Investigation Of Radio Frequency Heating As Pre-drying And Final-stage Drying Methods For Carrot Cubes

Carrot is rich in nutrients,containing plenty of vitamins,carotene and dietary fiber.Howerver,it is of high moisture content and easy to decay,thus leading to massive waste and loss.And it is not suitable for long-term storage,transportation and processing.Hence,develop an effective new drying method is important.Conventional drying methods such as hot air drying is of long processing time and low product quality,while vacuum freeze drying has high product quality but high energy consumption and long processing time.Hence,in this study,radio frequency(RF)technology was applied to both pre-treatment and final-stage drying of carrot cubes.Firstly,the feasibility of using hot-air assisted radio frequency(HA-RF)heating as a simultaneous dry-blanching and pre-dewatering method for carrot cubes was evaluated.The ultrasound-assisted osmosis dehydration(UOD)was improved by orthogonal test and used as comparison.The optimum pre-dewatering process was determined.Secondly,the drying characteristics,drying kinetics,moisture transfer of HA-RF as well as the sample quality after HA-RF were studied.Hot air(HA)drying and vacuum freeze drying(VF)were used as comparison.The optimal drying process was determined and vitamin C degradation kinetics was studied as well.(1)HA-RF treated at 8.0-8.6 cm electrode gap and 60°C hot air temperature was studied as a simultaneous blanching and pre-dewatering method,it was found that HA-RF at 8.3 cm electrode gap was the optimal parameter.In addition,the traditional osmotic dehydration process was optimized,of which the process parameters were 40%sucrose solution,10%salt solution and 1:4 solid-liquid ratio.By comparison,dehydration rate of UOD was higher than HA-RF,the water activity and color protection ability were also better than HA-RF,while HA-RF obtained higher hardness and vitamin C retention rate,the energy consumption was lower as well.Besides,peroxidase enzyme was inactivated along with the pre-dewatering process.Hence,HA-RF is a promising pre-dewatering method.(2)RF dry-blanching and UOD pre-dewatering were used as pre-treatment methods for final-stage drying.HA-RF treated at 6.0-7.0 cm electrode gap,3.5-4.5 cm sample thickness,and 60°C hot air temperature was studied as a final-stage drying method,and it was found that 6.5 cm electrode gap,4.0 cm sample thickness was the optimal parameter based on heating uniformity and drying efficiency.The average drying rate was 1.24 g/kg^-1 min^-1,the water diffusion coefficient was 7.39×10^-11 m²/min,and the heating uniformity is fine(TUI=0.03).By drying kinetics model simulation,it was found that the logarithmic model obtained the highest fitting degree(R²=0.99)to HA-RF drying process.In addition,the moisture transfer showed that during the HA-RF drying process,in addition to removing a large amount of free water,there was migration took place between the semi-bound water and the bound water.Besides,the internal and external signals of the carrot cubes decreased at the same time,thus indicating that HA-RF was a more uniform drying method.(3)Carrot cubes were dehydrated by HA drying,VF drying and HA-RF respectively until the final moisture content reduced to less than 10%.By evaluating the effects of different drying methods on sample quality,it was found that VF drying obtained the best sample quality,HA drying had the lowest sample quality,and HA-RF drying obtained comparable sample quality to VF drying with higher effiency.The final water activity of samples after HA-RF was about 0.41,hardness was 31.02 N,shrinkage rate was about 41%and sample had good brightness value and redness,46%vitamin C retention rate,no significant difference from vacuum freeze drying.During the HA-RF drying process,the degradation of vitamin C was consistent with the first-level reversible model,and the main reason for the loss of vitamin C was that AA degraded to DHAA in aerobic conditions.Besides,the reversible first order model predicted AA and DHAA degradation with activation energy of 40.54 and 35.83 k J/mol,respectively.Compared with other fruit and vegetable systems,the degradation activation energy of vitamin C in carrot cubes is larger and thus more stable during HA-RF thermal processing.Therefore,the optimum drying process for carrot cubes was to first RF dry-blanching,then UOD pre-dewatering(50%power,50?,30 min)by using a combination of 40%sucrose solution,10%salt solution and 1:4 solid-liquid ratio,and finally use HA-RF drying,the best process conditions were 60°C hot air,4.0cm sample thickness and 6.5 cm electrode gap.