The present study aimed at the identification, differentiation and characterization of red and white Cretan wine varieties, which are described with Protected Geographical Indication (PGI), using ultraviolet–visible absorption spectroscopy and fluorescence spectroscopy. Specifically, the grape variety, the wine aging process, the role of container type (barrel) and the addition of alternative antioxidants were investigated. The combination of spectroscopic results with machine learning-based models demonstrated the use of absorption spectroscopy as a facile and low-cost technique in wine analysis. In this study, a clear discrimination among grape varieties was revealed. Moreover, a grouping of samples according to aging period and container type of maturation was accomplished, for the first time.

Typical absorption spectra (UV - Vis - NIR)

UV 220 - 400 nm
Vis 400 - 700 nm
NIR 800 - 1800 nm

Typical fluorescence "contour maps" of the 5 Cretan varieties
(white wine: Assyrtiko, Vilana and red wine: Kotsifali, Mandilari, Liatiko)

Assyrtiko
Vilana
Kotsifali
Mandilari
Liatiko

Discrimination of grape varieties

  • Red wine varieties discrimination (Kotsifali, Liatiko, Mandilari) with UV absorption spectroscopy 220 – 400 nm.
PCA score plot
  • White wine varieties discrimination (Assyrtiko, Vilana) with UV absorption spectroscopy 220 – 400 nm.
PCA score plot

Discrimination of wines according to aging time

 

Red wine aging time discrimination (0, 3, 6 and 9 months) with fluorescence spectroscopy.

3-D score plot of Liatiko variety.

Discrimination of container type (barrel)

 

Maturation container discrimination with Vis absorption spectroscopy 400 – 700 nm of Liatiko variety.

3D PCA score plot of different maturation process.

Discrimination of antioxidants

Wine-makers use sulfites in various steps of winemaking, from fermentation to bottling. However, sulfites have been connected to human health problems, and despite the various attempts to replace or minimize their levels in the winemaking process, no significant and widely used commercial process with marketable proof has appeared so far. In recent years, significant efforts to produce healthier wines has led to the replacement or reduction of the addition of sulfites, using alternative substances or techniques. Resveratrol and related biophenols seem to be of great interest, since beyond their protective nature and contrary to sulfites they can positively affect consumer health (Kontaxakis et al., 2020). These bioactive phytochemicals are naturally produced in grapes as evolutionary acquired mechanisms against pathogens and UV irradiation. The research team of the Department of Agriculture, of the Hellenic Mediterranean University, in collaboration with «Alexakis SA» and «Domaine Paterianakis» wineries, investigated the possibility of replacing the addition of sulfur dioxide (SO2) by natural antioxidants. The addition of two concentrations of trans-resveratrol (30 and 60 ppm) and two concentrations of a mixture of phenolic substances extracted from winemaking by-products (60 and 120 ppm) was examined. Natural antioxidants were used as alternatives to the additional use of sulfuric anhydride after the fermentation in white (Vilana, Assyrtiko) and red (Kotsifali, Mantilari) wines. The effect on the antioxidant capacity and the physicochemical characteristics of the wines was studied during their storage for one year in the bottle. Classical wine analyses (OIV), determination of total phenolic components and determination of the antioxidant capacity of the wines (Rivero-Pérez et al., 2008; Waterhouse, 2001) were carried out to evaluate the effectiveness of the treatments in maintaining the quality of the wines. The addition of natural antioxidants, as an alternative to the addition of sulfuric anhydride, affected the characteristics of the wines, increasing the antioxidant capacity and the content of total phenolic components. The addition of natural antioxidants limited the quality degradation of the wines, limiting the volatile acidity in the wines to a degree proportional to the amount added. The winetasting evaluation of the experimental wines, it emerged that the addition of phenolic substance extracts affected the taste and aroma of the wines in a manner similar to a prolonged extraction during the fermentation period. In conclusion, the addition of antioxidant substances extracted from winemaking by-products can limit the need to add sulfuric anhydride to protect wines during their maturation and ageing.

120 wine samples were studied of 4 Cretan varieties (Assyrtiko, Vilana, Kotsifali, Mandilari) provided by 2 wineries (Alexakis and Paterianaki winery). The maturation process achieved with the addition of alternative antioxidants (phenols of 60 ppm and 120 ppm, resveratrol of 30 ppm and 60 ppm). The wine sampling took place over a period of 4 months, 8 months and 12 months during the years of 2019 and 2020.

Vilana - UV _ PLS - DA score plot
Vilana - UV _ PLS - DA loading plot
Vilana - UV _ Table
Assyrtiko - UV _ PLS - DA score plot
Assyrtiko - UV _ PLS -DA loading plot
Assyrtiko - UV _Table
Kotsifali - UV _ PCA score plot
Kotsifali - UV _ PCA loading plot
Mandilari - UV _ PLS-DA score plot
Mandilari - UV _ PLS-DA loading plot
Mandilari - UV_Table
Mandilari - Vis _ PLS-DA score plot
Mandilari - Vis _ PLS-DA loading plot
Mandilari - Vis_Table
Assyrtiko - Fluo _ PCA score plot
Assyrtiko - Fluo _ PC1-Loading Plot
Assyrtiko - Fluo _ PC3-Loading Plot
Vilana - Fluo _ PCA score plot
Vilana - Fluo _ PC1-Loading Plot
Vilana - Fluo _ PC2-Loading Plot
Mandilari - Fluo _ PCA score plot
Mandilari - Fluo _ PC1-Loading Plot
Mandilari - Fluo _ PC2-Loading Plot

Results from classical analyses

Replacing Sulfites in Wines with natural antioxidants - Resveratrol and winemaking by-products extracts

The possibility of replacing the addition of sulfur dioxide (SO2) in wine with natural antioxidants was investigated. Two concentrations of trans-resveratrol (RES 30 and 60 ppm) and two concentrations of a mixture of phenolic substances extracted from winemaking by-products (PHEN 60 and 120 ppm) were added. Natural antioxidants, RES and PHEN were used as alternatives to the additional use of sulfuric anhydride after the fermentation in white (Vilana, Assyrtiko) and red (Kotsifali, Mantilari) wines. The effect on the antioxidant capacity and the physicochemical characteristics of the wines was studied during their storage for one year in the bottle.

Classical wine analyses (OIV), determination of antioxidant capacity (see Graphs [A1], [B1], [C1] & [D1]) and determination of total phenolic components (see Graphs [A2], [B2], [C2] & [D2]) of white and red wines (analytical methods after Rivero-Pérez et al., 2008; Waterhouse, 2001) were carried out to evaluate the effectiveness of the treatments in maintaining the quality of the wines.

The addition of natural antioxidants, such as a) trans-Resveratrol (RES) and b) mixture of phenolic substances extracted from winemaking by-products (PHEN), as an alternative to the addition of sulfuric anhydride, affected the characteristics of the wines, increasing the antioxidant capacity (see Graphs [A1], [B1], [C1] & [D1]) and the content of total phenolic components (see Graphs [A2], [B2], [C2] & [D2]) in two years of wine production (2020 & 2021).

The addition of natural antioxidants a) trans-Resveratrol (RES) and b) mixture of phenolic substances extracted from winemaking by-products (PHEN), limited the quality degradation of the wines, limiting the volatile acidity in the wines to a degree proportional to the amount added (see Graphs [A3] & [B3]). Concerning the winetasting evaluation of experimental wines, appeared that the addition of phenolic substance extracts (PHEN), affected the taste and aroma of the wines in a manner similar to a prolonged extraction during the fermentation period.

In conclusion, the addition of antioxidant substances extracted from winemaking by-products can limit the need to add sulfuric anhydride to protect wines during their maturation and ageing.

Correlations of optical spectroscopic techniques with other analysis methods (e.g. classical analyses, etc.)

Correlation of Ultraviolet Absorption Spectroscopy with the concentration of phenolic ingredients

The results of a study on the ability of ultraviolet absorption spectroscopy 220-400nm in predicting the phenolic composition of wine are presented. The Folin-Ciocalteu method was used as a reference method, providing data on the overall phenolic composition of wine. The application of the Folin-Ciocalteu method was carried out by the scientific team of the Hellenic Mediterranean University (HMU). The absorption of wine in the ultraviolet region of the electromagnetic spectrum is directly related to phenolic components. Therefore, a strong correlation of the spectral characteristics of wine in ultraviolet with its phenolic composition is expected.

 

Comparison of the predicted value obtained from ultraviolet spectroscopy with the corresponding value by the Folin-Ciocalteu method

Correlation of Ultraviolet Absorption Spectroscopy with Tannins Concentration

The results of a study on the ability of UV absorption spectroscopy 220-400 nm in predicting the composition of wine in tannins are presented. The Ribereau-Gayon & Stonestreet method was used as a reference method. The application of the method was carried out by the scientific team of the Hellenic Mediterranean University (HMU). The absorption of wine in the ultraviolet region of the electromagnetic spectrum is directly related to tannins. Therefore, a strong correlation of the spectral characteristics of wine in ultraviolet with the concentration of tannins is expected.

 

Comparison of the predicted value obtained from ultraviolet absorption spectroscopy with the corresponding value from the Ribereau-Gayon & Stonestreet method

Correlation of Visible Absorption Spectroscopy with Anthocyanins Concentration

Comparison of the predicted value obtained from visible absorption spectroscopy with the corresponding value from the Somers method

Correlation of optical spectroscopic techniques with wine tasting

Astringency - fluorescence
Acidity - fluorescence

Correlation of optical spectroscopic techniques with wine structure analysis

Anthocyanins -Vis (400 nm - 700 nm)
Phenols - UV (220 nm - 400 nm)
Tannins - UV (220 nm - 400nm)
Total phenolic ingredients - UV (220 nm - 400 nm)
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