Antioxidant Activity of Carob Tree (Ceratonia siliqua L.) Leaf Extracts Obtained by Advanced Extraction Techniques

Abstract

Carob (Ceratonia siliqua L.) is a widely spread Mediterranean evergreen tree whose plant parts are rich in bioactive compounds with potential for application in functional food production. Carob leaves are the least explored part of the plant, and the main compounds of interest are polyphenols. Advanced extraction techniques, such as pressurized liquid (PLE), microwave-assisted (MAE) and ultrasound-assisted (UAE) extraction have not been sufficiently explored for their potential in extracting these compounds. Therefore, the aim of this paper was to optimize the parameters (temperature and time) of PLE, MAE and UAE of carob leaf polyphenols, characterize the individual compounds by ultra-high performance liquid chromatography tandem mass spectrometry (UPLC-MS2) and determine the antioxidant activity of the extracts. The optimal temperature and time were 160 °C/5 min for PLE, resulting in a total phenolic content of 68.21 mg of gallic acid equivalents (GAE) g−1. For MAE and UAE, the optimal conditions were 70 °C/10 min, under which total phenolic contents of 78.80 and 55.98 mg GAE g−1 were achieved, respectively. A total of 26 compounds, represented mainly by myricetin, quercetin-3-rhamnoside and gallic acid, were identified in extracts obtained by all three extraction techniques. All extracts showed high antioxidant activity (0.46–1.05 and 0.50–0.58 mmol TE g−1 for FRAP and DPPH, respectively), which correlated with the polyphenolic content and was affected by the concentration of ascorbic acid (0.03–0.52 mg mL−1), confirming that carob leaf extracts obtained by advanced extraction possess high potential for application in functional food products.