The importance of glassy biopolymer components in food

The glassy state is not just important in low moisture and frozen foods, wh ere it influences the physical and chemical stability and the crispness of some foodstuffs: glassy biopolymer components affect the physical propertie s of most food systems and low moisture biological systems. Glassy foods ar e not always fragile and crispy in texture. Therefore, the relationship bet ween 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. T he higher the rigidity of the chains, the lower the number of the chain seg ments 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 req uired 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 viscoel astic system results in an accumulation of mechanical energy (memory effect ), which can be released (elastic recovery) above the glass transition temp erature due to heating and/or addition of a plasticizer. The effects of mem ory and elastic recovery could be of particular importance for producing fo odstuffs which change their form, e.g, self-stirring dry foods and drinks o n re-hydration in hot water. The importance of glassy biopolymer ingredient s from the viewpoint of food formulation and processing is discussed.