Kinetics

Abstract

The equilibrium properties of a food depend on the ingredients selected and the conditions, so for example bread left in the oven will reach its equilibrium as a blackened crisp while bread left on the shelf will stale. However, by controlling the time and temperature during cooking and storage, the bread never reaches either of these undesirable states but is maintained in the correct nonequilibrium state. In fact most foods are in nonequilibrium states and we must understand both the thermodynamics, to understand if change is possible, and the kinetics, to understand how far it can proceed.

We will use kinetics in two main ways: first to answer the practical questions about processing and storage—if I can pasteurize milk in 30 min at 63 °C, how long must I process at 73 °C? What shelf life can I claim for my cookies? I know the vitamins advertised as healthy supplements in my granola bar will oxidize during storage— how much do I have to add to make sure that the claims on the label will still be valid if the product is stored for a year before it is sold? The second group of questions concerns the mechanism of reactions. The way the concentration of an ingredient changes with time and the way the rate is affected by the presence of other ingredients can be used to test the proposed mechanisms for the reaction. In this chapter, we will start by contrasting kinetic theory with thermodynamics. We will then look at how rates of change can be modeled in terms of the rate equations and briefly consider the relationships between kinetics and mechanism. We will show how the temperature dependence of rate can be used to infer some of the properties of the transition state and finally consider catalysis.