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To investigate the effects of different microwave drying conditions on the texture and nutritional quality of candied strawberries in a vacuum environment. This study investigated the effects of three different vacuum microwave treatment conditions (2.5 w/g-1.5 h, 3.5 w/g-1 h, 4.5 w/g-0.5 h) on moisture migration characteristics, texture, microstructure, color, anthocyanins, and phenolics of candied strawberries, which were evaluated by hot air drying. The results showed that the time required for vacuum microwave drying to dry candied strawberries to 20%±2% moisture content was significantly lower than that of hot air, and the higher the microwave power, the shorter the drying time. In addition, the water in the candied strawberries obtained after drying mainly appeared in two states, namely, bound water and water not easy to flow. The relaxation time T2 of strawberry candied strawberries obtained by vacuum microwave drying increased with the increase of microwave power, which indicated that the degree of freedom of water increased. Cross-sectional microscopic observation found that vacuum microwave drying caused cell wall rupture, and the higher the microwave power, the closer the site of fissure formation to the medullary core, and the bigger the fissure, and the smaller the porosity, whereas the cellular integrity and the porosity of the samples dried by the hot air were higher than that of microwave drying. In addition, the hardness, viscosity, and a* value of vacuum microwave-dried dried strawberry preserves were lower than those of hot air-dried samples, but the L* value, total sugar, anthocyanin, and phenolic content were higher. In conclusion, vacuum microwave drying had higher drying efficiency and could better retain the nutrients such as anthocyanins in candied strawberries.

In order to discover the moisture distribution and migration during the drying process of strawberries at constant or gradient temperatures and the effect on the quality of dried strawberries, the strawberries which had experienced osmotic dehydration were dried under constant temperatures (60 °C-12 h, flip it over when dry to 6 h) and gradient temperature (65 °C-2 h, 60 °C-2 h, 55 °C-2 h, flip it over, 65 °C-2 h，60 °C-2 h，55 °C-2 h) during hot air drying. Low-field nuclear magnetic resonance (LF-NMR) was used and the physicochemical (texture, VC, total phenols) and sensory qualities (flavor, texture, taste, color) of the preserved strawberries obtained by the two drying programs were compared. The results showed that the water content of the dry base of strawberries at both programs decreased with time extending gradually, but the rate of gradient temperature was higher than that of constant temperature drying. From the T2 inversion spectra and peak areas of LF-NMR, it illustrated that the free water was the main form of water in the osmotically dehydrated strawberries, and there were also a small amount of bound water and immobile water. With the drying procedure proceeded, the T2 peak of the strawberry gradually decreases, which indicates a decrease in the mobility of the water. However, the total relaxation peak area of constant temperature drying and gradient drying was reduced to 31.3% and 26.4% of the original at the end of the drying programs, which showed that the gradient drying had higher drying efficiency. Magnetic resonance imaging (MRI) showed that the water in strawberries was firstly enriched to the broken tip of the fruit and dispersed, then in the medium of the drying process, the peel took the main position for the water evaporation, and finally some water left in the pith. From the imaging results, the main difference between the two drying programs was that there was an obvious homogeneous wetness phenomenon in the strawberries after 6 h drying i.e. the water was redistributed evenly in the fruit at that time, while this process did not appeared in the gradient drying. In addition, the VC and total phenolic contents of dried strawberries were significantly higher (P