Effects of air impingement drying on the physicochemical characteristics and microstructure of desalted sea cucumber

Here air impingement drying (AID) was introduced to dry the desalted sea cucumber for less drying time and better quality of dried products. A systematic investigation was implemented to clarify the impacts of AID temperature (50, 60, and 70 ℃) and air velocity (4, 6, and 8 m/s) on the moisture distribution, moisture state, microstructure, hardness, and saponin content of desalted sea cucumber. Conventional hot air drying (HAD) was also taken as a control. The results revealed that the drying rate of desalted sea cucumbers increased significantly, as the drying temperature increased. The drying time of desalted sea cucumber was also reduced by 6.67%-33.33% at 6 m/s air velocity and various AID temperatures, compared with the HAD at 60 °C. The air velocity shared an insignificant effect on the drying time of desalted sea cucumbers (P>0.05) at the same temperature (60 ℃), particularly for the extended drying time (12-13 h) with the increased air velocity (4-8 m/s). The microstructure showed that the high AID temperature was beneficial to increase the porous structure of the material surface, leading to accelerated water migration. More and larger porous structures were observed on the surface of AID sea cucumber samples under the same conditions (the temperature was 60 ℃ and the air velocity was 6 m/s), compared with the HAD ones. As such, the AID drying rate was accelerated as well. However, the high air velocity was used to prevent the surface deformation (such as crusting) of desalted sea cucumber, thus reducing the structure porosity, which hindered the water migration and lowered the drying rate. The relaxation time of immobilized water in the AID sea cucumbers moved faster toward the short relaxation time, and then the peak amplitude decreased significantly, compared with the HAD. The low freedom of water molecules also led to a decrease in drying rate, with the extension of drying time. There was a weaker signal of proton density in the AID sea cucumbers. The water content was lower under the same conditions (drying time was 6 h). The water migration rate of AID sea cucumbers was higher than that of HAD, indicating that the AID was beneficial in shortening the drying time. The hardness of dried sea cucumber increased first and then decreased, with the increase in AID temperature and air velocity. The maximum hardness (494.25 N) was recorded at an AID temperature of 60 ℃ and 6 m/s air velocity. The saponin content (1.36-1.79 μg/g dry matter) of AID sea cucumbers increased with the increase in temperature, while there was no significant change in the air velocity. The saponin content of AID sea cucumber samples increased by 50% under the same conditions, compared with HAD. The temperature of 70 ℃ and the air velocity of 6 m/s were the better conditions for the AID sea cucumbers, in terms of drying efficiency and quality. The AID can be expected to improve the drying efficiency and ingredient retention rate of desalted sea cucumbers. The finding can provide theoretical reference and technical support for better drying quality.