Apple Pectic Gel Produced by Dehydration

Abstract

A novel, flexible-sheet like food, formed by the high methoxyl acid sugar gelation, was investigated to characterize drying-related properties. Product volume was reduced by 68% over the process, and the shrinkage was successfully modelled by assuming the volume reduction equal to the volume of water evaporated. The sorption isotherm was determined, and a new expression for the moisture content W as a function of water activity aw of the type W = C1*exp (C2*W^C3) was the most accurate for this J-shaped isohterm. The drying kinetics was studied at 50, 60 and 80ºC in a tray dryer. No constant drying rate period was found, and the drying curve was divided in high- and low-moisture zones. For high moistures, an internal-external control diffusive model coupling mass and heat transfer was applied, to obtain a mass transfer Biot number of 2.1. In the low moisture zone, a diffusive isothermal drying model for strict internal control was utilized. Diffusivities were about 10 ^(-9) m2/s at high moistures and were ten times lower at low moistures, although the activation energies were comparable (15,259 and 16,800 J/mol, respectively). The drying time at 60ºC was 6,67 h. The product scored four points out of five in a sensory evaluation of general acceptability.