Characteristics of moisture diffusivity and shrinkage evolution of in-hull almonds under hot air drying

Abstract

This study aimed to develop efficient drying methods for off-ground harvested almonds by investigating the drying kinetics, effective moisture diffusivity (D eff ), and shrinkage evolution of different almond components (hulls, shells, and kernels) during hot air drying of in-hull almonds. Experiments were conducted at 45, 60, and 75 °C with an air speed of 1.4 m/s, targeting a final moisture content of 10 % (dry basis). Moisture content and dimensional changes were recorded at each sampling point to derive drying curves, Deff, and shrinkage patterns. Results showed that the hull and shell exhibited typical drying behavior seen in agricultural materials, while the kernel showed delayed moisture reduction, particularly at 45 °C, where its moisture content remained relatively stable for approximately 720 min. Significant kernel drying occurred after 180 min at 60 and 75 °C. D eff increased from 9.70 × 10 −10 to 2.05 × 10 −9 m 2 /s for the hull, 4.71 × 10 −10 to 9.24 × 10 −10 m 2 /s for the shell, and 5.82 × 10 −11 to 1.45 × 10 −10 m 2 /s for the kernel across the tested temperatures. The dynamic variations in D eff with moisture content were effectively captured by third-order polynomial equations, achieving adjusted R 2 values above 0.973, 0.926, and 0.978 for hull, shell, and kernel, respectively. Non-isotropic shrinkage was observed among almond components. Minimal changes in sphericity and the absence of hull splitting indicate limited structural deformation under the tested drying conditions. These findings enhance the understanding of moisture transfer and shrinkage mechanisms during in-hull almond drying and support the development of predictive models and optimized drying strategies for off-ground harvested almonds.

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