Experimental and theoretical investigation of drying of some fruits and vegetables in an infrared dryer

In this study, effects of infrared heating power on drying characteristics of parsley, strawberry, red pepper, green pepper samples and dried quality parameters investigated. Experimental studies were carried out at 300, 400 and 500 W. Under these conditions, the drying times of untreated parsley, strawberry, red pepper, green pepper samples were found to be in the range of 14-33 min, 78-165 min, 66-110 min and 61-118 min, and effective moisture diffusivities to be in the range of 3.126 10-11 - 1.524 10-11 m2/s, 6.389 10-10 - 2.738 10-10 m2/s, 14.605 10-10 - 8.114 10-10 m2/s and 12.424 10-10 - 6.212 10-10 m2/s respectively. Drying rates and effective diffusivities were determined to be increasing with the increased infrared power. No changes were recorded in the drying times of the pretreated samples. Constant drying rate followed by a falling rate was observed in all drying experiments. The among models chosen from the literature, Page model was found to be the most suitable model with the highest values of R2, and the lowest values of the ? 2 and root mean square error (RMSE) under all drying conditions. The activation energies of the parsley, strawberry, red pepper, green pepper samples were found as 20.854, 4.871, 4.371, 5.421 W/g respectively. It was determined that rehydration capacities were increased with increasing infrared power. Rehydration capacities of the pretreated parsley, red pepper, green pepper samples were found to be higher than those of the untreated ones, however, that of strawberry treated with ethly oleate solution did not change. While, differences were recorded between the colour parameters of the pretreated and untreated samples, no significant effect of the infrared power on these parameters were detected. It was found that the vitamin C values of the untreated dried samples were lower than those of the raw ones, and the infrared drying power has an important role in the losses of vitamin C amount. On the other hand, it was seen that there were no differences between the vitamin C values of the pretreated and untreated samples.In order to determine the drying conditions which would provide the optimum amount of vitamin C in dried materials during infrared drying, Response Surface Methodology was used. For this purpose, the independent variables were chosen as raw matter amount (A, 10.3-18.7 g), infrared power (B, 316-484 W), drying time (C, 5.3-13.7 min) for parsley, raw matter amount (A, 83-117 g), infrared power (B, 316-484 W), drying time (C, 23-56 min) for strawberry, raw matter amount (A, 50-100 g), infrared power (B, 316-484 W), drying time (C, 29-49 min) for red pepper and green pepper, by using the three-level central composite rotate design (CCRD). Quadratic regression equations describing the effects of these factors on vitamin C amount were developed. Real values of the independent variables were found as A= 12.00 g, B= 350.01 W, C= 7.00 min for parsley, A= 90.00 g, B= 350.00 W, C= 30.00 min for strawberry, A= 60.00 g, B= 350.08 W, C= 33.00 min for red pepper, A= 73.98 g, B= 350.00 W,C= 33.00 min for green pepper. At these optimum points, vitamin C values of parsley, strawberry, red pepper, green pepper samples were recorded as 97.51 mg/100 g, 47.77 mg/100 g, 139.88 mg/100 g and 160.81 mg/100 g respectively.Keywords: Infrared drying, drying kinetics, kinetic model, effective moisture diffusivity, activation energy, rehydration capacity, colour change, Response Surface Methodology (RSM), vitamin C