Apple Pectic Gel Produced by Dehydration

A novel, flexible sheet-like food formed by the high methoxyl pectin-sugar-acid gelation during drying of apple puree was investigated to characterize drying-related properties. Product volume was reduced by 68% over the process, and this shrinkage was successfully modeled by assuming the volume reduction equal to the volume of water evaporated. The sorption isotherm at 25 degrees C was determined, and a new expression for the moisture content, W, as a function of water activity, a(w), of the type W = C-1 exp (C-2 a(w)(C3)) resulted as the most accurate for this J-shaped isotherm. The drying kinetics was studied at 50, 60, and 80 degrees C 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 mixed 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 varied around 1 x 10-9 m(2)/s for high moistures and were about ten times lower at low moistures, although the activation energies were comparable (15,259 and 16,800 J/mol, respectively). The drying time at 60 degrees C was 6.67 h. The product scored four points out of five in a sensory evaluation of general acceptability.