Soluble and hydrolyzable phenolic compounds in date fruits (Phoenix dactylifera L.) by UPLC-QTOF-MS/MS and UPLC-DAD

Phenolic compounds in plant foods exist in soluble, hydrolyzable, and insoluble forms. This study is the first to analyze both soluble (extracted using 80% methanol) and hydrolyzable (extracted using ethyl acetate after alkaline hydrolysis) phenolics in 18 date cultivars. The phenolic compounds were identified and quantified using ultra-high-performance liquid chromatography with quadrupole time-of-flight mass spectrometry and diode array detectors. A total of 59 peaks were detected, of which 45 were identified while 14 remained unidentified. The concentrations of the soluble and hydrolyzable phenolics ranged from 27.5–54.0 and 24–78.5mg/100g fresh weight, respectively. Gallic acid and ferulic acid predominated in the soluble and hydrolyzable fractions, quercetin and taxifolin were predominant in the soluble fraction, while caffeic acid and chlorogenic acid were abundant in the hydrolyzable fraction. The hydrolyzable phenolics were found to contribute significantly to the total phenolic content of date fruits pointing to the need to include them in the analysis of total phenolic compounds in foods.