Abstract
Radio frequency (RF) drying is a rapid dehydration technique that can be applied to perishable agricultural products. The objective of this research was to develop hot air-assisted RF drying protocols for in-shell walnuts. A pilot-scale, 27.12 MHz, 6 kW RF unit was used to study the drying process, including drying curves, drying kinetic models, and quality changes during RF drying and storage. Results showed that an electrode gap of 18.0 cm combined with hot air temperature of 50 °C provided an acceptable heating rate and stable sample temperatures during RF drying. Total drying times to reduce the whole walnut moisture content from 20 to 8.0% on dry basis required 240 and 100 min using hot air drying and RF/hot air combined drying, respectively. The drying curve for RF/hot air heating was fitted to five different kinetic models, and the best one was the Page model with R2 = 0.998. Quality analysis showed that there were no significant differences in peroxide value, free fatty acid, and colour between RF treatments and untreated controls during drying and storage periods (P > 0.05). RF heating can, therefore, provide a practical and effective method for drying in-shell walnuts with acceptable product quality.
Highlights
Walnut (Juglans regia L.), a member of the Juglandaceae family, is mostly cultivated in the countries of Northern hemisphere, such as China, USA, Iran, and Turkey
Walnut samples were placed in a vacuum oven (DZX-6020B, Nanrong Lab equipment Inc., Shanghai, China) at 100 ± 2°C and 21 kPa for 7 h, the samples were placed into a desiccator with CaSO4 (Calcium sulphate) for cooling, and the sample weight was recorded at room temperature
Similar trends were observed by Wang et al (2014b) who used hot air-assisted RF (HARF) drying for in-shell macadamia nuts
Summary
Walnut (Juglans regia L.), a member of the Juglandaceae family, is mostly cultivated in the countries of Northern hemisphere, such as China, USA, Iran, and Turkey. The moisture content (MC) of fresh walnuts is more than 20%, dry basis (d.b.), after harvesting. Walnuts are susceptible to decay and germination during storage due to high water and fat contents, and high lipase activities (Ma et al, 2013). Fresh walnuts must be dried to 8.0% and 5.0% (d.b.) MC of whole nuts and kernels, respectively, for long-term storage in ambient environment (Amaral et al, 2003). Developing an effective drying method for in-shell walnuts is a major postharvest processing concern in the nut industry
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