Determination of antioxidant capacity and phenolic content of chocolate by attenuated total reflectance-Fourier transformed-infrared spectroscopy

•FT-IR can predict antioxidant capacities, total phenol content and (+)-catechin content in chocolate.•The overall analysis for chocolate antioxidants was less than 2min with no sample pretreatment.•This FT-IR spectroscopic method is high-throughput and non-destructive. Antioxidant capacity and phenolic content of chocolate, containing different amounts of cacao (35–100%), were determined using attenuated total reflectance (ATR)-Fourier transformed-infrared (FT-IR) spectroscopy (4000–550cm−1). Antioxidant capacities were first characterized using DPPH (2,2-diphenyl-1-picrylhydrazyl) and ORAC (oxygen radical absorbance capacity) assays. Phenolic contents, including total phenol and procyanidins monomers, were quantified using the Folin–Ciocalteu assay and high performance liquid chromatography coupled with photodiode array detector (HPLC–DAD), respectively. Five partial least-squares regression (PLSR) models were constructed and cross-validated using FT-IR spectra from 18 types of chocolate and corresponding reference values determined using DPPH, ORAC, Folin–Ciocalteu, and HPLC assays. The models were validated using seven unknown samples of chocolate. PLSR models showed good prediction capability for DPPH [R2−P (prediction)=0.88, RMSEP (root mean squares error of prediction)=12.62μmol Trolox/g DFW], ORAC (R2−P=0.90, RMSEP=37.92), Folin–Ciocalteu (R2−P=0.88, RMSEP=5.08), and (+)-catechin (R2−P=0.86, RMSEP=0.10), but lacked accuracy in the prediction of (−)-epicatechin (R2−P=0.72, RMSEP=0.57). ATR-FT-IR spectroscopy can be used for rapid prediction of antioxidant capacity, total phenolic content, and (+)-catechin in chocolate.