Abstract
The management of oxygen during winemaking and aging is a big issue in order to achieve high quality wines. The correct amount of O2 improves aroma, astringency, bitterness and color, however an excess of oxygen promotes the appearance of yellow and brown colors and oxidative off-odors, while its absence leads to the formation of reductive aromas. Even thought our knowledge about the reactions occurring during wine oxidation are very rich and detailed, the scientific data about the wine behaviour under reductive storage is limited. The main objective of this work was to study the metabolomic changes of eight red wines caused by the storage under different oxidative and reductive conditions.
Eight red wines were stored under eight different conditions, which include a) micro-oxygenators at 25 ºC for 3 months; b) anoxic atmosphere at 25 oC for 1, 2 and 3 months; c) anoxic atmosphere at 35 oC for 3 months; and d) control. The following physicochemical analysis were made: LC-MS based metabolomic fingerprint, CIELab color, analyses of volatile sulfur compounds, redox potential, and basic oenological analysis.
Changes of concentration of H2S and methanethiol (higher amount of free forms under reductive conditions) and redox potential results showed the reliability of the sample set. Color of samples evolved in a different way depending on the storage conditions, getting darker the reduced samples. Metabolomic study revealed reactions with SO2 and direct linked tannin-anthocyanin (T-A) adducts were favoured under anoxia but in the presence of oxygen, reactions with acetaldehyde and ethyl-linked T-A and tannin-tannin (adducts) were the favoured. The reaction mechanism of these reactions favoured in absence of oxygen could explain the observed changes during reductive storage.
Ignacio Ontañón1*, Diego Sánchez1, Vania Sáez2, Fulvio Mattivi2,3, Vicente Ferreira1, Panagiotis Arapitsas2
1 Laboratorio de Análisis del Aroma y Enología. Departamento de Química Analítica. Facultad de Ciencias. Instituto Agroalimentario de Aragón –IA2- (Universidad de Zaragoza-CITA). C/ Pedro Cerbuna, 12. 50009. Zaragoza, Spain.
2 Research and Innovation Centre, Food Quality and Nutrition Department, Fondazione Edmund Mach, via E. Mach 1, 38010 San Michele all’Adige, Italy
3 Center Agriculture Food Environment, University of Trento, San Michele all’Adige, Italy
*Corresponding author: ionta@unizar.es
Article extracted from the presentation held during Enoforum Web Conference (23-25 February 2021)
Introduction
The management of oxygen during winemaking and aging is an issue that always worries the enologist. An adequate amount of oxygen has benefits on some sensory properties, color and stability of wines. 1,2 However, finding this optimum amount is not a trivial issue and an incorrect management could have undesirable consequences. An excess of oxygen has effects on the color of wines with the appearance of orange notes and also on some aromatic notes, as the decrease of fruity notes or the apparition of notes to boiled potato or honey. 1,3,4 The complete absence of oxygen is not free of problems and smells described as rotten eggs or putrefaction could appear due to the presence of hydrogen sulfide (H2S), methanethiol (MeSH) or another volatile sulfur compounds (VSCs). 1,5 These compounds are associated with off-odor problem known as reduced character.
The changes produced due to the presence of oxygen have been deeply studied and several mechanisms have been proposed to explain the typical sensory notes associated to the oxidation of wines. 6,7 In the case of reductive aging, the increases of concentration of free VSCs are well known, however good mechanisms to explain these changes have not been proposed yet.
The objective of this work was study changes produced in several red wines stored under different conditions of presence or absence of oxygen. Different parameters (VSCs, color and mDP) were measured and a metabolomic study was carried out to find the causes which explain the changes associated to different storage conditions
Materials and methods
Eight young red wines (five Tempranillo and three Grenache) were stored under different conditions. On one hand, a part of each wine was stored in lab-scale micro-oxygenators at 25ºC for 3 months. Some of these micro-oxygenators were full (FU), another half full (HF) and some of them were quarter full (QF). On the other hand, another part was divided in different vials and they were stored under anoxic conditions. Some of these vials were stored during 1, 2 and 3 months (1M, 2M and 3M) at 25ºC and some of them at 35ºC for 3 months (35). Control samples (CN) were stored under anoxia at 4ºC during 3 months (Figure 1).
After the storage, all samples were analyzed at the same time. VSCs were analysed by gas chromatograph-sulfur chemiluminescence detector (GC-SCD). 8 The CIELAB color parameters were measured according to the OIV-MA-AS2-11. The mean degree tannin polymerization (mDP) was calculated with the protocol described by Gris et al. 9 Samples were analyzed by LC-MS 10 to study the metabolomic changes produced by the different storages.
Results
As it was mentioned in the introduction, the concentration of VSCs increases under anoxia. It is also expected the concentration of their free forms decreases in presence of oxygen and so they are not perceived. Therefore, the measurement of VSCs was used to check the experiment run properly.
The Figure 2 demonstrates the typical behaviour of H2S and MeSH at the tested conditions, by using two examples (one Tempranillo and one Grenache) for each compound. As expected, the concentration of the two VSCs increased when samples were stored in absence of oxygen, with the highest concentration detected in samples stored under anoxia at 35ºC during 3 months. The results for samples stored in presence of oxygen were also normal, since the concentration of VSCs in oxidized samples was lower than in control samples, while H2S was below the limit of detection for all these samples.
The results of the wine color measurements, made according to the CIELAB OIV protocol, are shown in Figure 3. The evolution of lightness (L) was different for samples stored under anoxia than for oxidized samples. In the case of reduced samples, the difference between these samples and control samples was negative, and the lowest values were measured for samples stored at 35ºC. These results indicated that the wines stored under anoxia turn darker. On the opposite, the color of the oxidized samples turn lighter in respect to the control, especially for the Grenache wines.
Moreover, the Figure 3 shows the typical evolution of parameters a* and b* by using as example one of the wines. The oxidized samples had the highest changes, since were less red and more yellow. In addition, the reduced samples at 35ºC were very close to oxidized samples; while the wines stored under anoxia at 25ºC were closer to the control samples. This indicates that reduced samples at room temperature suffered lower changes.
The mean degree of polymerization (mDP) of condensed tannins was also measured after the storage, and the Figure 4 shows the percentage of mDP of each sample in respect to the control sample. A significant decrease was measured in the samples stored in presence of oxygen, however the samples under anoxia only suffered a slight decrease, and the lowest value in absence of oxygen was found in wines stored at 35ºC.
The metabolomic study revealed several reactions which behaviour depended by the presence or absence of oxygen. The formation of hydroxyethylsulfonate, the adduct of SO2 and acetaldehyde, 11 was one of them, and its concentration decreased strongly when wines were stored in presence of oxygen, as expected. However, this was not the only reaction in which SO2 and acetaldehyde played an important role, but both reacted with tannins (T), anthocyanins (A) and other compounds.
The acidic cleavage of tannins during wine aging is well known 1, and in our study we detected a higher concentration of reaction products between tannins and acetaldehyde, such as ethyl linked T-T or ethyl linked T-A, for the wine stored in presence of oxygen. Anthocyanins also reacted with acetaldehyde to produce different pyrano-A and phenyl-pyrano-A. The concentration of some sulfonated indoles increased in presence of oxygen as a consequence of the reaction between SO2 and indoles. The production of some indolic compounds was favoured too.
Since, the production of acetaldehyde under anoxia was limited due to the absence of oxygen; we noticed that tannins rearrangement was favoured at these storage cases, instead of having the production of ethyl bridge tannins. The stability of the mDP values for the anoxia samples, confirmed such a hypothesis.
Apart of the tannin rearrangement, other favoured reactions for the sample stored in anoxia were the formation of the sulfonated flavanols, the direct T-A addition, and the sulfonation of glutathione (Figure 5).
Our hypothesis is that the reactions of sulfonation and the direct T-A addition could be mediated by the presence of copper and iron. These metal cations would be reduced from copper (II) and iron (III) to copper(I) and iron (II). Then, these could react with some disulfides and polysulfanes present in wine and H2S, MeSH and other VSCs would be released and typical reductive notes could be detected.
Conclusions
The comparison between wines stored under anoxia and the same wines stored in presence of oxygen helped us to understand better the reductive storage chemical changes. Briefly, the reductive conditions:
- Favoured the production of volatile sulfur compounds.
- Delivered less light (darker) colored wines.
- Maintained the length of tannins polymerization, through rearrangements.
- Produced more sulfonated tannins
- Yielded more direct linked (and less ethyl linked) T-A pigments.
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