Discriminative analysis of free and esterified gallic acids in acorn shells by thermochemolysis-gas chromatography/mass spectrometry in the presence of organic alkalis

•Thermochemolysis-GC/MS revealed the presence of polyphenols in the wasted acorn shells.•The contents of free and esterified gallic acids were estimated by using two different organic alkalis.•Acorn shell extracts showed outstanding antioxidant activity corresponding to ascorbic acid.•Methanol extracts showed higher antioxidant activity than water extract because of the abundance of esterified gallic acids.

The presence of phenolic compounds, possessing outstanding antioxidant activity corresponding to ascorbic acid, in wasted acorn shells was revealed by means of thermochemolysis-gas chromatography/mass spectrometry (thermochemolysis-GC/MS) in the presence of organic alkalis. The chromatogram of intact acorn shell samples obtained by thermochemolysis-GC/MS using tetramethylammonium hydroxide (TMAH) showed a strong peak of 3,4,5-trimethoxybenzoic acid methyl ester. This product was also clearly observed on the chromatograms of the methanol and water extracts from the acorn shells, indicating that 3,4,5-trimethoxybenzoic acid methyl ester was derived from free and/or esterified gallic acids through the thermochemolysis reaction. Based on the peak intensities, the contents of gallic acid components were determined as ca. 27 wt% for the both extracts. Furthermore, the content of free gallic acid was selectively estimated with discriminating from its esterified counterpart by using tetramethylammonium acetate (TMAAc) as a methylating reagent. These results were also correlated with high antioxidant activity of the acorn shell extracts evaluated by a DPPH radical scavenging method.