Phenolic compound explorer: A mid-infrared spectroscopy database

Analyses of phenolic compounds can be time-consuming and not always very reliable. Mid-infrared (MIR) spectroscopy could be used as a non-destructive tool for food quality analysis and control. The MIR spectra of 36 standard phenolic compounds (13 phenolic acids [hydroxybenzoic and hydroxycinnamic acids], 23 flavonoids [flavonols, flavones, isoflavones, flavanols, and flavanones]) were collected using an Attenuated Total Reflectance (ATR) accessory. Data were collected to build a spectral database in order to identify spectral features of each family of phenolic compounds studied. The phenolic compound spectra showed common spectral bands associated with aromatic six-membered rings and phenol moieties. The bands were numerous between 1640 and 700 cm−1, leading to an overlap with other signals specific to different families. The methoxy group present in some phenolic compounds showed bands between 1470 and 950 cm−1. The phenolic acids were characterized mainly by bands of unsaturated carboxylic acids, esters and alkenes, whereas flavonoids were characterized mainly by bands associated with benzopyrylium, Benzo-γ-pyrone and 2-phenyl-3,4-dihydro-2H-chromen-3-ol vibrations. A comparison of flavonoids with phenolic acids showed that the latter could be distinguished by bands of carboxylic acids between 1755 and 1630 cm−1. Flavonoids were characterized by numerous peaks in the 1650-1400 cm−1 region and at spectral frequencies inferior to 1200 cm−1. Principal Component Analysis (PCA) was performed on the MIR spectra, resulting in good discrimination of the families of the studied flavonoids and phenolic acids. The spectral regions 1755-1400 cm−1 and 1000-870 cm−1 were of great importance in distinguishing the studied families of phenolic compounds. The study allows starting to build a MIR spectral database for phenolic compounds as bioactive components of food.