Influence of magnetic field-assisted freezing pretreatment on the freezing characteristics and nutritional quality of vacuum freeze-dried kiwifruit

• Magnetic field (MF) shortened the phase transition time druing kiwifruit freezing. • Appropriate MF treatment improved the hardness and flavor of dried kiwifruit. • The microstructure and antioxidant activiey of dried kiwifruit were modified by MF. • The rehydration ratio and color of dried kiwifruit treated by MF were not improved. The effects of magnetic field-assisted freezing pretreatments, including static magnetic fields (SMF) and alternating magnetic fields (AMF) with intensities of 0.5, 1.0, 1.5, and 2.0 mT, on the freezing and quality characteristics of freeze-dried kiwifruit slices were investigated. The results showed that the magnetic field-assisted freezing technique has unique advantages in improving the quality of freeze-dried kiwifruit crisps. The microstructure showed that by shortening the freezing time, the finest and uniform pore structure was formed in the 1.5 mT SMF-treated group, and the hardness (729.46±56.37 g) was significantly increased compared with the control group (509.17±45.10 g). The relative antioxidant capacity (RACI) index of 1.5 SMF (0.73) and 1.0 AMF (0.53) treatment groups was higher than control group. The total sensory evaluation scores were elevated in all magnetic field treatment groups, and the electronic nose analysis confirmed the reduced sulfide response value. However, the key challenge remains that the magnetic field treatment did not improve the color and rehydration of the crisps. This study confirms that magnetic field-assisted freezing can optimize the freezing process and product quality of kiwifruit slices, but different parameters need to be balanced for product characteristics to obtain the best overall quality.