Drying of black carrot treated with pulsed electric field (PEF) using different drying technologies and determination of some quality parameters

In this study, the technological and quality effects of drying black carrots (Daucus carota), to which Pulsed Electric Field (PEF) was applied as a pre-treatment, using different drying technologies were examined. Pretreatment was performed with a pulsed electric field (PEF) device with 24 kV power output. Pulse width was determined as 7 μs in the application parameters of the device. Since the distance between the plates where the pulsed electric field occurs is designed as 19 cm, the field strength per unit centimetre (24 kV/19 cm) is 1.26 kV/cm. Dosage adjustment for pretreatment of the samples at different strengths was made over specific energy. In the application, four different energies, 1 kj/kg, 3 kj/kg, 6 kj/kg and 9 kj/kg, were applied to the groups except the control group. Black carrots were pre-treated by passing through a belt system designed in industrial size so that solid products can pass through. In the machine with a maximum working capacity of 7,5 tons/hour, raw materials were passed with a belt speed of 0.075 m/s. During this pre-treatment, only specific energy values were changed between the groups. The pretreated black carrot was cut into 4 mm thick slices and dried with three different drying technologies. The drying technologies were hot air drying (AD), microwave vacuum drying (MWVD) and freeze drying (FD). AD processes were carried out at 70°C at an air velocity of 1 m/s. MWVD processes were carried out under vacuum at 8.3 kW power. Different process parameters were studied in preliminary trials. The drying process was continued with a minimum moisture content below 5% and the total time was recorded. The last drying technology, FD, was carried out at a drying temperature of -20°C under an absolute pressure of 0.8 mbar. As a result, a total of 15 different drying processes were carried out using 1 control group, 4 different pretreatment values and 3 different drying technologies. Physicochemical analysis of the samples obtained, determination of apparent volume, bulk density, real volume, real density, porosity, determination of rehydration rate, determination of total monomeric anthocyanins, determination of anthocyanin degradation criteria, determination of anthocyanin profile (HPLC), Total phenolic content determination, antioxidant capacity determination, texture profile analysis, scanning electron microscopy (SEM) determination, effect of PEF pretreatment on drying time, effect of PEF pretreatment on extraction yield were analyzed. After drying, the shortest drying time was observed in the MWVD and the longest drying time was observed in the groups dried with FD. The decrease in drying times between the groups is in parallel with the increase in VEA power. Drying times in the control group without VEA were 6.68 hours, 0.55 hours and 16.95 hours for AD, MWVD and FD, respectively. It was observed that drying times decreased in all 3 different drying technologies with the increase of PEF pretreatments applied. The percentages of decrease in drying times at 1,-3,-6 and 9 kj/kg power applied as pretreatment were calculated as 5.86%, 8.35%, 10.22%, 11.47% for AD, 9.09%, 9.09%, 21.21%, 24.24% for MWVD and 14.16%, 20.26%, 21.14%, 25.47% for FD, respectively. The highest reduction in drying time was calculated as 25.47% in the FD group with VEA 9 kj/kg. The rehydration rates of the samples were measured in the range of 351%-721.9%. The lowest rehydration rates were 351% and 382% in VEA-1 MWVD and VEA-3 AD samples, respectively. The highest rehydration rate was measured in FD samples with 721.9%. Since the highest amount of porosity was detected in FD samples, the highest rehydration rate was measured in FD samples. The lowest anthocyanin content was 37.57, 47.31, 50.10 mg/100 g dm in PEF-9 AD, PEF-9 MWVD and PEF-6 MWVD samples, respectively; the highest anthocyanin content was 146.12, 143.33 and 140.55 mg/100 g dm in PEF-3 FD, PEF-1 FD and PEF-0 FD samples, respectively. The lowest DPPH values were 16.07%, 20.2% and 23.26% in PEF-9 AD, PEF-9 MWVD and PEF-9 FD samples, respectively; the highest values were 55.14%, 48.9% and 46.17% in PEF-0 FD, PEF-0 MWVD and PEF-0 AD samples, respectively. The lowest total phenolic content (TFMM) was 86.80, 96.94 and 174.93 mg GAE/100 g dm in PEF-9 MWVD, PEF-9 AD and PEF-6 MWVD samples, respectively; the highest was 476.76, 452.58 and 422.17 mg GAE/100 g dm in PEF-0 MWVD, PEF-0 FD and PEF-1 FD samples, respectively. Keywords: Black Carrot, Pulsed Electric Field, Microwave Vacuum Drying, Conventional Drying, Freeze Drying.