Drying kinetics of Nephelium Lappaceum (Rambutan) using osmotic dehydration

Nephelium Lappaceum or Rambutan is a seasonal fruits that is very popular in Southeast Asia region. It has short storage life, about 4 to 5 days after harvesting. In this study, an oven was used to dry Rambutan and the effect of chemical pre-treatment on the Rambutan was investigated. Two different sets of pre-treatment were used, namely set B (treatment by using calcium chloride, malic, ascorbic and citric acid, sucrose and trehalose) and C (treatment by using sucrose and citric acid). Untreated sample (Set A) was prepared as a control. Drying temperature of 50^oC was used throughout the experiment for 24 hours drying time. Weights of the samples for each set were measured at interval of 10 min, 15 min, 30 min and 1 hour. Drying rate (DR) and moisture ratio (MR) were calculated from the results. Calculated DR and MR were fitted to five different thin layer mathematical models namely, Lewis Model, Page Model, Handerson and Pabis Model, Logarithmic Model and Two-term Model. The value of coefficient of determination (r²), reduced sum square error (SSE) and root mean square error (RMSE) were compared in order to find the best mathematical model for drying kinetics of Rambutan. The results showed that Logarithmic Model gives the highest r², lowest SSE and RMSE value for set A (r² = 0.9972, SSE =0.000285 and RMSE = 0.000.016881), set B (r² = 0.9950, SSE = 0.000535 and RMSE = 0.023138) and set C (r² = 0.9950, SSE = 0.000624 and RMSE = 0.024989). Thus, Logarithmic model is the best model that suit to represent drying characteristics of Rambutan. The effective moisture diffusivity also estimated by using Fick´s diffusion model; set A (3.11 × 10^-10), set B (3.32 × 10^-10) and set C (2.99 × 10^-10).