Drying characteristics and quality of Stevia rebaudiana leaves by far-infrared radiation

Abstract

The far-infrared drying technique was used to dry Stevia rebaudiana leaves, and the drying rate, drying kinetics, the glycoside content of dried Stevia rebaudiana leaves and surface structure of the dried material were explored at different drying temperatures (45 °C、50 °C、55 °C、60 °C、65 °C), irradiation height (60 mm、120 mm、180 mm、240 mm、300 mm). Under different conditions, the moisture rate decreased and the drying rate increased significantly with the increase in drying temperature and the decrease in irradiation height, Weibull model fits the drying curves well (R2 > 0.99). The range of the estimated water diffusion coefficient Dcal was 0.841 8 × 10−4–2.945 3 × 10−4 m2 s−1, and the range of the effective water diffusion coefficient Deff was 0.835 9 × 10−6–3.629 1 × 10−6 m2 s−1. The activation energy was 28.79 kJ mol−1 determined by Arrhenius equation. By comparing the glycoside content of dried products under different drying conditions, it was found that far-infrared drying could better retain the glycoside content. The drying temperature have the batter effect on active ingredients of samples when it increased from 45 °C to 55 °C, the stevioside and rebaudioside A increased from 7.01 to 7.88 mg to 7.44 and 8.58 mg, respectively. Far-infrared drying technology could increase the number of micropores on the surface and inside the materials and improve the quality of dried products. This study explored the law of water transfer during the drying process of Stevia rebaudiana leaves and provided a theoretical reference for the research and application of Stevia rebaudiana leaf drying technology.