Abstract:
Abstract: Green prickly ash is one of the main flavoring spices in Chinese food, with a strong numb taste and medicinal effects, such as antipruritic, vasodilation, and lowering blood pressure. The initial moisture content of green prickly ash is up to 66%-70% (w.b.), easy to cause browning reaction and mildew during storage and transportation. Drying has also been a necessary step before deep processing of green prickly ash, in order to ensure the quality of products while prolonging the shelf life. A pulsed vacuum drying can keep the material in a circulating state of alternating vacuum and atmospheric pressure, where the microscopic pores inside the material are continuously squeezed or expanded to form micro-channels, which significantly improves the moisture migration rate for high drying efficiency. The vacuum environment is also beneficial to reducing the browning reaction and the loss of flavor components during drying. In this study, an investigation was made to clarify the effects of drying temperature (55, 60, 65 and 70℃), holding time of vacuum (5, 10, 15, and 20 min), and holding time of atmospheric pressure (1, 3, 5, 7 min) on the drying characteristics and quality attributes of green prickly ash under a pulsed vacuum drying. The hot air drying was treated as the controlled group. A Weibull model was used to simulate the drying curves of green prickly ash. A single factor experiment was performed on the Box-Behnken platform. A response surface optimization was conducted, where five indicators were included of the average drying rate, color, aperture opening ratio, volatile oil and amide content of green pepper. The results showed that the Weibull model accurately simulated the vacuum pulsating drying. The pulsed vacuum drying of green prickly ash behaved a brief rise in the early stage, and then a slow-down drying stage. Both drying temperature and holding time of atmospheric pressure posed a significant effect on the drying rate of green prickly ash (P<0.05). Specifically, the drying rate was higher, as the holding time was shorter at atmospheric pressure. The drying rate, color and aperture opening ratio were improved, while the browning reaction and the loss of volatile oil content were reduced, as the drying temperature increased. Meanwhile, the amide content decreased with the rise of drying temperature. Once the temperature exceeded 70°C, a significant downward trend occurred in the quality indicators of green prickly ash. The influencing factors in green pepper drying were ranked as follows: drying temperature, vacuum holding time, atmospheric pressure holding time. The optimized processing parameters of pulsed vacuum drying were fixed at the drying temperature of 61.4℃, the vacuum holding time of 5 min, and the atmospheric pressure holding time of 5 min. The comprehensive score was up to the maximum of 8.06 in the response surface model under this condition. Three verification tests were carried out to take the average value. The relative deviation between the test and predicted value was only 2.6% in the response surface model, indicating that the optimization data was reliable. A verification test was determined as the average drying rate of 0.81 g/(g·min), ΔE value of 6.31, aperture opening ratio of 92.3%, volatile oil content of 0.72 mL/10 g, and amide content of 27.65 mg/g. The optimal processing parameters of green prickly ash can provide a sound reference for the practical application of similar materials under pulsed vacuum drying.