Influence of sweeping frequency ultrasonic pretreatment on pulsed vacuum drying characteristics and microstructure of okra based on real‐time monitoring

Abstract

AbstractSweeping frequency ultrasonic (SFU) pretreatment was utilized to promote pulsed vacuum drying (PVD) process of okra. Drying characteristics and microstructure of okra were studied. Then mechanism of SFU pretreatment was revealed by acoustic signal monitoring technologies in time and frequency domains and bubble oscillations. Results showed that core temperature (57–58°C) of PVD, rehydration ratio (3.16%), hardness (23.90 N) and, fracturability (3.15 mm) were the highest, while moisture ratio was the smallest in SFU pretreated okra group at 30 min during the ultrasonic pretreatment process. It was related to changes of microstructure. SFU pretreated okra cells got folded, intercellular space was large, and the tunnel structure was generated until 30 min. However, excessive ultrasound caused the structural collapse of cells; the tunnel structure was congested, which was not beneficial to moisture transfer during PVD process. Effects of SFU pretreatment were further explained by monitored data of acoustic signals in time and frequency domains. The maximum voltage amplitude (40.6 mV) and space peak temporal peak acoustic intensity (2.76 × 106 W/m2) were the highest at 30 min during the pretreatment process. The corresponding SFU signals had a wide frequency domain and fluctuation range of energy, and cavitation effects of ultrasonic bubbles was facilitated.Practical applicationsPVD is an innovative drying technology. However, okra structure is complex, which is against moisture transfer during the process of drying. Hence, SFU pretreatment was added before drying. The tunnel structure of okra generated by the pretreatment could support moisture transfer suggesting a rapid rate of drying. The best quality of dried okra was obtained at 30 min during the pretreatment process, which makes for long storage and preparation of functional snacks. Acoustic signals were analyzed by real‐time monitoring technique during pretreatment of okra, and the cavitation intensity of SFU fields was revealed. Thus, the fundamental basis for accurate ultrasonic processing is provided.