Control of microwave assisted macadamia drying

We studied the microwave-assisted macadamia drying, using surface temperature control and without surface temperature control as well as hot air drying, and assessed the drying time, drying rate, colour values, hardness and water activity of nuts and efficiency. The surface temperature of macadamia nuts was controlled between 60 and 80 degrees C using a thermal radiation detector. For drying without the surface temperature control, drying temperatures in the range of 40-60 degrees C and microwave power were controlled in the range of 1.6-4.4 Wg(-1). We found that increase in microwave power and temperature had a positive effect with shorter drying time and increased drying rate. Microwave assisted hot air drying by surface temperature control led to a better quality product in respect of kernel brightness and suitable a(w) for storage, but a harder kernel. In addition, drying without surface temperature control had the highest energy efficiency followed by surface temperature control and hot air only. Considering quality, the recommended condition was drying by surface temperature control at 60 degrees C with 60 degrees C hot air combined with microwave.