Preheating an oven is an essential part of baking. Doing so is an insurance from temperature drops. Preheating also provides time for the metal walls of the oven and air the reach the required temperature.
If we look inside the oven, the hot metals act as a store of heat. This is known as the “heat sink”, which radiates heat inward to prevent the temperature from fluctuating.
During baking, it is sometimes inevitable to open the oven door. Well, actually, it is a common mistake, especially when baking cakes. Because the rush of cold air may cause the cake to sink, making it to go flat, and giving you a denser cake. Furthermore, there is moisture loss that encourages the formation of a hard crust sooner.
But these undesirable results do not apply for all baked products. One good example is bagel. Before they are placed in the oven, this baked good of Jewish origin are boiled in water prior to baking. So its structure will not be affected significantly if the door is opened halfway through. Well, unless the bagel is boiled in water for longer than necessary—it will immediately go flat out once taken out of the water.
It is safer to open the door once the product has stopped rising and the crust has formed.
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In-any-case, it is a general rule to keep the oven door closed when baking and have the oven properly preheated to prevent uneven baking. This applies to breads, pastries, and cakes. But how long does it take to properly preheat the oven though?
HOW LONG DOES IT TAKE TO PREHEAT AN OVEN
A fully preheated oven will ensure that the temperature drop is short lived when the door is opened. Well, provided that you closed the door promptly.
What the small heating element inside does is work to reheat the air if the walls are cold. But hot walls will help in recovering the correct air temperature immediately.
The thing is air heats quicker than metal. So if you set the oven at 360°F (180°C), preheated it for 15 minutes, and the thermostat shows the correct air temperature, the oven walls may still be cool. In most cases, 30 minutes or more must be allowed to properly preheat the oven if set at 360°F (180°C) and a “heat sink” to form. However, the power of the oven and the size must be taken into account.
On average, an oven will take around 20 minutes to fully preheat. Although the thermostat light will let you know by turning off, allow at least 10 minutes before you actually bake. Why? The oven will detect lower and higher temperatures, but the temperature indicated is the average. So if you allow the oven to take more time, the fluctuations will become smaller. Hence, the temperature becomes more accurate.
