Drying Monukka grapes with air-impingement jet technique and quality analysis

An experimental device for drying grapes was designed according the principle of air-impingement jet technology. For this device, the hot air temperature and velocity were automatically controlled, the relative humidity of heat media could be monitored, and moreover, all of the data could be collected and saved. Based on this device, the drying technique and drying quality of Monukka grapes were studied. Experimental results showed that the drying curves of Monukka included three stages of pre-heating, constant rate drying and retarded drying. As the increasing of impingement temperature and hot air velocity, the drying rate of constant rate drying stage was improved, the drying time of this stage was shortened, but those factors were insignificant in retarded drying stage. There were no significant differences in the quality indexes of raisins at the range of experimental drying temperatures and velocities. The kinetics of the degradation of ascorbic acid was also studied under various drying temperatures. The pilot equipment of air-impingement jet dryer was developed and put into operation. The optimum technological parameters were air temperature of 65℃, air velocity of jet nozzle at 20 m/s, drying for 29 h. The moisture content (dry matter, %) of raisins is 17.1%, soluble solid content is 77.8%, titratable acidity is 2.3%, and ascorbic acid content is 6.74×10^-2 mg/g.