Effect of stirring in the osmotic dehydration of chestnut using glycerol solutions

Abstract

The use of osmotic dehydration like a previous step to drying process could be an interesting option in order to reduce costs or to preserve the characteristics of food material. This work studies the osmotic dehydration operation of chestnuts ( Castanea sativa M.) with aqueous solutions of glycerol (25, 35, 45 and 60 g/100 g) and glycerol and salt (35 and 9.7 g/100 g, respectively), at different soaking times (1, 2, 4 and 8 h) at 20 °C. In order to evaluate the hydrodynamic effect of osmotic medium on the mass transfer rate, static and dynamic conditions have been tested (0, 40 and 110 rpm). The process was monitored employing several parameters related to moisture content and solid gain changes. Water loss rate using binary solutions increases with glycerol concentration and no dependency was detected with stirring level using glycerol concentration higher than 35 g/100 g. WL/SG ratio increases with glycerol concentration and linear behaviour between WL/SG and water activity of solution was established. NMC contents decrease with glycerol concentration up to 0.78 at 8 h of osmotic dehydration and NSC contents increase with glycerol concentration up to 1.12. WL kinetics using ternary solutions composed by glycerol and sodium chloride solutions were similar to the obtained with glycerol solutions at the same water activity range, but SG kinetics is improved by the acquisition of sodium chloride by the chestnut. In this way, the WL/SG ratio and NMC are lower. Model proposed by Peleg was satisfactorily employed to fit the experimental NMC and NSC data for both binary and ternary solutions considering that equilibrium value is no dependent on stirring level.