Effect of glass transitions on the rheological properties of food polymers

The rheological properties of food products are highly dependent on the physical state of the structural matrix, which ranges from the glassy state, the glass-rubber transition region, the rubbery plateau region, the rubbery flow region, and viscous flow region. Depending upon the combination of temperature and/or moisture content, the viscosity could be anywhere from 10(15) mPa.s for glassy material and 1 mPa.s for flowable, low viscosity liquid. During the glass transition range alone, the rheological properties can change as large as 1000 times. The rheological properties of ingredients, in-process intermediates, and finished products also affect the processing techniques, textural quality, physical and chemical stability during storage. In this paper, the similarity in theological behaviors between synthetic and food polymers, as affected by degree of crystallinity, cross-linking, plasticization, and phase separation is reviewed. Various mathematical equations for modeling the changes in theological properties during glass transitions are also discussed. The advantages and shortcomings of each model will also be compared.