FT-NIR spectroscopy as a tool for valorization of spent coffee grounds: Application to assessment of antioxidant properties

Spent coffee grounds (SCGs) are a significant residue produced annually worldwide. This by-product contains high levels of bioactive compounds, such as chlorogenic acid and flavonoids that have recognized antioxidant properties. Recovery of these compounds previously for utilization in energy production is an interesting valorization approach, since they can be further incorporated in food products. A rapid, clean and reliable technique is required to discriminate SCG samples with high content of total phenolics, and total flavonoids as well as to assess their antioxidant capacity. In this work, Fourier-transform near infrared (FT-NIR) spectroscopy was used to determine the antioxidant capacity, total phenolic and total flavonoid contents of several SCG samples (n=101). The antioxidant capacity of SCGs was determined by the ABTS assay, using the direct procedure that measures both soluble and insoluble antioxidant fractions and by the indirect approach that assesses the antioxidants present in SCG ethanolic extracts. FT-NIR spectra were calibrated against total flavonoid and total phenolic contents and the antioxidant capacity using PLS modeling. Spectral wavelength selection and latent variables were optimized for the lowest root mean square errors. All PLS models' results showed good linearity with an R2 for an independent test set of 0.93, 0.96, 0.95 and 0.95 for antioxidant capacity of SCGs, antioxidant capacity of SCG ethanolic extracts, and total flavonoid and total phenolic contents, respectively. The range error ratio (RER) was higher than 13 for all models (higher than 16 for three of them). Results confirmed that NIR spectroscopy is a promising technique for routine assessment of these parameters in SCGs and is a viable and advantageous alternative to chemical procedures involving laborious extractions. ► Spent coffee grounds (SCG) are a valuable source of bioactive compounds. ► Rapid technique is required for quantitative discrimination of SCG samples. ► NIR spectroscopy was proved as a valuable tool for assessment of TAC, TPC and TFC. ► High-throughput technique is proposed as alternative to time-consuming chemical assays.