Effects of radio frequency assisted hot air drying on the texture and microstructure of jujube slices

Abstract: The purpose of this research is to deal with the surface hardening and crust formation, as well as the low crispness of jujube slices during hot-air drying (HAD). A radiofrequency (RF) heat treatment was also applied to the HAD jujube slices. An influence mechanism was proposed for the texture and microstructure of hot-dried jujube slices at different temperatures (55 ℃, 65 ℃, and 75 ℃) under the RF heat treatment. The results showed that the hardness and chewiness of jujube slices decreased first and then increased, whereas, the springiness, cohesiveness, and resilience increased first and then decreased, while the adhesiveness increased with the decrease of moisture ratio during the drying process. The hardness, crispness, chewiness, springiness, and cohesiveness of hot air-dried jujube slices decreased gradually, with the increase of temperature, but, the drying temperature had no significant effect on the resilience. The hardness and crispness of hot air-dried jujube slices after applied RF heat treatment decreased by 12.6%-18.7%, 13.8%-20.4%, respectively, while the springiness and cohesiveness increased by 12.2%-25.3% and 5.9%-13.2%, respectively. A correlation analysis showed that there was a significant positive correlation between the springiness and cohesiveness of jujube slices (P < 0.01). The hardness of jujube slices presented a positive correlation with the chewiness and crispness (P < 0.05) during different drying. A principal component analysis (PCA) was used to establish a new model for the texture quality assessment. As such, the highest score and the best texture quality were achieved in the dried product at 75 ℃ during RF-assisted HAD (RF+HAD). Microstructural characterization showed that the tissues of fresh jujube slices were composed of tightly connected cells with quite similar in size, indicating a clear cell edge. Furthermore, the cell structure of jujube slices deformed over time during the hot-air drying, which was mainly driven by the moisture removal from the cell fluid followed by the turgor loss. The cell deformation during drying was divided into two stages. In the first stage, the cell wall shrank gradually, as the turgor pressure decreased, leading to the reduced area, perimeter, and diameter in the product. In the second stage, the turgor reached the minimum, as the drying further continued, where the water of cell fluid was fully removed. Correspondingly, a higher level of warping and wrinkling occurred in the cell wall to accommodate the lower volumes of cell fluid. The larger micropore radius and more pores were found in the jujube slices after RF heat treatment, compared with the HAD. The porous structure greatly contributed to improving the evaporation and diffusion of water in the jujube slices, further changing the texture of hot-dried jujube slices. Temperatures also posed a great impact on the internal structure of jujube slices. Specifically, the cells of jujube slices experienced more significant shrinkage, with the increase of drying temperature during drying. Therefore, the microstructure evolution can be expected to clarify the influence mechanism of the texture (such as the hardness, crispness, springiness, and cohesiveness) of hot-air dried jujube slices after RF heat treatment. This finding can provide a strong reference for the process control of drying quality in the food products during radiofrequency-assisted hot air drying.