Non-destructive measurement of antioxidant activity and water content in chili powder (Capsicum annuum L.) using near-infrared spectroscopy

Chili (Capsicum annuum L.) is a potential source of antioxidants due to its high vitamin and lutein contents. The information on antioxidant activity and water content of chili are needed by the chili-based industries which utilise chili's natural antioxidant content for various purposes in health, food, or herbal products. These information are often obtained through destructive testing which damages the tested materials, and also requires a relatively long analysis time. Therefore, the present work aimed to non-destructively measure the antioxidant activity and water content of chili powder using near-infrared spectroscopy (NIS) combined with partial least square (PLS) analysis. A total of 84 samples of chili powder from four different colour groups were used in the present work. Approximately 70% of the samples were used for calibration, while the rest were used for validation. Samples' reflectance data were acquired using NIRFlex N500 Fiber Optic Solids Cells at wavelengths of 1000 – 2500 nm. The antioxidants activity and water content were also analysed destructively for comparison/reference data. PLS was then performed on the spectrum and reference using the Unscrambler X version 11.0 software. There were also three pre-treatments used to improve the spectrum, i.e., multiplicative scatter correction, first derivative Savitzky Golay, and detrending. The best calibration model was successfully obtained when the antioxidant activity spectrum was processed using the de-trending pre-treatment with six PLS factors. The coefficient of determination (R²) and residual predictive deviation (RPD) values for this were 0.83 and 2.60, respectively. Additionally, the best calibration model was successfully obtained when the water content spectrum was processed using the first derivative Savitzky Golay pretreatment with five PLS factors. The R² and RPD values for this were 0.96 and 2.25, respectively. Both models were observed to be accurate, and have the potential to be used for measuring the antioxidant activity and water content of chili powder samples.