The importance of glassy biopolymer components in food.

The glassy state is not just important in low moisture and frozen foods, where it influences the physical and chemical stability and the crispness of some foodstuffs: glassy biopolymer components affect the physical properties of most food systems and low moisture biological systems. Glassy foods are not always fragile and crispy in texture. Therefore, the relationship between mechanical behaviour and molecular dynamics in low-moisture biopolymer systems will be considered. Vitrification of a macromolecular system only requires a mutual fixation of a certain proportion of the chain segments. The higher the rigidity of the chains, the lower the number of the chain segments which must be mutually fixed to vitrify the system. Mechanical stress can help the thermal movement to re-activate the motion of mutually fixed segments and involves a long deformation of glassy material. The stress required to effect long deformation presumably increases up to the strength of the material as the temperature decreases from the glass transition to the temperatures of brittleness (crispness). Vitrification of a loaded viscoelastic system results in an accumulation of mechanical energy (memory effect), which can be released (elastic recovery) above the glass transition temperature due to heating and or addition of a plasticizer. The effects of memory and elastic recovery could be of particular importance for producing foodstuffs which change their form, e.g. self-stirring dry foods and drinks on re-hydration in hot water. The importance of glassy biopolymer ingredients from the viewpoint of food formulation and processing is discussed.