4
Reaction Kinetics
Chemical kinetics is generally discussed with respect to reactions between
molecules (or ions or radicals) in a gas phase or in a very dilute solution. In
foods, we often have other situations. The system never is gaseous, it is
rarely very dilute, and it may have more than one phase containing
reactants. Changes may occur within molecules, especially macromolecules.
Reactions may be between particles, causing, for instance, their aggregation.
Numerous other changes may occur, such as phase transitions, leading to a
change in rheological properties, color, or other perceptible property. In
nearly all such cases we are greatly interested in the rate at which these
processes occur. This we cannot derive from the bond energies involved or
from other thermodynamic considerations: these may tell us what the
driving force is, but in general the rate results from a driving force divided
by a resistance, and the resistance may be very large or highly variable.
In this chapter, we will recall some basic aspects of chemical reaction
kinetics in solution, starting from an oversimplified point of view and
gradually bringing in more complications. We will not discuss theory aimed
at explaining reaction rates on a molecular level (molecular reaction
dynamics). Other rate processes will be discussed in Chapters 5 and 13.