Pore formation in apple during air-drying as a function of temperature: Porosity and pore-size distribution

Characteristics of air-dried apple produced by drying at 50, 80 and 105°C were studied by measuring apparent porosity, open pore porosity and pore-size distribution with a helium gas pycnometer and mercury porosimetry. Apparent porosity increased with the decrease of moisture content as well as with the increase of drying temperature. Fresh apple showed negligible closed pores, whereas drying caused collapse of surface pores, and consequently reduced open pores. Pore-size distribution for fresh apple showed two sharp peaks, one at 5.8 μm and another at 3.6 μm. Apple rings, dried for 20 h, showed only one sharp peak at 3.6 μm while samples dried for 30 h showed three peaks, at 10, 5.8 and 3.6 μm, respectively. Application of high pressure (0.12-414 MPa) resulted on negligible intrusion of mercury into the sample. Fractal analysis indicated the formation of more micro-pores on the surface caused by air-drying.