Baking cookies Baking cookies

Baking cookies

When you set the baking parameters of the conveyor oven is important to understand not so much the temperature as the heat transfer process. The essence of the final production of pastry is baking and drying. Produced in ancient times was a dried biscuit slices of bread (crumbs) suitable for seafarers as food with a long shelf life.
Later, from simple types of dough consisting mainly of flour and water, the flat billets for baking and drying began to be molded, getting products known as biscuits. Their drying was very important to ensure a long shelf life. Past cooks, making products from flour, fat and sugar, found that if small test buns are baked in a conventional hot oven and removed when they acquire a good color and stable texture, they will not be dry enough in the center to be completely cooled Crispy. Putting these baked blanks back into the oven at a slightly lower temperature for drying improved the consumer properties and increased the shelf life of the finished products. This, apparently, led to the appearance of the name "biscuit", meaning "twice baked." Baking first in a colder oven for a longer time allows the products to dry, but leads to poor coloration of the surface and poor development of the structure. The idea of ​​separating the regulation of humidity from regulating the development of internal structure and color returned with the advent of modern electronic technology.
Baking involves heating the dough. Heat transfer is carried out by convection, heat conduction and radiation, and these processes are difficult to understand and evaluate if only temperatures can be measured in the furnace in some places. Heat (heat) and temperature are not the same. Heat is energy, measured, for example, in calories. The result of the heating is a change in temperature, which is measured in degrees (° C). To illustrate the situation, analogy with climatic conditions can be useful. Imagine a man standing on a sandy beach on a hot, sunny day. This person is warm (due to heat radiation from the sun); The soles of his feet feel the heat of the sand (thanks to the thermal conductivity), and the air around the person warms it by convection, since the air is slightly, but moves. The temperature of air and sand is the same. A person sweats and loses moisture is a natural process that serves to cool the body, as the body takes a lot of heat to evaporate water. If the sun goes down behind a cloud, a person feels that it's not so hot, as the heat transfer by radiation becomes lower, but the temperature of the surrounding air does not change. If a person is standing on a towel or a wooden board, his feet will not be so hot, as heat transfer due to thermal conductivity will not be so fast, but the surface temperature has not changed. If the wind starts to blow, the person will become hotter (but the air temperature has not changed), and he will begin to lose more moisture (although an increase in evaporation can make a person feel that it has become cooler).
When baking, we can measure only the temperature in the oven without difficulty, and the amount of heat transferred is related to the difference in temperatures of the two bodies, however a very hot surface (for example, burners) will give more radiant heat than the internal surface of the furnace, the steel tape of the hearth will allow due to the thermal conductivity Give the test piece more heat than the wire mesh tape, and moving air in the furnace will provide significantly greater heat transfer due to convection than stationary. Therefore, considering the temperature profiles in the oven when baking, it is also necessary to take into account the mechanisms and the values ​​of the heat transfer that occurs.
The simplest furnace is a heated box with a door. The temperature in it can be set, but when the door opens to place test billets in the oven, the temperature drops slightly, and then gradually rises to the previous preset level. Biscuits are now mostly baked in conveyor stoves, which can be compared to a long box with an entrance on one side and an exit on the other. Blanks move inside on tape or on sheets. The great difference between a static furnace and a conveyor belt is that in a conveyor furnace, the temperature and heat transfer conditions can be varied along the length of the oven, i.e., actually during the baking period. Therefore, in one part of the furnace there may be higher temperatures, strong radiation and weak convection, and in the other - a lower temperature and strong convection. To develop the structure, color the surface and the mass fraction of moisture in the baked cookies, the rate of heat transfer to the test billet is very important, and the skill of the oven is precisely in its choice.
When baking, a lot of water evaporates from the test billet. If the furnace has direct gas heating, a lot of water is generated in the furnace from the burnt gas. The amount of water vapor in the furnace atmosphere is regulated by the exhaust system. If the atmosphere in the first part of the furnace contains a lot of water vapor, some water can condense on the surface of the cold test pieces when they enter the furnace. This leads to the following. First, the surface becomes wet, which can lead to the dissolution of sugars, etc., and secondly, the heat released during the transition of water from the vapor state to the liquid (latent heat) heats the surface of the dough, and some of this heat Due to thermal conductivity, will fall into the center of the test billet. Soon this water evaporates again, but the moment when the surface becomes dry and firm will come later than if there were less water vapor in the furnace atmosphere. To heat 1 g water at 1 ° C, 1 calories of energy are required. To convert 1 g of water at 100 ° C (boiling point) to steam, 540 cal is required, and vice versa, when 1 g of steam at 100 ° C condenses, it releases 540 cal. This is the latent heat of evaporation, which indicates that a very large amount of energy is required to dry the cookie.
The temperature profile in the oven cookie does not give the full information necessary to know exactly how the proceeds pastries. The recorded temperatures are useful only as part of the overall characteristics of the conditions in the furnace.
The duration of baking cookies is very small and amounts to 2,5-15 min with an average value of about 6 min, and in a static furnace it is usually impossible to quickly change the temperature, and therefore the results of baking in these furnaces often do not coincide with conveyor furnaces. Experimentation to determine the optimum baking conditions in a conveyor furnace requires constant feeding of the same test pieces, which is potentially a long and expensive procedure. This leads to the fact that the possibility to change the conditions in the oven with baking is not always used to achieve optimal conditions for a certain type of product. Baking often remains a kind of "magic".
The conditions necessary for different types of products are different, since the development of the structure and the amount of moisture to be removed depend on the formulation. Although the cost of fuel for heating the furnace is a small fraction of the total cost of production of MCI (usually about 5%), there is an increasing desire to increase the efficiency of the furnace and use less fuel. When designing the furnace, one should take into account not only the efficiency of heat transfer and the maintenance of stable conditions, but also the design, since the furnace is a large complex of equipment that can be very expensive. Heat recovery systems for utilizing used heat from furnace chimneys are not very common, because the heat produced is of "low grade". Nevertheless, more concern arises about the pollution of the atmosphere, and until recently, to regulate the release of heat from the baking oven, not much has been done. If there is a requirement to clean gases from the furnace, it can be expected that the newly created systems will include heat recovery.
There are many different constructions of furnaces, and constantly new developments take place. It is impossible to exhaustively describe the operation of all types of furnaces and manage, because it would have required many technical details. Therefore, below we will try to give a general description of the changes taking place in baking MKI, and show how these changes can be accomplished in the furnaces of different designs using the main types of fuel - gas, petroleum products and electricity.
Changes dough pieces for baking
When baking dough pieces can be seen three major changes:
  • a significant reduction in the density of the product associated with the development of an open pore structure;
  • reduction of the moisture content of up to 1-4%;
  • changing surface staining (reflectance).
Although these changes are considered to be independent of each other and consistent (when the product is moving in an oven at about the above order), it will be shown that there is significant overlap of physical and chemical changes. For convenience, we consider them separately. The changes depending on the duration of baking are shown in Fig. 38.1.
38.1Fig. 38.1. Generalized changes in baking dough pieces
The development of structure
The development of the structure occurs mainly in the first quarter or a third of the baking period (Figure 38.2). All changes taking place depend on the temperature, some characteristics of the formulation and the shape of the test billet. When baking, gas bubbles and water vapor form, which expand and lead to a significant reduction in the density of the dough. It is the open porous structure that gives the liver a pleasant texture. The development of the structure is often called "lifting in the furnace." The conditions for achieving the maximum lift maintained further during baking are not fully understood, but changes in the test billet include:
  • heating the starch to its gelatinization and protein denaturation;
  • the formation of gas by chemical leavening agent;
  • expansion of the gas bubbles formed as a result of increase in temperature and the increase in their vapor pressure;
  • gap and fusion (coalescence) of some of these bubbles;
  • moisture loss from the surface of the product due to the evaporation of moisture followed by migration to the surface and continuing access to the furnace atmosphere;
  • increasing the concentration of the sugar solution with increasing temperature;
  • reduction consistency sugar solutions and fat with increasing temperature.
38.2Fig. 38.2. Changes in baking [8]
The most important changes are related to the formation of gas bubbles and their expansion in an environment that at first becomes softer and more elastic, and then more dense and rigid. Many studies have shown that an increase in volume due to the gas released by the disintegrant does not explain the amount of lifting of the products in the furnace. The gas released by the disintegrant can explain up to half the volume increase. The formation of water vapor also contributes, because when water turns into steam, there is a sharp increase in its volume. Although the expansion in essence should be due to water vapor, the term steam is misleading, since this concept should be associated with water vapor at a temperature above 100 ° C. The answer gives an examination of the physical essence of water vapor pressure. In Fig. 38.3 shows an increase in the volume of air (or other gases - for example, carbon dioxide) - dry and saturated with water vapor. Since the structure of the product is fixed long before reaching the test blank 100 ° C, it is now clear how the water vapor participates in the expansion.
38.3Fig. 38.3. Expansion depending on air temperature and dry air, saturated steam
This significant increase in volume due to increased vapor pressure with increasing temperature of the test structure is limited, since due to the surface tension in small bubbles pressure much higher than large ones. Thus, with increasing temperature in the test there is a very unstable physical situation, as there is a delicate balance between a) extension that before rupture may undergo softening complex starch / protein / water / sugar, b) merge (coalescence) of bubbles and c) increase stiffness as solidification of the gel.
With the inevitable loss of water vapor through the surface of the test billet, a dense crust is first formed on it. In addition, when the surface dries, the heat does not pass through the dough due to heat conduction as easily. The center heats up more slowly, and this slows down the development of gas bubbles. This means that heating the center of the test billet at the initial stage of baking (before drying and hardening of the surface) is very important. Therefore, it can be assumed that radiant heat and heat due to thermal conductivity from the tapping tape is relatively important for heating the center of the test billet at the beginning of the baking process.
Form the structure of the bubbles tend to be the largest in the center of the dough pieces and the smallest - in the peripheral layers where the crust is formed. An extreme example of such a structure can be seen in pita (Arabic bread with "pocket"). This product is prepared from a thin piece of dough, which is placed in a very hot and dry kiln. The crust is formed rapidly, and the dough swells almost to form a ball, and then there is its rupture and subsidence. The product thus has two layers of crust to break a line passing through the center (like an empty shell oysters).
To obtain a more uniform texture, the formation of the crust should be delayed, and at the center of the test billet, the vesicles should merge as little as possible. This example of the opposite action is the baking of special breadcrumbs (for sausage stuffing), where very slow heating and minimal formation of gas bubbles are combined. Since a small amount of baking powder is used (or it is not used at all), the crust is formed very slowly, the gas bubbles are very small and their fusion is insignificant. The structure becomes rigid, and the size of the bubbles and, consequently, the texture is very uniform throughout the baked and dried material.
There are two main forms of the structure of MKI: structures that require the formation of more or less the same bubbles, and the structure where a few large cavities are formed. Examples of structures of the second type are water cookies, cream crackers and flaky cookies. The baking conditions necessary to obtain these two types of structures are very different and are determined by the formation of a different number and type of gas bubbles, which then expand due to water vapor. Large gas bubbles in crackers and puff pastry are formed from breaks in the test formed by layers of fat, or layers of less damp dough formed during lamination or lamination. Rapid heating of the test piece leads to a large expansion of these long flat pores, which causes a bubbling and layered (flocculent) structure.
To obtain a more uniform structure with rounded cells, for most other types of articles, it is necessary to achieve a large degree of expansion to a significant fixation of the structure, which prevents further expansion. As already mentioned, the fixation of the structure occurs when the starch matrix is ​​gelatinized, protein denaturation and solidification due to loss of moisture. The moisture output from the surface of the cookie is related to the temperature, heat flow and water vapor pressure (humidity) on the surface. The latent heat of evaporation of water is great, so it takes a lot of heat to evaporate it. The concept of humidity in the furnace atmosphere can lead to a misunderstanding of baking conditions. No matter how much steam is present in the furnace atmosphere, which is at a temperature above 100 ° C, moisture will always be released through the surface of the test billet. Moisture output is delayed only if the surface temperature of the test billet is below 100 ° C and the microclimate on the surface is saturated with water vapor. In the first part of the furnace, where the structure develops, it is necessary that the heat enters the test billet as quickly as possible with minimal loss of moisture from the surface. These conditions will be considered below.
The starch gelatinization takes place in the temperature range of 52-99 ° C and to some extent depends on the duration. The proteins are denatured and coagulated at a temperature above about 70 ° C. Gas from chemical disintegrators is released at a significant rate when the temperature reaches about 65 ° C. From Fig. 38.3 shows that the increase in volume due to water vapor at a temperature above 70 ° C accelerates very quickly. The fats used for baking completely melt at temperatures well below the above. When different parts of the test piece reach temperatures above about 65 ° C, expansion and loss of elasticity occur simultaneously. If the expansion is too large, the structure breaks, and if it is too small, a tight closed structure is formed.
The development of the structure depends on a constant controlled increase in gas expansion until the gel matrix of starch and proteins stabilizes (does not solidify). There is always some limitation of the magnitude of the expansion due to the fusion of gas bubbles, which leads to a less stable structure. Destruction will also occur if the expansion occurs too quickly (if too much gas is released from chemical disintegrants) or the rate of heat transfer is too high. Both of these lead to an increase in the formation of large gas bubbles, less strong compared to small gas bubbles. If the dough undergoes a temperature reduction (for example, when passing from a warm zone to a colder one before the matrix solidifies), the gas bubbles decrease, and it is unlikely that the process of their increase can occur before the matrix solidifies.
Concluding the discussion of the structure, it should be noted the importance of impaling the test piece. Creating an air passage through the dough piece contributes to the formation of crust, which reduces the probability of formation of large bubbles, such as in pita bread (see. Above). The more and the faster rise in gas formation test workpiece temperature, the more important prick dough. If the holes of impaling located too far from each other, are often formed bubbles.
Products with a high content of fat and sugar have a lower dough moisture. This means that the proteinaceous substances are not sufficiently hydrated, and therefore the process of gluten formation occurs to a lesser extent. In addition, when the dough is heated, there is not enough water to gelatinize a large amount of starch. The structure is basically a matrix of sugar or some kind of iris mass. With increasing temperature, this matrix becomes softer, rather than stabilized; It is fixed only on cooling after the product leaves the oven, and therefore when baking a dough with a high sugar content, a very large expansion is observed, followed by considerable subsidence. The spreadability of the test billet and subsidence are the cause of the cracked surface of the product (for example, Ginger Nuts or crispy biscuits). The amount of spreadability for a particular formulation can be controlled to some extent by the temperature in the oven and the state of the belt tap. The dough, which is very delicate when baked, can not be baked on wire mesh, since the dough is deeply entrapped in it (it is best to use a flat steel tape). Some types of dough can be baked on a dense woven wire mesh.
moisture Reduction
Ideally, moisture loss should occur after fixing the structure of the workpiece, but this, naturally, can not be achieved throughout the entire volume of the test billet. Moisture can only escape through its surface, and therefore, due to the action of capillary forces and diffusion, migration of moisture to the surface must occur. Both of these phenomena are accelerated by temperature gradients, so at this stage of baking, the entire product must be heated rapidly to 100 ° C. If the surface heats up too much and dries quickly (for example, when there is a lot of air in the oven), the color change occurs prematurely, and therefore it is difficult to dry the biscuit sufficiently without excess surface staining.
When the gel from starch and protein loses moisture, some shrinkage occurs, and therefore some loss is inevitable products of lifting (Fig. 38.2). In most cases, due to the destruction of the gas bubbles and this fusion loss is small compared to the settling of the internal structure, but if continue heating, shrinkage will continue until carbonization.
The moisture gradient in the test billet increases during drying, and since the biscuit dries, the structure containing starch and proteins shrinks. While the cookie is hot, its structure is flexible enough to withstand the shrinkage stresses, but if a large moisture gradient persists after the cookie exits the oven, a phenomenon known as "product cracking" can occur. When the cookie is cooled, the moisture levels out, moving from the wetter areas to the drier ones, and the shrinkage stresses that develop here can cause cracking - this is cracking. The best way to prevent cracking is to ensure a low moisture content of the products (so that the moisture gradient values ​​are small).
Products with a large amount of fat or sugar have a plastic structure, and when the cookies cooled voltage are less pronounced in connection with which the crackers, and other kinds of semisweet biscuit with a relatively low content of fat and sugar is important to control the moisture content.
The required moisture content of the cookies is determined by two main factors. When the content is too low, it will have a biscuit "burnt" taste may be too dark. A too high moisture content will not crispy structure may cracking products during storage and reduction products consumer properties, particularly taste, can occur faster.
color change
Although baking occurs when a yellowish-brownish tinge appears, the term "color" is used here to refer only to darkening - reducing the reflectivity of the product surface. Color changes occur for a number of reasons. Maillard reaction (see section 10.6) - non-enzymatic formation of substances of golden brown color - involves the interaction of reducing sugars with proteins, in which substances with attractive reddish-brown hues are formed. This reaction occurs at a temperature of about 150-160 ° C and only in the presence of moisture. It is not possible to reheat the baked cookies to significantly enhance the color of the surface as a result of the Mayar reaction. While the test billet is relatively moist, to achieve these high temperatures, radiation heating is more important than convection heating. Color also develops due to dextrinization of starch and caramelization of sugars. At even higher temperatures, the product is charred or burnt.
If the structure is very open products, moisture migration to the surface is slower, so the local increase in the surface temperature, and hence the staining can be accomplished easily. For example, a well-developed puff pastry will turn lighter than the dense layer structure. An excess of alkali, typically resulting from too much sodium bicarbonate in the formulation to cause total yellowish staining of the entire product inside, that in cases where no other staining unattractive.
With continued drying coloration occurs due to these changes in thinner or more exposed parts of the product, which is accompanied by the development of bitterness. If this process continues inside the cookie, it is characterized by defects (damage). This cookie has a bitter and unpleasant to him.
The conditions in the furnace
The furnace is a hot chamber or tunnel designed to supply the desired heat flux to the test billets and to remove moisture. Heating is provided by burning fuel (gas, oil products) or using electricity. Heat is transmitted in three ways: by radiation, due to heat conduction and convection. All three methods are always in effect, although these or other technical means are used to separately enhance the effect of each of them. The furnace design should ensure fast and accurate temperature control under variable loads and heating in one of the three ways. All furnaces have some drawbacks, and the situation is aggravated by a lack of understanding of the effect of each heat transfer method at various stages of baking.
Test billets are placed on a surface, which is usually a steel sheet or a metal wire mesh. Upon entering the furnace, the test piece is exposed to heat as a result of a combination of thermal conductivity (through the feed belt), convection (from hot air moving in the furnace), and radiation (from hot furnace surfaces and hot burners). Emitted (infrared) heat with a certain wavelength does not penetrate deep into the test piece, and its quantity reaching the product is inversely proportional to the distance from the hot surface. A large fraction of the heat absorbed by the test billet is the radiated heat, but it is naturally larger where there are very hot surfaces (for example, burners), and if the furnace chamber is small (if the roof is low). The most effective way of heat transfer is convection, but moving hot air takes away moisture and very quickly dries the surface of the dough.
When evaporation from the moisture product and expand the cold air that enters the furnace with the product, the pressure in the furnace increases. In the case of furnaces with direct (internal chamber) heating, where the gas burns in baking chambers, the pressure increases even more. This pressure should be reduced, so air ducts (hogs) are provided with an outlet to the atmosphere. If the pressure increase corresponds exactly to the exit velocity from the ducts, the atmosphere in the furnace soon becomes very saturated with water vapor. It is often called humidity, but since the temperature in the furnace is higher than 100 ° C, the relative humidity used in measuring the moisture content in meteorology is not applicable here. The amount of moisture present should be determined by the ratio of the mass of moisture and the mass of the air present (for example, the mass of water to a given air mass), or through the dew point, the temperature to which air must be cooled to start condensation.
The release of hot gases through the air ducts causes heat loss, and therefore much effort has been exerted to create means limiting the rate of escape to a minimum. Although during the baking process the drying of cookies takes place in a kiln with a very humid atmosphere, it should be understood what exactly happens in this case. On the surface of the cold test billet entering the humid atmosphere, water condenses, which retards the drying of the surface and requires for its evaporation a large amount of heat entering the billet due to thermal conductivity and very quickly heating it. Then, as the temperature of the dough rises, this water evaporates again, slowing the rate at which the surface temperature rises. Moistening the surface of the dough at the entrance to the furnace, where the atmosphere of the furnace is saturated with water vapor, contributes to a general rise in the temperature of the test billet and maintains the elasticity of the "crust" longer than in a dry atmosphere. Nevertheless, excessive condensation on the surface of the dough can cause uneven staining and the appearance of spots in the areas of sugar dissolution. If the moisture film is not excessive, dissolving the sugar on the surface later when baking can give the product a pleasant appearance or gloss.
Humidity in the furnace can be specifically increased by injecting steam. Drying test blank surface may be delayed by spraying with water before being placed in the oven. As for the water vapor content in the furnace atmosphere, except wet furnace atmosphere influence is difficult to be sure of its advantages or disadvantages for baking. Reducing the speed of the gas outlet of the furnace to increase the humidity in it can also reduce the convection and radiation level of the burner (because their power is reduced), and hence the rate of heat transfer.
Theoretically, the moisture content in the furnace atmosphere should not affect the rise in the furnace, however some tests show that it decreases with a high moisture content. One aspect of this issue is the nature of the open burning of gas in a very humid atmosphere of the furnace. Although air is mixed with the gas before its combustion, oxygen must be present to completely burn around the flame. If the atmosphere of the furnace is very wet, there is a possibility of a lack of oxygen, and consequently, with the change in the flame, the amount of heat radiated by it decreases. Probably, an unconscious understanding of this caused the operators to insist that for the baking of crackers open chimney flaps are necessary in the first zone of furnaces with direct (internal chamber) heating. The position of the flaps, apparently, should be based on determining the oxygen content in the furnace, and not on the "humidity".
It would be ideal if the test piece could be heated evenly and quickly to lose a large amount of moisture. This means that the movement of air near the surface of the test billet should be minimal. In traditional ovens it is impossible to heat the surface quickly enough (and at a sufficiently low temperature to prevent staining) only by means of thermal conductivity and radiation. To promote heat transfer due to thermal conductivity, it is highly desirable to preheat it before placing the test pieces on the tape.
After the dough piece has expanded and its structure is stabilized, it is necessary to direct the process to remove moisture. Air movement will maintain this favorable temperature and humidity test workpiece surface. When dried products, however, the heat transfer from the surface to the center of a wet by conduction becomes more difficult, and there is high humidity gradient. The drying step (before the surface is dry) the temperature can be set so as to achieve the desired coloration of the product.
It is important that during baking, the atmosphere in the oven is kept constant across the oven, otherwise the test billets, once baked, will turn into products of different thickness, shape or color when entering the oven. In many furnaces, the movement of gases is not ideal. A common problem is that the atmosphere that replaces the air leaving the air ducts enters the furnace mainly from the two ends of the furnace. Thus, considerable cooling takes place due to the incoming air, and the effective length of the furnace due to this decreases. In addition, usually in the oven on one side of it are provided inspection hatches (glazed or not). If, as a result of poor sealing, etc., air penetrates through these holes, this side of the furnace will be colder. In addition to controlling the same heating across the belt tape (there is no temperature skewing), it is also necessary to provide different heating from below and from above, which will allow to regulate the staining of the upper and lower surfaces of the products, as well as their shape.
Typical temperature baking parameters
As already mentioned, the baking process is dependent on the heat transfer and temperature - but one of its determinants.
All ovens and is possible to control the temperature and air outlet, and in many - and air velocity (forced air circulation in the furnace chamber). Typically, devices that measure the temperature in the furnace chamber and controlling the heating system can not detect the temperature around the dough pieces, due to which it is impossible to give here a robust set of baking temperatures that are suitable for a particular cookie. The resulting temperatures are useful only as part of the registration conditions in the furnace.
If the baking speed should be higher, the temperature should be longer, and the duration of the baking depends on the type of biscuit and its characteristics (especially on the weight and thickness of the test billet). To consider this issue, we assume that each furnace has three independent controlled zones (usually more than one, and very rarely less) with means for regulating the relative value of heat transfer from above and below. When searching for optimal baking parameters, it is necessary to take into account the operating mechanisms and to regulate the conditions in the furnace gradually and methodically, registering the results and the specified parameters. It can be found that by setting suitable parameters, it is necessary to periodically adjust them, as the external climatic conditions influence the air outlet from the ducts, etc. The search for and maintenance of optimum baking conditions usually requires the production of products for a long time.
Biscuit dough obtained foliation or with chemical disintegrants
Of these types of test, you need to remove a lot of water. It is necessary to obtain a very open structure that, when using a puff or laminated dough, will be laminated or flaked. The best structure is obtained for large and very large values ​​of heat transfer at the beginning of the furnace. The significant contribution of radiant heat in the first zone appears to be important, therefore it is important to use a considerable amount of burners for its transmission, and in part the open output ensures good air movement. To increase the heating effect due to thermal conductivity in the first part of the furnace, some preheating of the belt tape can be useful. The taping tape is usually made of light wire mesh, but in the US, a wire cloth is used, woven from a wire of large cross-section, and its strong preheating. Obtaining optimum heat transfer without furnaces with direct gas heating of the first zone is unlikely.
Cookies at the furnace outlet should be crispy and rigid as well to reduce the risk of cracking the moisture content should not exceed 1,5%. Large zones of turbulence in the subsequent moisture removal improves. For the regulatory framework and the necessary removal of moisture from the center of the pastry need to prick the dough pieces. baking duration is usually about 2,5-5 minutes. The faster baking, and hence the higher the heat transfer in the first zone, the more open the structure of the finished product.
Achieving a smooth flat form provided controlled heating biscuits ratios above and below the first zone. cookies Center is curved toward a stronger heat source.
Typical temperatures (depending on furnace type) to make cream crackers (in zones) 250,290,250 ° C with a duration baking 3,0 min and snack crackers ( "snack») - 200,250,240 ° C under baking duration 4,5 min.
Protracted sweet biscuits
When baking this cookie to be removed a lot of water, but not as much of crackers, and usually require "sredneotkrytaya" structure. Most structures are obtained by gradually increasing heat transfer with a maximum in the center of the oven. The contribution of radiant heat in the first zone is likely to be useful, but it can use the cookie oven completely heated indirectly. Preheat the tape hearth is not required.
A large amount of water vapor in the front of the oven gives the product a good surface gloss. Cookies can be baked on a steel tape or wire grid, but the grid is more common, as it can help to achieve greater speeds baking. To control structure and the necessary removal of moisture from the center of the pastry need to prick the dough pieces.
baking duration is usually 5-7 minutes. Upon exiting the furnace crunchy cookie, a hard and crumbly, the moisture content should not exceed 1,5% to reduce the risk of cracking. Large air turbulence in the following areas improves the removal of moisture.
Typical temperatures (depending on oven type) for biscuits Tea Finger up 140, 200, 200 ° C (duration baking 6,0 minutes), and for cookies Rich Tea - 150, 210,240 ° C (baking duration 7,0 minutes).
Cookies from pastry with a low fat and sugar
Dough for this cookie is not required to remove large amounts of water, and a desired pattern is usually not very open. Good structure when I get more or less the same throughout the heating furnace. How important is the contribution of radiant heat - it is not clear, but for the production of these biscuits are suitable all types of furnaces.
Cookies can be baked on a steel tape or wire grid, but the grid is more common because it is possible to achieve a large baking speeds. Steel tapes are usually given in a hollow biscuit bottom, which does not occur on the wire belts. Typically, baking is some spreading of the dough pieces and wire mesh better limits the size of products than steel tape. Prick dough makes possible higher rate of baking.
baking duration is usually 6-13 minutes, but it depends essentially on the thickness of the dough pieces. Biscuits at the exit from the furnace can be either solid and soft, and the moisture content thereof is usually about 2,5%. Air movement improves the removal of moisture.
Typical temperatures (depending on oven type) for the type of shortbread Shortbread make 205,230,230 ° C (duration baking 11 minutes), and for Digestive biscuits - 180,240,170 ° C (baking duration 7,0 minutes).
Cookies from pastry with high fat and sugar content of the dough primarily for wire cutting and spritz cookies
Because these types of dough to be removed a little water, and the structure is not very important. Good structure obtained by more or less the same throughout the heating furnace. For these types of baking cookies can be used all types of furnaces. High moisture content in the first zone allows better development and spreading of which after settling can give an attractive surface with cracks.
The pastry should be baked on a steel tape as usual dough piece during baking "spreads". The dough becomes soft in the oven, and flows in a wire mesh, making it difficult for the removal of baked cookies and contaminating the feed. Prick dough pieces are extremely rare.
baking duration is usually 8-12 minutes, and essentially depends on the thickness of the dough pieces. Cookies at the exit of the furnace is typically soft, plastic and requires cooling before removing the ribbon. The moisture content is not essential and is usually about 2,5%. removing moisture promotes the movement of air in the subsequent zones.
Typical baking temperatures are (depending on the type of oven) for Ginger Nuts - 150, 180, 180 ° C with duration of baking 8,5 minutes and for Choc Chip Cookie - 185,185,170 ° C when baking duration 12 minutes.
Types of furnaces
Currently, MCI mostly baked in conveyor furnaces, but many small producers baked products on trays, placed in a stationary furnace, which can be one of the following types:
  • installing on the fixed trays by means of the baking oven or a shovel plate;
  • of cradle conveyor furnace in which the trays are placed on the platform, rotating in a horizontal plane in a closed furnace;
  • deck oven, where the trays are placed on racks, which then is rolled into the furnace and rotated in a vertical plane in a closed oven.
Most kilns to ensure even heat distribution usually provide for forced convection.
There is a message on the conveyor oven, built in the city of 1810, which used the walking belt of wire mesh, but it was unsuccessful. Nevertheless, conveyor ovens in the UK have been introduced on a cookie factories around 1849-1851 years., But almost to the end of the century, they are not widespread.
Tunnel kilns until about 1950-ies. Remained relatively short. In 1972, conveyor kilns with a length of about 19 m appeared. Initially, chains played the role of ribbon on which baking trays were placed, which were removed after leaving the furnace. Later, when long steel rolling became available (at the beginning of the 1930-ies), continuous tapes were introduced. From the beginning, these tapes were only steel width about 600 mm, but soon the standard width was 32 in. (About 800 mm), and for some products wire mesh of various shapes began to be used. Currently, the standard width is 1000 or 1200 mm, and even wider tapes are used.
In the first furnaces, the inner lining with refractory bricks allowed to store a significant amount of heat, which helped to reduce the temperature changes of the furnace during the "batch" movement of the products. To heat these bricks, it took some time, when the furnace was lit, and for a long time - for cooling, that is, with increasing and decreasing heating, considerable inertia was observed. Therefore, baking was regulated mainly by changing the duration. It was important that a significant amount of heat could be transferred to the product in the oven with little convection. Furnaces with internal brick lining are now very rare. The transition to lighter structures, insulated with mineral wool or fiberglass, with increased convection to improve the transfer of heat from the gas burner or hot channels has reduced the cost of furnaces and significantly improved the regulation of temperature in the furnace.
The question of the ideal design of a baking oven for a long time was the subject of controversy and numerous attempts were made to create mathematical models of heat distribution in the furnace. The fact that there are many designs, each of which is called the best one, suggests that we have not yet reached an optimal design. Difficulties are mainly due to the inability to measure the temperature of the test billet in the oven or the microclimate around it. Until now, the role of heat coming to the product in the form of radiation and by convection is not exactly known. Some adjustment of the lifting amount of the article can be achieved by changing the conditions in the first part of the furnace, but the magnitude of these changes and the optimal parameters for each type of biscuit are far from being determined. More careful measurements of the conditions in the furnace and the parameters of the products obtained under these conditions are necessary.
There oven for baking MKI, operating on different fuels (gas and oil products of different quality) and electric furnace, heat dissipating directly or indirectly in the furnace chamber. only gas, light oil with a low sulfur content and the electricity can be used for direct heating of the furnace atmosphere. Heat from other petroleum products to be transferred indirectly through heat exchangers.
The use of electricity allows some other types of heat transfer. Furthermore wire heating elements, which can be red-hot-hot, high-frequency radiation (dielectric heating), microwave or induction heating of the metal parts (e.g., bands hearth furnaces) can be used for power transmission. Types of electric heating will be discussed below.
The main types of heating systems, ovens for pastry
Furnaces direct action
In furnaces with direct gas heating (DGF, Digest Gas Fired), there are many belt burners above and below the conveyor belt. Carbureted gas and air are supplied to each burner, the pressure of this mixture determining the power released. To ensure uniform heating across the tape, there are various flame size adjustment devices. DGF-furnaces can be equipped with an additional system of forming air flows, which increases the rate of heat transfer. The top of the furnace chamber is usually low, and the burners are located as close as possible to the belt of the hearth. This means that a significant fraction of the heat reaching the product is radiant heat. Electric furnaces are similar to DGF-ovens, but the heaters in them are electric.
Direct-acting furnaces with forced convection. In each zone of the furnace there is a large heater, and the combustion products are fed into the mixing chambers located above and below the belt. It is possible to control the blowing speed and the ratio of hot air circulating above and below the tape. To maintain uniform airflow, the furnace chamber arch is usually higher than in a direct gas heating furnace. This means that in furnaces with forced convection, the share of radiant heat in the overall heat transfer is less, but the conditions for heat transfer and the temperature conditions across the furnace chamber are more homogeneous.
Stoves convective-radiative action. Hot gases from this zone passes through the burner pipe, located above and below the ribbon and then discharged through the other tube and pass over the tubes in the first direction of the belt. The first pipe to radiate heat cookies, and then released from the air makes them convection air currents. To maximize the action of radiation emitting tube as close as possible to the tape.
Furnaces indirect action
indirect oven with forced convection ovens analogous direct forced convection, but the heat exchanger is adjacent to the zone heater heats air passing through the mixing chamber into the furnace chamber.
Oven indirect (Cyclotherm). Hot gases pass through the pipe above and below the belt and are returned to the heater. The combustion products are held in the furnace chamber. The furnace chamber and over hot pipes a separate air circulation system.
Furnaces mixed action
These stoves are a combination of the two types described above. Widespread mixed bed furnace consists of a first zone with direct gas heating and subsequent two or more zones with forced convection. The principle of operation is to allocate at the start of baking of maximum power and a large amount of radiant heat and then to provide a large amount of heat by convection ovens in the part where the drying takes place.
If combustion products are released into the atmosphere of the furnace, it greatly increases the amount of water vapor, and becomes necessary to increase drainage of the chamber. In ovens always provides the ability to control the amount of heat dissipated in each zone of the furnace, and the ratio of the amount of heat supplied to the top and bottom of the product.
Furnaces with indirect heating are usually divided in length into large zones and equipped with several large heaters. Directly heated furnaces are usually equipped with a large number of small heaters to regulate the regimes grouped in similar large furnace zones. In such furnaces, individual heaters (both above and below the tape) can be turned off. To control and redirect hot gases to different parts of the kiln chamber or through ducts to the atmosphere in the furnace, dampers are provided. In addition to furnaces with direct and indirect heating, there are structures with enhanced convection or thermal radiation. In successive zones, some kind of heat transfer can prevail, and mixed-effect furnaces in different zones can use different heat sources (for example, electric radiating panels).
Furnaces of all types usually have many control points that provide a huge range of possibilities, but make it difficult to choose. In many cases, the problem is aggravated by the fact that the control means are calibrated quite roughly and are located along the entire length of the furnace. For example, a flap can be simply installed on the outlet duct. The lever that turns this damper to regulate the amount of gas flowing through the duct can have a linear scale graded from 0 to 10, but with values ​​from 0 to 5, the output can be about 10-80%, and for 5 to 10 - about 80- 100%.
The number of independently controlled zones and their length should ideally be designed to correspond to the type baked product and the time during which the product is in a certain area. To simplify the design and reduce manufacturing costs typically make all zones of the same length, and the power delivered to each of them varies. The first zone should have maximum power, as this is where the tape must be heated hearth, and the dough pieces brought to the temperature of the beginning of the removal of moisture.
Hot gases from the baking chamber rises through the ducts due to natural convection, but usually a fan is used additionally. Nevertheless, the outside temperature, speed and direction of the wind affect the rate of gas escape from the duct. The movement of hot gases in the furnace chamber is important for the uniformity of heat transfer, and it is interesting that, as far as the author knows, nowhere in the direct heating furnaces is there a diversion (flue gas) from above and below. Even in the case of forced evacuation, it is always assumed that hot gases should be discharged from above the furnace chamber, which, unfortunately, often affects the movement of air and heat transfer along the edges of the belt tape.
Kiln determined by its length and baking duration necessary to obtain the desired product structure, color and moisture content. For most baking products rate determined by the time necessary for satisfactorily drying. For a very approximate calculations can be
using the following loading test for different choices provided adequate power in the furnace:
wire cutting dough

16-18 kg / hour / m2 tape;

shortcrust pastry

18-20 kg / hour / m2 tape;

sweet dough

16-22 kg / hour / m2 tape;

dough for cream crackers

-22 Kg / hour / m2 tape;

dough for soda crackers

22-25 kg / hour / m2 tape.

furnace efficiency may be calculated on the basis of the fuel quantity measurement with a known calorific value, the predetermined period of burning, the mass loss due to evaporation of water and increasing the temperature of product ingredients.
For satisfactory baking crackers and cookies water indirectly heated oven typically do not have sufficient capacity in the first zone. To make them more versatile, these furnaces can be supplemented with direct gas heating or electrical components.
the transfer belt usually have a width of 800,1000 and 1200 mm, although they may have other dimensions. There are various types of tapes having various degrees of transparency, durability and weight (Fig. 38.4). Sheet steel belts can be thick or 1,2 1,3 mm weight per square meter of about 9 kg. There perforating
38.4 Fig. 38.4. Various types of feed belt
Steel strips with holes of specified diameter are expensive, but with the strength and durability of solid steel belts they have an improved ventilation of the base of the product. There are also a variety of wire tapes - from very light ones with a square hole mesh (for example, 5 x 5, etc., where the numbers indicate the number of interlaces per inch, with a square meter weight of about 3,5 kg), to loop wire, providing better support for the product, Durability and great flexibility on the final shafts. Heavy flattened wire meshes with chevron weave have a maximum mass of square meters of tapes - about 19 kg. These tapes are especially popular in the US, but it should be noted that a large capacity is required to heat the tape at the furnace charging opening. Preparing the tapes for work and care for them will be discussed in the next section.
At the ends of the furnace, there are end rollers. At the exit of the furnace, the shaft is equipped with a drive, and at the loading end there is a tensioning device that keeps the tape taut, but not so tight that it is damaged (especially with light wire strips). The diameter of these shafts is sufficient to ensure that the bands and their joints are not stretched during bending, and their shaft axes can be inclined to facilitate adjustment. To prevent slipping, it is sometimes necessary to cover the drive shaft with a fibrous refractory material. Tapes in the oven are supported on metal or graphite strips or rolls, which are located close enough to prevent significant sagging of the tape between them.
The distance to which the feed belt extends beyond the confines of the kiln chamber on each side depends on how the product is placed on the tape, and also on what cooling is required before removing the products. Casing-off or wire-cut products require a significant amount of space for placing forming equipment over the belt. Products with a high sugar content or delicate elegant products require a long exit - perhaps with air supplied by a fan or by spraying water under the tape to allow the products to harden before they are removed from the tape.
Available furnace length about 150 m, but the average length of the furnace is about 60 m with capacity of about 1,2 tons of biscuits per hour, or about 100-ounce packages per minute.
Products are removed from the tape with a special knife, which should allow you to clean the products and transfer them with minimal disruption to the relative location. This ensures a good supply to the packaging machine or subsequent processing of the products. The removal knife can be a thin blade made of steel or a solid synthetic material or a wire scraper; For the removal of different products, different types of cutting knives are used. Biscuits with a low fat content at the time of removal are usually quite stiff, and so the knife can be placed slightly below the bend of the tape passing around the drive shaft; More flexible types of cookies in some cases should be removed with completely flat knives (to prevent cross-section cracking). It is important that the knife does not damage and does not scratch the surface of the tape. Cleaning the tape to remove particles or fat on it is a separate, later operation, on which we will stop in the section 38.6.3.
In view of the growing concern of energy saving schemes were developed by some of the low-grade waste heat lost in the furnace ducts. The main problem with this is that the gases exiting characterized by high humidity and may include many alkali metal and resinous materials (derivatives of sulfur-containing compounds and degradation products fuel chemical disintegrants biscuits, for example, ammonium bicarbonate, and volatile fractions of fats present in the test).
The growth of electricity use for baking
Electric furnaces are likely to play an increasingly important role in the future, as the burning of hydrocarbons such as oil and gas will decrease. At present, electric furnaces are similar to direct gas heating furnaces and a number of heaters in transverse pipes in the furnace chamber located above and below the belt. The power of these heaters can be regulated by means of very economical thyristor switches. The strength of the currents used is very high, and the replacement of heaters is very expensive. To improve the heat transfer to the product in the furnace, it is desirable to have some turbulence, but since there are no combustion products, the tap can be very accurately controlled. It is possible to have a number of incandescent heaters giving intense thermal radiation for coloring the products.
Interest in the use of microwaves and radio-frequency radiation has increased in order to speed up baking and increase its efficiency. It is understood that the radiation penetrating the test billet or partially baked product has a significant advantage over the heat acting only on the surface. At least theoretically, heating the test piece contributes to a more even development of the structure. APVBaker company proposed to use microwave ovens in standard furnaces, which heat later in the baking process (for faster drying) both test billets and the products themselves. Sasib Bakery offers the use of radio frequency radiation to accelerate drying in the zones of the traditional furnace following the first.
Dryers for drying with the help of radio frequency radiation after baking are now widely distributed. There is information that using after baking drying with heating currents of high frequency, you can achieve an increase in yield by 33%. The product, baked to the desired structure and color, is transferred to a non-metallic tape and passes through a relatively short device. The cookie at the outlet of the device has the required humidity, and the moisture gradient between the center and the surface is very low, which significantly reduces the likelihood of cracking after cooling.
Radio frequency (RF) dryers are commonly used frequency 27,12 MHz, which is specifically allocated to international agreements for such purposes. Available RF oven at 25,40,50,60,75 and 85 kW. RF blocks have efficiency 65-72% (conversion efficiency of electric power consumption in the energy transferred articles).
The frequency of the microwaves used is 2450 MHz. Microwave heating increases with the distance from the tape, since the field on the tape is zero. The tape, however, gives good heating due to thermal conductivity. The energy of microwaves is used in the first zone for rapid heating of the test billet, in the middle zones - to stop the release of loosening gas, and in the latter zones - to increase the rate of moisture removal. It is reported that with the reasonable use of microwave heating it is possible to reduce the duration of baking, regulate the thickness of products and the moisture content, and also reduce the moisture gradients in the baked product. The energy received from microwave heating should be used in combination with traditional heating, as the latter determines the development of color and taste of finished products.
Preparing tapes hearth and care
Preparing a New Tape
Before using for baking, a new tape must be prepared. Preparation includes conditioning and cleaning. Cleaning consists mainly in the removal of mineral oil and dirt, and in the case of wire tapes it can be performed by simply wiping the tape with a clean cloth after heating it to about 150 ° C. Steel bands (including perforated) to create a clean shiny surface, to which the products will not stick when baking, require more attention. The tape is traditionally heated to about 150 ° C and applied to it with grease, the tape is heated again in the oven, and then the fat is wiped off the oven with a clean cloth. Surface irregularities due to scratches, pitting corrosion of the metal or carbon coating from the test particles often causes sticking of the products.
Grease the new tapes to prevent sticking
The conditions causing the products to stick to the tape when baking are not always clear, but a thin film of vegetable oil usually prevents the sticking of sticky substances (such as syrup or dairy products) that appear when baking on product bases. Products that are low in fat and high in sugar or eggs can, before being placed on the tape of products, require special coating. Various lubricating devices are produced on the basis of vegetable fats or waxes (for example, bees). The fat film must be minimally necessary and homogeneous, but even in this case, it is possible that the products are unacceptably spreading when the sugars are melting or dissolving when heated. This spreading can be limited by additional strewing onto the oil film of flour or starch before placing it on the product tape. It may be easier to apply oil mixtures and
The frequency of the microwaves used is 2450 MHz. Microwave heating increases with the distance from the tape, since the field on the tape is zero. The tape, however, gives good heating due to thermal conductivity. The energy of microwaves is used in the first zone for rapid heating of the test billet, in the middle zones - to stop the release of loosening gas, and in the latter zones - to increase the rate of moisture removal. It is reported that with the reasonable use of microwave heating it is possible to reduce the duration of baking, regulate the thickness of products and the moisture content, and also reduce the moisture gradients in the baked product. The energy received from microwave heating should be used in combination with traditional heating, as the latter determines the development of color and taste of finished products.
Preparing tapes hearth and care
Preparing a New Tape
Before using for baking, a new tape must be prepared. Preparation includes conditioning and cleaning. Cleaning consists mainly in the removal of mineral oil and dirt, and in the case of wire tapes it can be performed by simply wiping the tape with a clean cloth after heating it to about 150 ° C. Steel bands (including perforated) to create a clean shiny surface, to which the products will not stick when baking, require more attention. The tape is traditionally heated to about 150 ° C and applied to it with grease, the tape is heated again in the oven, and then the fat is wiped off the oven with a clean cloth. Surface irregularities due to scratches, pitting corrosion of the metal or carbon coating from the test particles often causes sticking of the products.
Grease the new tapes to prevent sticking
The conditions causing the products to stick to the tape when baking are not always clear, but a thin film of vegetable oil usually prevents the sticking of sticky substances (such as syrup or dairy products) that appear when baking on product bases. Products that are low in fat and high in sugar or eggs can, before being placed on the tape of products, require special coating. Various lubricating devices are produced on the basis of vegetable fats or waxes (for example, bees). The fat film should be minimally necessary and homogeneous, but even in this case, it is possible that the products are unacceptably spread when the sugars are melted or dissolved during heating. This spreading can be limited by additional strewing onto the oil film of flour or starch before placing it on the product tape. It may be easier to apply oil and cereal mixtures than each of these ingredients individually. Proprietary blends of specially selected tapes for lubrication are produced, but their application is very difficult.
Cleaning the feed belt
Regardless of whether tapes are needed in the coating, it is important to keep the tape clean and the oven includes special cleaners. After removing the products from the tape, a scraper should be used to remove adhering large particles from the tape. In addition, it may also be necessary to thoroughly process the tape with rotating wire or additional fabric brushes. "Too thorough" cleaning can remove the necessary coating of the tape, and very greasy brushes can simply redistribute "dirt" without removing it. When the tape returns, the bottom of the oven is supported on the shafts. If these shafts are too polluted by the settled crumbs of cookies and grease, the deposition of additional dirt can degrade the action of the brushes.
If baked products with a high content of fatty foods or sugar-containing products on wire tapes, carbon will settle in the mesh holes. If it is separated, but not properly removed with brushes, black particles will adhere to the bases of the baked goods. Therefore, it is better to clean the wire strips before baking starts or after it starts. In case of too much settling on the carbon tape, it can be removed from the warm tape by scrapers, manually or (in exceptional cases) by treatment with a caustic soda. When using the latter method, it is important that caustic soda be applied very carefully and then carefully removed before washing the next batch. In all likelihood, after such processing, it will be necessary to restore the state of the tape.
ribbons for Care
When heated, the tapes expand, and to maintain flat surfaces and reduce stress, which can lead to deformation and problems with the control of the tape, it is very important to maintain uniform heating across the tape along the entire length of the furnace. After finishing the baking and turning off the heaters, the tape should continue to move until the temperature drops to about 100 ° C or lower for steel tapes and no more than 150 ° C for wire. Only then can the tape drive be stopped. The cooling period is very important, and during the cooling process, do not open the inspection hatches on one side of the oven, as one side will cool faster than the other. Cooling will accelerate if the gas removal flaps and doors at the beginning and at the end of the furnace are fully open.
If the tape is fed during baking due to a power failure should be stopped, two problems can arise. First, the products quickly overheat and can ignite, and secondly, the tape will become excessively hot. Therefore, means should be provided for automatically extinguishing the heating in the oven for such a case, and all measures should be taken to immediately unwind the tape. In some cases, a backup power source (for example, batteries) may be provided, but in any case a handle must be attached to the drive shaft, and in the event of a power outage, it is necessary to rewind the tape immediately. It is also important to have means for safely collecting burnt or burning products, It is necessary to continue moving until the power supply is switched on or the tape is cooled sufficiently to stop.
Rust can quickly destroy the belt surface, and therefore if the humid atmosphere in an oven, and especially if it is not used regularly, it is advisable to cover the surface of the fat or oil belt. In addition, to prevent rust coating the inner surface of the tape must sometimes be very sparse lubricate it with grease (such as silicon or graphite). Too much grease can cause the drive to slip on the shaft end.
Measurement and control of the baking process
It has been shown above how the heating and temperature parameters influence the development of the structure of the product, the change in its moisture content and the staining. In furnaces of various types, means are provided for setting the temperatures of individual zones, for changing the degree of turbulence in the air, for setting heat directed to the top and bottom of the belt and for setting the level of removal of gases. Section 38.4 provides some information on the typical temperature profile for the production of different types of products. Below we will dwell on how to ensure optimal conditions in a particular stage of baking and how to maintain these conditions.
Although cookie ovens have several independently controlled areas, there is usually significant air movement from one zone to another, since the supply of air for replacement is not specifically regulated. Quite often it is possible to observe a strong draft in the furnace - both from the side of the charging hole, and at the exit of the furnace. This reduces the effective length of the furnace, since cold air enters the furnace. The integrity of the zone is important for regulation. Such a design provides for monitoring the pressures in each zone and maintaining them the same. Fresh air entering the furnace to replace the air removed in each zone enters the furnace and heats up before entering the furnace chamber. The advantages for precise control are obvious - the only danger is that the sudden drop in temperature experienced by the test piece as it passes from one zone to another can affect the structure of the products, causing subsidence.
furnace controls are usually not too accurately calibrated, and often, in spite of the linearity of their scales, they are nonlinear. This is particularly true, for example, valves for gas withdrawal (see. 38.5 section). Very little furnaces equipped with some indication of humidity or air speed, and in most cases thermometers or thermocouples used to determine the state of the furnace atmosphere are few and are located so that they provide very little information about the conditions immediately adjacent to bake the dough pieces.
Since the furnace has only one inspection hatch on the clear zone that possibility baker for visual inspection of the conditions in the furnace are very limited. Therefore, the baker has to rely on his experience to controlling emerging from the furnace product, determine if its different zone of the furnace work as he wants. In fact, "the best" of the furnace shall be established by trial and error, and experienced operators make the necessary settings changes daily or administered empirically.
Conditions near products can be measured by moving the sensor or series of sensors through the furnace and recording temperature changes. The simplest version is the use of a thermocouple at the end of a light two-wire wire insulated with heat-resistant plastic. This thermocouple moves along the furnace, and then quickly returns back (like a spinning rod) through the entire furnace or disconnects from the recording device. The advantage of this method is the rapid acquisition of the temperature profile and the fact that subsequent profiles can be quickly recorded without loss of time. The drawback is that the wire is often spoiled due to heat and must be very flexible.
Other devices for recording the temperature in the furnace have been developed on the basis of the use of recording electronic devices placed in insulated casings. Such instruments can record temperatures in several places simultaneously, pairs of sensors with dry and wet beads can be used to estimate the water vapor content, and small anemometers are used to record air velocities. The main difficulty in this case is the effects of conditions in the furnace, from which the electronics should be protected, and an extremely small space for passage of the device (sometimes not exceeding 30 mm).
At high working temperatures the difference in temperature between the wet and dry thermometers used to evaluate the content of water vapor is very small with relatively large and possibly significant changes in the moisture content, and at the same time sufficiently accurate devices with a reliable supply of water to the wet bulb temperature is extremely difficult to create.
A constant evaluation of the water vapor content can be performed by sampling the atmosphere of the furnace through heated pipes (to prevent condensation) or through the outlet duct (chimney) and passing the samples through the appropriate sensors. Calibration and maintenance of these sensors is a serious problem, since dust, fatty smoke and various corrosive gases are also present in the furnace atmosphere. The moisture content in the furnace atmosphere, and even more so in its various locations, is important for the rate of release of gases, and therefore for the thermal efficiency of the furnace. The location of the gas outlet points and, probably, the quality of the baking, are also related to the content of water vapor, so the management of "moisture" is a useful requirement. If reliable control of "humidity" is possible and the positive effect of maintaining a given "humidity" on the quality of biscuit baking can be shown, an obvious means of controlling the level would be the connection with the gas discharge valves. In all likelihood, the best method is to use variable speed ventilators for venting.
Measurement of the temperature and the method of heat exposure to the product (conduction, convection and radiation) - this is a serious problem, and despite numerous attempts to create a mathematical model and thoroughly implement the experimental baking, their meaning is unclear. Note also the special equipment as described in section 5.8.5. Our incomplete understanding of the regulatory environment of the furnace due to two main problems:
along the entire length of the oven has many controls, the state of which it is difficult to register and the effects of which are very interrelated;
still there is little progress in the continuous measurements of the parameters of baked products, which must be matched position regulators and values ​​of the measured parameters of the furnace.
Currently, microelectronics and centralization of funds furnace control, remote damper actuators, and so on. D. Allows a much better record data. If you understand the relationship of desired conditions and the effects of various means of regulation, you can create highly sophisticated control loops.
Since we know that the rise of the dough in the oven occurs in the initial stage of baking, and the color of the product acquires later, it seems reasonable to analyze the ways of measuring these and probably other properties - for example, the length and width of the products at the end of the respective zones, Ovens, where information for management purposes is somewhat outdated. For this, it would be good to consider the use of furnace zones separated from one another by tunnels for observation. The value of the temperature of the ribbon when placing blanks on it requires further research and, probably, control, as we can not yet be sure that it is better for the quality of the product and the efficiency of baking - heavy or lightweight tapes.
These measurement and control problems involved many experts, but it would be extremely unfair not to highlight the great work on the automation of furnaces for the production of cookies and computer management of [1-4].
oil spraying after baking
Many savory crackers, as well as some other products coated with sprayed hot oil. Immediately after removal from the product of the hearth belt passes through the apparatus in which they sprayed warm vegetable oil. Oil flows from the injection nozzles with rotating disks or by static charges. All methods of the last feed, usually lead to contamination of the surroundings due to the spraying of fine droplets of oil.
The coating of oil applied only on the upper or both surfaces and making 8-18% of the mass of the cookie significantly improves the appearance of the surface of the products, its color and slightly improves the palatability. In some cases, a flavored oil is applied which, for a spicy and lingering sweet pastry, is useful for flavoring and taste, which would be lost by adding this oil to the dough before the baking step. The main problem with flavored oil is that it contaminates the cooling conveyors used when leaving the oil spraying device and the smell can spread in the packaging area of ​​the factory.
The oil used for spraying, is particularly susceptible to rancidity, since it is sprayed in a hot state and readily oxidized. On oil product is in the form of a surface film, which is also ideal for the oxidation. Therefore, it is recommended to use a fat or oil resistant to oxidation and are well is coconut oil because it is low in unsaturated fatty acids. It is easy to acquire, and it is much cheaper than a specially prepared resistant to fat oxidation.
Literature
  1. WADE, P. and WATKIN, D. А. (1968) Biscuit Automation Part IV - Some Results Obtained With the Biscuit Sampling and Automatic Measuring Equipment, C&CFRA (FMBRA) Report 12.
  2. LAWSON, R. and BARRON, L. F. (1970) Biscuit Automation Part VI — Mathematical Modelling of a Pilot Scale Travelling Oven, C&CFRA (FMBRA) Report 38.
  3. CORNFORD, S. J. (1979) The Biscuit Oven Temperature Recorder Mark III, C&CFRA (FMBRA) Bulletin No. 4, 147.
  4. LAWSON, R. and JABBLE, S. S. (1979) Further Moves Towards a Fully Automatic Semi-Sweet Biscuit Plant, C&CFRA (FMBRA) Report 85.
Moreliterature
  1. WADE, P. AND BOLD, E. R. (1968) Investigation of The Baking of Semi Sweet Biscuits, Part I — Some Factors Affecting the Thickness of the Finished Biscuit, C&CFRA (FMBRA) Report 14.
  2. HODGE, D. G. and WADE, P. (1968) Investigation of The Baking of Semi Sweet Biscuits, Part II Changes Occurring in the Temperature and Thickness of Dough Pieces During Baking, C&CFRA (FMBRA) Report 22.
  3. HOLLAND, J. M. (1979) Increasing Productivity by Dielectric Heating, BCMA Technical Conference.
  4. MOWKRAY, W. R. (1981) Technology of the «hot box», Food Manufacture, October.
  5. LAWSON, R., MILLER, A. R. and THACKER, D. (1986) Heat transfer in biscuit baking Part I: The effects of radiant energy on semi-sweet biscuits, C&CFRA (FMBRA) Report 132.
  6. Strayfield International Ltd (1986) An array of applications are evolving for radio frequency drying. Food Eng. Int’1. November.
  7. MACFARLANE, I. (1989) Measurement of oven conditions and types of ovens used in the biscuit industry. Biscuit Seminar, ZDS Solingen, Germany, December.
  8. MANLEY, D. J. R. (1998) Biscuit, Cookie and Cracker Manufacturing Manuals, 4. Baking and cooling of biscuits, Woodhead Publishing, Cambridge.
  9. LAWSON, R. (1994) Mathematical modelling of cookie and cracker ovens, in The science of cookie and cracker production, edited by H. Faridi, Chapman & Hall, London.
Last modified on Tuesday, October 27 2015 15: 43
Vladimir Zanizdra

Founder Baker-Group.net site. More than 25-years of experience in the confectionery industry. More than 20-five years of management experience. Experience in the organization and design of the production from scratch. Site: baker-group.net/contacts.html El. mail This e-mail address is protected from spam bots. You need JavaScript enabled to view.

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