An investigation into the drying of apricots using solar energy : A thesis presented in partial fulfilment of the requirements for the degree of Master of Engineering in Agricultural Engineering at the University of Canterbury, New Zealand

Abstract

In this thesis a drying model APRICOT_DRYING was developed using an empirical equation for drying constant k derived from a set of detailed experiments. The preliminary model verification produced some reasonable results for the assumptions considered. Solar drying was found to remove a significant amount of moisture during the simulated apricot drying.

Initially, a resolution IV two-level fractional factorial experimental design was used to identify relevant variables affecting the drying time of apricots by estimating their main effects and interactions between any two of them. The variables considered and the levels of each were air temperature, 50 and 60°C; relative humidity, 30 and 40% ; air speed, 2 and 5 m/s; sulphiting duration, 50 and 90 minutes; fruit size, 50 and 60 mm diameter; blanching, 30 seconds and none at all; and microwaving, with and without. Out of these seven variables, only microwaving was eliminated in succeeding experiments for lack of significance. The drying time to 25 % moisture content wet basis increased with the increase in relative humidity, fruit size, and air speed. On the other hand, an increase in air temperature and sulphur content shortened the drying time. Blanching and microwaving decreased the drying time as well.

The effect of air speed on the drying time of apricots was further investigated for the range of 1.985 to 4.955 m/s during the '88-'89 apricot season. This was triggered by the seemingly uncharacteristic trend observed from the previous experiments. It seemed to appear that an optimum air speed which may produce the shortest drying time lies somewhere between 2 and 3.5 m/s for relative humidity range of 30 to 40 % at 55°C dry bulb temperature. In another set of trials during the same season and using a modified tumble dryer, the generalized thin-layer drying equation was developed by studying the drying characteristics of apricots for an air temperature range of 35 to 55°C, initial moisture content of 85.7% wet basis, and air speed range of 0.75 to 2.0 m/s. A significant function expressing the drying constant k in terms of air temperature and speed was derived.