22-1

S. KASAPIS and I. Al-Marhoobi. Food Science and Nutrition, Sultan Qaboos University, P.O. Box 34, Al-Khod, Muscat, 123, Oman

In synthetic polymer research, the equation of Williams, Landel and Ferry (WLF) has provided a mainstay in utility and the theory of vitrification of amorphous systems. It has been reported that the WLF equation is not applicable to polysaccharide networks due to development of intermolecular associations, resulting in a sharp change in the distribution of relaxation times during the coil to helix transition.

Our objective is to show that sugars cause massive changes in the nature of the structures formed by commercial polysaccharides at normal level of use (< 1%), which can be followed by the WLF equation.

The structural properties of mixtures of k-carrageenan, agarose and gellan with sugars at a total level of solids between 60 and 87% were followed by measuring the storage and loss modulus (G' and G", respectively). Samples were cooled/heated at a scan rate of 1°C/min between the temperature range of 90 to -60°C thus covering the transformation from a viscous solution to a hard glass. Modulus values soared to 10e10 Pa with the loss modulus passing through a maximum of 10e8.5 Pa. At this stage, the trace of G' overtakes that of G" thus clearly demarcating the end of the glass transition and the beginning of the glassy state. Mechanical spectra (10e-1 to 10e2 rad/s) were taken at regular temperature intervals and the time-temperature superposition principle was used to create composite curves of twenty decades of frequency.

The combined free volume/WLF theory was applied to the composite curves to define the rheological glass transition temperature (Tg). Thus, small additions of polysaccharide shifted dramatically the Tg of sugar preparations to higher temperatures. The Tg acquires physical significance as it demarcates the passage from the glass transition with free-volume molecular processes to a glassy state where energetic barriers to rotation become dominant.