Formation of Underpressure in an Apple Cylinder during Convective Drying

Abstract

The aim of the study was to experimentally determine drying-related shrinkage and internal pressure formation during convective drying of a cylindrical sample of apple tissue. Simultaneous measurements of mean moisture content, internal material temperature at four points, axial pressure inside the sample, and sample shrinkage were performed on an axially rotating apple cylinder, continuously weighed using telemetry. The sample was dried at three temperatures, 40, 50, and 60°C, and one air velocity. Development of negative pressure inside the sample reaching −90 kPa was observed. Radial shrinkage of the sample reached 50% on drying completion. In order to simulate the process, a model incorporating differential balance of stresses with heat and mass transfer balances was constructed and solved. The dependence of Young's modulus on moisture content was measured. The simulated results satisfactorily described the drying curve, temperature curve, and pressure magnitude but underestimated shrinkage.