Design, fabrication and testing of a screw conveyor dryer for roasting cashew nuts

Experimental prototype screw conveyor dryer with rice husk-fired furnace as heat source was specifically designed and fabricated for dry roasting cashew nuts. The performance characteristics of the developed system was evaluated in terms of the quality indexes of extracted cashew kernels, maximum compressive load and weight loss of roasted cashew nuts and cashew nut shell liquid (CNSL) yield. The operating conditions of the experimental unit such cylinder surface temperature, exposure time and feed rate with two levels of initial moisture content of raw cashew nuts were investigated. Results showed that quality indexes of extracted cashew kernels i.e. whole kernel yield, kernel whiteness and no. of kernels/kg were strongly dependent on operating conditions and initial moisture content of raw cashew nuts. The effect on maximum compressive load and weight loss of roasted cashew nuts was also recorded significant. Roasting cashew nuts of high moisture content in the experimental screw conveyor dryer at 300 deg C surface temperature for 2.0 minutes exposure time in 12 kg/min feed rate gave the highest whole kernel yield with an acceptable kernel whiteness. In general, high-temperature short-time conduction heating of cashew nuts resulted in better quality of cashew kernels and accelerated the operation. On the other hand, the extraction of shell liquid during conduction heating of cashew nuts in the experimental unit was insignificant due to kernel quality deterioration and CNSL vaporization. While extraction of CNSL from shells in the experimental dryer was found possible although the yield was lower compared to mechanical expellers. Finally, roasting cashew nuts in the experimental screw conveyor dryer was comparably better than hot-oil bath method in terms of the resulted kernel quality, capacity and safety of operations.