Osmotic dehydration of litchi pulp as a pretreatment for drying processes

Abstract

Effects of osmotic dehydration on mass transfer properties such as water loss (W), solute gain (S) and weight reduction (G) during osmotic dehydration were investigated in order to determine the usefulness of this technique as pre-treatment for further drying of litchi pulp.  The effects of variations in sucrose (50% and 60 % w/w) and salt concentrations (10% w/w), solution temperature, and length of immersion time on the moisture removal of the product and its organoleptic characteristics’ on osmosis were analyzed.  About 80% of the water loss occurred between 4-6 h under most of the conditions.  Longer treatment time in high concentrations of sucrose resulted in a very soft product, which is difficult to handle and unsuitable for further drying.  Increasing concentration at the same temperature did not cause significant increments in weight change.  After osmotic treatment, the pulp was dried in a tray dryer at 70℃ for 10 h.  Osmotic treatment was responsible for increasing drying rate in a subsequent convective tray drying.

Keywords: osmotic dehydration, cabinet drying, rehydration ratio, pretreatment, solute gain, weight loss