Abstract
Stevia leaves, which are commonly used as a natural sweetener in food products, have increased in importance for antioxidant delivery due to their high content of phenolic compounds. In this study, the influence of the drying process on stevia leaves, with regards to phenolic content and antioxidant activity during drying kinetics 40 °C for 7 h, was studied. The effect of solvent concentration and extraction time using a 32 factorial design on total phenol content (TPC), and on antioxidant activity of extracts obtained from dried stevia leaves, by ultrasound assisted extraction (UAE) as alternative method was evaluated. Steviol glycosides contents were also evaluated by a conventional and UAE method. Phenols identification, quantification and purification were performed by Ultra Performance Liquid Chromatography-Electrospray Ionization-Mass Spectrometry (UPLC-ESI-MS), Ultra Performance Liquid Chromatography-Photodiode Array (UPLC-PDA) and advanced automated flash purification, respectively. Drying time affected the moisture content of stevia leaves. A constant weight was reached after six hours of drying, and higher antioxidant activity was observed, while the highest TPC was obtained after seven hours of drying. The highest TPC (91.57 ± 8.8 mg GAE/g dw) and antioxidant activity (603.24 ± 3.5 μmol TE/g dw) in UAE method was obtained when ethanol 50% at 5 min was used. Steviol glycosides extracted by UAE were recorded with a content of 93.18 ± 1.36 mg/g dw and 98.97 ± 1.75 mg/g dw for stevioside and rebaudioside A respectively. Six phenolic compounds including four phenolic acids and two flavonoids were identified and quantified by UPLC-PDA, and confirmed by ESI-MS reporting its fragmentation pattern. Diosmin and chlorogenic acid were the most abundant compounds with values of 2032.36 μg/mL and 434.95 μg/mL respectively. As a novelty we found that the antioxidant activity evaluated in partially purified fractions suggested that biological activity might be attributed to the synergistic effect of the six phenols present in the stevia leaves extract. In addition to its sweeting properties, stevia leaves constitute a potential source of polyphenolic compounds, with antioxidant activity that could be used as a food additive.
Highlights
Stevia rebaudiana is an herbaceous perennial plant that is composed of leaves containing a natural source of diterpenic glycosides, allowing them to be extensively used as a non-caloric sugar substitute which is 250 to 300 times sweeter than sucrose [1]
The drying kinetic of stevia leaves was performed at 40 ◦ C throughout the entire process to more accurately evaluate the effect of the heating time on the moisture content reduction, phenol content and antioxidant activity
Castillo-Téllez et al [7] agreed that the best time to collect and store stevia leaves is when they become brittle with 10–12% moisture content
Summary
Stevia rebaudiana is an herbaceous perennial plant that is composed of leaves containing a natural source of diterpenic glycosides, allowing them to be extensively used as a non-caloric sugar substitute which is 250 to 300 times sweeter than sucrose [1]. It is traditionally used in non-alcoholic beverages, bakery products and chewing gum [2]. Stevia leaves extracts contains phytochemicals, such as phenols which are the main compounds responsible of the antioxidant activity of the extracts [1,3]. Lemus-Mondaca et al [9] claimed there was an increase in total phenol content (TPC)
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