INVESTIGATION OF GELATION PHENOMENA OF SOME POLYSACCHARIDES BY ULTRASONIC SPECTROSCOPY

The gelation process of two polysaccharides, amylose and alginate, has been investigated by means of ultrasonic spectroscopy. This method allows an immediate analysis of the acoustic wave propagating through the medium by means of Fast Fourier Transform of the signals. Measurements were performed within the frequency range 15-100 MHz. The absorption and the velocity of the ultrasonic wave can therefore be obtained as a function of time over a given frequency range. The results were compared to classical theological measurements in oscillatory shear at a low strain amplitude and, in case of amylose, to turbidity variations. In the course of amylose gelation, variations in the absorption of the ultrasonic signal paralleled those of turbidity in the medium and took place at a much earlier stage than the one at which the increase in G' and G '' can be detected. In the case of alginate, absorption variations paralleled more closely those of the viscoelastic parameters G' and G ''. The differences can be interpreted in relation to the mechanisms of gelation. Amylose gelation takes place through a phase separation. Ultrasound appears extremely sensitive to such changes in the material. The method shows the first steps in the phase separation phenomenon. This suggests that variations in the ultrasonic absorption can be taken as a means to detect the earlier steps of the gelation process of starchy materials. Gelation of alginate is a calcium-mediated process, in which case, ultrasounds provide similar information as theological measurements.