ANALYSES OF TEMPERATURE AND MOISTURE PROFILES DURING APPLE DRYING

An experimental system has been designed to investigate the drying rate, temperature and moisture profiles developed within food materials during forced convective air drying. Moisture profiles were determined using the gamma ray densitometry, and embedded thermocouples were used to sense the local temperatures. The overall drying rate was measured using an electronic balance. A microcomputer and a computer front end were implemented for data acquisition and experimental control. An evaporation front was observed in a narrow region of the apple specimen, this front moved into the sample as drying progressed. The amount of apple shrinkage was determined to be linearly related to the receding distance of the evaporation front.;The experimental results from various drying conditions were collected and analyzed. The theoretical pseudo wet bulb temperature calculated from the drying air conditions agreed very well with the experimental results. The apple drying rate fell in the falling rate period when apples were dried at a relatively low humidity. The air humidity has the greatest influence of any of the drying conditions on the pseudo wet bulb temperature and on the time period of the first falling rate period. The temperature dependence of the drying rate followed an Arrhenius type of equation.;The heat and mass transfer coefficients were evaluated from the temperature profile, moisture profile and the drying rate curves. A linear relationship exists between the Nusselt number and the square root of the Reynolds number. A relationship of the Nusselt number, Prandtl number and Reynolds number in the drying period of 0 to 3 hours was developed, i.e., Nu = 0.719;A mathematical model based on the principles of heat and mass transfer in a porous solid was formulated to simulate the drying phenomena of apples. The Crank-Nicolson implicit finite difference scheme with the Tridiagonal Matrix Algorithm (TDMA) was employed to solve the parabolic partial differential equations.