Effect of mono- and dual frequency power ultrasound assisted enzymolysis on the degree of hydrolysis and ACE inhibitory activity of Stevia protein hydrolysates

Purpose: To study the effects of ultrasound pretreatment at different frequencies and working modes, including mono frequency ultrasound (MFU) and dual-frequency ultrasound (DFU), on the degree of hydrolysis (DH) and angiotensin-I-converting enzyme (ACE) inhibitory effect of Stevia protein (SP). Methods: Protein from Stevia leaves was extracted by water (50 g suspended in 1000 mL). The hydrolysis of stevia protein extract (SPE) was carried out using alcalase at 5 % enzyme substrate (E/S) ratio. A study was then carried out to investigate its microstructure and morphology using scanning electron microscopy (SEM). Results: The results showed that ultrasound pretreatment did not increase DH of SP significantly (p > 0.05). However, the highest ACE inhibitory activity of stevia protein hydrolysate was obtained at DFU level (20/50 KHz). Overall, ultrasonic frequency mode had a significant influence on ACE inhibitory activity. Conclusion: The frequency selection of ultrasound pretreatment of SP is essential for the preparation of ACE inhibitory peptide

Effect of drying methods on drying kinetics, energy features, thermophysical and microstructural properties of Stevia rebaudiana

BACKGROUND: Stevia leaves were subjected to convective hot-air, infrared and vacuum drying at 40, 60 and 80 °C, followed byan assessment of thermophysical properties and microstructure, along with drying kinetics modelling and evaluation of energyfeatures for all drying operations.RESULTS: Effective moisture diffusivity (Deff) showed dependency on temperature with values ranging from 1.08 × 10−12 to7.43 × 10−12 m2 s−1 for convective drying, from 0.71 × 10−12 to 6.60 × 10−12 m2 s−1 for infrared drying, and from1.29 × 10−12 to 5.39 × 10−12 m2 s−1 for vacuum drying. The thermal properties of the dried Stevia leaves under different dryingconditions showed values of density, specific heat, thermal diffusivity, thermal conductivity and thermal effusivity rangingfrom 95.6 to 116.2 kg m−3, 3050 to 3900 J kg−1 K−1, 4.28 × 10−7 to 5.60 × 10−7 m2 s−1, 0.16 to 0.23 W m−1 K−1 and 244 to305 W s0.5 m−2 K−1, respectively. As for microstructure, convective hot-air drying showed better preserved leaf characteristics,compared to infrared- and vacuum-drying, whereby scanning electron microscopy (SEM) image analysis also revealed noticeable differences at higher temperatures. Statistical analysis showed that the Midilli?Kuçuk model fitted best the experimentaldata of drying curves (0.961 < r2 < 0.999, 0.000064 < SSE < 0.005359, and 0.000074 < χ 2 < 0.006278). Comparison of thedrying methods with respect to energy features showed that convective drying at 80 °C led to lowest specific energy consumption (61.86 kW h kg−1) with highest efficiency (8.5%).CONCLUSION: The results of this study contribute to a better understanding of the drying behaviour and showed that thermophysical properties of dried Stevia leaves and energy features are affected by drying methods.Fil: Lemus Mondaca, Roberto. Universidad de Chile. Facultad de Ciencias Físicas y Matemáticas; ChileFil: Zura Bravo, Liliana. Universidad de Chile. Facultad de Ciencias Físicas y Matemáticas; ChileFil: Ah Hen, Kong. Universidad Austral de Chile; ChileFil: Di Scala, Karina Cecilia. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Departamento de Ingeniería Química. Grupo de Investigación en Ingeniería en Alimentos; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata; Argentin