Quantification of Corn Adulteration in Wet and Dry-Processed Peaberry Ground Roasted Coffees by UV-Vis Spectroscopy and Chemometrics

Abstract

In this present research, a spectroscopic method based on UV-Vis spectroscopy is utilized to quantify the level of corn adulteration in peaberry ground roasted coffee by chemometrics. Peaberry coffee with two types of bean processing of wet and dry-processed methods was used and intentionally adulterated by corn with a 10-50% level of adulteration. UV-Vis spectral data are obtained for aqueous samples in the range between 250 and 400 nm with a 1 nm interval. Three multivariate regression methods, including partial least squares regression (PLSR), multiple linear regression (MLR), and principal component regression (PCR), are used to predict the level of corn adulteration. The result shows that all individual regression models using individual wet and dry samples are better than that of global regression models using combined wet and dry samples. The best calibration model for individual wet and dry and combined samples is obtained for the PLSR model with a coefficient of determination in the range of 0.83-0.93 and RMSE below 6% (w/w) for calibration and validation. However, the error prediction in terms of RMSEP and bias were highly increased when the individual regression model was used to predict the level of corn adulteration with differences in the bean processing method. The obtained results demonstrate that the use of the global PLSR model is better in predicting the level of corn adulteration. The error prediction for this global model is acceptable with low RMSEP and bias for both individual and combined prediction samples. The obtained RPD_p and RER_p in prediction for the global PLSR model are more than two and five for individual and combined samples, respectively. The proposed method using UV-Vis spectroscopy with a global PLSR model can be applied to quantify the level of corn adulteration in peaberry ground roasted coffee with different bean processing methods.

Keywords: UV–Vis spectroscopy; adulteration; authentication; dry bean processing; global model; individual model; multiple linear regression; partial least squares regression; peaberry coffee; wet bean processing.

Figure 1

The visual appearance of peaberry dry-processed (A) and wet-processed (B) coffee with 10–50% of corn adulteration.

Figure 2

Spectral data of peaberry wet and dry-processed coffee with 10–50% of corn adulteration in the range between 250 and 400 nm: (a) raw spectra; (b) pre-processed spectra (MAS + SNV + SG1d).

Figure 3

Plot of the first two principal components by PCA in the range between 250 and 400 nm: (a) raw spectra; (b) pre-processed spectra (MAS + SNV + SG1d).

Figure 4

The plot of x-loading calculated by PCA in the range between 250 and 400 nm using pre-processed spectra.

Figure 5

Actual versus predicted values of corn adulteration (% w/w) in peaberry coffee samples for the best PLSR model on (a) individual wet calibration samples, (b) individual dry calibration samples, and (c) combined calibration samples.