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What’s an oven? [Collection]


Humans began to use fire about a million years ago, in that backward hunting and gathering era, our ancestors could only use dead branches and leaves to make fire and bake simple ingredients directly.
It was only in the Stone Age that humans began to use stone to make cooking utensils to use, stone has good insulation, so in modern times people are still using stone slabs to bake pasta.

If the direct use of firewood to bake ingredients is simple and crude, then the use of kilns to bake food is full of technical content. This enclosed space made of clay can gather a lot of heat energy transmitted by the firewood, and the walls of the oven will transfer this heat to the food efficiently, giving it a burnt taste on the outside and tender on the inside. The emergence of the kiln also facilitated the bread-making technology from Egypt to the world.

By the 19th century, the introduction of electric ovens, finally let people out of the inefficient wood-fired baking method, making the food processing industry into the fast lane. Of course, all this thanks to Faraday, Tesla and other scientists to the contribution of electricity. This kind of electricity can be used in the oven, enough to overturn the ancient Egyptian cognition, can not think of baking ingredients has become so simple and fast.

Throughout history, people baked food has gone through three main stages of change, respectively, charcoal bare baked, kiln and electric ovens.
Although electric ovens have far surpassed the first two in terms of convenience, control, energy efficiency and environmental protection, but today’s charcoal grill, kiln bread is still quite popular, which is the reason for the handmade sentiment, publicity stunts。

In any case, the electric oven has become the mainstream baking equipment is undoubtedly. Although the design of the oven is becoming more and more humane and intelligent, but unfortunately, many people do not actually understand it.
The so-called understanding of the oven’s temperament is not as simple as measuring a temperature difference. Starting from the power on, to the aroma of the food out of the oven, the principles involved in electricity, thermodynamics, materials science, chemistry and so on.
But these theoretical knowledge is not boring, and even very interesting, because they are closely related to our daily life. In the following, I will explain the working principle of electric oven, heat transfer, temperature distribution, load-bearing appliances, and application examples in an in-depth manner.

  • Working principle

Strip away those “smart”, “Internet” and other flashy veneer, the skeleton of the electric oven is a tin box plus a few heating tubes. Unlike burning firewood, its mission is to convert electrical energy into thermal energy.
Heat pipe is the core component to complete this mission, it uses to the principle of the thermal effect of electricity, heat and the square of the current through the conductor, resistance is proportional, that is, the greater the current, the higher the heating power.

Figure out the above points of knowledge, you will understand why some ovens are rated at 220v, some are 380v, and the current used in the oven is also 10a and 16a. The purpose of increasing the voltage or current is obvious: to increase the power output of the oven per unit of time and improve the efficiency of baking, which is especially obvious in larger ovens.

In modern electric ovens, the heating tubes are mainly made of quartz and stainless steel. Quartz tube heating efficiency, low cost, the disadvantage is not strong and durable enough; stainless steel tube is generally filled with magnesium oxide powder, in the thermal conductivity, durability are very good performance, but the cost is slightly higher. In the early days due to cost reasons, many low-end ovens are using quartz heating tubes, but now stainless steel tubes have gradually taken the place of quartz tubes.

When the heat pipe is energized, the temperature in the middle position tends to rise faster than the temperature at the ends, so designing multiple heat pipes in a parallel structure from left to right can easily lead to uneven heating of food. If a single M-type heat pipe is used, this problem can be improved to a large extent.

When the oven is first preheated, the heaters will be red because they are running at full power, but when the temperature reaches the set value, the heaters will become energized at intervals, so they will not reproduce the red state. However, in some M-type heating tube ovens, the heating tube is more densely distributed, and will be energized at intervals during preheating, so it is normal not to glow red.
Therefore, a good oven must have a stable and accurate temperature control system, so how does this temperature control system work?

First of all, the temperature sensor probe should be installed at a location near the heat pipe, so that the heat pipe can accurately monitor the degree of heat. Like the heat pipe, there are good and bad induction probes. The main difference between mechanical temperature control and electronic temperature control ovens is reflected in the induction probe above, mechanical ovens use bimetal, electronic ovens use thermistors, which is the business mouth high NTC (negative temperature coefficient), the sensitivity of the latter is several times the former.

Second, in addition to being equipped with a high-performance sensing probe, you also need to have a system to control the heating tube power on and off. Temperature is a delayed parameter, just as you need to wait 5 minutes before taking a reading when measuring your body temperature.
Of course, the oven’s sensing probe doesn’t take that long, but traditional temperature control systems tend to have some delay and bias. To get a more stable temperature profile, it is necessary to introduce PID control, which can effectively solve the impact of the time lag of the sensed temperature. Nowadays, this PID control is basically only equipped on electronic ovens.

Simply put, the complete workflow of the electric oven is that when the oven starts to power up, the temperature of the heat pipe gradually rises and exceeds the set temperature, the control system can monitor this signal through the sensor probe, and immediately cut off the power to the heat pipe to prevent the temperature in the oven continues to rise.
When the temperature drops to a certain value after a period of power failure, the induction probe sends a signal to the control system to continue to energize the heat pipe, and so on and so forth.
The test is how to make the amplitude of this curve smaller, so that it can be approximated as a straight line.

  • Heat transfer

Before you can study the temperature distribution in an oven, you need to understand heat transfer. In fact, this knowledge has been learned in school. There are three main ways of heat transfer, which are radiation, convection and conduction.

Radiation is the origin of everything in the world, and most of the energy we currently consume comes from solar energy.

Sun selflessly delivers its energy to us in a constant stream by means of radiation. In fact it is not just sunlight, but objects that normally emit light, such as flames, light bulbs, lightning, etc. also radiate heat, only this heat is too small compared to sunlight for us to feel it. The essence of radiation is a kind of electromagnetic wave, this kind of ray is similar to microwave and X-ray.

The main characteristic of convection is reflected in the word “flow”, which refers to the fluid.
What is a fluid? It is an object that can flow, such as gas, liquid, the most common in daily life is air and water. I’m sure you will immediately think of smart, boiled food is the use of water convection heat transfer, and air conditioning is the use of air to transfer heat.
Convection needs to have two basic conditions, one is a fluid, the second is a temperature difference.

Conduction is also a very common way of heat transfer, and the reason why we can feel the temperature when we touch an object is the result of conduction. Conduction often occurs between solids, the higher the temperature of the object, the stronger the thermal motion of the molecules, while the low temperature of the object molecular movement is slow. By making sufficient contact, the side with strong molecular motion will drive the weak side, or the slow side will slow down the fast side to achieve a state of temperature averaging.

Guess how many of these ways of heat transfer occur in the oven? The answer is that all three occur.

In an ordinary flat oven, the lower heating tube will continuously create hot air to rise to provide temperature for the baking pan (or mold), and at the same time radiation is also being carried out, the baking pan will conduct heat to the food, this process occurs convection, radiation and conduction. Similarly, the upper heating tube also plays the same role, but the hot air will only rise, so the effect of the upper heating tube through convection heating ingredients is very limited, in order to solve this objective defect, manufacturers in the control of the upper heating tube will intentionally raise the temperature.

In addition, the radiation effect of the upper heating tube will be more obvious, because we bake most of the desserts will not be covered at the top. Not only does the heat pipe emit radiation to the ingredients, but when the temperature of the four walls of the oven increases, it also has a weak radiation effect on the ingredients.

The reason why a flat oven cannot bake with multiple layers is simple. In the case of having multiple layers of food, relying on the heating tube to heat straight up and down, both convection and radiation cannot penetrate the baking pan and the thick air layer in the middle, so except for the top of the top baking pan and the bottom of the bottom baking pan, the other layers can hardly be heated, which is the same principle as the thermos.
Although many flat ovens have a hot air circulation function, unfortunately, it does not have a heating wire in the fan opening, so it can only make the heat in the oven evenly dispersed, but can not output high temperature hot air like a real air oven.

Air oven was created to solve the problem of inefficient baking in flat ovens.
The main heat transfer method of a real air oven is convection. The back of the oven is equipped with an oversized fan and heating wire, which can send out a constant stream of hot air and convect it evenly to each layer of ingredients, making the heat transfer efficiency very high.
As a result, the temperature and time for baking the same food in an air oven can be reduced. However, the performance of the air oven is not perfect, because its strong hot air can cause the surface of the ingredients to be dry, which is not suitable for certain desserts that need moist taste.

In addition, there are air ovens and flat ovens in one oven and steam ovens, their heat transfer is more diverse, you have to analyze according to the set baking mode, the basic principle has been mentioned above, so I will not repeat it.

  • Temperature distribution

Even with high-tech additions such as M heat pipes, NTC induction probes and hot air circulation, it is impossible to achieve an absolutely uniform distribution of oven temperature over time and space. The problem of uneven distribution over time is explained in the section on working principles.
The uneven distribution in space is mainly reflected in the horizontal and vertical directions.

Heat pipes of a flat oven are at the top and bottom, whether convection or radiation, the amount of heat the food gets is inversely proportional to the distance from the food to the heat pipes.
In other words, the closer the food is to the heat pipe, the higher the heat it can obtain, and the farther the distance, the lower the degree of heat. This principle explains why the temperature of small ovens is not easy to control. For example, at a fixed temperature and time, a cake with the top 4 inches from the heat pipe bakes with good color.
When you change the distance to 2.4 inches, it may be that the degree of color on it is just a little darker; but when you reduce the distance to 0.8 cm, perhaps the top becomes charred.

When you have a flat oven with uneven temperature in the horizontal direction, for example, a whole tray of cookies on the left side and the right side of the color is obviously different, this problem can certainly be solved by switching positions in the middle. However, most of the reasons for this phenomenon are uneven temperature distribution of the heat pipe, the best thing to do is to change an oven.

The problem of darker coloring may also occur with ingredients on the side of the air oven close to the fan, as the hot air cools down as the distance traveled increases. Of course the specifics are up to you, and if this deviation occurs when baking a certain food, it will be necessary to switch directions midway through the baking process next time to get even coloring on all the food.

In general, large ovens have a more even temperature distribution than small ovens, and air ovens in turn are more even than flat ovens.
I recommend using an oven with a capacity of at least 40 liters for home baking to get a more balanced temperature. Of course, you also need to learn to cite the relationship between the internal temperature of the oven and its position.
Use an oven thermometer to test out the temperature difference between locations, and make a scientific temperature setting based on the maturity of the ingredients from the previous baking session, combined with the recipe requirements.

To test the oven temperature with a thermometer, do this
If your oven has independent upper and lower heat functions, first put the thermometer into the top layer of the oven (if the top layer is too close to the heat pipe, then move down a layer), turn off the lower heat and turn on the upper heat alone, set the temperature between 160-200 ℃ (this is the more common temperature range for baking), after a few minutes the thermometer reading is stable, and then see what the difference between the reading and the set temperature, which is the temperature difference of the upper heat.
Similarly, you can also test the temperature difference of the lower fire.

The advantage of measuring the temperature difference separately is that you can more accurately control the temperature of the upper and lower flames, in baking chiffon cake, puffs and other temperature-sensitive desserts, you can make targeted adjustments to ensure the success rate.

If it is a low version of the oven does not have independent temperature control function of the upper and lower fire, or is a real air oven oven (no upper and lower heat pipe), then you can only put the thermometer on the top, middle and bottom to measure, calculate the average of the three temperatures, and then subtracted from the set temperature, is your oven temperature difference.

Even if your oven can not be independently controlled by the upper and lower heat, but there are still some techniques to achieve the upper and lower heat different temperature heating.

For example, by changing the level of the baking pan, the closer the baking pan is to one end of the heat pipe, the higher the heating temperature at that end will be. In addition, there is a cover tin foil, the bottom of the double baking tray and other sharp operation, can effectively achieve a single side of the cooling, to ensure that you try again and again.

However, the most effective way is to “buy buy buy”! Choose an oven with independent temperature control for both the upper and lower tubes, as well as with a fan oven, even temperature control. Put the kitchen can also enhance the class.

  • Carrying apparatus

No matter what ingredients are baked in the electric oven, you can not leave the bearing apparatus, including molds, baking trays, baking nets and so on. In fact, the baking tray is also considered a kind of mold, but it has a relatively large heat area. Depending on the material, shape and color, the thermal conductivity of the mold will also vary.

Main materials of molds are copper, aluminum alloy, carbon steel, stainless steel, silicone, etc.

Copper has excellent thermal conductivity and can bake a hard and brittle skin, but it is less used because of its high price.

Aluminum alloy material generally with the coating or anodizing treatment, it is characterized by better thermal conductivity, light material, easy to shape, and cheap, so it is widely used.
The main components of Carbon steel are iron and carbon, also with non-stick coating, compared with aluminum alloy thermal conductivity is slightly worse, but the manufacturing cost is very low, so can occupy a place in the market.

Stainless steel this material has hard, corrosion resistant, high temperature resistant, easy to clean and many other advantages, but as a mold, its thermal conductivity is relatively poor, and the manufacturing cost is high, so it is not popular.

Silicone is a new type of polymer material, with the advantages of good elasticity, light quality, easy to release the mold, but because of the poor thermal conductivity will cause the dessert is not easy to color, and is not resistant to high temperature (more than 250℃).

Molds used in current baking come in different shapes, including round cake molds, hollow chiffon molds, square toast boxes, and flat muffin liners, which determine the heating efficiency of the molds is different. In the case of the same material, the more closed the mold is, the less likely the moisture inside will evaporate and the less radiation the ingredients will absorb, so the heating efficiency will be slower.
Volume of the mold is too large, the above situation will also occur, then you can only reduce the temperature and extend the baking time to ensure that the ingredients are cooked through.
Hollow mold is designed to increase the heating area and improve the baking efficiency of the improvement program.

In addition to material and shape, the color of the mold can also affect heat transfer, which is caused by the fact that different colors can absorb heat to different degrees. What you need to know is that the darker the color of the mold, the higher the heat absorbed by radiation.
A black mold will then heat up much faster than a silver mold. So you need to learn to adjust the reasonable baking temperature according to the different color of the mold.

In general, the bottom of the mold should be supported by a baking net, not a baking tray.
Because the baking pan and the mold are not tightly together, just like the ground can support a four-legged table, but in fact the contact area between them is negligible, and the contact area will affect the efficiency of heat transfer between the two objects.
If you use a baking net, the bottom of the heat pipe is convection, radiation in the form of direct heat transfer to the mold, less baking pan this intermediary, heating efficiency is naturally much higher.

  • Application Examples

The essence of baking food is to raise the temperature of the food so that the molecules inside it undergo chemical changes.
An important factor affecting the heating efficiency is the water content, because the boiling temperature of water is 100°C. As long as a certain part of the food contains water, the temperature at that location will not exceed this value, and then the Merad and caramelization reactions that cause the food to ripen will not be possible.
Oil and sugar have the ability to block the movement of water molecules, so the higher the content of oil and sugar, the less likely the water will boil, and the longer it will take to heat.

To achieve a state of inner cookedness and outer not burnt is very much a test of the baker’s skill and experience. The good thing is that recipe writers will provide us with a set of applicable baking solutions, so that we will not be like headless flies.
The three most typical recipes are cakes, bread and cookies, which have different ratios of water, oil and sugar, so we have to design a reasonable plan according to the local conditions.

In cake batter, the liquid content is usually much higher than the flour, and the ratio of oil to sugar is also higher.
This type of batter is highly liquid and requires semi-enclosed molds to be loaded. When the mold is large, it needs to be baked at low temperature for a long time to slowly allow the moisture to evaporate away. Obviously, the moisture evaporates much faster on the outside than on the inside, and only when the surface of the cake becomes dry does it begin to ripen. Cake batter is the most unstable of the three types of food, and it is very easy to deflate when left to stand for a long time. Therefore, the oven should be fully preheated in advance and then baked in time, which will prevent the bottom from depositing and the cake will be more easily cooked.

Compared to the large amount of water in the cake batter, the bread dough has a moderate amount of water and less oil and sugar. Since the dough is basically solid and not liquid, in most cases no mold is used, only a baking sheet is needed.
Molded bread like toast, thanks to the water retention capacity of the toast case, the finished bread retains more moisture and has a softer texture, but at the cost of a longer baking time. In addition, most breads that do not require molds heat up more efficiently than cakes, provided of course that the volume is the same.
Those low-sugar or no-sugar European-style bread, which usually contains more water and lacks the nutrients for the browning reaction, need to be heated at a higher temperature to color, and with the steam, it will produce a crispy outside and soft inside effect.

In addition to the three more common types of food mentioned above, there are two more extreme cases of baking: the low-temperature water bath method and pizza, which requires ultra-high temperatures.

Water bath method is generally used in cheesecakes and gourmets’ cakes, which are desserts that need to be baked at low temperatures.
As long as the water in the baking pan is always present, the temperature delivered to the mold by the lower heat will not exceed 100℃, and the heating of the upper heat will be affected by the water vapor. The bottom and sides of desserts with this heating method are less likely to ripen and expand very slowly without large air holes. During the baking process, the oven is misted with water, which keeps the cake moist overall.
Water bath method utilizes a layer-by-layer conduction method in which the baking pan conducts heat to the water, which in turn conducts heat to the mold, and finally the mold conducts heat to the batter. In the process of several conduction, the heat will gradually decay, so the water bath method requires the baking pan and water to be fully preheated until the water appears to boil in the baking pan, which is the best time to send it to the oven.

Pizza first originated in Italy, the local people like to use the kiln to bake their specialties. The temperature of the charcoal fire in the kiln is as high as 400 degrees Celsius, making the pizza cooked in a very short time, to achieve a crispy crust, tender and juicy inside the state.
Home made pizza naturally only use electric oven, but the ordinary oven is the limit of 250 ℃, how to simulate the effect of the kiln? Remember I mentioned earlier that the slab can maintain high temperatures, so you can use an electric oven with a baking slab this combination. When the slab is fully preheated, it can deliver a lot of heat to the dough at the bottom of the pizza in a short period of time, making the crust extremely crispy, while the cheese topping at the top can avoid losing its soft and drawn effect after a long time of baking.

Baking solutions for other foods are basically much the same, and can be considered based on its water, oil and sugar content.

  • Summary

Although an oven is nothing more than a box that generates heat, the entire process of baking food using electricity involves a lot of common sense physics and chemistry.
To be proficient in your oven, in addition to mastering the theoretical basis of how the oven works, heat transfer, temperature distribution, load-bearing appliances, and application examples, you must also change your strategy according to the actual situation.

Perhaps this is the dumbest way to learn to bake, but it’s also the most useful. Without the spirit of research, you can’t do baking well.
Do not always expect others to tell you exactly what to do, but learn to tell yourself what to do, after continuous experimentation and summary, over time you will be able to be on your own.

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