ENERGY REQUIREMENT AND QUALITY ASPECTS OF CHINESE JUJUBE (ZIZYPHUS JUJUBA MILLER) IN HOT AIR DRYING FOLLOWED BY MICROWAVE DRYING

Chinese jujube samples were dried in hot air at 60C, and then switched into microwave drying of 45, 65 and 90 W at moisture contents of 50, 40 and 30%, respectively. Results indicated that the energy consumptions of combined drying at a transition point of 50% were all lower than that of microwave drying by itself at three power levels, which might be ascribed to the reduction of initial heat-up region in microwave drying of combined drying. As compared with hot air drying alone, the combined drying can save energy consumption up to about 55%. Combined drying was higher in the rehydration capacity of the dried products than microwave drying alone or hot air drying alone, and is also beneficial to the preservation of vitamin C and less browning. Hot air drying followed by microwave drying of 45 W at transition point of 50% was best in terms of higher product quality with less energy consumption. PRACTICAL APPLICATIONS Jujube is a fruit distributed to over 30 countries around the world. Drying of jujube is energy intensive. Hot air drying alone takes too much time, requires more energy consumption and degrades the product quality, and microwave drying alone will result in more browning of the product. The combined drying - hot air drying followed by microwave drying - is a promising way to lower energy consumption and to raise product quality.