Kinetics of Physical Changes

Physical changes in foods are very important in relation to quality. Many foods are dispersions: they contain (colloidal) particles. Examples are fat globules in an aqueous phase (as in milk), water globules in a fat phase (as in margarine), foam bubbles in an aqueous phase (as in beer foam), fat crystals in an oil phase (as in butter and margarine), protein particles in an aqueous phase (as in milk), starch granules (as in bread), etc. Quality changes arise from changes in such dispersions. Most colloidal systems are inherently unstable in a thermodynamic sense, in other words the systems are not in equilibrium; they may, however, be kinetically stable. For instance, emulsions will tend to separate into an oil and water phase, which is the result of coalescence. Aggregation may occur, of emulsion droplets, of protein particles, of crystals, etc. Sometimes these processes are desirable, for instance, coalescence in the case of churning of cream, coagulation of casein micelles in cheese curd formation, formation of sugar crystals in sugar manufacture, etc. Mostly, however, such changes will result in quality loss. Examples are aggregation and sedimentation of fruit particles in a fruit juice, serum separation in tomato ketchup, creaming of fat globules in milk, sugar crystallization in candy bars, and many more examples could be given. In this chapter we will discuss how some of these changes can be described kinetically. The size scale of particles present in foods may span up to six orders of magnitude; the structural elements range from low molecular weight molecules to high molecular weight compounds to colloids to microorganisms, i.e., from nanometers to some hundred micrometers. In general, the larger the particles, the longer the time scales involved at which something happens (such as a molecular reaction or aggregation of colloidal particles). We need to remark that kinetics of physical changes is not an easy topic to discuss. Most of the available models are developed for dilute, ‘‘clean’’ systems, and foods are all but ‘‘clean’’ and dilute, as remarked before. One has to be careful therefore in applying models to describe physical changes in foods. Much active research is going on in this field and it is beyond the scope of this book to discuss all these developments; the interested reader will find some references at the end of this chapter. We will limit ourselves therefore to some phenomena that are important for food quality such as diffusion, aggregation and coalescence. Nevertheless, it is stressed that the length of this chapter is inversely proportional to its importance for food quality.