The Evolution of Mechanical Properties and Cellular Structure of Apples During Freeze Drying Combined with Hot Air Drying (FD-HAD) Process

Apples are one of the most popular fruits in the world and have a significant share in domestic and international fruit production. Drying is a common method used to extend the shelf life of apples. However, it also induces irregular morphological changes in apples, which are essential to maintaining the structural integrity of the material. Therefore, it is necessary to understand the effect of cellular changes at the microscopic level on the macroscopic deformation of the material during drying. In this paper, the evolution of cell wall pectin fractions and viscoelastic properties of apples during freeze drying combined with hot air drying was investigated. The findings indicated that during the HAD stage, a decrease in the relaxation modulus (E1) of the samples was observed in the compression tests when the sample temperature was significantly higher than the glass transition temperature (Tg). As the difference between the two decreased, the samples exhibited increased stiffness and higher E1. The results of the pectin content analysis showed that the HAD process accelerated the loss and degradation of water-soluble pectin in the samples with high moisture content at the transition point. Simultaneously, the esterification degree of chelator-soluble pectin increased, leading to a reduction in the support provided to the cellular structure of the samples, which consequently affected their mechanical properties. These findings may provide valuable information for the application of freeze drying combined with hot air drying in the efficient processing of dried fruit and vegetable products.