Radio frequency-assisted hot air drying of carrots for the production of carrot powder

Radio frequency-assisted hot air drying (RF-HAD) was carried out to produce carrot powders. Effect of electrode gap, sample thickness and sample weight on RF-HAD efficiency was investigated. 90 mm of electrode gap, 2.5 cm of sample thickness and 600 g of sample weight were selected as RF-HAD parameters in order to obtain acceptable drying rate and avoid excessive deformation. RF-HAD kinetics were studied and compared with hot air drying (HAD). RF-HAD increased drying rate and reduced drying time by 45%. dehydrated vegetables powder

Also, the quality of carrot powders obtained after RF-HAD was evaluated and compared with those produced by air and freeze drying. The results showed that the samples dried with RF-HAD had greater contents of β-carotene, ascorbic acid, and total phenols, higher antioxidant activity and improved rehydration capacity compared to HAD.

Drying of vegetables is one of the most commonly used preservation processes that provide economical post-harvest management and increased shelf-life. Conventional drying techniques such as HAD may cause textural changes, loss of color,aroma and taste, decrease in nutritional quality and rehydration properties due to high temperature and long drying time. Freeze drying (FD) is effective, but it is not preferred for reasons such as high energy consumption and operating costs compared to other drying processes.

Recently, the increasing demand of consumers to high quality dried products and rising energy costs have triggered the investigation and development for innovative drying techniques. Recent studies focus on the fourth generation drying methods such as microwave (MW), infrared (IR), and radio frequency (RF) to meet the request for high quality dried products. Among them, RF heating standalone or in combination with other conventional drying techniques has been studied as an advanced drying technique for different types of agricultural products including kiwifruits, hazelnuts ; apple; banana ;

For HAD, the drying rate was much slower than RF-HAD which resulted in a longer drying time. In addition, similar to the RF-HAD, with the decreasing moisture content, the drying rate decreased gradually and the HAD occurred only in falling rate period. Lower drying rates of HAD can be attributed to its heating mechanism. During HAD, thermal energy is first transferred to the sample surface in the early stage of drying and then to the inner regions in the late stage of drying.

This causes moisture to be removed from the free surfaces at different times, affecting the drying process. Unlike hot air, RF energy penetrates to the sample’s parenchyma since the dielectric material (water) is homogenously distributed in the entire volume. This mechanism provides internal and external moisture removal to occur simultaneously and faster drying process.Nahimana and Zhang (2011) observed similar results for microwave vacuum drying and HAD of carrots.

In this study, the efficacy of RF-HAD was evaluated to produce qualitative carrot powders by investigating the effect of process parameters on the drying characteristics of samples. RF-HAD at 90 mm of electrode gap, 2.5 cm of sample thickness and 600 g of sample weight had an acceptable drying rate. To explain the drying kinetics of carrot samples, the experimental data obtained after RF-HAD was fitted to seven mostly used drying models and compared with HAD. The best fitting model was the two-term model for both RF-HAD and HAD of carrots.