Description
The aim of the research was to perform a comparative study between a near-infrared (NIR) spectrophotometer and a thermobalance (currently widely used in coffee company laboratories) for rapid analysis of the moisture content of roasted beans and ground coffee. The accuracy of the NIR models has been compared with the accuracy of the measurements performed on the same samples with the thermobalance. As a reference, the determination of the water content was carried out also with an official method in a stove at 103 °C.
A benchtop diode array NIR spectrophotometer (Corona Extreme, Zeiss, Germany) for process measurement in the range 950-1650 nm (resolution of 10 nm) equipped with a rotating acquisition system suitable for non-homogeneous samples was used. A total of 266 samples (roasted beans and ground coffee samples) from different coffee blends were analyzed. The data analysis was planned following a threefold approach: (i) exploratory analysis of the spectral data through Principal Components Analysis (PCA); (ii) calculation of predictive models for quantitative estimation of the moisture content using the PLS technique; (iii) comparison between the analytical tools through statistical analysis carried out on the thermobalance data in order to assess the accuracy of the two methods with respect to the stove reference data.
The models calibrated starting from NIR data on ground coffee provided the best results (R2val = 0.96, RMSEP = 0.15%). Very good results were achieved also for the model starting from spectra of roasted coffee beans (R2val = 0.95, RMSEP = 0.16%). A correlation analysis was performed for the two methods with the official method in the stove. Correlation coefficients with the stove data of r = 0.98 and r = 0.96 were achieved for NIR prediction on ground coffee and thermobalance measurements, respectively. For both the methods correlations with the reference one were highly significant (p < 0.01). In conclusion, NIR spectroscopy is a reliable method both for the use in the laboratory to replace the thermobalance and potentially for on-line moisture content measurements on different coffee matrices.
Keywords: coffee monitoring, non-destructive, water content, stove, chemometrics
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