**3.1 Improve the hot air drying process**

Elevating the drying temperature is a potential approach to reduce the drying time and improve the efficiency of the current HA drying practices. However, keeping the nuts at high temperatures for long times is not desirable, as it may induce significant quality deterioration to nuts, such as lipid oxidation and browning [23, 49]. Enhancing the drying rates by intensifying the heat and moisture transfer during the drying while maintaining the product qualities has become the key approach. Since the hull and shell have much higher IMCs than the kernel [2, 42], most thermal energy in the heated air is consumed for raising the temperature and evaporating moisture in the hull and shell, particularly in initial drying stages [50, 51]. As the results, the temperature increase in kernels is significantly slowed down, and the moisture removal from kernel to the environment is greatly restricted due to the reverse moisture gradient from the hull to the kernel [50]. Based on these characteristics, Chen [15] developed a new drying strategy by using high temperature heating in the beginning of drying to quickly heat up and partially dry the hull and shell; at the same time, due to the relatively low thermal conductivity of the kernel and shell, the kernels should not be over-heated; then before kernels reached a temperature that was high enough to cause significant oil quality deterioration, drying temperature was decreased to finish the drying. With the aids of experimental studies and mathematical modeling of the heat and moisture transfer (**Figure 5**) during the drying, the feasibility of this new drying strategy was verified [51, 52]. Drying in-shell walnuts by HA with step-down temperatures reduced the drying time and energy consumption by up to 40% and 24%, respectively, and obtained similar oil quality and kernel color in the dried products compared to the conventional practice. Similarly, using HA drying with step-down temperature and tempering for in-hull almonds significantly reduced the drying time and did not affect the quality of the dried almonds in terms of oil quality or incidence of concealed damage [13].

Sorting the nuts into different groups based on IMCs first and then drying them separately is another strategy of improving the drying efficiency, moisture uniformity and product quality. Khir et al. [42] studied the correlation between the MCs of walnuts and terminal velocities and developed a sorting method called the 'air knife'. The walnuts were separated into low and high MC groups and dried separately, which resulted in 18–28% energy saving compared to without sorting. Meanwhile, the uniformity of moisture in the dried products was greatly improved. This technology has been commercialized and installed in-line in walnut hulling and drying facilities in California (**Figure 6**). Similarly, the correlations between

#### **Figure 5.**

*Modeling results showing: (A) schematic diagram of pilot-scale column dryer; (B) mesh grid of the modeled system; (C) distribution of moisture and (D) temperature profile within the column and within single walnuts the moisture and temperature distribution within single walnut located in a pilot-scale column dryer [51, 52].*

**Figure 6.**

terminal velocities of pistachios and almonds with their MCs have also been studied [44, 53]. Chen et al. [13, 54] have shown that in-hull almonds, in-shell almonds and loose hulls at harvest could be separated based on the thickness and terminal velocities of different groups, and suggested sorting and removing almond hulls prior to drying benefited the improvement of moisture uniformity and energy saving.
