Experimental and Numerical Study of Superheated Steam Drying for Sliced Avocado

Superheated steam drying (SSD) of sliced avocado is studied by both experiment and theory. An experimental system is built to evaluate the effects of drying temperature and gas velocity on the evaporation rate and color change of the product. Experiments are conducted for ripened Booth avocado from Daklak province of Vietnam. Experimental results show that evaporation is faster at higher temperatures and velocities, but the effect of temperature decreases at higher velocities. At temperatures below 130 °C, the product is not brown, but at a temperature in the range of 130–150 °C, the dried sample is burned. The experimental model is fitted with high accuracy to predict the moisture rate as a function of time. The evaporation processes of SSD are also compared with hot air drying at the same drying conditions. To extend the application ability of the result, the theoretical model is developed on the basis of Fick's law in which the spatial distributions of temperature and moisture are taken into account. The effective diffusivity is determined by an optimization tool with good agreement. This parameter is constant for all drying conditions, so the model can be applied to a huge range of drying conditions. The numerical results show that the temperature is almost uniform in the sample because of high thermal conductivity, but the moisture gradient is significant.