Kinetics of sun and air drying of different varieties of seedless grapes

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Abstract
The drying kinetics and the quality parameters of seedless grapes were evaluated in a screening test carried out to select the variety or varieties best suited to the production of dried grapes. Experiments were carried out on a pilot scale by sun drying and air convection drying under controlled conditions and with the grapes pretreated by ethyloleate dipping. The kinetics data, analysed according to a simplified form of Fick's law, allow the calculation of a rate constant, which we tried to correlate to the grapes' geometrical parameters: i.e., the berries' average radius and skin thickness. A systematic computing procedure showed that the following simple relationships were well correlated to the experimental values and could be used to forecast drying kinetics: Sun drying: K= 0. 05–12.8XR2XTh Air drying: K= 0. 42–87.8XR2XTh where:K= rate constant, (per hr),R= average berries; radius (cm),Th = average skin thickness (cm × 10−3).The following scale of drying speeds for the varieties was found:Ruby > Emerald > Thompson > Delight > Rodiwith a two‐fold decrease of the rate constant value from Ruby to Rodi, which are respectively the fastest and the slowest drying varieties. Composition and colour data show that experimental samples compare favourably with commercial samples of dried grapes, as a consequence of faster drying due to the dipping pretreatment and careful control of drying conditions.

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