Water as a plasticizer: physico-chemical aspects of low-moisture polymeric systems

Abstract

Introduction ‘Water is the most ubiquitous plasticizer in our world.’ It has become well established that plasticization by water affects the glass-to-rubber transition temperatures ( T g ) of many synthetic and natural amorphous polymers (particularly at low moisture contents), and that T g depression can be advantageous or disadvantageous to material properties, processing, and stability. Eisenberg has stated that ‘the glass transition is perhaps the most important single parameter which one needs to know before one can decide on the application of the many non-crystalline (synthetic) polymers that are now available.’ Karel has noted that ‘water is the most important… plasticizer for hydrophilic food components.’ The physicochemical effect of water, as a plasticizer, on the T g of starch and other amorphous or partially-crystalline (PC) polymeric food materials has been increasingly discussed in several recent reviews and reports, dating back to the pioneering doctoral research of van den Berg. The critical role of water as a plasticizer of amorphous materials (both water-soluble and water-sensitive ones) has been a focal point of our research, and has developed into a central theme during six years of an active industrial program in food polymer science. Recently reported studies from our laboratories were based on thermal and thermomechanical analysis methods used to illustrate and characterize the polymer physico-chemical properties of various food ingredients and products (e.g. starch and rice; gelatin; gluten; frozen aqueous solutions of small sugars, derivatized sugars, polyols, and starch hydrolysis products (SHPs); and ‘intermediate moisture food’ (IMF) carbohydrate systems, all of which were described as systems of amorphous or PC polymers, oligomers, and/or monomers, soluble in and/or plasticized by water.