Comparative analyses of three dehydration methods on drying characteristics and oil quality of in-shell walnuts

ABSTRACT

Developing an effective drying method for in-shell walnuts (Juglans regia L.) is a major postharvest processing concern in the nut industry. Three drying methods, including hot air drying (AD), vacuum drying (VD), and hot air-assisted radio frequency drying (ARFD), were experimentally compared and analyzed. The changes in lipid oxidation attributes, fatty acid composition, total antioxidant capacity (TAC), and total phenolic concentration (TPC) of walnuts were determined after dehydration and during storage. The results showed that the drying time required for in-shell walnuts using ARFD was the shortest (138 min), followed by VD (185 min) and AD required the longest time (300 min). Particularly, AD resulted in the highest lipid oxidation, followed by VD and ARFD. The walnuts treated by ARFD contained more unsaturated fatty acid than those treated by AD. Moreover, both the reduced power assay and free radical scavenging capacity tests showed that ARFD and VD had little effect on the TAC and TPC of walnuts during the drying process and storage. Overall, ARFD provides an effective and rapid drying method for in-shell walnuts.

KEYWORDS:

• Antioxidant activity
• hot air
• lipid oxidation
• radio frequency
• storage
• vacuum

Additional information

Funding

This research was conducted in the College of Mechanical and Electronic Engineering, Northwest A&F University and Sino-US Joint Research Centre for Food Safety, and supported by research grants from the General Program of National Natural Science Foundation of China (No. 31371853), Open Fund from Zhejiang Academy of Agricultural Sciences (No. 2010DS700124-ZM1605), and National Key Research and Development Program of China (2016YFD0401000). The authors thank Bo Zhang, Bo Ling and Rui Li for their assistance during the experiments.