Discrimination of Apple Liqueurs (Nalewka) Using a Voltammetric Electronic Tongue, UV-Vis and Raman Spectroscopy

Abstract

The capability of a phthalocyanine-based voltammetric electronic tongue to analyze strong alcoholic beverages has been evaluated and compared with the performance of spectroscopic techniques coupled to chemometrics. Nalewka Polish liqueurs prepared from five apple varieties have been used as a model of strong liqueurs. Principal Component Analysis has demonstrated that the best discrimination between liqueurs prepared from different apple varieties is achieved using the e-tongue and UV-Vis spectroscopy. Raman spectra coupled to chemometrics have not been efficient in discriminating liqueurs. The calculated Euclidean distances and the k-Nearest Neighbors algorithm (kNN) confirmed these results. The main advantage of the e-tongue is that, using PLS-1, good correlations have been found simultaneously with the phenolic content measured by the Folin–Ciocalteu method (R2 of 0.97 in calibration and R2 of 0.93 in validation) and also with the density, a marker of the alcoholic content method (R2 of 0.93 in calibration and R2 of 0.88 in validation). UV-Vis coupled with chemometrics has shown good correlations only with the phenolic content (R2 of 0.99 in calibration and R2 of 0.99 in validation) but correlations with the alcoholic content were low. Raman coupled with chemometrics has shown good correlations only with density (R2 of 0.96 in calibration and R2 of 0.85 in validation). In summary, from the three holistic methods evaluated to analyze strong alcoholic liqueurs, the voltammetric electronic tongue using phthalocyanines as sensing elements is superior to Raman or UV-Vis techniques because it shows an excellent discrimination capability and remarkable correlations with both antioxidant capacity and alcoholic content—the most important parameters to be measured in this type of liqueurs.

Analysis of red wines using an electronic tongue and infrared spectroscopy. Correlations with phenolic content and color parameters

“…CIELab coordinates were determined by measuring the transmittance of the wine every 10 nm over the visible spectrum (from 380 to 780 nm) using the illuminant D65 and 10ᵒ standard observer, following the CIE recommendations (Commission Internationale of L'Eclairage) (Sliwinska et al, 2016). These parameters are: a* (redness or -a*: greenness), b* (yellowness or -b*: blueness), L* (lightness), C* (chroma or saturation) and h* (hue angle).…”

Voltammetric Electronic Tongues in Food Analysis

“…As already pointed out, voltammetric electronic tongues can provide valuable qualitative information about food samples when combined with a chemometric method of pattern recognition. Table 1 summarizes some relevant applications found in references [14,15,16,17] and [28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71] focused on the characterization, classification and authentication of food products.…”

“…As a first example of sample discrimination with voltammetric electronic tongues, we will briefly discuss the work by M. Sliwinska et al, published in [43] and focused on apple liqueurs (Nalewka). In this study, an array of four working electrodes was used, including an unmodified carbon paste electrode (C-CPE) and three electrodes chemically modified with cobalt, iron and zinc phthalocyanines (denoted as CoPc–CPE, FePc–CPE and ZnPc–CPE, respectively).…”

“…Apple varieties: Ligol (black), Kosztela (red), Grey Reinette (blue), Rubin (green), Cox Orange (purple). Reproduced from [43].…”

“…Scores plot obtained in the PCA treatment of cyclic voltammetric (CV) data (see Figure 2) acquired with four working electrodes in three replicates of five types of apple liqueur (from Ligol, Kosztela, Grey Reinette, Rubin and Cox Orange apples). Reproduced from [43]. …”