| Kiwifruit is a kind of berry with high nutritional value,which is rich in minerals,vitamins,polyphenols and other functional ingredients beneficial to human body.However,kiwifruit tends to become soft and rotten after harvest,and hence,it is valuable to develop its preservation and processing technology.Processing fresh kiwifruit into dried products could be an effective method to extend its shelf life,simultaneously reduce its storage and transportation costs.In view of the research status of kiwifruit drying,In view of the disadvantages of long drying time and unsatisfactory product quality of kiwifruit by conventional hot air drying,direct contact ultrasound technology and far-infrared radiation(FIR)technology were applied to kiwifruit drying in this study.FIR drying could realize the internal heating of kiwifruit and improve its heating condition,while the application of direct contact ultrasonic could effectively reduce the resistance of mass transfer inside the material and accelerate the diffusion of water.The combination of the two drying technologies could theoretically achieve the purpose of high-efficiency and high-quality drying of kiwifruit.Therefore,the objective of this study was to investigate the drying characteristics of ultrasound assisted FIR drying of kiwifruit slices,and discusses the influence of drying conditions on the bioactive components of dried products.The research could provide a reference for the research and development of kiwifruit drying technology and high-quality processing of kiwifruit.The main contents of this study are as follows:1.The effect of different color fixatives on the PPO activity of kiwifruit was studied by spectrophotometry.Based on the single factor experiment and response surface methodology,the optimum condition for discoloration prevention of kiwifruit slices before drying was selected.The results showed that 4-hexylresorcinol,L-cysteine,D-sodium erythorbate and citric acid had significant inhibition on PPO activity.The optimal condition for discoloration prevention was: the mass fractions of4-hexylresorcinol,L-cysteine,D-sodium isoascorbate and citric acid were 0.021%,0.020%,0.208% and 0.311%,respectively.2.In order to reveal the basic law of FIR drying of kiwifruit,the effects of FIR temperature on drying characteristics of kiwifruit slices was discussed.The results showed that drying time reduced by 60% and average drying rate increased by 150% as FIR temperature increased from 120 to 280℃.SEM observation verified that porous structure of kiwifruit tissue could be formed by increasing FIR temperature.LF-NMR technology was applied to analyze the influence of FIR temperature on internal water states and moisture migration of kiwifruits during drying.The results showed that the water states in kiwifruits included free water,immobilized water and bound water,and higher FIR temperature could accelerate the migration and removal of moisture.3.In order to explore the enhancement effect of direct contact ultrasound during far-infrared radiation drying of kiwifruit,the drying characteristics and moisture transfer rule of kiwifruit slices during ultrasound assisted FIR drying were investigated.The results showed that the application of ultrasound in FIR drying could significantly improve the drying rate,and shorten the drying time by 13～40%.SEM observation illustrated that higher ultrasonic power could produce looser microstructure accompanied by more and larger channels,which was conducive for moisture migration.The results of LF-NMR showed that ultrasound application in FIR drying could promote the evaporation and diffusion of water.The removal time for free water at ultrasonic power of 54 W was 57% shorter than that without ultrasound application,and higher ultrasonic power could accelerate the migration and removal of immobilized water and bound water.MRI images showed the relative proton intensity decreased faster at higher ultrasonic power,elucidated that higher ultrasonic power could promote outward migration of water during FIR drying,so as to facilitate the removal of water and accelerate the drying process.4.The bioactive components and antioxidant activities of dried kiwifruit were investigated based on spectrophotometry,amino acid analyzer and high performance liquid chromatography,in order to explore the effects of drying conditions on the quality of dried products.The results showed that ultrasound application in FIR drying could improve the retention of bioactive components in terms of phenols,vitamin C,glutathione,free amino acids and triterpenes.Properly increasing ultrasonic power could effectively shorten the drying time,which was conducive to reduce the loss of thermal sensitive components such as phenols,vitamin C and glutathione.However,excessive strong ultrasound might cause the cell disruption and the release of intracellular content,resulting in the loss of bioactive components.Kiwifruit slice dried at ultrasonic power of 36 W showed greater antioxidant capacity,while the sample dried without ultrasound application showed the weakest antioxidant capacity.The optimum process parameters were determined as radiation temperature of 200℃ and ultrasonic power of 36 W by adopting comprehensively weighted grading method.With this condition,the drying time could be greatly reduced,and the retention of bioactive components could be improved to the largest extent. |
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