Free and bound phenolic compounds in barley (Hordeum vulgare L.) flours: Evaluation of the extraction capability of different solvent mixtures and pressurized liquid methods by micellar electrokinetic chromatography and spectrophotometry

Phenolic compounds exist in free and bound forms in cereals. The efficiency, reliability and suitability of recovering free phenolic compounds from barley by conventional, solid–liquid and pressurized solvent extractions, using different mixtures and methods, were tested. The extraction recovery of bound phenolics was evaluated using alkaline and acid hydrolyses. This study illustrates a rapid application of micellar electrokinetic chromatography for the analysis of free and bound phenolic compounds in barley samples. After developing a capillary electrophoresis optimization plan, barley phenols were analyzed within 5.5 min, using a buffer containing 20 mM sodium tetraborate, 10 mM sodium dodecyl sulfate and 5 mM KH2PO4 (pH 9), a 40 cm × 50 μm capillary, 30 kV and 30 °C. The selectivity of the extraction methods in recovering phenolic classes was evaluated by capillary electrophoresis and compared with spectrophotometric measurements. Electropherograms of free phenolic extracts showed flavan-3-ol compounds, proanthocyanidins and hydrolysable tannins. Aqueous acetone and aqueous ethanol solvents extracted the highest amount of catechins and hydrolysable tannins, respectively. The extraction yield of bound phenolic compounds (especially hydroxycinnamic acids) increased when the digestion time for alkaline hydrolysis was prolonged.