Effects of contact ultrasound & far‐infrared radiation strengthening drying on water migration and quality characteristics of taro slices

This study explored the effects of contact ultrasound strengthening hot air drying (CU-HAD), far-infrared radiation drying (FIRD) and contact ultrasound strengthening far-infrared radiation drying (CU-FIRD) on the moisture state and distribution, microstructure, quality characteristics of taro. The results showed that the drying times of CU-HAD, FIRD and CU-FIRD were reduced by 26.32%, 47.37% and 63.16%, compared with single HAD, respectively. During the drying process, the free water was removed from the taro slices, the immobile water increased first and then decreased, and the T2 relaxation distribution curves shifted to the left; Moreover, with the progress of drying, the water content of taro slices continued to decline, and the signal of magnetic resonance imaging (MRI) decreased. As the change of water content was closely related to the internal structure of taro slices, the application of both CU and FIR produced looser tissue structure and larger micro-tunnels. And loose structure led to low hardness. Furthermore, compared with single HAD, the rehydration ratio, color difference, total phenolic content, DPPH radical scavenging activity and ABTS radical scavenging activity of CU-FIRD were significantly better than FIRD and CU-HAD. The relative contents of heterocyclics and esters in the taro slices dried by CU-FIRD method were similar to those of CU-HAD and more abundant than those of FIRD, and the relative contents of aldehydes and olefins were richer than those of the other two drying methods, which had a positive impact on the flavor. So CU-FIRD is suitable and promising for the production of dried taro products. Novelty impact statement Both CU and FIR applied in HAD could significantly accelerate water removal and improve product quality. CU-FIRD could affect the microstructure of both sides of taro slices, and achieve quicker moisture removal comparing with single application of CU or FIR. CU-FIRD could combine the advantages of CU and FIRD, and realize higher nutrient values and better antioxidant activities.