Antiplasticization of Food Polymer Systems by Low Molecular Mass Diluents

摘要

Low molecular mass (LMM) compounds or diluents, acting as external plasticizers, are generally used to enhance the workability, flexibility, ductility, and/or distensibility of synthetic glassy polymers. However, at low concentrations, the presence of many of such diluents may instead cause a polymer-diluent blend to become stiffer or more brittle than the neat polymer—a phenomenon termed antiplasticization—even though the glass transition temperature (T_g) is depressed. Such a phenomenon appears not to be confined to synthetic polymers. Most food and other biomaterials may be regarded as partially amorphous, partially crystalline metastable polymeric systems, that display remarkable fundamental and generic similarities to synthetic polymers. Thus, LMM diluents (notably water, sugars, and polyols), which normally act as plasticizers, may exert effects on the mechanical properties of reduced-moisture food systems similarly in ways that reflect antiplasticization rather than plasticization. LMM diluents are therefore neither intrinsically plasticizers nor antiplasticizers. Besides alterations in properties of the polymer-diluent blend, abrupt changes in properties of the diluent itself may be indirect reflections of mechanical antiplasticization of the polymer. Although the physical manifestation of antiplasticization by LMM diluents is undisputed, the actual mechanisms involved remain unresolved. This review presents an overview of this important fundamental phenomenon that can simultaneously and profoundly influence various physical and functional properties, and hence quality and acceptability, of certain food products. Attention is focused on water (which is undeniably the most important diluent in foods) and glycerol (a commonly used humectant), and their interactive plasticizing-antiplasticizing effects.