MICROWAVE-ENHANCED DENATURATION OF SUGAR-BEET TISSUES

Microwave-enhanced sugar beet denaturation was investigated by 1) meas uring the temperature response of cossettes due to the 'thermal effect ' of microwave power; 2) examining the effect of the microwave dose (M D) on the effective diffusivity of sugar through beet tissues, D-e; 3) determining the most appropriate MD for microwave denaturation of bee ts; and 4) dissecting the impact of microwave radiation on the structu re of beet cells through transmission electron microscopy. Statistical analysis showed that the temperature response of cossettes contained in a cylinder (diameter 150 mm) and heated at 2450 MHz was similar to that of distilled water microwaved under the same conditions (alpha=0. 001). A quadratic response function was found to be a very good approx imation to the measured relationship of temperature Versus MD (alpha=0 .01). The effective diffusivity, D-e, increased rapidly with MD in the range between 216 and 768 kJ/kg. Above this range D-e remained almost unchanged [(1.63 +/- 0.06) x 10(-10) m(2)/s]. The relationship of D-e versus MD was closely approximated by a sigmoid-shaped asymptotic fun ction. Beet samples started to lose their stiffness (resilience) from the MD of 1170 kJ/kg and up due to moisture vaporization. By the crite ria of highest possible D-e and the least possible deterioration of co ssette texture, the best MD for microwave denaturation of beets appear ed to be within the range from 768 to 1170 kJ/kg. Considering the disi ntegration of cell walls caused by microwave radiation at higher MDs, it was better to maintain a MDs of 768 kJ/kg. Electron micrographs sho wed that microwave power had substantial effects on the structure of b eet cells. Similar microstructural changes were observed for beet cell s denatured by scalding.