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

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 (4000-550 cm−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) = 12.62 μmol 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.