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

Abstract

Sweeping 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 applications PVD 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.

Evaluation of the effect of ultrasonic pretreatment on vacuum far‐infrared drying characteristics and quality of Angelica sinensis based on entropy weight‐coefficient of variation method

“…This is due to the cavitation effect of ultrasonic power, which causes the rapid growth and rapid rupture of microbubbles and produces strong shock wave energy. It changes the structure of tissue cells and forms a more uniform and regular porous structure, indicating that ultrasonic pretreatment can effectively inhibit the shrinkage deformation and fracture collapse of the internal structure of the material (Aksoya et al., 2019; Zhang et al., 2020). It is worth noting that the pretreatment time also had significant effects on the microstructure of the material, compared with the pretreatment time of 20 min (Figure 7c); the micropore channels in the sample after ultrasonic treatment for 40 min (Figure 7d) shrink and rupture, which may be due to the rapid change of temperature and humidity gradient stress in the sample caused by long‐term ultrasonic treatment, the surface hardening phenomenon, the strong local stress, and the destruction of the microstructure under the action of extrusion and shear.…”

“…Ultrasound, as a kind of periodic mechanical acoustic wave, is a nonthermal treatment technology (Rashid et al., 2019). Due to its unique cavitation, mechanical, and thermal effects, it has remarkable advantages in improving the quality of dried products and reducing energy consumption in the drying process (Xu et al., 2021; Zhang et al., 2020). Bantle and Eikevik (2014) found that the frequency range of 20–40 kHz was the best in ultrasonic‐assisted convection drying.…”

Theoretical and experimental analysis of the drying kinetics of okra using infrared dryer

“…Furthermore, the elevated temperature employed in convection hot air (HA) processes often leads to the degradation of crucial nutritional components and the alteration of color (An et al, 2016;Ozyalcin & Kipcak, 2021). Several effective drying techniques for okra have been documented, such as oven drying, vacuum freeze drying, and heat pump drying (Jiang et al, 2017;Kipcak & _ Ismail, 2021;Wang et al, 2019;Xu et al, 2020Xu et al, , 2021Zhang et al, 2021). The processes above are characterized by their high costs, substantial energy consumption, and notable impact on the nutritional and sensory attributes of the product.…”

Real-time monitoring system for quality monitoring of jujube slice during drying process

“…Tomas-Egea et al [8] applied the infrared thermography to monitor the drying process of hot air and microwave combined drying of potato. Zhang et al [9] studied the influence of sweeping frequency ultrasonic pretreatment on pulsed vacuum drying characteristics and microstructure of okra based on real-time monitoring. Zhao et al [10] developed a novel online moisture monitoring method for vacuum drying lithium-ion battery powder.…”