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

Abstract

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.

Identification of bitter compounds from dried fruit of Ziziphus jujuba cv. Junzao

“…If so, the formation mechanism of the bitterness of jujube fruit might be partially explained to occur during processing and storage. Maillard reaction [20] have been generally considered as a vital reaction for formation of jujube flavour and brown flesh colour, but there is no evidence to date whether Maillard reactions resulted in the bitterness of jujube fruits during processing and storage. So, future studies should pay attention to the Maillard reaction's effect on jujube flavour.…”

Changes of Physical Properties during Microwave Processing of Sagon

“…5. As mentioned earlier, the nonenzymatic browning reaction might be the main reason for this brown color formation because of either sugar oxidation or unsaturated bond formation between sugar and protein group (Fang et al 2011). It means that the sagon powder having higher sugar content exhibited darker with corresponding increasing redness and yellowness.…”

“…But the browning process was clear and faster when sagon powder was heated at higher microwave power. Higher browning degree at higher microwave power may be due to nonenzymatic Maillard reaction and brown pigment formation, which may happen either from sugar oxidation or because of unsaturated aldehyde or ketone formation through the reaction of sugar protein (Rhim et al 1989;Lopez et al 1997;Maskan 2000;Fang et al 2011). It is also noticeable that sagon powder processed at high microwave power also possessed high standard errors.…”

“…Research reveals that microwave drying is best suited at falling rate period or at low moisture content state to finished drying when it is the least efficient portion for conventional drying system (Andrés et al 2004;Fang et al 2011). As sagon ingredients are relatively in low moisture content, microwave drying or cooking is, therefore, best method to remove moisture besides other benefits.…”

Effects of Magnetron Arrangement and Power Combination of Microwave on Drying Uniformity of Carrot

“…This improvement not only expands the application of microwave drying but gradually improves the technology to alleviate process and equipment constraints. [1] These innovative combining techniques have been widely used in drying pumpkin, [7] jujube, [8] purple sweet potato, [9] apple, [10] lettuce cubes, [11] carrot, [12] and mushrooms. [13] Though microwave heating is widely used, it has a significant disadvantage of hot spots depending on sample geometry [1,14,15] due to its nonuniformity of heating.…”