The physical factors involved in the drying of sultana grapes

Abstract

The various physical factors involved in the drying of sultana grapes have been determined from measurements of drying rate under definite conditions of temperature, humidity, and air flow, in a specially constructed oven. Ninety-five per cent. of weight lost is water, 2 per cent. is carbon dioxide produced at an almost constant rate during drying, and 3 per cent. unassigned loss. Drying occurs in three distinct stages: (1) the grape retains its regular ellipsoidal shape by an elastic contraction of the skin; (2) the skin commences to wrinkle in the range of 20-50 per cent, loss in original weight; (3) the drying rate decreases markedly, beginning when approximately 95 per cent. of total loss of weight has occurred and when probably all the free water has been removed from the system. For each of these stages there is a linear relation between time and log [c / (c – w) 1, where c is the total loss in weight on complete drying (approximately initial water content), and w is the loss in weight at time t. A drying constant has been calculated from this logarithmic function for the initial stage of drying, and has been used for determining the mechanism of drying. Water movements through the parenchymal cells are fast compared with those through the skin. Since various dipping treatments give different drying rates there is no rate-controlling diffusion through the stationary air layer a t the grape surface. The drying rate is controlled by the diffusion of water through the waxy cuticle, and is inversely proportional to the amount of cuticle present as determined by extraction with chloroform. The elastic contraction of the skin causes an increase in the thickness of the cuticle which reduces the permeability. The heat required for the evaporation of water and for heating the grape to the equilibrium drying temperature is transferred from the surroundings to the grape. The rate of drying increases rapidly with rise in temperature owing to the vapour pressure of water and the permeability of the cuticle increasing with the temperature.