Department of Food, Environmental and Nutritional Sciences (DeFENS), Università degli Studi di Milano, Via G. Celoria 2, Milan, Italy
daniela.fracassetti@unimi.it; antonio.tirelli@unimi.it
Introduction
During the storage of wine, some changes can occur affecting the composition, characteristics and quality of wine. The storage conditions can modulate the reaction kinetics leading the proper and expected evolution of wine. Among the factors affecting these reactions, the influence of temperature is well known to the wine producers and, for these reasons, the wine is usually kept under 15°C during the storage. The stages of commercialization, from the winery to the table, can play an important role on the overall characteristics of wine in case of improper conditions of storage are applied. One of the factors that besides temperature can affect the wine quality is represented by the light. Certain reactions induced by light can be responsible for a detrimental decay of wine. The photo-degradation of organic acids, i.e. tartaric acid, catalysed by iron, produces glyoxylic acid and the following browning as a consequence of xanthylium ion formation (Grant-Preece et al., 2018). Moreover, the presence of riboflavin (RF), a strong photosensitizer compound, can cause the appearance of off-flavours. When RF is exposed to light, it reaches a high energy state and two photo-degradative mechanisms can take place. In Type II pathway, the oxygen can take part in the photo-degradative reactions generating singlet oxygen, an electrophile able to react with alkenes, amines, sulfides (Foote, 1976; DeRosa & Crutchley, 2002). In Type I pathway, RF can be reduced in presence of electron-donor compounds in which methionine (Met) is included (Figure 1). The oxidation of this amino acid leads to methional that is unstable and photo-reactive and, through the retro-Michael reaction, generates methanethiol (MeSH) and acrolein. Two molecules of MeSH yield dimethyl disulfide (DMDS) (Maujean & Seguin, 1983a). The formation of these sulphur compounds is associated to a defect known sunlight flavour or as light-struck taste (LST) (Maujean, Haye, & Feuillat, 1978; Maujean & Seguin, 1983b; Andrés-Lacueva, Mattivi, & Tonon, 1998). Both MeSH and DMDS show low perception thresholds (2-10 μg/L for MeSH and 20–45 μg/L for DMDS) and they confer rotten egg- or cabbage-like odours and cooked-cabbage or onion-like notes, respectively for MeSH and DMDS (Fracassetti and Vigentini, 2018).
Riboflavin occurs in law amount in grape (few tens μg/L of grape juice) (Riberau-Gayon et al., 2006) and it increases during the alcoholic fermentation up to around 150-200 μg/L (Mattivi et al., 2000; Fracassetti et al., 2017) or more (Ournac, 1968). A concentration of RF below 50-80 μg/L can limit the risk of LST appearance (Pichler, 1996; Mattivi et al., 2000; Fracassetti et al., 2019a). Methionine is usually found at 3-4 mg/L (Amerine and Ough, 1980; Riberau-Gayon et al., 2006), a molar concentration 40-folds higher than that one of RF. The appearance of LST is responsible for relevant economical losses and waste of product (Cáceres-Mella et al., 2014). A recent survey regarding 230 Italian wineries showed that the occurrence of this defect weighs on 6.5% of the production of white and rosé wines. As a consequence, oenological strategies are necessary to overcome the formation of this defect. Contemporarily, a further comprehension of the chemical mechanisms behind the LST formation can support specific oenological treatments as well as clarify the reasons why certain white wines can be more susceptible than others (Mattivi et al., 2000).
Oenological strategy (i): choice of fermenting yeast
Ournac (1968) firstly proved the ability of Saccharomyces cerevisiae in releasing RF during the alcoholic fermentation. The author determined RF in wines produced from 4 different grape varieties and fermented with 3 different S. cerevisiae strains; this vitamin was detected in concentration in the range 210-259 μg/L, while a wider range was found when spontaneous fermentations were carried out (110-250 μg/L). The metabolic pathway in S. cerevisiae included RIB5 gene encoding for RF synthase enzyme as last step of RF synthesis (Santos et al., 1995). The yeast-mediated production of RF is affected by both temperature and the presence of other vitamins (Pichler 1996, Paalme et al. 2014). We investigated the RF release from 15 commercial Saccharomyces strains of oenological use at the end of vinification of a Chardonnay must containing negligible amount of RF (5 μg/L). The tested yeast strains showed diverse ability in releasing RF which occurred in the range 29-170 μg/L (Figure 2) and no correlation in RF production was found among S. cerevisiae and S. bayanus strains (Fracassetti et al., 2017).
These data support the influence played by the fermenting yeast used for the alcoholic fermentation on RF content. In particular, three of them produced less about 30 μg/L RF (Figure 2).
Oenological strategy (ii): shading the wine
The physical barrier protecting the wine against the light is represented by the glass. The wavelengths having the major impact on RF photo-degradation range from 370 nm to 440 nm (Grant-Preece et al., 2017). At these wavelengths, the clear glass does not adsorb the light. Increased protection is offered by the antique green glass, while the amber one showed the complete adsorption of light wavelengths with detrimental effect on RF (Figure 3) (Maury et al., 2010; Clark et al., 2011). The addition of dyeing additives into the glass for filtering the damaging wavelengths is the other sustainable strategy to shade the wine.
Certain plastic films are also used for covering the bottles (17% of the wineries interviewed). These protective devices are able to adsorb the light wavelengths causing RF photo-degradation.
Oenological strategy (iii): removal of riboflavin
The three actors of the appearance of LST in white wine are RF, Met and, surely, light. Keeping under control the concentrations of both RF and Met, the occurrence of this fault can be limited. As a consequence, the removal of RF can be a challenge as the aroma characteristics of wine should be preserved. Certain adjuvants show the ability to deplete RF and bentonite was firstly proposed (Pichler, 1996) as this fining agent in already commonly used for the protein stabilization. Based on these promising results, we investigated the effectiveness of some adjuvants, including six types of bentonite, two type of charcoal, zeolite, polyvinylpolypyrrolidone (PVPP), kaolin, egg albumin (at pH 6.7 and 10.4) and a colloidal suspension of pure silica. These adjuvants were firstly tested in model wine and, among them, bentonite, charcoal and zeolite effectively deplete RF in model solution up to 60%, 100% and 50%, respectively. A negligible removal was found with the treatment with PVPP, kaolin, silica and egg albumin (Fracassetti et al., 2017). Due to the promising results obtained with bentonite, charcoal and zeolite, they were also tested in white wine. Their effectiveness decreased in comparison to model wine and the depletion of RF resulted 10% with zeolite (1 g/L), 25% with bentonite (1 g/L) and 60% with charcoal (50 mg/L). The latter was also tested at double concentration (100 mg/L) and RF removal was 70%. Moreover, we did not find an increase in RF depletion by longer treatments with charcoal (up to 24 hours), either in synthetic wine or in white wine (Figure 4).
The effectiveness of charcoal in removing RF proved to be lowered in wine and a suitable dosage should be optimised under winemaking conditions. Moreover, an attentive treatment with charcoal should be done due to its negative effect on depleting aromatic compounds.
Oenological strategy (vi): use of tannins
The use of phenols is an emerging practice to limit the appearance of LST. The antioxidant properties of phenolics are well known and their ability in quenching single oxygen was also reported (Brivida et al., 1993; Lagues et al., 2017). Flavan-3-ols showed to prevent the formation of this fault (Maujean & Seguin, 1983b). Nevertheless, any phenol addition to wine has to be carefully evaluated as these compounds affect the perception of both bitterness and astringency (Robichaud and Noble, 1990). Condensed tannins were previously tested and they resulted effective against LST maybe due to their capacity in acting as a shield (Maujean and Seguin, 1983b). In some preliminary trials we found that hydrolysable tannins were also affective since LST was not perceived. We tested hydrolysable tannins, from chestnut, oak and nut gall, in model wine considering both oxic and anoxic conditions. Interestingly, the hydrolysable tannins exerted a protective effect since both Met lost and the volatiles sulphur compounds formed, namely MeSH, DMDS and dimethyl trisulphide (DMTS), were lower, in particular when the nut gall tannins were added. The protective effect resulted furtherly evident in anoxic condition that favoured the appearance of LST (Fracassetti et al., 2019a). This was confirmed by the sensory analysis: the perception of the “cooked cabbage” descriptor was lower under oxic condition as well as in the samples added with hydrolysable tannins (Figure 5)
Thanks to the positive influence of hydrolysable tannins, their use represents a novel and sustainable approach to prevent the detrimental effect deriving from the photo-degradation of RF and Met. Further investigations will evaluate the effectiveness of other gallic acid-rich tannins in preventing the LST in bottled white wine.
Evolution of light-struck taste during storage
The detrimental consequence of LST appearance has been well demonstrated and consolidated (Maujean and Seguin, 1983a,b; Mattivi et al., 2000; Fracassetti et al., 2019a). However, some questions come up: does LST persist unaltered over time? Can the light exposure lead to stable compounds over the storage in the dark? Based on these questions, we evaluated the evolution of LST in model wine and young white wine, both containing RF and Met, exposed to light and stored in the dark for 24 months. The possible protective effect against the LST of certain antioxidants, including sulphur dioxide, glutathione and chestnut tannins was assessed, by adding them either individually or in different combinations.
In model wine, the combined addition of the three antioxidants was the most effective since negligible amounts of MeSH and DMTS were detected, whereas DMDS was not found. The addition of sulphur dioxide limits the formation of DMTS whose role remains to be clarified. Considering the three antioxidants, the order of effectiveness in preventing the formation of LST was sulphur dioxide> chestnut tannin> glutathione (Fracassetti et al., 2019c). In young white wine, variable amounts of MeSH and DMDS were present, depending on the added antioxidant. Contrary to what was observed in the model wine, DMTS was not detected. In presence of chestnut tannin and glutathione, alone or added in combination with sulphur dioxide, MeSH and DMDS were not detected or they were found in concentrations below their perception threshold. Considering the three antioxidants added individually, the order of effectiveness in preventing the formation of the LST was chestnut tannin > glutathione> sulfur dioxide (Fracassetti et al., 2019b). The formation of sotolon was also considered as marker of oxidative reactions leading to the atypical aging of white wine (Lavigne et al., 2008). In a previous study, this compound was revealed in wine samples added with phenol-based preparations (Fracassetti et al., 2016). Low concentration values of sotolon were found in young white wine (< 0.03 µg/L – 3.94±0.74 µg/L), about a half of its perception threshold which was reported to be 7-8 µg/L in white wine (Guichard et al., 1993).
The evolution of LST during storage is an important aspect that allows a better understanding of post-bottling changes. The addition of antioxidants has shown to have a protective effect. In particular, the hydrolysable tannins resulted affective against the formation of LST in white wine. The use of hydrolysable tannins before bottling could effectively prevent this wine fault. At the same time, the oxidative phenomena responsible for sensory alterations have not been observed. Further investigations will be carried out under real production conditions.
Oenological tannins: how they can prevent the photo-degradation?
Maujean and Seguin (1983b) suggested that the ability in counteracting LST was due to the shielding property of condensed tannins. In our study, negligible variation of absorbance value was found (< 0.02 AU) and consequently, the light shielding property could be excluded (Fracassetti et al., 2019a). We suggested that the protective effect of hydrolysable tannins might be related to the ability of hydrolysable tannins in quenching the singlet oxygen originating from Type II pathway. Moreover, it was proposed that the limited degradation of Met in presence of tannins might be due to their competition with Met in Type I pathway (Figure 1) (Fracassetti et al., 2019a). In order to better understand the role of hydrolysable tannins on photo-degradative reactions, an approach based on Nuclear Magnetic Resonance (NMR) technique was applied.
Model solution containing RF (0.2 mM) and Met (2 mM) was adjusted at pH 3±0.1 with nitric acid 0.01 N and then exposed to light in a NMR tube up to 8 cycles of irradiation-acquisition for a total of 600 seconds under light, in the absence and in the presence of gallic acid (2 mM). NMR spectra were acquired immediately after the light exposure. The assays were carried out in air and nitrogen conditions. The latter was obtained by sparging nitrogen for 15 minutes in both stock solutions and NMR tube.
Gallic acid was chosen as model phenolic because it is the constitutive unit of gall nut tannins (Obreque-Slíer et al., 2009; Vignault et al., 2018). By comparing the NMR spectra obtained for the sample maintained in the dark with those after light exposure (Figure 6), whether in the absence (Figure 6B) or in the presence (Figure 6C) of gallic acid, a new signal was observed at 2.64 ppm. This signal was assigned to the S(O)CH3 moiety of methionine sulfoxide. The latter was described among the compounds from the photo-induced oxidation of Met in presence of a photosensitizer compound (Barata-Vallejo et al., 2010). The kinetic rate constant of Met sulfoxide formation was determined and it resulted in a 2-fold decrease in the presence of gallic acid (Fracassetti et al., 2020). The ability of gallic acid to quench singlet oxygen was evidenced since the formation of Met sulfoxide was slowed down.
Similarly, methionine sulfoxide was found in nitrogen condition, even if to a lesser extent as lower responses were found. In particular, when gallic acid was added, negligible amounts of methionine sulfoxide were detected. Moreover, the signal assigned to RF was not revealed anymore in the absence of gallic acid. Considering the formation kinetic of methionine sulfoxide, it resulted 10-folds slower in nitrogen condition in comparison to air condition (Fracassetti et al., 2020).
The NMR approach developed allows to elucidate the photo-degradation mechanisms in an aqueous environment in a short time. Moreover, NMR resulted a suitable technique, using Met sulfoxide as a marker compound to investigate the photo-induced oxidation mechanisms occurring between RF and Met. Gallic acid acts as photo-protectors since it was also able to inhibit RF photo-degradation. Gallic acid can also quench the singlet oxygen deriving from the passage from singlet RF to triplet RF (Figure 1).
Influence of other wine components: the case of transition metals
Wine is a complex matrix where several reactions occur. Besides the photo-degradation of RF, other light-induced reactions can take place. This is the case of the photo-degradation of tartaric acid catalysed by iron, known as photo-Fenton process (Grant-Preece et al., 2017). This reaction causes the formation of glyoxal and, consequently, xanthylium ions (Figure 6) which are involved into the browning phenomena of white wine (Li et al., 2008). The Fenton reaction can involve other organic acids (i.e. dihydroxyfumaric acid) able to react with wine phenolics (i.e. catechin) further favouring the formation of xanthylium derivatives. The levels of the latter pigments can increase if copper is also present (Clark et al., 2008). Copper is also an oxidation catalyst affecting, with iron, the general oxidative status of wine.
To the best of our knowledge, no study was carried out to understand the impact of iron and copper on photo-degradation of RF and Met. We evaluated their effect by a Response Surface Methodology (SRM) approach considering the influence of oxygen as well (Mastro, 2020).
Fifteen runs were prepared in which different concentrations of iron (0 mg/L, 5 mg/L and 10 mg/L), copper (0 mg/L, 0.25 mg/L and 0.5 mg/L) and oxygen (0 mg/L, 3 mg/L and 8 mg/L) were present following the Box-Behnken experimental plan. The study was done in model solution as well as in presence of catechin (100 mg/L) and caffeic acid (70 mg/L), used as model phenolics.
It is interesting to note that Met loss was higher in absence of phenols (average: -29%) and among the two ones considered, caffeic acid (average: -4%) resulted more effective against the degradation of methionine in comparison to catechin (average: -9%). Methionine sulfoxide and methionine sulfone were also determined as they are two of the known compounds deriving from the oxidation of Met (Barata-Vallejo et al., 2010). Their concentrations were negligible (methionine sulfoxide < 0.5 mg/L; methionine sulfone < 0.05 mg/L) supporting the idea that other unknown compounds originate from the oxidation of Met (Barata-Vallejo et al., 2010). Similarly, from data of total sulphur compounds (sum of MeSH, DMDS and DMTS formed) appeared that phenolics limited their formation with average lower values in presence of caffeic acid (min: 0.06 µg/L; max 3.65 µg/L). The sulphur compounds were higher in presence of catechin (min: 0.00 µg/L; max 17.65 µg/L) and relevant concentrations were detected in model solution (min: 5.35 µg/L; max 150.25 µg/L). These differences evidenced the protective effect of phenols in the experimental conditions applied. In general, the presence of copper did not seem to prevent the formation of sulphur compounds, especially in case of DMTS. The addition of iron affected the content of sulphur compounds as well. This is probably due to the Strecker degradation that can be catalysed by both transition metals (Yaylayan, 2003) and the formation of methional can be enhanced. The statistical analysis evidenced that the formation of sulphur compounds was significantly affected only by oxygen without phenolics, while the transition metals can significantly affect the formation of sulphur compounds with phenolics. In the presence of both catechin and caffeic acid, copper was negatively correlated with sulphur compounds, while iron was positively correlated when caffeic acid was present. Nevertheless, the transition metals played a role in the perception of LST. The copper resulted positively correlated with the sensory data in presence of catechin, maybe because higher levels of polysulfide compounds were formed. On the contrary, in model wine and in the presence of caffeic acid the interaction copper*oxygen was positively correlated and the interaction iron*oxygen was negatively correlated (Mastro, 2020).
The results suggest that the oxygen concentration at bottling can play an important role on the appearance of LST. Iron and copper can favour the formation of sulphur compounds as they catalyse the Strecker degradation. The presence of phenolic acids could protect the wine against the appearance of LST maybe as a consequence of their reaction with singlet oxygen and sulphur compounds. Flavan-3-ols in anoxic condition and in presence of both transition metals could prevent LST.
Conclusions and future perspectives
The studies carried out on LST have clarified and deepened the photo-degradation mechanisms and suitable, novel and sustainable approaches have been applied to counteract the appearance of LST throughout the entire winemaking process, from the fermentation step up to commercialization. Further researches will be related to the formation of LST in white wine and we are looking for chemical, oenological and microbiological strategies allowing to prevent the formation of LST in still and sparkling wines thanks to the Enofotoshield project that has been recently funded by European Agricultural Fund for Rural Development (EAFRD).
Acknowledgments
The studies have been supported by Piano di Sostegno alla Ricerca – Linea 2, Università degli Studi di Milano, and European Agricultural Fund for Rural Development (Enofotoshield project; D.d.s. 1 luglio 2019 – n. 9551, B.U.R.L. Serie Ordinaria n. 27 – 04 luglio 2019). The research team working on this topic includes Prof. Sara Limbo, Prof. Luisa Pellegrino, Prof. Ileana Vigentini, Prof. Roberto Foschino and Prof. Enzio Ragg (DeFENS, Università degli Studi di Milano), Prof. Davide Ballabio (Department of Earth and Environmental Sciences, Università di Milano-Bicocca), Dr. Alessandra Di Canito (post-doc) and Dr. Rebecca Bodon (research fellow). We are grateful with Dr. Maria Manara (DalCin) for providing yeast and hydrolysable tannin samples and the students for their technical support (Jordi Encinas, Eleonora Piredda, Andrea Baratti, Melissa Mastro and Natalia Palmowska).
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