English abstract
Drying rate of apple rings increased with the increase of air-drying temperature, thus indicating the possibility of crust formation at higher temperature. Characteristics of air-dried apple produced by drying at 50, 80 and 105°C were studied by measuring true, apparent and material density, apparent and open pore porosity, and pore size
distribution with helium gas pycnometer and mercury porosimetry. Apparent porosity increased with thdecrease of moisture content as well as with the increase of drying temperature. Fresh apple showed negligible closed pores, whereas drying ca caused collapse of surface pores and consequently forming high closed pores. The shrinkage-expansion coefficient increased with the increase of drying temperature, indicating enhancement of pore formation at higher temperature. Pore-size distribution for fresh apple showed two sharp peaks, one at 5.8 um and another one at 3.6 um, respectively. Apple rings dried for 20 h showed only one sharp peak at 3.6 um, and 30 h dried sample showed 3 peaks at 10, 5.8 and 3.6 um, respectively. Hysteresis was observed in intrusion and extrusion curves due to the different path followed by extrusion curve compared to the intrusion path. Application of high pressure (0.12-414 MPa) resulted in very negligible intrusion of mercury in the sample. Fractal analysis indicated the formation of more micropores due to air drying.
