Cooking provides many examples of different types of chemical reactions. Weak bonds require much less energy to break and manipulation of weak bonds in the kitchen occurs at lower temperatures of boiling water or below. Covalent bonds are stronger and require more thermal energy to be broken and rearranged. Baking and frying temperatures supply the high thermal energy to make new molecules that have new tastes and colors.
Chemical Bonds Release Energy when Made and Require Kinetic Energy to be Broken
Bond formation is reflected in the energy released when two atoms or molecules collide and are held together by the lower energy of their new configuration, i.e. energy is required to break the bond and separate the atoms/molecules. Water molecules are in constant motion and the higher the temperature, the greater the energy of motion, kinetic energy. Colliding water molecules can momentarily form hydrogen bonds, by partially sharing a hydrogen atom between the oxygens of two molecules. This bond represents a lower energy configuration for the two bonded molecules and energy was released. Other water molecules collide with the bonded molecules, contribute their kinetic energy, the hydrogen bond is broken and the less energetic molecules scatter.
Covalent Bond Rearrangements Produce New Molecules
Covalent bonds release much more energy in their formation than weak bonds, such as the hydrogen bonds involved in hydrophilic and hydrophobic interactions. Rearranging molecules by breaking and reforming covalent bonds, requires much higher temperatures well above room temperature or boiling. That is why baking, grilling or frying is needed to provide new flavors and colors unavailable through boiling. Proteins in egg white can be whipped into meringues or gelled by boiling, but frying or baking produces browning and new flavors. Frying rearranges the covalent bonds in the egg white proteins to create new molecules with new properties.
Activation Energy for Reactions Comes from Thermal Energy
Molecules are constantly colliding with their neighbors and the collisions result in intense vibrations as the atoms of a molecule are shaken against the limits of their bonds. The energy of collisions is related to the energy of motion, kinetic energy, of the molecules, which is called the temperature. Higher temperature means a higher average kinetic energy of the molecules. Reactions involving higher bond energies will take place at higher temperatures. The rate of the reaction will be dependent on the fraction of the molecules that have a high enough kinetic energy to break the bonds involved in the reaction. The energy required for a reaction to take place is called the activation energy and the activation energy is provided by the thermal energy.
Food Preparation Illustrates Weak versus Covalent Bonds, and the Thermal Energy Needed For Reactions
The preparation of food involves breaking and forming chemical bonds by various forms of heating to increase the thermal energy available. Whipping, dissolving and boiling can break weak, hydrogen bonds, but high thermal energies of frying, baking and grilling are needed to change covalent bonds to produce new colors and flavors.
References:
Alberts, B. et al. 2008. Molecular Biology of the Cell, 5th ed., Garland Science.
Campbell, N.E, et al. 2007. Biology, 8th ed., Benjamin Cummings.
Join the Conversation