Effects of pretreatment on characteristics and qualities of Chinese jujube drying by segmented intermittent microwave coupled with hot air

Abstract: This research was aimed to explore the optimal drying process of Chinese jujube by segmented intermittent microwave coupled with hot air drying (IM&AD) and to study the influence of different drying pretreatment technologies on drying rate, unit energy consumption and quality of jujube. The drying pretreatment technologies included dipping in alkaline emulsion ethyl oleate solution followed by slow freezing at -18 ℃ (AEEO + freeing), high pressure carbon dioxide (HPCD), and hot water blanching (HWB). Firstly, to obtain the best drying process, intermittent ratio in the first drying stage (IR1), transition point in moisture content between the 2 stages of drying (TPMC), and intermittent ratio in the second drying stage (IR2) were set as the independent variables, and the drying time, unit energy consumption and vitamin C content after drying were set as the dependent variables. According to central composite design of the response surface technology, the obtained optimal process parameters were IR1 of 4 (the ratio of runtime to stand-by time of microwave oven was 5 s : 15 s), TPMC of 1, and IR2 of 6 (5 s : 25 s). On this condition, the drying time was the shortest (625.27 min), the unit energy consumption was the minimum (5 128 kJ) and the vitamin C content after drying was the maximum (419 mg/(100 g)). Then, to know the effect of drying pretreatment technologies on quality, Chinese jujube samples were dried respectively by 3 pretreatment technologies combined with the best drying process. The results implied that the special epidermis structure of Chinese jujube may be damaged during drying pretreatments, and especially the method of AEEO + freeing led to the moisture in Chinese jujube much more easily diffused and evaporated. So the drying rate was obviously improved and the energy consumption was reduced. Correspondingly, with regard to drying rate, the AEEO + freeing groups were the maximum, followed by the HPCD groups and the HWB groups, and the control groups were the minimum. The unit energy consumption was different: The AEEO + freeing groups were the minimum and the HWB groups were the maximum. There were rich amino acids, saccharides, vitamin C and polyphenols in Chinese jujube. So the browning occurred easily during pretreatment and drying process including enzymatic browning and non-enzymatic browning. The results showed that the HWB groups could inhibit enzyme activity and reduce the enzymatic browning degree, while the AEEO + freeing and HPCD groups would enhance enzymatic browning degree by breaking the cell membrane. So the browning degree of HWB groups was the minimum (P<0.05). In addition, a closely positive correlation was observed between the browning and drying time. The brightness of Chinese jujube changed obviously after drying process compared with the control group, but there were no significant differences between the 3 pretreatment groups (P>0.05). The red was the characteristic color of Chinese jujube, and it became darker with drying time continuing due to oxidation reaction. The HPCD pretreatment resulted in the highest vitamin C content, total flavonoids and phenolics retention, and oxidation resistance value. It was mainly due to the acidic condition provided by carbon dioxide in HPCD, which could make vitamin C, flavonoids and phenolics more stable than others. Meanwhile, these components had a potent antioxidant activity, so the HPCD groups had a stronger antioxidant ability. The results show that the IM&AD combined with the HPCD is advanced technology to dry Chinese jujube, and the data in this study can promote this technology to be applied widely.