Moisture Distribution and Migration of Preserved Strawberries During Hot Air Drying at Constant and Gradient Temperature

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, V_C, 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 V_C and total phenolic contents of dried strawberries were significantly higher (P<0.05) using the gradient program than those under constant temperature drying, which were 1.89 and 1.14 times higher (P<0.05). Moreover, significantly lower (P<0.05) values of the viscosity, hardness, and chewiness were obtained by the persevered strawberries. For the sensory scores, the gradient program resulted in considerably better (P<0.05) texture, appearance, taste, and overall ratings for dried strawberries than constant temperature drying which were 31.3%, 41.3%, and 40.2%. Overall, the drying efficiency of step drying was higher and the quality of the dried strawberries obtained was better.