**3.2 Novel thermal and non-thermal drying technologies**

Infrared (IR) heating has the advantage of high heating intensity compared to convective heating. IR radiation could penetrate 2–5 mm depth into food surface [55], which matches the typical thickness of the nut hull and shell. Therefore, IR heating was an ideal technology to pre-dry the tree nuts. A commercial-scale sequential infrared and hot air (SIRHA) drying technology (**Figure 6**) was developed by Chen [15] and Atungulu et al. [40], which had 14.2 ton/h throughput and achieved 13.6–26.5% drying time reduction and 10–20% energy savings, respectively. Venkitasamy et al. [22, 23] used SIRHA drying for pistachios and almonds, and

#### *Processing of Tree Nuts DOI: http://dx.doi.org/10.5772/intechopen.102623*

achieved 9% and 40%, respectively, compared with HA drying only. Additionally, research has shown that the percentage of shell splitting for pistachios increased with drying rates, and thus spray-rinsing with water and then IR drying could effectively improve the shell splitting [56].

Heat pump drying utilizes the thermal energy in the air flowing out of the dryer by dehumidifying and condensing the water vapor containing in the outflow air, retrieving the enthalpy of heat evaporation, then circulating the heat back to the dryer inlet [57]. HA heat pump drying was used for walnuts and higher drying efficiency was achieved without affecting the product quality [58, 59]. IR-assisted heat pump drying for walnuts reduced 20% drying time and 10% energy consumption compared to HA drying [60]. Solar heat pump drying reduced the drying time and increased the energy utilization ratio of pistachios [61, 62].

Dielectric heating such as microwave (MW, wavelength range: 300 MHz– 300 GHz) and radiofrequency (RF, wavelength range: 3 kHz–300 MHz) can penetrate the foods. The electromagnetic wave activates the water molecules through dipole rotation and/or ionic polarization, generating heat volumetrically within foods, while non-polar molecules are not affected [63]. HA drying at 50°C assisted with RF heating at 27.12 MHz and 6 kW reduced 58.3% drying time of walnuts compared to HA drying only [64]. Intermittent MW drying could be used to dry pistachios without affecting the nut quality [65].

Some non-thermal technologies have also been studied as assistance to conventional drying processes. For example, high-power ultrasound treatments improved the drying rate and energy efficiency of pistachios [66]. The drying time of almonds was reduced by 58.33% with 40 min ultrasound treatment [67]. Such phenomena should be attributed to the enhancement of moisture transfer by the molecule vibrations induced by ultrasonic field. Under vacuum, the boiling point of water and vapor pressure in the drying chamber were significantly lower, and thus increase the driving force of moisture transfer during the drying process [12]. Vacuum drying was able to dry walnuts in a shorter time compared with conventional practice [68]. IR- and MW-assisted vacuum drying improved the drying efficiency of almonds [69].
