| Freeze drying(FD)is a process in which water in the form of ice under low pressure is removed from materials by sublimation.Freeze drying is a well-known technique to preserve the nutritive quality of fruits.However,processing time and energy consumption of FD process are high.Besides,the soft texture and high hygroscopicity of low-density fruits and vegetables were usually occurred in freeze-dried products.The purpose of this work was to investigate the effects of pretreatments and combined drying on the drying characteristics and quality of apple chips.Here are the main conclusions:(1)The effects of combined freeze drying on energy consumption,physicochemical quality of apple chips were investigated.The results showed that combined drying improved the hardness of apple chips.Compared with FD products,the hardness of the hot pump combined freeze-dried apple chips increased by 35%,crispness increased by 94%,hygroscopicity decreased by 12.2%(2 h),the retention rate of major phenolic compounds was 70.8%,and the aroma of the product was closer to that of FD samples compared with other combined drying methods.In general,the quality of apple chips can be effectively improved by using heat pump drying to water ratio 0.4 and then using freeze drying to the final water content.(2)In order to investigate the effects of freezing rate and freeze-thaw cycles(FTC)pretreatments on the drying characteristics and quality of FD apple chips,the apple slices were treated with different freezing temperature and 1-3 times freeze-thaw cycles pretreatments.The results showed that the freezing rate was increased with decreasing of freezing temperature and increasing of FTC times.According to the analysis of drying characteristics of apple slices,it was found that the Page model could better represent the drying process of apple chips with different temperatures and different freeze-thaw cycles.Compared with the-20?pre-freezing samples,the apple chips treated by FTC-1 had a more uniform and larger pore structure,the pore volume was increased by 37.2%,energy consumption was decreased by 14.6%,and crispness was increased by 117.6%.(3)We investigated the impacts of five saccharides osmotic agents on the kinetics of mass transfer during the osmotic dehydration,drying characteristics,water sorption isotherms,texture properties and microstructure of freeze-dried apple slices.The results demonstrated that the water loss efficiency of monosaccharides was higher than that of the disaccharides and oligosaccharide,based on k1 of the Peleg model(fructose:k1=155.0,glucose:k1=139.0,sucrose:k1=199.0,maltose:k1=270.2,isomaltooligosaccharids(IMO):k1=351.5).Besides,the drying rate of osmotic samples(0.372 g·g-1·h-1-0.557 g·g-1·h-1)was lower than untreated sample(0.654 g·g-1·h-1)at the primary drying.A significant positive correlation between water loss,solid gain and hardness(WL:R2=0.888,SG:R2=0.857)was confirmed.The hardness of apple chips could be controlled by osmotic agents and WL/SG.(4)The effects of ultrasound-assisted osmotic dehydration(UOD)with different frequency(40 k Hz,80 k Hz,40+80 k Hz)and intensity(25 W·L-1,50 W·L-1,75 W·L-1)on the kinetics of mass transfer,phenolic loss and antioxidant properties of apple cubes were investigated.The results demonstrated that the mass transfer efficiency of solid gain(SG)was higher under dual-frequency(?values were 0.413)than 40 k Hz frequency(0.707)and 80 k Hz frequency(0.793)under the same intensity.While the SG of75 W·L-1 UOD samples is supposed to increase by 62.5%compared with 25 W·L-1UOD at 60 min under the same frequency.Notably,the loss of phenolic under 75 W·L-1 UOD was 18.9%,substantially higher than that of the 25 W·L-1 UOD,which was 1.2-1.8%. |
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