Effects of ultrasonic pretreatment on the microwave drying quality of shepherd's purse (Capsella bursa-pastoris)

Shepherd's purse is one types of the most favorite vegetable food with an excellent source of bioactive compounds, such as polyphenols, flavonoids, vitamins and chlorophyll. However, it can deteriorate rapidly during storage, due to the high percentage of moisture. Microwave drying is one of the most efficient and rapid methods to reduce the water activity of food. However, nonuniform temperatures and corona discharges can degrade the product quality during microwave drying. Ultrasonic waves can be expected to serve as the pretreatment on the mass transfer for food drying in recent years. This study aims to explore the effect of ultrasonic pretreatment on the drying quality of shepherd's purse. Four ultrasonic pretreatments (4, 6, 8 and 10 W/g) were set before the microwave drying. A systematic investigation was then made on the mass transfer, microstructure, water distribution, water state and enzyme activity of the shepherd's purse after the ultrasonic pretreatment. Then, further analysis was carried out on the drying characteristics, color, rehydration property, nutrients (chlorophyll, total phenols, total flavonoids and ascorbic acid) and flavor behavior of the samples after microwave drying. The results showed that the ultrasonic pretreatment significantly decreased the activities of peroxidase (POD) and polyphenol oxidase (PPO) in shepherd's purse, whereas, there was an increase in the loss rate of dry matter and the response value of richness, salty, acidity. At the same time, there were increasing response values of nitrogen and oxygen compounds, sulfur compounds, and aromatic components of dried products. The relaxation of low-field nuclear magnetic resonance showed that there were three states of water (T₂₁, T₂₂ and T₂₃) in the shepherd's purse, which represented the bound, immobile, and free water. The moisture freedom of the shepherd's purse increased at first and then decreased after ultrasonic treatment. But compared with fresh samples, there was an increase in the moisture freedom of the shepherd's purse that was treated with ultrasound. The cavitation and mechanical properties of ultrasound enhanced the internal mass transfer. Compared with the control sample, the enzyme activity of POD and PPO of shepherd's purse after ultrasonic pretreatment decreased by 24.59%-52.46% and 62.86%-94.29%, respectively, and the total color difference decreased by 23.92%-55.07%. Once the ultrasonic power was 6 W/g, there was the intact cell morphology of shepherd's purse, together with the high effective water diffusion coefficient, rehydration ratio, chlorophyll, total phenols and total flavonoid content of samples. Compared with the control group, the total chlorophyll content, total flavonoid content and total phenol content of the dried products increased by 11.02%, 7.12% and 3.63%, when the ultrasonic power was 6 W/g. The Page model shared the high fitting degree, which was the most suitable for the microwave drying kinetics of the shepherd's purse. The drying rates of the shepherd's purse increased first and then decreased, according to the drying kinetics models. The results showed that ultrasonic pretreatment could accelerate the microwave drying rate by changing the microstructure and water distribution of shepherd's purse, and maintain the color and nutritional quality. This finding can provide a theoretical basis for the application of ultrasonic pretreatment on microwave drying of shepherd's purse.