Basic Thermodynamics

Abstract

We expect food to change over time; recipes suggest cooking times, packaging states a shelf life, and we will pay more for a 10-year-old than a 5-year-old whiskey. Some changes occur over a fraction of a second and others over several years; some improve the quality of the food, while others harm it. Whatever the mechanisms involved, controlling changes in foods to optimize quality and ensure safety is the primary task of the food technologist. We must answer two important and distinct questions about change—what can happen, and will it happen fast enough to be relevant to the food we eat? The first question is concerned with the thermodynamics of the system and the second with the kinetics. If we observe a change, then we know that it is both thermodynamically possible and kinetically viable. If we see nothing, it could be either thermodynamically impossible or thermodynamically possible but kinetically too slow to be important. For example, during baking, bread will brown rapidly, i.e., we can conclude that the browning reaction is both thermodynamically possible and kinetically viable. However, if the same dough is held at room temperature, it remains the same pale color over several weeks; either the reaction is thermodynamically impossible under these conditions or thermodynamically possible but too slow to be seen.