Submitted to: Food and Bioprocess Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/15/2013
Publication Date: 1/15/2014
Publication URL: http://handle.nal.usda.gov/10113/58241
Citation: Wang, Y., Zhang, L., Johnson, J.A., Gao, M., Tang, J., Powers, J.R., Wang, S. 2014. Developing hot air assisted radio frequency drying for in-shell Macadamia nuts. Food and Bioprocess Technology. 7(1):278-288. Interpretive Summary: Macadamia nuts are one of the most valuable edible nuts in the world with a high mono-unsaturated fatty acid content that may lower the risk of the heart disease. A major problem for macadamia nut producers is nut loss caused by mold and germination due to high moisture contents. Freshly harvested macadamia nuts have a moisture content of about 33%, which must be reduced to about 1.5% kernel moisture for safe storage. The industry standard drying process requires more than one month, including drying on-farm for 3-4 weeks and hot air drying for 6 days. It’s desirable to develop a novel drying process to shorten drying time and retain more nutrients in the treated nuts. Dielectric drying using radio frequency energy in combination with hot forced air has been proposed to improve conventional drying methods for preserving agricultural products. To make dielectric drying practical and acceptable by the macadamia nut industry, the radio frequency/hot air drying process must provide uniform heating without damaging nut quality. A pilot-scale 27.12 MHz and 6 kW radio frequency system was used to determine the optimal operational conditions and the resulting drying curve and quality attributes of the treated nuts. The results showed that an electrode gap of 15.5 cm and a hot air temperature of 50°C provided an acceptable heating rate and stable sample temperatures, and were used for further drying tests. The drying curves showed that radio frequency/hot air drying took 360 min to reach the target moisture content of 3.0% for whole nuts while hot air alone took more than twice as long (750 min). Nut quality parameters (peroxide values and free fatty acid levels) increased with the drying time both for hot air and radio frequency/hot air drying but remained within the range required by the nut industry. Radio frequency/hot air drying shows great potential to reduce nut loss, improve nut quality and reduce processing times by providing a rapid, uniform and environmentally friendly drying technology for the macadamia nut industry.
Technical Abstract: Dehydration offers a means of preserving foods in a stable and safe condition as it reduces water activity and extends shelf-life of perishable agricultural products. The purpose of this study was to develop radio frequency (RF) drying protocols for in-shell macadamia nuts based on conventional hot air drying. A pilot-scale 27.12 MHz and 6 kW RF system was used to determine the operational parameters, the drying curve and the quality attributes of the treated nuts. The results showed that an electrode gap of 15.5 cm and a hot air temperature of 50°C provided an acceptable heating rate and stable sample temperatures, and were used for further drying tests. The drying curves showed an exponential decay and required 750 min and 360 min to achieve the final moisture content of 0.030 kg water/kg dry solid (3.0% dry basis) in whole nuts for hot air and RF heating, respectively. The drying kinetics of the nuts were described well by the Page model for hot air drying but by the Logarithmic model for RF/hot air drying. Peroxide value and free fatty acid increased with the drying time both for hot air and RF drying but remained within acceptable range required by the nut industry. The RF treatment shows potential to provide rapid, uniform and environmentally friendly drying technology for the nut industry.