Hussein JB, Soji MY, Abiona OO and Oke MO
The drying characteristics of osmotically pretreated red onion slices via hot air oven at different drying temperatures (40°C to 80°C) and 1.5 m/s air velocity was investigated. Fresh onion bulb was manually sliced (4 ± 0.2 mm) and pretreated in 20% osmotic solution (sucrose, salt and water in ratio 1:1:4 weight in grams) for 2 hours. Eight drying models (Page, Logarithmic, Henderson and Pabis, Newton (Lewis), Wang and Singh, Parabolic, Midilli and Prakrash and Kumar) were used to optimize the goodness of fit to the experimental data. The models were compared using coefficient of determination (R2), chi-square (χ2) and root mean square error (RMSE). Some selected functional properties of the dried onion were also examined. It took 660 min, 600 min, 390 min, 300 min and 180 min at 40°C, 50°C, 60°C, 70°C and 80°C respectively to dry dehydrated red onion from 82.14% (wet basis) moisture content to constant equilibrium moisture content. The drying took place only in a falling rate period. The Midilli model was found to best fit the experimental data compared to other models. The effective moisture diffusivity values were found to be 1.76838 × 10-10 m2/s for 40°C; 1.98 × 10-10 m2/s for 50°C; 3.11 × 10-10 m2/s for 60°C; 4.26 × 10-10 m2/s for 70°C and 7.03 × 10-10 m2/s for 80°C respectively. The activation energy (Ea) was found to be 29.67 KJ/mol. The results of these measures have confirmed the consistency of the developed model to describe satisfactorily the drying characteristics of red onion slices at different drying temperature.