moisture content and the hydration capacity of gluten, therefore it characterizes flour more objectively and is more closely correlated with protein content. Dry gluten content (in%): in premium flour - 9.4-10, ZG 1st - 10.2-12.7; 2nd - 8.7-11.7.
Baking ball from 2 g of gluten makes it possible to predict the volumetric yield of bread to a certain extent. A good quality gluten ball has a volume of 4.5-5.5 cm 3, and the ratio of its height to diameter is 1.1-1.2.
Ball spreadability from 10 g of raw gluten, determined at a temperature of 30 ° C, for one, two and three hours of proofing, fairly objectively reflects the quality and indirectly indicates the activity of proteolytic enzymes. The diameter of the balls (half the sum of two perpendicular measurements) of average quality gluten is approximately equal (in mm): at the beginning of determination - about 30; after 1 hour - from 40 to 50; after 2 hours - from 50 to 55; after 3 hours - from 55 to bO.
Characterization of gluten quality can be carried out using instruments, the most common is the IDK-1 gluten deformation meter, in which a force P = 1.18 N is applied to a gluten ball weighing 4 g for 30 s. The deeper the instrument punch is immersed in the gluten, the she is weaker. I. M. Reuter gives the following gradation of gluten quality (H def - quality criteria in instrument units): strong - 60-70, average - 71-80, satisfactory - 81-100, weak - more than 100. If the result obtained on the IDK -1, multiplied by 0.2, you get the extensibility of gluten in centimeters.
Thus, studying the quality of gluten using standard and additional methods allows us to characterize its properties quite objectively and comprehensively. However, the process of gluten laundering is influenced by many factors, including temperature and water hardness, duration of laundering, amount of water consumed, etc. In addition, gluten-wine proteins are isolated from the natural environment, and therefore their properties do not completely coincide with the behavior of their in the test. Therefore, although studying gluten is somewhat faster and easier, determining the strength of flour based on the properties of the dough gives more reliable results.
Proteolytic enzymes are the second component of the protein-proteinase complex; in healthy wheat grain they have relatively low activity. However, in defective grain and flour made from it it increases sharply. Proteases, acting on gluten, reduce its elasticity and increase fluidity. Proteolysis is not always accompanied by the formation of free amino acids, i.e., destruction of the primary protein structure. In the initial stage, proteolysis affects the tertiary and quaternary structures of the protein molecule, causing its disaggregation and the formation of polypeptides.
Inhibit (slow down) proteolysis oxidizing agents capable of oxidizing sulfhydryl groups to disulfide groups.
Proteolysis activators are reducing agents that destroy disulfide bridges between protein molecules and thereby weaken gluten. Flour and yeast, especially old ones, contain the tripeptide glutathione, which has a strong reducing effect. The amino acid cysteine has the same property. Special studies of the activity of proteolytic enzymes when evaluating flour are not carried out. Their activities are judged by the quality of gluten and the structural and mechanical properties of the dough.
Characteristics of the “strength” of flour based on the structural and mechanical (rheological) properties of the dough. The dough is a hydrated colloidal complex - a polydispersoid. It has a certain internal structure and peculiar continuously changing structural and mechanical properties. Methods that allow them to be characterized simultaneously characterize the “strength” of flour.
Determining the “strength” of flour by the spreadability of a ball of yeast-free dough suggested by prof. L. Ya-Auerman. Using this method, dough is kneaded with a moisture content of 46.3%; 100 g of dough is rolled into a ball and kept for one, two and three hours, taking into account not only the properties of gluten, but also the total effect of protein substances, proteolytic enzymes and non-starch polysaccharides on the rheological properties of the dough. After 3 hours of resting, the diameter of a ball of dough made from strong flour increases to no more than 83 mm, medium - to 97, weak - more than 97 mm.
Determination of the “strength” of flour based on the consistency of the dough is carried out using a consistometer (penetrometer). At the same time, the structural and mechanical properties of the dough are studied, which are used to judge the activity of proteolytic enzymes that cause gluten disaggregation and a decrease in its elasticity. For testing, dough is kneaded at a constant moisture content for each type of flour. Keep it in a thermostat at a temperature of 35 °C for 60, 120 and 180 minutes (Ko, Keo, Ki20 and Kieo) and determine the depth of punching of the dough with a punch under the influence of a force P = 50 g (0.49 N). The deeper the punch is immersed in the dough, the weaker the flour and the higher the K value in the standard units of the device. So, in good quality 1st grade flour, Ko does not exceed 100, Kbo - up to 120, Ki20 - up to 150 and Kieo - up to 180.
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