Heat Transfer

In reality, to create the perfect baked goods does not only require knowledge on the chemistry behind baking, it is equally important to delve into the physics governing how the doughs and batters transform into delightful baked goods. Understanding the mechanisms of heat transfer — radiation, conduction, convection and induction — ensures that the transformation is exactly how you anticipated it to be.

Radiation

Radiation is the rapid heat transfer from a warmer object to the surface of a cooler object by electromagnetic (EM) radiation. When the particles on the cooler surface absorb EM radiation, they vibrate rapidly, then colliding with neighbouring particles to transfer energy. Appliances which utilises radiation for heat transfer include toasters, boilers, infrared heat lamps, conventional ovens, microwave ovens and dielectric ovens. Dull and dark materials have the highest emissivity so they are often used in cooking appliances. Emissivity is the relative amount of heat radiated by different materials, with dull black material having an emissivity of 1.

In conventional ovens, the hot oven walls radiate heat, so products nearest to the walls would receive the most heat. To prevent uneven baking, the product should be placed in the centre of the oven, away from the walls. The direction of pans should also be rotated halfway through baking. Only the surface of the baked goods is heated by infrared (IR) radiation.

In microwave ovens, the magnetron generates microwaves which is a tube present in the oven. Microwaves can penetrate many types of cookware and the surface of food more easily than IR radiations, so microwaves could penetrate further into the food than IR radiations, hence heating with microwaves is relatively fast. However, different substances absorb energy transferred by microwaves differently, so food tends to be heated unevenly.

Dielectric ovens radiate radio waves which could penetrate even deeper into food. Therefore, dielectric ovens can bake bread dough quickly and evenly, producing bread with the same colour and texture throughout. They are also used to bake Japanese bread crumbs (panko), making them uniformly white, light and crispy. This is used to prepare tempura and other fried food.

Conduction

Conduction is when vibrating particles from the hotter region collide with particles in the cooler region, transferring kinetic energy. If burned-on food is present on cookware, it would cause heat to be radiated unevenly on the surface of the cookware. Hence, removing burned-on food on cookware can ensure even cooking and browning.

Various methods can be used to slow down heat transfer by conduction. Water has low heat conductivity and a low specific heat capacity which is the amount of energy required to raise 1kg of a substance by 1°C. So water bath is often used to bake custard and cheesecake which are best baked slowly and evenly. Air has a even lower heat conductivity as gas particles are far apart so they do not collide as frequently. Double sheet pans trap a layer of air between the two sheet pans, slowing heat transfer. This can prevent the bottom of cookies from burning. Double boiler in which a bowl/pot is placed over a large pot of simmering water can trap an insulating pocket of air between the boiling water and the product, slowing heat transfer. This is used to gently warm products that could be damaged by high heat, such as egg whites, chocolate and fondant.

Materials that can conduct heat quickly have a high thermal conductivity. Solids have higher thermal conductivity than liquid or gas as particles are packed closer together. Metals tend to have a really high thermal conductivity as delocalised electrons are mobile to transfer thermal energy through the metallic lattice. Materials that are poor conductors of heat are called insulators.

Convection

Convection involves the movement of heat through fluids, including liquid and gas where particles could flow past each other. As warmer particles have more kinetic energy and vibrate more, the particles become less dense and would rise above the cooler particles. Together the constant upward movement of warmer particles and the downward movement of cooler particles create a convection current. The movement of particles can be accelerated if the fluid is stirred.

In convection ovens, the movement of particles in the fluid is sped up, Some convection ovens have fans that blow hot air, forcing the movement of air throughout the oven. Reel and rotating ovens move the product through the air, causing hot air to move more rapidly towards the cooler surfaces of the baked goods. Convection ovens are suitable for heavy doughs such as cookies but are not suitable for cakes, muffins, sponge cakes, soufflés, custards and cheesecakes. Cakes and muffins can take on an asymmetric shape; sponge cakes and soufflés can lose volume; custards and cheesecakes can easily over-bake. If a convection oven is used, remember to lower the baking temperature and shorten the baking time. The number of times and the amount of times that the oven door is opened should be minimised, this is to prevent convection currents from forming between the cooler air in the kitchen and the warmer air in the oven. In addition, avoid overloading the oven to ensure that there is space between pans for air to circulate.

Induction

Induction heating uses electromagnetic fields to directly heat pans made of magnetic materials, providing a fast and energy-efficient cooking method. Coils below smooth-top ceramic surfaces generate a strong magnetic field, causing molecules in the pan to rapidly flip. The vibrating particles then collide with one another to transfer heat via conduction. For this to work, pans must have a flat bottom so that more molecules could be flipped by the EM fields at a given amount of time. Advantages of induction cooking include reduced heat lost to the surrounding due to the pan being heated directly, faster and better temperature regulation. 

Understanding the mechanisms of different methods of heat transfer and utilising them to your advantage could greatly improve baking results, ensuring even cooking, proper texture, and delightful flavours in your baked goods:)

Reference

• Figoni, P.L. (2010) How Baking Works: Exploring the Fundamentals of Baking Science. 3^rd ed. Providence, Rhode Island: Wiley.