Extraction of some bioactive compounds from fenugreek seeds by ultrasonic-assisted extraction process, characterization of obtained compounds and determination of their stability

In this thesis, phenolic compounds and dietary fiber from fenugreek seeds were extracted by classical and ultrasonic-assisted extraction (UAE) techniques at different process parameters. The phenolic compounds obtained at different extraction conditions were characterized by determination of total phenolic compounds, antioxidant activity, and total flavonoid and saponin contents. For the classical extraction and UAE of phenolic compounds, the process variables (solvent mixture percentage, solid/solvent ratio, ultrasonic amplitude, and extraction time) were optimized to ensure the highest antioxidant activity. The total phenolic compound, flavonoid, and saponin contents of the phenolic extracts obtained by UAE treatment (13.26 g/L solid/solvent ratio, 54.72% ultrasonic amplitude, and 59.71 minutes of extraction time) under optimum conditions are >25% higher, and the antioxidant activity (5.02 Trolox/g dry sample) was 21% higher, with half the processing time compared to classical extraction. Moreover, the thermal stability of extracts obtained at the optimum conditions in both extraction techniques were determined at different temperatures (60-100°C) and pH values (pH 3.0-9.0), and the highest thermal stability was obtained at pH 3.0, while the lowest was at pH 9.0. The thermal stability of extracts obtained by UAE technique was higher than by classical extraction at all process conditions. For dietary fiber extraction from fenugreek seeds, classical extraction and UAE were carried out in alkaline medium, and the process variables (solid/solvent ratio, NaOH concentration, ultrasonic amplitude, and extraction time) were optimized to ensure the highest extraction yield. Soluble and insoluble dietary fibers were isolated from the extracts obtained at the different extraction conditions, and the samples' physicochemical (water holding capacity, oil holding capacity, swelling capacity) and functional (glucose adsorption capacity, α-amylase inhibition capacity) properties were determined. The optimum conditions ensuring the highest total dietary fiber yield (79.35%) for UAE technique were determined as 52.3 g/L solid/solvent ratio, 57.86% ultrasonic amplitude, and 24.68 minutes of extraction time. The time to reach the highest extraction yield was 4.8 times shorter in the UAE process compared to classical extraction. For both extraction techniques, the characteristic properties of insoluble dietary fibers obtained under different extraction conditions were higher than for soluble dietary fibers. By monitoring the physicochemical and functional properties at two different storage conditions (6 months at 25°C and 40% RH, 3 months at 38°C and 90% RH), the storage stability of the dietary fibers obtained by both extraction methods were determined. It was confirmed that the storage stability of dietary fibers obtained by UAE is higher than the samples obtained by classical extraction.