(213c) Why Is the Mustard in the Fridge?  a Fun Assignment on the Thermodynamics of Food Safety

Activity is often introduced in chemical engineering thermodynamics courses in the context of modeling non-ideal solutions in vapor-liquid equilibrium, but this is only the first of many useful applications. One of these applications is estimation of osmotic pressure, often in the context of drinking water purification or creation of isotonic medical saline solution. In both of these examples, the activity of water in solution (a_w) is the key value in the analysis. For both my thermodynamics and applied food science courses, I created an assignment that focuses on yet another important application of a_w: food safety. Prior to refrigeration, extremes of osmotic pressure were used to prevent or reduce microbial spoilage of foods through addition of salt, sugar, acid, or a combination thereof. This assignment, which works both in-class and as homework, starts with published food labels and a few assumed molecular masses and results in fairly accurate determinations of whether or not a given food should be “shelf stable” without refrigeration. The assignment can also be extended to ask students to invent a recipe for preserving fruits and vegetables that result in answers remarkably similar to traditional versions of jam, raisins, and pickles.