Microstructural changes, water migration, and oil release in Camellia oleifera seeds during microwave vacuum drying

Abstract

Microwave vacuum drying, due to the high efficiency of dielectric heating and vacuum's temperature-friendly conditions, has gained increasing attention. This study examined the moisture migration behavior of Camellia oleifera seeds during microwave vacuum drying and its effects on oil release and product quality. Results showed pronounced moisture variation, with the drying rate rising initially and then declining. The effective moisture diffusion coefficient was 1.358 × 10⁻⁸ m²/s, and the Page model accurately described the drying kinetics. Oil content first increased, peaking at 15 min with a 43.06 % rise over the initial value, before declining with prolonged drying. Bound and immobile water decreased steadily. Structural deformation of the seed shell and kernel intensified with drying time and moisture loss, leading to a denser pore distribution per unit area, along with visible oil exudation on the seed surface. X-ray micro-CT analysis confirmed outward moisture migration from the interior, beginning with small core pores that enlarged and merged over time. Total and open porosity increased continuously, while closed porosity rose to 5.67 ± 0.27 % at 10 min before declining. Correlation analysis emphasized the dominant role of moisture variation in pore formation. Overall, these findings provide new insights into the drying behavior of Camellia oleifera seeds under microwave vacuum conditions.