Innovatively identifying naringin and hesperidin by using terahertz spectroscopy and evaluating flavonoids extracts from waste orange peels by coupling with multivariate analysis

Fourier transform far-infrared (FT-FIR) was exploited to evaluate the flavonoids content of orange extracts using Soxhlet extraction. The absorbance fingerprint spectra of hesperidin and naringin were identified in the range of 0.5-7 THz. The absorption peaks of hesperidin were obtained at 1.53, 1.85, 2.46, 3.36, 4.51, 5.35, 6.51, and 7.55 THz, respectively. For naringin, the absorption peaks were observed at 0.75, 1.50, 2.29, 3.76, 5.18, 6.48, and 8.36 THz, respectively. The perceptible absorption peaks at 3.76, 4.40, and 5.18 THz can be observed from fingerprint spectra of naringin whilst those peaks cannot be detected from the hesperidin THz spectrum. The noticeable absorption peaks at 3.36, 4.51, 5.35, and 8.19 THz can be obtained from fingerprint spectra of hesperidin whereas they are indiscernible from the naringin THz spectrum. Pre-treatments such as normalization, standard normal variate (SNV), multiplicative scatter correction (MSC), 1st and 2nd derivation were employed to enhance the robustness of the model performance. Partial least squares regression model performed well to explain the relationship between THz spectra and concentrations of hesperidin and naringin, with the determination coefficients of prediction up to 0.99 (pre-treated by 1st deviation) for hesperidin and 0.97 for naringin, respectively. This study addressed the detection of hesperidin and naringin by using terahertz spectroscopy, which could potentially simplify the procedures to detect the flavonoids of orange extracts from waste orange peels.